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Farmers have been disrupting the carbon cycle ever since humans started converting forests to agricultural lands. But are there farming practices that can be applied to increase the carbon storage in the soil and subsequently counteract increasing carbon dioxide levels in the atmosphere? In this study I investigate if soil organic matter (SOM) and soil organic carbon (SOC) change with longer interruption between tillage events. The study was conducted by studying SOM concentrations and SOC pools in eight fields with different time since tillage (1 to 14 years). I found that SOM concentrations increased in the O horizon of the studied soil in response to increased time since tillage. Here, SOM concentrations were on average around 13 % one year after tillage, while fourteen-year-old farmland had a concentration around 15 %. In similar, SOC pool increased from around 0.1 kg C m-2 in the O horizon of 1 year old soil to 0.33 kg C m-2 14 years after tillage. While both SOM concentrations and SOC pools increased in the O horizon over time since tillage, the SOM concentration and SOC pools decreased in the subsoil. I found no net sequestering of SOC in response to less frequent tillage in comparison to more frequency tillage. My conclusion is that limiting tillage to 14-year cycles is not enough to increase carbon sequestration. 

Soil quality standards are based on partitioning and toxicity data for laboratory-spiked reference soils, instead of real world, historically contaminated soils, which would be more representative. Here 21 diverse historically contaminated soils from Sweden, Belgium, and France were obtained, and the soil-porewater partitioning along with the bioaccumulation in exposed worms (Enchytraeus crypticus) of native polycyclic aromatic compounds (PACs) were quantified. The native PACs investigated were polycyclic aromatic hydrocarbons (PAHs) and, for the first time to be included in such a study, oxygenated-PAHs (oxy-PAHs) and nitrogen containing heterocyclic PACs (N-PACs). The passive sampler polyoxymethylene (POM) was used to measure the equilibrium freely dissolved porewater concentration, C-pw, of all PACs. The obtained organic carbon normalized partitioning coefficients, K-TOC, show that sorption of these native PACs is much stronger than observed in laboratory-spiked soils (typically by factors 10 to 100), which has been reported previously for PAHs but here for the first time for oxy-PAHs and N-PACs. A recently developed K-TOC model for historically contaminated sediments predicted the 597 unique, native K-TOC values in this study within a factor 30 for 100% of the data and a factor 3 for 58% of the data, without calibration. This model assumes that TOC in pyrogenic-impacted areas sorbs similarly to coal tar, rather than octanol as typically assumed. Black carbon (BC) inclusive partitioning models exhibited substantially poorer performance. Regarding bioaccumulation, C-pw combined with liposome-water partition coefficients corresponded better with measured worm lipid concentrations, C-lipid (within a factor 10 for 85% of all PACs and soils), than C-pw combined with octanol-water partition coefficients (within a factor 10 for 76% of all PACs and soils). E. crypticus mortality and reproducibility were also quantified. No enhanced mortality was observed in the 21 historically contaminated soils despite expectations from PAH spiked reference soils. Worm reproducibility weakly correlated to C-lipid of PACs, though the contributing influence of metal concentrations and soil texture could not be taken into account. The good agreement of POM-derived C-pw with independent soil and lipid partitioning models further supports that soil risk assessments would improve by accounting for bioavailability. Strategies for including bioavailability in soil risk assessment are presented.

Boreal forests are often strongly nitrogen (N) limited. However, human activities are leading to increased N inputs into these ecosystems, through atmospheric N deposition and forest fertilization. N input into boreal forests can promote net primary productivity, increase herbivore and pathogen damage, and shift plant species composition and community structure. Genetic diversity has been suggested as a key mechanism to promote a plant species’ stability within communities in response to environmental change. Within any plant population, specific traits (e.g. growth and defense traits) can vary substantially among individuals, and a greater variation in traits may increase chances for the persistence of at least some individuals of a population, when environmental conditions change. One aspect of plant chemistry that can greatly vary among different genotypes (GTs) are condensed tannin (CTs). These secondary metabolites have been suggested to affect plant performance in many ways, e.g. through influencing plant growth, the interactions of plants with herbivores and pathogens, and through affecting litter decomposition, and hence the return of nutrients to plants. To investigate how genotypic variation in foliar CT production may mediate the effects that anthropogenic N enrichment can have on plant performance and litter decomposition, I performed a series of experiments. For these experiments, aspen (Populus tremula) GTs with contrasting abilities to produce foliar CTs (i.e. low- vs. high-tannin producers) were grown under 3 N conditions, representing ambient N (+0 kg ha^-1), upper level atmospheric N deposition (+15 kg ha^-1), and forest fertilization rates (+150 kg ha^-1). This general experimental set-up was once established in a field-like environment, from which natural enemies were excluded, and once in a field, in which enemies were present. In my first two studies, I investigated tissue chemistry and plant performance in both environments. I observed that foliar CT levels decreased in response to N in the enemy‑free environment (study I), but increased with added N when enemies were present (study II). These opposing responses to N may be explained by differences in soil N availability in the two environments, or by induction of CTs after enemy attack. Enemy damage generally increased in response to N, and was higher in low-tannin than in high-tannin plants across all N levels. Plant growth of high‑tannin plants was restricted under ambient and low N conditions, probably due to a trade-off between growth and defense. This growth constraint for high‑tannin plants was weakened, when high amounts of N were added (study I and II), and when enemy levels were sufficiently high, so that benefits gained through defense could outweigh the costs of defense production (study II). Despite those general responses of low- and high‑tannin producers to added N, I also observed a number of individual responses of GTs to N addition, which in some case were not connected to the intrinsic ability of the GTs to produce foliar CTs. In study III, gene expression levels in young leaves and phenolic pools of the plants that were grown in the enemy‑free environment were studied. This study revealed that gene control over the regulation of the phenylpropanoid pathway (PPP) was distributed across the entire pathway. Moreover, PPP gene expression was higher in high-tannin GTs than in low‑tannin GTs, particularly under ambient N. At the low N level, gene expressions declined for both low- and high-tannin producers, whereas at the high N level expression at the beginning and the end of the PPP was upregulated and difference between tannin groups disappeared. Furthermore, this study showed that phenolic pools were frequently uncorrelated, and that phenolic pools were only to some extent related to tannin production and gene expression. In study IV, I investigated the decomposability of litter from the field plants. I found that N enrichment generally decreased mass loss, but there was substantial genetic variation in decomposition rates, and GTs were differentially responsive to added N. Study IV further showed that CTs only had a weak effect on decomposition, and other traits, such as specific leaf area and the lignin:N ratio, could better explain genotypic difference in mass loss. Furthermore, N addition caused a shift in which traits most strongly influenced decomposition rates. Collectively, the result of these studies highlight the importance of genetic diversity to promote the stability of species in environments that experience anthropogenic change.

In boreal ecosystems plant production is often limited by low availability of nitrogen. Nitrogen retention in below-ground organic pools plays an important role in restricting recirculation to plants and thereby hampers forest production. Saprotrophic fungi are commonly assigned to different decomposer strategies, but how these relate to nitrogen cycling remains to be understood. Decomposition of Scots pine needle litter was studied in axenic microcosms with the ligninolytic litter decomposing basidiomycete Gymnopus androsaceus or the stress tolerant ascomycete Chalara longipes. Changes in chemical composition were followed by C-13 CP/MAS NMR spectroscopy and nitrogen dynamics was assessed by the addition of a N-15 tracer. Decomposition by C. longipes resulted in nitrogen retention in non-hydrolysable organic matter, enriched in aromatic and alkylic compounds, whereas the ligninolytic G. androsaceus was able to access this pool, counteracting nitrogen retention. Our observations suggest that differences in decomposing strategies between fungal species play an important role in regulating nitrogen retention and release during litter decomposition, implying that fungal community composition may impact nitrogen cycling at the ecosystem level.

1. Earlier snowmelt at high latitudes advances above-ground plant phenology, thereby affecting water, nutrient and carbon cycles. Despite the key role of fine roots in these ecosystem processes, phenological responses to earlier snowmelt have never been assessed below-ground. 2. We experimentally advanced snowmelt in two contrasting plant community types (heath and meadow) in northern Sweden and measured above- and below-ground phenology (leaf-out, flowering and fine root growth). We expected earlier snowmelt to advance both above- and below-ground phenology, and shrub-dominated heath to be more responsive than meadow. 3. Snow melted on average 9 days earlier in the manipulated plots than in controls, and soil temperatures were on average 0.9 degrees C higher during the snowmelt period of 3 weeks. This resulted in small advances in above-ground phenology, but contrary to our expectations, root phenology was unresponsive, with root growth generally starting before leaf-out. These responses to the snowmelt treatment were similar in both plant community types, despite strong differences in dominating plant functional types and root properties, such as root length and turnover. 4. The lack of a response in root phenology, despite warmer soil temperatures and above-ground phenological advances, adds evidence that above-ground plant responses might not be directly translated to below-ground plant responses, and that our understanding of factors driving below-ground phenology is still limited, although of major importance for water, nutrient and carbon cycling.

Arctic soils play an important role in Earth's climate system, as they store large amounts of carbon that, if released, could strongly increase greenhouse gas levels in our atmosphere. Most research to date has focused on how the turnover of organic matter in these soils is regulated by abiotic factors, and few studies have considered the potential role of biotic regulation. However, arctic soils are currently missing important groups of soil organisms, and here, we highlight recent empirical evidence that soil organisms' presence or absence is key to understanding and predicting future climate feedbacks from arctic soils. We propose that the arrival of soil organisms into arctic soils may introduce "novel functions", resulting in increased rates of, for example, nitrification, methanogenesis, litter fragmentation, or bioturbation, and thereby alleviate functional limitations of the current community. This alleviation can greatly enhance decomposition rates, in parity with effects predicted due to increasing temperatures. We base this argument on a series of emerging experimental evidence suggesting that the dispersal of until-then absent micro-, meso-, and macroorganisms (i.e. from bacteria to earthworms) into new regions and newly thawed soil layers can drastically affect soil functioning. These new observations make us question the current view that neglects organism-driven "alleviation effects"when predicting future feedbacks between arctic ecosystems and our planet's climate. We therefore advocate for an updated framework in which soil biota and the functions by which they influence ecosystem processes become essential when predicting the fate of soil functions in warming arctic ecosystems.

Climate change is affecting the species composition and functioning of Arctic and sub-Arctic plant and soil communities. Here we studied patterns in soil microarthropod (springtails and mites) communities across a gradient of increasing elevation that spanned 450 m, across which mean temperature declined by approximately 2.5 degrees C, in sub-Arctic Sweden. Across this gradient we characterized microarthropod communities in each of two types of vegetation, i.e., heath and meadow, to determine whether their responses to declining temperature differed with vegetation type. Mite abundance declined with increasing elevation, while springtail abundance showed the opposite response. Springtail communities were dominated by larger species at higher elevation. Mite abundance was unaffected by vegetation type, while springtail abundance was 53% higher in the heath than meadow vegetation across the gradient. Springtails but not mites responded differently to elevation in heath and meadow vegetation; hemi-edaphic species dominated in the heath at higher elevation while epiedaphic species dominated in the meadow. Our results suggest that sub-Arctic mite and springtail communities will likely respond in contrasting ways to changes in vegetation and soil properties resulting from climate warming.

Enrichment of forest soils with inorganic nitrogen (N) tends to inhibit oxidative enzyme expression by microbes and reduces plant litter and soil organic matter decomposition rates. Without further explanation than is currently presented in the scientific literature, we argue that upregulation of oxidative enzymes seems a more competitive response to prolonged N enrichment at high rates than the observed downregulation. Thus, as it stands, observed responses are inconsistent with predicted responses. In this article, we present a hypothesis that resolves this conflict. We suggest that high rates of N addition alter the competitive balance between enzymatic lignin mineralisation and non-enzymatic lignin oxidation. Using metatransciptomics and chemical assays to examine boreal forest soils, we found that N addition suppressed peroxidase activity, but not iron reduction activity (involved in non-enzymatic lignin oxidation). Our hypothesis seems positioned as a parsimonious and empirically consistent working model that warrants further testing.

Reliable manipulation of soil organic matter (SOM) – a necessity for optimal land management – is constrained by our limited mechanistic understanding of SOM formation. Here we propose a novel mechanistic element that may contribute to SOM dynamics, supplementing existing frameworks, based on evolutionary-ecological rather than chemical or physical limitations to decomposition. We argue that decomposition of some substrates may be constrained by spatial competition from opportunists. We describe and test a mathematical model based on our framework, providing a proof-of-concept that substrate can, in principle, be spared decomposition and accumulate even when it is physically and chemically accessible. Our framework can help explain a variety of SOM dynamics, including priming and the suppression of decomposition by nitrogen addition, as well as the typical composition of SOM. An augmented mechanistic framework for understanding SOM dynamics can help guide targeted empirical study, which in turn can contribute to more optimised land management.

This document lays out a strategy for the development of SEAD – A Strategic Environmental Archaeology Database, which will facilitate the digitisation and accessibility augmentation of MAL’s existing data from nearly thirty years of work in the fields of archaeology and environmental science. SEAD will also provide a framework for the entry of data from all future research and consultancy work at MAL, and allow guest researchers and external partners to contribute to, and work with the same data. The planned system will be implemented at both local and internet levels, and be designed with an aim towards broadening its scope with external partners in the future. SEAD will be made available online in order to increase the ease of access to environmental archaeology data and encourage an expansion of both the discipline and Sweden’s role in it. This is inline with current EU strategies on enhancing research infrastructure, and providing a greater insight into human-environment interactions for long term planning.

Global vegetation regimes vary in belowground carbon (C) and nitrogen (N) dynamics. However, disentangling large-scale climatic controls from the effects of intrinsic plant–soil–microbial feedbacks on belowground processes is challenging. In local gradients with similar pedo-climatic conditions, effects of plant–microbial feedbacks may be isolated from large-scale drivers. Across a subarctic–alpine mosaic of historic grazing fields and surrounding heath and birch forest, we evaluated whether vegetation-specific plant–microbial feedbacks involved contrasting N cycling characteristics and C and N stocks in the organic topsoil. We sequenced soil fungi, quantified functional genes within the inorganic N cycle, and measured 15N natural abundance. In grassland soils, large N stocks and low C : N ratios associated with fungal saprotrophs, archaeal ammonia oxidizers, and bacteria capable of respiratory ammonification, indicating maintained inorganic N cycling a century after abandoned reindeer grazing. Toward forest and heath, increasing abundance of mycorrhizal fungi co-occurred with transition to organic N cycling. However, ectomycorrhizal fungal decomposers correlated with small soil N and C stocks in forest, while root-associated ascomycetes associated with small N but large C stocks in heath, uncoupling C and N storage across vegetation types. We propose that contrasting, positive plant–microbial feedbacks stabilize vegetation trajectories, resulting in diverging soil C : N ratios at the landscape scale.

As global climate change advances, shifts in winter precipitation are becoming more common in high latitude ecosystems, resulting in less insulating snow cover and deeper soil frost. Long-term alterations to soil frost can impact on ecosystem processes such as decomposition, microbial activity and vegetation dynamics. In this study we utilized the longest running, well-characterized soil frost manipulation experiment in a boreal forest. We measured nematode family composition and feeding group abundances at four different soil layer depths from plots that had been subjected to deep soil frost for one and 11 years. The overall abundance of nematodes and the different feeding groups were unaffected by deep soil frost. However, a higher Maturity Index was weakly associated with deep soil frost (indicative of lower nutrient enrichment and more persister nematode (i.e., K-strategist) families), likely due to the loss of nutrients and reduced inputs from inhibited decomposition. Multivariate and regression analyses showed that most nematode families were weakly associated with dominant understory plant species and strongly associated with soil organic matter (SOM). This is probably the result of higher resource availability in the control plots, which is favorable to the nematode community. These results indicate that the nematode community was more strongly driven by the long-term indirect effects of deep soil frost on SOM as opposed to the direct effects. Our findings highlight that the indirect effects of altered winter precipitation and soil frost patterns may be more important than direct winter climate effects. Further, such indirect effects on SOM and the plant community that may affect the nematode community can only be seen in long-term experiments. Finally, given the critical role nematodes play in soil food webs and carbon and nutrient cycling, our results demonstrate the necessity of considering the response of nematodes to global climate change in boreal forest soils. 

Approximately 40% of earth's carbon (C) stored in land vegetation and soil is within the boreal region. This large C pool is subjected to substantial removals and transformations during periodic wildfire. Fire-altered C, commonly known as pyrogenic carbon (PyC), plays a significant role in forest ecosystem functioning and composes a considerable fraction of C transport to limnic and oceanic sediments. While PyC stores are beginning to be quantified globally, knowledge is lacking regarding the drivers of their production and transport across ecosystems. This study used the chemo-thermal oxidation at 375°C (CTO-375) method to isolate a particularly refractory subset of PyC compounds, here called black carbon (BC), finding an average increase of 11.6 g BC m⁻² at 1 year postfire in 50 separate wildfires occurring in Sweden during 2018. These increases could not be linked to proposed drivers, however BC storage in 50 additional nearby unburnt soils related strongly to soil mass while its proportion of the larger C pool related negatively to soil C:N. Fire approximately doubled BC stocks in the mineral layer but had no significant effect on BC in the organic layer where it was likely produced. Suppressed decomposition rates and low heating during fire in mineral subsoil relative to upper layers suggests potential removals of the doubled mineral layer BC are more likely transported out of the soil system than degraded in situ. Therefore, mineral soils are suggested to be an important storage pool for BC that can buffer short-term (production in fire) and long-term (cross-ecosystem transport) BC cycling.

Temperature profoundly affects saprotrophic respiration rates, and carbon quality theory predicts that the rates' temperature Sensitivity should increase as the quality of the carbon source declines. However, reported relationships between saprotrophic respiration responses to temperature and carbon quality vary widely. Some of this variability may arise from confounding effects related to both substrate quality and substrate availability. The importance of these variables, as well as substrate diffusion and uptake rates, for the temperature sensitivity of saprotrophic respiration has been validated theoretically, but not empirically demonstrated. Thus, we tested effects of varying substrate uptake rates on the temperature sensitivity of organic carbon degradation. For this purpose we created a model system using the organic layer (O-horizon), of a boreal forest soil, specifically to test effects of varying monomer uptake and release rates. The addition of both monomers and polymers generally increased the temperature sensitivity of saprotrophic respiration. In response to added monomers, there was a linear increase in the temperature sensitivity of both substrate-induced respiration and the specific growth rate with increasing rate of substrate uptake as indicated by the CO2 production at 14 degrees C. Both of these responses diverge from those predicted by the carbon quality theory, but they provide the first empirical evidence consistent with model predictions demonstrating increased temperature sensitivity with increased uptake rate of carbon monomers over the cell membrane. These results may explain why organic material of higher carbon quality induces higher temperature responses than lower carbon quality compounds, without contradicting carbon quality theory. 

Phosphorus is the main limiting nutrient for plant growth in large areas of the world and the availability of phosphorus to plants and microbes can be strongly affected by soil properties. Even though the phosphorus cycle has been studied extensively, much remains unknown about the key processes governing phosphorus availability in different environments.

In this thesis the complex dynamics of soil phosphorus and its availability were studied by relating various phosphorus fractions and soil characteristics to microbial respiration kinetics. The soils used represent a range of aluminium, iron, carbon and total phosphorus content, and were located in four different biomes: subtropical forest, warm temperate forest, boreal forest and tundra.

The results showed that NaOH extractable phosphorus, a fraction previously considered to be available to plants only over long time scales, can be accessed by microbes in days or weeks. Microbial phosphorus availability was not related to aluminium or iron content in any of the studied systems, not even in highly weathered soils with high aluminium and iron content. This is in contrast with other studies of soils with high sorption capacity and shows the variability of factors that govern phosphorus availability in different environments.

In the boreal forest chronosequence, no difference could be seen with age in total phosphorus content or concentrations of occluded phosphorus forms. However, there were lower concentrations of labile phosphorus forms in older systems, which were correlated with a decrease in microbial respiration. This was most likely related to organic matter quality in the system, and not to geochemical factors.

Phosphorus availability was linked to differences in topography (water regime) and vegetation in the tundra ecosystems. The results suggest that the availability of phosphorus, both for microbes and plants, was lower on the meadow vegetation sites compared to the two types of heath vegetation.

Many factors are important for phosphorus availability in soils, but these results suggest that microbes can access less available phosphorus if not restricted by carbon, and this may be important in regard to forest management practices as well as effects of environmental change.

• Below and aboveground vegetation dynamics are crucial in understanding how climate warming may affect terrestrial ecosystem carbon cycling. In contrast to aboveground biomass, the response of belowground biomass to long-term warming has been poorly studied.
• Here, we characterized the impacts of decadal geothermal warming at two levels (on average +3.3°C and +7.9°C) on below and aboveground plant biomass stocks and production in a subarctic grassland.
• Soil warming did not change standing root biomass and even decreased fine root production and reduced aboveground biomass and production. Decadal soil warming also did not significantly alter the root–shoot ratio. The linear stepwise regression model suggested that following 10 yr of soil warming, temperature was no longer the direct driver of these responses, but losses of soil N were. Soil N losses, due to warming-induced decreases in organic matter and water retention capacity, were identified as key driver of the decreased above and belowground production. The reduction in fine root production was accompanied by thinner roots with increased specific root area.
• These results indicate that after a decade of soil warming, plant productivity in the studied subarctic grassland was affected by soil warming mainly by the reduction in soil N.

Aluminium and Fe fractions were obtained in samples from two temperate podzols by selective extraction with NaOH (Al-n, Fe-n), Na-dithionite-citrate (Al-d, Fe-d), acid NH4-oxalate (Al-o, Fe-o) and Na-pyrophosphate (Al-p, Fe-p) following the traditional fractionation procedures, and also by the use of the chlorides of K (Al-K), La (Al-La) and Cu (Al-cu) as non-buffered extractants for Al. Carbon content was also determined in the Na-pyrophosphate extract (C-p). Soil sampling was done at high-resolution to allow a more detailed characterization of the vertical processes than the traditional sampling by whole soil horizons. Results showed that Al-p and Fe-p make a large proportion of the Alo and Fe-o meaning that organoaluminic complexes dominated in the "active" metal pool instead of inorganic compounds. The degree of metal saturation of soil organic matter (estimated by the (Al-p Fe-p)/C-p molar ratios) increases with depth, especially in the uppermost samples of spodic horizons (Bhs1) where it increases up to 0.1. Aluminium dominates in the adsorption positions of the organic matter in the spodic horizon (Fe-p/Al-p ratios <0.5), except in the Bhs1 horizon (ratios > 1), indicating that the immobilization of Fe containing complexes occurs 10-15 cm above that of Al The highly stable Al-OM complexes accounted on average for 60% of the organoaluminic associations (>70% in the Bhs horizons). The moderately stable complexes predominate in A horizons (57-77% in ACB1 and 37-48% in ACB2) and the largest proportions of low stability complexes were found in the uppermost samples of the spodic horizons (Bhs1) of both soils (9-21%), together with the highest Fep contents and a decrease in pH values. From a stepwise multiple regression model it is suggested that pH is the main variable accounting for the stability of Al-OM compounds together with C and organically bound Fe contents. It is suggested that the illuviation of unsaturated organic acids lower the pH in upper spodic horizons, leading to the complexation of metals from formerly precipitated organometallic complexes and/or leading to their redissolution, enabling their migration to deeper soil layers. Iron complexes would be less soluble at soil pH, resulting in a differentiation of an upper Fe-rich Bhs1 horizon and a lower Bhs2 Al-rich horizon. The depth variation in C accumulation was found to be related to the proportion of highly stable Al-OM fraction.

Tundra ecosystems hold large stocks of soil organic matter (SOM), likely due to low temperatures limiting rates of microbial SOM decomposition more than those of SOM accumulation from plant primary productivity and microbial necromass inputs. Here we test the hypotheses that distinct tundra vegetation types and their carbon supply to characteristic rhizosphere microbes determine SOM cycling independent of temperature. In the subarctic Scandes, we used a three-way factorial design with paired heath and meadow vegetation at each of two elevations, and with each combination of vegetation type and elevation subjected during one growing season to either ambient light (i.e., ambient plant productivity), or 95% shading (i.e., reduced plant productivity). We assessed potential above- and belowground ecosystem linkages by uni- and multivariate analyses of variance, and structural equation modelling. We observed direct coupling between tundra vegetation type and microbial community composition and function, which underpinned the ecosystem's potential for SOM storage. Greater primary productivity at low elevation and ambient light supported higher microbial biomass and nitrogen immobilisation, with lower microbial mass-specific enzymatic activity and SOM humification. Congruently, larger SOM at lower elevation and in heath sustained fungal-dominated microbial communities, which were less substrate-limited, and invested less into enzymatic SOM mineralisation, owing to a greater carbon-use efficiency (CUE). Our results highlight the importance of tundra plant community characteristics (i.e., productivity and vegetation type), via their effects on soil microbial community size, structure and physiology, as essential drivers of SOM turnover. The here documented concerted patterns in above- and belowground ecosystem functioning is strongly supportive of using plant community characteristics as surrogates for assessing tundra carbon storage potential and its evolution under climate and vegetation changes.

Background: The dynamics of phosphorus (P) in the environment is important for regulating nutrient cycles in natural and managed ecosystems and an integral part in assessing biological resilience against environmental change. Organic P (P-o) compounds play key roles in biological and ecosystems function in the terrestrial environment being critical to cell function, growth and reproduction.

Scope: We asked a group of experts to consider the global issues associated with P-o in the terrestrial environment, methodological strengths and weaknesses, benefits to be gained from understanding the P-o cycle, and to set priorities for P-o research.

Conclusions: We identified seven key opportunities for P-o research including: the need for integrated, quality controlled and functionally based methodologies; assessment of stoichiometry with other elements in organic matter; understanding the dynamics of P-o in natural and managed systems; the role of microorganisms in controlling P-o cycles; the implications of nanoparticles in the environment and the need for better modelling and communication of the research. Each priority is discussed and a statement of intent for the P-o research community is made that highlights there are key contributions to be made toward understanding biogeochemical cycles, dynamics and function of natural ecosystems and the management of agricultural systems.

Alterations in fire activity due to climate change and fire suppression may have profound effects on the balance between storage and release of carbon (C) and associated volatile elements. Stored soil mercury (Hg) is known to volatilize due to wildfires and this could substantially affect the land air exchange of Hg; conversely the absence of fires and human disturbance may increase the time period over which Hg is sequestered. Here we show for a wildfire chronosequence spanning over more than 5000 years in boreal forest in northern Sweden that belowground inventories of total Hg are strongly related to soil humus C accumulation (R-2 = 0.94, p < 0.001). Our data clearly show that northern boreal forest soils have a strong sink capacity for Hg, and indicate that the sequestered Hg is bound in soil organic matter pools accumulating over millennia. Our results also suggest that more than half of the Hg stock in the sites with the longest time since fire originates from deposition predating the onset of large-scale anthropogenic emissions. This study emphasizes the importance of boreal forest humus soils for Hg storage and reveals that this pool is likely to persist over millennial time scales in the prolonged absence of fire.

Boreal forests are frequently subjected to disturbances, including wildfire and clear-cutting. While these disturbances can cause soil carbon (C) losses, the long-term accumulation dynamics of soil C stocks during subsequent stand development is controlled by biological processes related to the balance of net primary production (NPP) and outputs via heterotrophic respiration and leaching, many of which remain poorly understood. We review the biological processes suggested to influence soil C accumulation in boreal forests. Our review indicates that median C accumulation rates following wildfire and clear-cutting are similar (0.15 and 0.20 Mg ha⁻¹ year⁻¹, respectively), however, variation between studies is extremely high. Further, while many individual studies show linear increases in soil C stocks through time after disturbance, there are indications that C stock recovery is fastest early to mid-succession (e.g. 15–80 years) and then slows as forests mature (e.g. >100 years). We indicate that the rapid build-up of soil C in younger stands appears not only driven by higher plant production, but also by a high rate of mycorrhizal hyphal production, and mycorrhizal suppression of saprotrophs. As stands mature, the balance between reductions in plant and mycorrhizal production, increasing plant litter recalcitrance, and ectomycorrhizal decomposers and saprotrophs have been highlighted as key controls on soil C accumulation rates. While some of these controls appear well understood (e.g. temporal patterns in NPP, changes in aboveground litter quality), many others remain research frontiers. Notably, very little data exists describing and comparing successional patterns of root production, mycorrhizal functional traits, mycorrhizal-saprotroph interactions, or C outputs via heterotrophic respiration and dissolved organic C following different disturbances. We argue that these less frequently described controls require attention, as they will be key not only for understanding ecosystem C balances, but also for representing these dynamics more accurately in soil organic C and Earth system models.

Boreal forests store approximately 470 Pg of carbon (C) in the soil, and rates of soil C accumulation are significantly enhanced by long-term nitrogen (N) enrichment. Dissecting the compositional profile of soils could help better understand the potential mechanisms driving changes in C cycling under enriched N conditions.

We examined the impacts of long-term N addition on the chemical composition of soil organic matter (SOM) in a mature boreal forest. Two large experimental plots (15 ha each) were established: a control and a fertilised plot. The latter received NH4NO3 fertilizer at an average rate of 75 kg N ha−1 year−1 for 12 years. While the centre of this plot received the prescribed amounts of fertilizer, the year-to-year variation in distribution of fertilizer around the designated edges of the plot created a gradient in N-loading. Along this gradient, a compositional shift in SOM in the organic horizon was assessed using two methods: pyrolysis-gas chromatography/mass spectrometry (GC/MS) and solid-state 13C nuclear magnetic resonance spectroscopy (13C NMR).

Both of these methods revealed that the chemical composition of SOM changed with increasing N loading, with an increased fraction of lignin derivatives (i.e., aromatic, methoxy/N-alkyl C) relative to that of carbohydrate (i.e., O-alkyl C), accompanied by increased soil C mass (kg m−2) at the fertilised plot. Also, the relative abundance of N compounds in the pyrolysis products increased with the N loading, mainly due to increased methyl N-acetyl-α-D-glucosaminide in the F/H horizon, plausibly of microbial origin. Microbial N processing likely contributed to soil accumulation of fertilizer-derived N.

Our results corroborate the hypothesis that addition of inorganic N suppresses enzymatic white-rot decomposition relative to non-enzymatic brown-rot oxidation. Taken together, our study suggests that N enrichment leads to a selective accumulation of lignin-derived compounds and points to a key role of such compounds for N-induced SOM accumulation.

• Half of the biological activity in forest soils is supported by recent tree photosynthate, but no study has traced in detail this flux of carbon from the canopy to soil microorganisms in the field.

• Using 13CO2, we pulse-labelled over 1.5 h a 50-m2 patch of 4-m-tall boreal Pinus sylvestris forest in a 200-m3 chamber.

• Tracer levels peaked after 24 h in soluble carbohydrates in the phloem at a height of 0.3 m, after 2–4 d in soil respiratory efflux, after 4–7 d in ectomycorrhizal roots, and after 2–4 d in soil microbial cytoplasm. Carbon in the active pool in needles, in soluble carbohydrates in phloem and in soil respiratory efflux had half-lives of 22, 17 and 35 h, respectively. Carbon in soil microbial cytoplasm had a half-life of 280 h, while the carbon in ectomycorrhizal root tips turned over much more slowly. Simultaneous labelling of the soil with showed that the ectomycorrhizal roots, which were the strongest sinks for photosynthate, were also the most active sinks for soil nitrogen.

• These observations highlight the close temporal coupling between tree canopy photosynthesis and a significant fraction of soil activity in forests.

In the nineteenth century, numerous settlements were established in the alpine region of Fennoscandia (the Scandes), an area that later became a major international scene for Arctic research. Here we raise awareness of this era and show that earthworm-driven bioturbation in “pristine” soils around contemporary Arctic research infrastructure is caused by soil fauna left behind during early land use. We use soil preserved under an alpine settlement to highlight that soils were not bioturbated when the first house was built at a site where bioturbation is now widespread. A review of archived material with unique site-specific chronology constrained the onset of bioturbation to the post-1871 era. Our results suggest that small-scale land use introduced earthworms that now thrive far beyond the realms of former cultivated fields. The legacy of soil fauna from this example of “ecological imperialism” still lingers and should be considered when studying soils of the Scandes.

Earthworms are among the most crucial species for global food production and soil fertility. However, in soils and ecosystems that have evolved without worms, their introduction can lead to significant ecological change. Due to the eradication of soil fauna during the last glacial cycle, and slow recolonization, high-latitude soils generally lack large earthworms. But this situation is about to change as several species of earthworms are spread to northern habitats through human-mediated dispersal.

In this thesis, I investigate the impacts on plant communities and carbon cycling that results from the dispersal of earthworms—primarily Lumbricus and Aporrectodea spp.—in the Fennoscandian Arctic. To achieve this, I combined data from a four-year mesocosm study with observations from earthworm-invaded soils in the Fennoscandian mountain range. My findings indicate that earthworm presence can make tundra ecosystems more graminoid-rich, and cause preferential grazing by rodents, likely due to the higher nitrogen content in plants growing in more fertile soil.

My research has revealed that earthworms play a significant role in stimulating tundra plant biomass growth, particularly belowground. I attribute this increase in plant biomass to the extended growing season facilitated by earthworm activity and more plant available nitrogen. This growth enhancement was consistent across different vegetation types but only led to an increase in net ecosystem carbon (C) uptake in dwarf shrub-dominated tundra. In contrast, in meadow tundra, earthworms had no net effect on the ecosystem C pool, due to an increased mineralization of soil organic carbon (SOC), which counterbalanced the enhanced plant carbon sequestration.

Furthermore, using species distribution modelling, I confirmed that earthworm dispersal in the Fennoscandian Mountains is likely driven by human vectors. I estimate that approximately 7% of this region currently consists of habitats that are both climatically suitable and prone to human-mediated earthworm dispersal.

Over the last decade, an increasing number of studies have used soundscapes to address diverse ecological questions. Sound represents one of the few sources of information capable of providing in situ insights into processes occurring within opaque soil matrices. To date, the use of soundscapes for soil macrofauna monitoring has been experimentally tested only in controlled laboratory environments. Here we assess the validity of laboratory predictions and explore the use of soil soundscape proxies for monitoring soil macrofauna (i.e., earthworm) activities in an outdoor context. In a common garden experiment in northern Sweden, we constructed outdoor mesocosm plots (N = 36) containing two different Arctic vegetation types (meadow and heath) and introduced earthworms to half of these plots. Earthworms substantially altered the ambient soil soundscape under both vegetation types, as measured by both traditional soundscape indices and frequency band power levels, although their acoustic impacts were expressed differently in heath versus meadow soils. While these findings support the as-of-yet untapped promise of using belowground soundscape analyses to monitor soil ecosystem health, direct acoustic emissions from earthworm activities appear to be an unlikely proxy for tracking worm activities at daily timescales. Instead, earthworms indirectly altered the soil soundscape by ‘re-engineering’ the soil matrix: an effect that was dependent on vegetation type. Our findings suggest that long-term (i.e., seasonal) earthworm activities in natural soil settings can likely be monitored indirectly via their impacts on soundscape measures and acoustic indices. Analyzing soil soundscapes may enable larger-scale monitoring of high-latitude soils and is directly applicable to the specific case of earthworm invasions within Arctic soils, which has recently been identified as a potential threat to the resilience of high-latitude ecosystems. Soil soundscapes could also offer a novel means to monitor soils and soil-plant-faunal interactions in situ across diverse pedogenic, agronomic, and ecological systems.

Human introductions have resulted in earthworms establishing in the Arctic, species known to cause cascading ecosystem change. However, few quantitative outdoor experiments have been performed that describe how these soil modifying earthworms are reshaping structures in tundra soils. In this study, we used three-dimensional (3-D) X-ray images of soil cores (approximately 10 cm diameter, 20 cm height, N = 48) to assess how earthworms (Aporrectodea sp. and Lumbricus sp.) affect soil structure and macropore networks in an outdoor mesocosm experiment that lasted four summers. Effects were assessed in both shrub-dominated (heath) and herb-dominated (meadow) tundra. Earthworms almost doubled the macroporosity in meadow soils and tripled macroporosity in heath. Interestingly, the fractal dimension of macropores decreased in response to earthworm burrowing in both systems, indicating that the presence of earthworms reduced the geometric complexity in comparison to other pore-generating processes active in the tundra. Observed effects on soil structure occurred along with a dramatically reduced soil moisture content, which was observed the first winter after earthworm introduction in the meadow. Our findings suggest that predictions of future changes in vegetation and soil carbon pools in the Arctic should include major impacts on soil properties that earthworms induce.

Plants do not grow in isolation; they interact with diverse microorganisms in their habitat.The development of techniques to identify and quantify the microbial diversity associated with plantscontributes to our understanding of the complexity of environmental influences to which plants areexposed. Identifying interactions which are beneficial to plants can enable us to promote healthygrowth with the minimal application of agrochemicals. Beneficial plant–microbial interactionsassist plants in acquiring inaccessible nutrients to promote plant growth and help them to copewith various stresses and pathogens. An increased knowledge of plant–microbial diversity can beapplied to meet the growing demand for biofertilizers for use in organic agriculture. This reviewhighlights the beneficial effects of soil–microbiota and biofertilizers on improving plant health andcrop yields. We propose that a multi–omics approach is appropriate to evaluate viability in thecontext of sustainable agriculture.

Soil mixing by earthworms can have a large impact on the fate of nutrients and pollutants and on the soil's ability to sequester carbon. Nevertheless, methods to quantify earthworm ingestion and egestion under field conditions are largely lacking. Soils of the Fennoscandian tundra offer a special possibility for such quantifications, as these soils commonly lack burrowing macrofauna and exhibit a well-defined O horizon with low bulk density on top of a mineral soil with higher density. Since ingestion-egestion mixes the two soil layers, the temporal changes in the bulk density profile of such soils may be useful for estimating field ingestion rates. In this study, we applied a model for earthworm burrowing through soil ingestion to observed changes in soil densities occurring in a mesocosm experiment carried out in the arctic during four summers with intact soil. The earthworms present in the mesocosms were Aporrectodea trapezoides, Aporrectodea tuberculata, Aporrectodea rosea, Lumbricus rubellus and Lumbricus Terrestris (fourth season only). We show that changes in soil density profiles can indeed be used to infer earthworm ingestion rates that are realistic in comparison to literature values. Although uncertainties in parameter values were sometimes large, the results from this study suggest that soil turnover rates and endogeic earthworm soil ingestion rates in tundra heath and meadow soils may be as high as those reported for temperate conditions. Such large ingestion rates can explain observed large morphological changes in arctic soils where dispersing earthworms have resulted in complete inmixing of the organic layer into the mineral soil. Our approach is applicable to soil profiles with marked vertical differences in bulk density such as the soils of the Fennoscandian tundra where earthworms are currently dispersing into new areas and to layered repacked soil samples that are incubated in the field.

Hydrothermal carbonization and dry pyrolysis transform sewage sludge (SS) into nitrogen- (N) and phosphorus- (P) rich hydrochars (Hyd) and pyrochars (Py), respectively, which may act as slow-release fertilizers with carbon (C) sequestration potential. Whereas this has been mostly studied with short-term experiments, this study focused on the cycling of char-derived N, P and C after ageing during a second grass cropping cycle. Lolium perenne was grown for 3 months in pots on soil mixed with ¹³C and ¹⁵N-enriched SS, Hyd or Py and allowed to age during a first cropping period of 10-month incubation. The δ¹⁵ of the plants confirmed that even during the second cropping, N derived from the amendments was plant accessible. Higher uptake of N from Hyd than from Py is explained by the lower biodegradability of the latter. Plant growth during the second cropping period was associated with a decrease of total P in all treatments, but only the soils with Hyd and Py evidenced an increase of Olsen P. Thus, during the second cropping, more insoluble P was mobilized from the carbonized residues than P needed for plant growth. Compared to control soils prepared with and without KNO₃, higher biomass production was yielded with the amended soils. Hyd proved to have the highest longer-term N mobilization potential. Following the change in δ ¹³C of the soil, we observed that during the second incubation, independently of their aromaticity, all amendments and the native soil organic matter had comparable turnover rates, although the amount of organic matter with slower turnover added with the amendment increased with aromaticity. A rough estimation of the impact of thermal treatment of SS on its C-sequestration potential revealed no major differences between char types. The higher fertilization capacity of Hyd, however, indicates that it is a good candidate for soil amendment as long-term fertilization is combined with a long-term increase of the SOC pool.

Highlights

• Medium-term fertilization and C-sequestration potential of chars from sewage sludge were tested.
• Hydrochar (Hyd) and pyrochar (Py) provide N and P for plant growth during a second cropping period.
• Compared to soils with and without KNO₃, Hyd and Py increase plant productivity of the second crop.
• On a long-term scale, native SOM, amended SS, its aged Hyd and Py show comparable turnover rates.

Nitrogen fixation associated with cryptogams is potentially very important in arctic and subarctic terrestrial ecosystems, as it is a source of new nitrogen (N) into these highly N limited systems. Moss-, lichen- and legume-associated N-2 fixation was studied with high frequency (every second week) during spring, summer, autumn and early winter to uncover the seasonal variation in input of atmospheric N-2 to a subarctic heath with an altered climate. We estimated N-2 fixation from ethylene production by acetylene reduction assay in situ in a field experiment with the treatments: long- vs. short-term summer warming using plastic tents and litter addition (simulating expansion of the birch forest). N-2 fixation activity was measured from late April to mid November and 33 % of all N-2 was fixed outside the vascular plant growing season (Jun-Aug). This substantial amount underlines the importance of N-2 fixation in the cold period. Warming increased N-2 fixation two- to fivefold during late spring. However, long-term summer warming tended to decrease N-2 fixation outside the treatment (tents present) period. Litter alone did not alter N-2 fixation but in combination with warming N-2 fixation increased, probably because N-2 fixation became phosphorus limited under higher temperatures, which was alleviated by the P supply from the litter. In subarctic heath, the current N-2 fixation period extends far beyond the vascular plant growing season. Climate warming and indirect effects such as vegetation changes affect the process of N-2 fixation in different directions and thereby complicate predictions of future N cycling.

Pharmaceutical compounds often coexist in mixtures rather than as individual entities. However, little is known about their co-adsorption and co-mobility in soil and groundwater. In this study, we investigated the adsorption of a quinolone antibiotic (nalidixic acid, NA) and an anti-inflammatory agent (niflumic acid, NFA) onto two soils from France and Sweden in water-saturated soil columns. Despite its lower hydrophobicity, adsorption of NA is much greater than NFA, which can be ascribed to the presence of both carbonyl and carboxylic groups in NA molecule. The data suggest that adsorption to soil components can mainly take place through hydrogen bonding and surface complexation mechanisms, prevailing over hydrophobic interactions. Accordingly, more sorption of NA and NFA was observed in the Swedish soil because it contains more clay content, and much higher Al and Fe contents than the French soil. Injection of NA/NFA mixture in the column did not modify the breakthrough behavior compared to single systems, although cooperative adsorption was observed under static batch conditions. Ca²⁺ inhibited NA adsorption by forming a soluble NA-Ca²⁺ complex but promoted NFA adsorption both in single and binary systems. The mobility in soil columns was well predicted using a new transport model that accounts for both kinetics and binding reactions of NA and NFA to soil constituents. This work will help in accurately predicting the mobility of coexisting pharmaceutical compounds in soils.

The Avaldsnes site was intensively studied employing soil chemistry, magnetic susceptibility (352 samples), and soil micromorphology (74 thin sections), complemented by SEM/EDS investigations on thin sections; a pollen profile and a single organic chemistry sample were also studied. Six archaeological areas were investigated employing samples from both profile and grid-sampled areas.

This study was conducted to evaluate the effect of compost produced with agricultural residues and oat-based biochar, iron oxide and halloysite nanoparticles as additives of the process of composting on soil chemical properties, nutrient status and growth of ryegrass Lolium perenne L. For this, a 90-day mesocosm experiment was carried out under greenhouse conditions. Bare soil and a basal fertilization treatment were compared to soils amended with nonadditive compost (NA compost), compost supplied with oat-based biochar (Bioch compost), iron oxide nanoparticles (Fe compost), and halloysite nanoparticles (Ha compost). Compost supplied with nanoparticles and biochar combined were also considered. The incorporation of compost with or without additives increased the content of total C and N in soil, with N diminishing (total and mineral forms) and C/N modifications after 90 days. The addition of compost and co-composted treatments also increased the total contents of main nutrients such as Ca, K, P and S. Furthermore, the supply of additives into composting did not increase the concentration of trace toxic elements. At the end of the experiment, plant biomass increased by the addition of the different organic amendments, with the highest shoot biomass in soils amended with compost supplied with nanoparticles. These results suggest that the addition of compost based on agricultural residues with additives such as halloysite or biochar improves chemical properties and nutritional status of soil that favor and increase plant growth of Lollium perenne stablished in soils from the Mediterranean Region.

For measurement of the time lag between photosynthesis and CO2 efflux from soil, the carbon isotope pulse-labeling technique is considered as the most suitable. However, an interference from the abiotic tracer CO2 component is identified as a key difficulty for obtaining accurate results with this technique. Guidelines on how to reduce this interference are therefore urgently needed. The flux of abiotic (CO2)-C-13 tracer into soil during the labeling stage, and its return to atmosphere during the monitoring stage was modeled numerically, and the labeling stage also analytically. The controls of the abiotic interference were investigated using these models. The amount of the abiotic tracer component and the time distribution of its rate of return to the atmosphere, were predicted by these models. The main model parameters were D-m (=the ratio between the soil (CO2)-C-13 diffusivity and the retardation factor), and the (CO2)-C-13 concentration at the soil atmosphere interface during the labeling stage (S-13), while background (CO2)-C-13 soil production parameters were unnecessary. The presented models guide the selection of experimental parameters for minimization of the abiotic interference. With parameterization for a particular case, the present numerical model provides a preliminary order-of-magnitude estimate of the abiotic component, which would indicate if this interference is of significance.

Some heavy metals in farmland soil can be transported into the waterbody, affecting the water quality and sediment at the watershed outlet, which can be used to determine the historical loss pattern. Cd is a typical heavy metal leached from farmland that is related to phosphate fertilizers and carries serious environmental risk. The spatial-vertical pattern of Cd in soil and the vertical trend of Cd in the river sediment core were analyzed, which showed the migration and accumulation of Cd in the watershed. To prevent watershed Cd loss, biochar was employed, and leaching experiments were conducted to investigate the Cd loss from soil depending on the initial concentration. Four rainfall intensities, 1.25 mm/h, 2.50 mm/h, 5.00 mm/h, and 10.00 mm/h, were used to simulate typical rainfall scenarios for the study area. Biochar was prepared from corn straw after pretreatment with ammonium dihydrogen phosphate (ADP) and pyrolysis at 400 °C under anoxic conditions. To identify the effects of biochar amendment on Cd migration, the biochar was mixed with soil for 90 days at concentrations of 0%, 0.5%, 1.0%, 3.0%, and 5.0% soil by weight. The results showed that the Cd leaching load increased as the initial load and rainfall intensity increased and that eluviation caused surface Cd to diffuse to the deep soils. The biochar application caused more of the heavy metals to be immobilized in the amended soil rather than transported into the waterbody. The sorption efficiency of the biochar for Cd increased as the addition level increased to 3%, which showed better performance than the 5% addition level under some initial concentration and rainfall conditions. The research indicated that biochar is a potential material to prevent diffuse heavy metal pollution and that a lower addition makes the application more feasible.

The expected shortage of global phosphate has enforced the search for alternative resources for P fertilizers. Therefore, the present study focuses on the turnover of phosphorus (P) of hydrochars and pyrochars derived from sewage sludge (SS) in soils during plant growth. We designed a pot experiment in which Lolium perenne L. was allowed to grow on a Calcic Cambisol amended with SS-derived chars. Hydrothermal carbonization (HTC) yielded the SS-hydrochars (200^◦C, 260^◦C; 30 min, 3 h), whereas the SS-pyrochars were obtained after dry pyrolysis (600^◦C, 1 h). Increasing severity of HTC lowered the recovery of total P (P_T) from the feedstock to 76%. The Olsen-P diminished from 4% P_T in the untreated sludge to 1% P_T in the hydrochars, whereas the pyrochars exhibited an Olsen-P between 3 and 6%. At the end of the pot experiment, the soils amended with pyrochars and with hydrochars produced at 200^◦C contained more Olsen-P than the unamended soils, proving that P-rich chars can indeed serve as a P fertilizer. Part of the P sequestered in the chars turned into a mobile form during the experiment. After addition of our chars, the soil pH remained alkaline, allowing the conclusion that P could not have been solubilized through just abiotic processes. We suggest that biological and biochemical processes are involved in this mobilization. This work demonstrates that, in order to evaluate the efficiency of an organic amendment as a P fertilizer, the knowledge of their P availability alone is not sufficient and a better understanding of the biochemical processes involved in the cycling of its immobilized P is certainly required. 

Plant litter decomposition is a key process for carbon dynamics and nutrient cycling in terrestrial ecosystems. The interaction between litter properties, climatic conditions and soil attributes, influences the activity of microorganisms responsible for litter mineralization. So far, studies using standardized litters to investigate the response of bacterial and fungal communities under different environmental conditions are scarce, especially along wide geographic ranges.

We used a standardized protocol to investigate the diversity of bacteria and fungi in plant litter with the aim of: (i) comparing the microbial communities of native and exotic litters with the community of local soil along a European transect from northern Finland to southern Italy, (ii) defining whether and to what extent, litter types with different traits represent selective substrates for microbial communities, (iii) disentangling the abiotic drivers of microbial diversity, and (iv) correlating the microbial diversity and species co-occurrences patterns with litter mass loss.

We buried native litter and three exotic standardized litters (Deschampsia cespitosa, rooibos tea and green tea) at 12 European study sites. We determined litter mass loss after 94 days. We used an automated molecular DNA-based fingerprinting (ARISA) to profile the bacterial and fungal communities of each litter type and soil (180 samples in total).

Microbial communities in native and exotic litters differed from local soil assemblages. Green tea and D. cespitosa litter represented more selective substrates compared to native litter and rooibos. Soil moisture and soil temperature were the major drivers of microbial community structure at larger scales, though with varying patterns according to litter type. Soil attributes (i.e. moisture and C/N ratios) better explained the differences in microbial abundances than litter type. Green tea degraded faster than all other litter types and accounted for the largest number of positive co-occurrences among microbial taxa. Litter mass loss was positively correlated with fungal evenness and with the percentage of positive co-occurrences between fungi.

Our findings suggest that the microbial community at larger scales reflects the complex interplay between litter type and soil attributes, with the latter exerting a major influence. Mass loss patterns are in part determined by inter- and intra-kingdom interactions and fungal diversity.

Soil life supports the functioning and biodiversity of terrestrial ecosystems. Springtails (Collembola) are among the most abundant soil arthropods regulating soil fertility and flow of energy through above- and belowground food webs. However, the global distribution of springtail diversity and density, and how these relate to energy fluxes remains unknown. Here, using a global dataset representing 2470 sites, we estimate the total soil springtail biomass at 27.5 megatons carbon, which is threefold higher than wild terrestrial vertebrates, and record peak densities up to 2 million individuals per square meter in the tundra. Despite a 20-fold biomass difference between the tundra and the tropics, springtail energy use (community metabolism) remains similar across the latitudinal gradient, owing to the changes in temperature with latitude. Neither springtail density nor community metabolism is predicted by local species richness, which is high in the tropics, but comparably high in some temperate forests and even tundra. Changes in springtail activity may emerge from latitudinal gradients in temperature, predation and resource limitation in soil communities. Contrasting relationships of biomass, diversity and activity of springtail communities with temperature suggest that climate warming will alter fundamental soil biodiversity metrics in different directions, potentially restructuring terrestrial food webs and affecting soil functioning.

In seasonal freeze-thaw zones of NE China, the policy-oriented land management has caused successive land use conversions of native woodland, dry cropland and paddy field for food security. Controls of soil property factors on soil dissolved organic carbon (DOC) dynamics might vary with deforestation. This study aimed to test performance of soil properties interpreting DOC variability along soil profile following a vegetation succession of native forest, rainfed crops (maize-soybean rotations) and paddy rice in an observation area of the Sanjiang Plain. The linear mixed effects model evaluated relative importance of soil properties with comparisons of adjusting and not adjusting for random effects of land use and soil depth as subject variables. The modeling results revealed presence of consistent soil property factors indicating DOC dynamics before and after deforestation. When excluding interferences of land uses and soil layers, interpretations of soil properties were weakened. Soil moisture and bulk density predominantly accounted for DOC variability across land uses, presenting greater estimated effects (0.69 and -0.64, respectively) over those of total nitrogen, soil organic carbon and hydrolyzable nitrogen (0.49, 0.44 and 0.31, respectively). But no soil property factor indicated DOC variability with soil depth. Further research is needed to understand why indications of soil moisture and bulk density on DOC dynamics would differ between horizontal and vertical.

Policy-oriented successive land use conversion intensively occurred in seasonally frozen zones of China during the past five decades. However, responses of soil carbon (C) and nitrogen (N) to land use conversion under cold temperate climates are not fully understood. The objective was to characterize C and N variations following a succession of forest, dryland and paddy. Soil cores were collected for 6 layers with a 10 cm increment from three adjacent chronosequences to determine concentrations of soil organic carbon (SOC), total nitrogen (TN), dissolved organic carbon (DOC) and alkaline hydrolysable nitrogen (HN). Analysis of variance with multivariate general linear model was operated on data sets. Significant losses of SOC and TN storages subject to land use conversion were merely confined within 0 - 10 cm layer, decreasing by 16 % and 38 % for forest to dryland and by 23 % and 43 % for forest to paddy, respectively. Cultivation also influenced SOC and TN stocks at 20 - 40 cm depth for dryland and 20 - 60 cm depth for paddy with increases by 38 Mg C ha(-1) and 2.8 Mg N ha(-1) for forest to dryland, and by 56 Mg C ha(-1) and 4.1 Mg N ha(-1) for forest to paddy, respectively. Successive land use conversion from forest to cropland affected C and N levels in deeper layers, demonstrating the high potentials of subsoil in sequestrating C and N. The extents of cultivation-induced SOC and TN redistribution along soil profile varied among different agricultural systems. DOC and HN changes interpreted SOC and TN changes with land use, presenting high involvements of soluble compartments in SOC and TN variations. The net variation in SOC/TN ratio effectively indicated C and N changes when dryland was converted to paddy.

In mountain ecosystems with marked seasonality, climate change can affect various processes in soils, potentially modifying long-term key soil services via change in soil organic carbon (C) storage. Based on a four-year soil transplantation experiment in Swiss subalpine grasslands, we investigated how imposed climate warming and reduced precipitation modified the drivers of soil carbon enzyme potential activities across winter and summer seasons. Specifically, we used structural equation models (SEMs) to identify biotic (microbial community structure, abundance and activity) and abiotic (quantity and quality of organic matter resources) drivers of soil C-enzymes (hydrolase and oxidase) in two seasons under two different climate scenarios. We found contrasting impacts of the climate manipulation on the drivers of C-enzymes between winter and summer. In winter, no direct effect of climate manipulation (reduced rainfall and warming) on enzyme activity was observed. Yet, climate indirectly down-regulated enzyme activity through a decrease in the availability of water extractable organic carbon (WEOC) labile resources. During summer, reduced soil moisture induced by the climate manipulation directly reduced soil microbial biomass, which led to a decrease in C-enzyme activity. In general, across both seasons, neither microbial community structure, nor organic matter quality were strong determinants of enzymatic activity. In particular organic matter recalcitrance (aromaticity) was not found as a general driver of either hydrolase or oxidase C-enzyme potential activities, though we did observe higher C enzyme activities led to an increase of particulate organic matter recalcitrance in the summer season. Overall, our results highlight the seasonality of climate change effects on soil organic matter enzymatic decomposition, providing a comprehensive picture of seasonal potential cause and effect relationships governing C mineralization in subalpine grasslands.

Primary minerals that enter soils through bedrock weathering and atmospheric deposition can generate poorly crystalline minerals (PCM) that preferentially associate with soil organic carbon (SOC). These associations hinder microbial decomposition and the release of CO2 from soils to the atmosphere, making them a critical geochemical control on terrestrial carbon abundance and persistence. Studies that explore these relationships are typically derived from soil chronosequences that experience negligible erosion and thus do not readily translate to eroding landscapes. Here, we propose a theoretical framework to estimate steady-state PCM density and stocks for hilly and mountainous settings by coupling geochemical and geomorphic mass balance equations that account for soil production from bedrock and dust, soil erosion, PCM formation from weathering, and the transformation of PCMs into crystalline phases. We calculate an optimal erosion rate for maximum PCM abundance that arises because PCMs are limited by insufficient weathering at faster erosion rates and loss via “ripening” into more crystalline forms at slower erosion rates. The optimal erosion rate for modeled hilltop soil is modulated by reaction rate constants that govern the efficiency of primary mineral weathering and PCM ripening. By comparing our analysis with global compilations of erosion and soil production rates derived from cosmogenic nuclides, we show that landscapes with slow-to-moderate erosion rates may be optimal for harboring abundant PCM stocks that can facilitate SOC sequestration and limit turnover. Given the growing array of erosion-topography metrics and the widespread availability of high-resolution topographic data, our framework demonstrates how weathering and critical zone processes can be coupled to inform landscape prioritization for persistent SOC storage potential across a broad range of spatial and temporal scales.

Boreal peatlands are key high-latitude ecosystem types and act as a carbon (C) sink storing an estimated 25% of the world's soil C. These environments are currently seeing the most substantial changing climate, especially during the winter. CO₂ emissions during the winter can correspond to 80% of the growing season's net CO₂ assimilation. Yet, our conceptual understanding of the controls on microbial metabolic activity in peat soils at temperatures ≤0 °C is poor. We used stable isotope probing of peat samples and tracked the fate of ¹³C-glucose using ¹³C-NMR. We show that microorganisms in frozen boreal peat soils utilize monomeric C-substrates to sustain both catabolic and anabolic metabolism at temperatures down to −5 °C. The ¹³C-substrate was transformed into ¹³C–CO₂, different metabolites, and incorporated into membrane phospholipid fatty acids. The 16S rRNA-based community analyses revealed the activity at −3 °C changes the composition of the bacterial community over relevant timescales. Below 0 °C, small temperature changes have strong effects on process rates and small differences in winter soil temperature may affect C dynamics of northern peatlands. Understanding biological processes at low and below zero temperatures are central for the overall functioning of these systems representing one of the world's major soil C pools.

Arctic tundra active-layer soils are at risk of soil organic carbon (SOC) depletion and degradation upon global climate warming because they are in a stage of relatively early decomposition. Non-growing season (NGS) warming is particularly pronounced, and observed increases of CO2 emissions during experimentally warmed NGSs give concern for great SOC losses to the atmosphere. Here, we used snow fences in Arctic Spitsbergen dwarf shrub tundra to simulate 1.86 degrees C NGS warming for 9 consecutive years, while growing season temperatures remained unchanged. In the snow fence treatment, the 4-11 cm thick A-horizon had a 2% lower SOC concentration and a 0.48 kg Cm-2 smaller pool size than the controls, indicating SOC pool depletion. The snow fence treatment's A-horizon's alkyl/O-alkyl ratio was also significantly increased, indicating an advance of SOC degradation. The underlying 5 cm of B/C-horizon did not show these effects. Our results support the hypothesis that SOC depletion and degradation are connected to the long-term transience of observed ecosystem respiration (ER) increases upon soil warming. We suggest that the bulk of warming induced ER increases may originate from surface and not deep active layer or permafrost horizons. The observed losses of SOC might be significant for the ecosystem in question, but are in magnitude comparatively small relative to anthropogenic greenhouse gas enrichment of the atmosphere. We conclude that a positive feedback of carbon losses from surface soils of Arctic dwarf shrub tundra to anthropogenic forcing will be minor, but not negligible.

Three strains of Gram‐negative bacterium, Rhizobium, were developed by gamma (γ)‐irradiation random mutagenesis. The developed strains were evaluated for their augmented features for symbiotic association, nitrogen fixation, and crop yield ofthree leguminous plants—chickpea, field‐pea, and lentil—in agricultural fields of the northern Indian state of Haryana. Crops treated with developed mutants exhibited significant improvement in plant features and the yield of crops when compared tothe control‐uninoculated crops and crops grown with indigenous or commercial crop‐specific strains of Rhizobium. This improvement was attributed to generated mutants, MbPrRz1 (on chickpea), MbPrRz2 (on lentil), and MbPrRz3 (on field‐pea). Additionally, the cocultured symbiotic response of MbPrRz1 and MbPrRz2 mutants was found to be more pronounced on allthree crops. The statistical analysis using Pearson's correlation coefficients revealed that nodulation and plant biomass were the most related parameters of crop yield. Among the effectiveness of developed mutants, MbPrRz1 yielded the best results for allthree tested crops. Moreover, the developed mutants enhanced macro‐ and micronutrients of the experimental fields whencompared with fields harboring the indigenous rhizobial community. These developed mutants were further genetically characterized, predominantly expressing nitrogen fixation marker, nifH, and appeared to belong to Mesorhizobium ciceri (MbPrRz1) and Rhizobium leguminosarum (both MbPrRz2 and MbPrRz3). In summary, this study highlights the potential ofdeveloped Rhizobium mutants as effective biofertilizers for sustainable agriculture, showcasing their ability to enhancesymbiotic relationships, crop yield, and soil fertility.

Soil organic matter (SOM) is important in maintaining soil fertility and other ecosystem functions. Yet, land management in intensive agriculture has caused SOM level to decrease, with knock-on effects for soil fertility and quality. Therefore, land management options that ensure that SOM is not depleted and that soil functions are better sustained are of increasing interest. However, there is limited knowledge on how different land managements affect the composition of SOM and associated microbial functional profiles. Twelve long-term field experiments, covering a wide range of climatic zones and soil types, were selected in Sweden. They focused on the role of combining ley in crop rotations with the manure application (livestock farm), as opposed to the management without ley and receiving only inorganic fertilizer (arable farm). In ten out of the 12 study sites, livestock farm management tended to have higher proportions of aliphatic and double bonded C groups, as estimated by mid-infrared spectroscopy. This was further confirmed by 13C NMR analysis, which found greater proportions of O-alkyl and di-O-alkyl groups and less aromatic C in livestock farm than arable farm management in five of the eight sites analyzed. The changes in SOM composition were reflected in microbial functional profiles across many sites: soils from livestock farm management utilized more carbohydrates and amino acids, while polymer and aromatic compounds were associated with arable farm management. Overall, shifts in both microbial functional profiles and SOM composition showed great consistency across geographical and climatic zones. Livestock farm management maintained higher levels of microbial functional diversity and were associated with higher proportions of “reactive” C functional groups. Our investigation demonstrates that livestock farm management could maintain soil fertility over the long-term via the changes in SOM composition and the regulation of microbial functional profiles.

Earthworms and soil microorganisms contribute to soil health, quality, and fertility, but their importance in agricultural soils is often underestimated. This study aims at examining whether and to what extent the presence of earthworms (Eisenia sp.) affected the (a) soil bacterial community composition, (b) litter decomposition, and (c) plant growth (Brassica oleracea L., broccoli; Vicia faba L., faba bean). We performed a mesocosm experiment in which plants were grown outdoors for four months with or without earthworms. Soil bacterial community structure was evaluated by a 16S rRNA-based metabarcoding approach. Litter decomposition rates were determined by using the tea bag index (TBI) and litter bags (olive residues). Earthworm numbers almost doubled throughout the experimental period. Independently of the plant species, earthworm presence had a significant impact on the structure of soil bacterial community, in terms of enhanced α- and β-diversity (especially that of Proteobacteria, Bacteroidota, Myxococcota, and Verrucomicrobia) and increased 16S rRNA gene abundance (+89% in broccoli and +223% in faba bean). Microbial decomposition (TBI) was enhanced in the treatments with earthworms, and showed a significantly higher decomposition rate constant (k_TBI) and a lower stabilization factor (S_TBI), whereas decomposition in the litter bags (d_litter) increased by about 6% in broccoli and 5% in faba bean. Earthworms significantly enhanced root growth (in terms of total length and fresh weight) of both plant species. Our results show the strong influence of earthworms and crop identity in shaping soil chemico-physical properties, soil bacterial community, litter decomposition and plant growth. These findings could be used for developing nature-based solutions that ensure the long-term biological sustainability of soil agro- and natural ecosystems.

Organic soils in boreal ecosystems and peatlands represent a huge global carbon pool and their composition strongly affects soil properties. Nevertheless, the characterization of soil organic matter (SOM) molecular composition, which is essential for elucidating soil carbon processes and turnover, is not easily achieved, and further advances in the area are greatly needed. Two dimensional (2D) liquid state H-1-C-13 nuclear magnetic resonance (NMR) spectroscopy has been used on dimethyl sulfoxide (DMSO) extracts of SOM to achieve molecular level characterization, with signals from many identifiable molecular groups observable. Here we show that a simple and fast sample preparation allows acquisition of 2D H-1-C-13 NMR spectra from extracts of plant litter and organic layers in boreal ecosystems, with fast data acquisition. Our 2D NMR spectra revealed several differences in the DMSO extracts of different tree litter samples, O-horizons of forest soil, peat-forming moss (Sphagnum) and peat. The results mirror established differences between OM in soils and litter of different forest ecosystems (e.g. between deciduous and coniferous litter) but also provide indications for research to untangle previously conflicting results (e.g. cutin degradation in soil or carbohydrate degradation in peat). Thus, combination of 2D NMR methods can greatly improve analysis of litter composition and SOM composition, thereby facilitating the elucidation of their roles in biogeochemical and ecological processes that are critical for foreseeing feedback mechanisms for SOM turnover as a result of global environmental change.