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Phosphorus availability and microbial respiration across biomes:  from plantation forest to tundra
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Place, publisher, year, edition, pages
Umeå: Umeå universitet, Institutionen för ekologi, miljö och geovetenskap , 2010. , 32 + 4 papers p.
Keyword [en]
phosphorus availability, microbial bioassay, soil respiration, microbial growth rate, Hedley fractionation, soil sorption, weathered soils, boreal forest, subarctic and tundra
URN: urn:nbn:se:umu:diva-33732ISBN: 978-91-7264-989-1OAI: diva2:317688
Public defence
2010-05-27, Institutet för Rymdfysik, Aulan, Rymdcampus 1, Kiruna, 09:00 (English)
Available from: 2010-05-06 Created: 2010-05-04 Last updated: 2010-05-06Bibliographically approved
List of papers
1. Microbial responses to P addition in six South African forest soils
Open this publication in new window or tab >>Microbial responses to P addition in six South African forest soils
Show others...
2010 (English)In: Plant and Soil, ISSN 0032-079X, E-ISSN 1573-5036, Vol. 329, no 1/2, 209-225 p.Article in journal (Refereed) Published
Abstract [en]

Forests growing on highly weathered soils are often phosphorus (P) limited and competition between geochemical and biological sinks affects their soil P dynamics. In an attempt to elucidate the factors controlling the relative importance of these two sinks, we investigated the relationship of between soil microbial growth kinetics and soil chemical properties following amendments with C, N and P in six South African forest soils. Microbial growth kinetics were determined from respiration curves derived from measurements of CO2 effluxes from soil samples in laboratory incubations. We found that microbial growth rates after C + N additions were positively related to NaOH-extractable P and decreased with soil depth, whereas the lag time (the time between substrate addition and exponential growth) was negatively related to extractable P. However, the growth rate and lag time were unrelated to the soil’s sorption properties or Al and Fe contents. Our results indicate that at least some of the NaOH-extractable inorganic P may be biologically available within a relatively short time (days to weeks) and might be more labile than previously thought. Our results also show that microbial utilization of C + N only seemed to be constrained by P in the deeper part of the soil profiles.

Place, publisher, year, edition, pages
Springer, 2010
Phosphorus availability, Microbial bioassay, Weathered soils, Soil respiration, Microbial growth rate, Hedley fractionation
National Category
Biological Sciences
urn:nbn:se:umu:diva-33733 (URN)10.1007/s11104-009-0146-3 (DOI)000275543300016 ()
Available from: 2010-05-04 Created: 2010-05-04 Last updated: 2012-10-02Bibliographically approved
2. Microbial indices of P availability across a forest productivity gradient in South Africa
Open this publication in new window or tab >>Microbial indices of P availability across a forest productivity gradient in South Africa
(English)Manuscript (preprint) (Other (popular science, discussion, etc.))
Abstract [en]

Soil microorganisms have the capability to solubilize different fractions of phosphorus (P) and can probably access P fractions unavailable to plants in the short term. However, there are few studies available that attempt to estimate the potentially available microbial P. We compare a wet-chemical extraction procedure for P with a microbial bioassay based on respiration kinetics after amending carbon (C), nitrogen (N), and sub-saturation levels of P in laboratory incubations across a plantation forest productivity gradient in South Africa. We found that the estimated microbial available P was at least 10-fold higher than P fractions conventionally defined as easily plant available based on wet-chemical methods. The microbial P was strongly positively related to sorbed P (i.e. NaOH extractable P) P (r2=0.63, p<0.001) and indicates that this P fraction contributes to the microbial P utilization within a relatively short time frame (<300 hrs) when C and N are not limiting. This was further emphasized by the change in respiration kinetics when the amendment of C and N were combined with phosphate. Addition of phosphate-P increased the growth and maximum respiration (max resp) rates and decreased the amount of time needed to reach max resp in comparison to amendments with only C and N. Our study indicates that sorbed P), a dominant P fraction in highly weathered soils, is most likely accessible to microorganisms.

Phosphorus availability, weathered soil, soil respiration, microbial bioassay, Hedley fractionation, plantation forest
urn:nbn:se:umu:diva-33735 (URN)
Available from: 2010-05-04 Created: 2010-05-04 Last updated: 2010-05-06
3. Soil phosphorus and microbial response to a long-term wildfire chronosequence in northern Sweden
Open this publication in new window or tab >>Soil phosphorus and microbial response to a long-term wildfire chronosequence in northern Sweden
2009 (English)In: Biogeochemistry, ISSN 0168-2563, E-ISSN 1573-515X, Vol. 95, no 2/3, 199-213 p.Article in journal (Refereed) Published
Abstract [en]

In the prolonged absence of major disturbances, ecosystems may enter a stage of retrogression, which is characterized by decreased ecosystem process rates both above and belowground, and often reduced availability of phosphorus (P). Disturbance through wildfire can increase soil P losses through leaching or erosion, but in the long-term absence of fire, soil P could potentially become increasingly bound in more stable forms that are less available to microbes. We studied forms of P and microbial respiration kinetics in the humus layer of a group of islands that vary considerably in wildfire frequency (40–5,300 years since last fire), and which are known to enter retrogression in the prolonged absence of fire. We found a decrease in labile P with decreasing fire frequency but no change in total P. Soil microorganisms responded more strongly to N than to P addition, and microbial biomass N:P ratios remained unchanged across the gradient. However, the concentration of labile P was the best predictor of microbial respiration responses across the islands, and this provides some evidence that declining access to P could contribute to the decline in soil microbial activity during retrogression. Our results show that even though N is arguably the main limiting nutrient during retrogression in this chronosequence, long term absence of fire also causes a decline in P availability which negatively affects microbial activity. This in turn could potentially impair microbially driven processes such as decomposition and mineralization and further contribute to the reduced availability of soil nutrients during retrogression.

Place, publisher, year, edition, pages
Springer, 2009
Boreal forest, Microbial respiration, Phosphorus, Retrogression, Succession, Wild fire
urn:nbn:se:umu:diva-33734 (URN)10.1007/s10533-009-9331-y (DOI)
Available from: 2010-05-04 Created: 2010-05-04 Last updated: 2010-05-21Bibliographically approved
4. Phosphorus availability and microbial respiration across different tundra vegetation types
Open this publication in new window or tab >>Phosphorus availability and microbial respiration across different tundra vegetation types
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Phosphorus (P) is an important nutrient in tundra ecosystems that co-limits or in some cases limits primary production. The availability of P is largely driven by soil characteristics, e.g., pH, organic carbon, and abundance of P-sorbing elements such as aluminium (Al) or iron (Fe). We tested how vegetation and soil properties relate to P availability across different tundra vegetation types. The different soil P fractions in the organic top soil were measured and plant foliar nitrogen (N) to P ratio was used as an indicator of plant nutrient status. Microbial and plant bioassays were used to study microbial respiration kinetics and plant biomass response to carbon, N, and P amendments. The distribution of P fractions differed significantly across vegetation types; labile fractions of P were less abundant in meadow sites compared to heath sites. Calcium-phosphates seemed to be an important P-fraction in meadows, but were only found in lower concentrations in the heath. There were only small differences in sorbed P between the vegetation types and this corroborated with the distribution of Al+Fe. Plant N: P ratios and the plant bioassay indicated decreasing P availability from dry heath to mesic heath to mesic meadow. Our results showed that vegetation type was related to soil chemistry and P availability; however, in contrast to other studies, this effect was not related to redistribution of Fe and Al. We conclude that in this tundra ecosystem plants are generally co-limited by N and P, and P availability varies between different vegetation types, which is reflected in both above- and belowground ecosystem processes.

Phosphorus availability, subarctic tundra, Hedley fractionation, soil respiration
urn:nbn:se:umu:diva-33736 (URN)
Available from: 2010-05-04 Created: 2010-05-04 Last updated: 2010-05-06

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