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Decomposition rates are commonly assessed from mass loss in litterbags or teabags, but underestimation may occur if organic matter (OM) enters the bags during decomposition, e.g. through colonization by fungal hyphae. We used C4-plant (lemongrass) teabags and an isotopic mass-balance approach to quantify external OM inputs to decomposing litter during a 24-month field study in two C3-plant ecosystems, a temperate forest and a heath tundra, with contrasting climates and vegetation. After 24 months, OM inputs represented 4% of recovered mass in the tundra and 22% in the forest, corresponding to an underestimation of decomposition by 3% to 6%, with stronger biases in the forest due to accelerated litter processing and belowground activity. The use of litter- or teabags containing plant material with isotopic signatures distinct from the studied system enables the quantification of OM inputs entering the bags during litter decomposition, thereby improving the comparability of decomposition rates across ecosystems.

Empirical in-situ measurements of ecosystem carbon dioxide respiration (Reco) in high-latitude ecosystems remain limited, yet they are essential for understanding how tundra carbon cycling responds to climate warming across different environmental contexts and for reducing uncertainties in upscaled carbon budgets and carbon–climate feedbacks. Here, we present the TundraFlux Database, which to date is the most comprehensive synthesis of tundra Reco responses to experimental warming. The database compiles over 24 000 daily-aggregated in-situ Reco measurements from control and warmed plots with open-top chambers at 64 Arctic and alpine tundra sites across 12 countries. By coupling Reco measurements with extensive metadata on climate, vegetation, and soil characteristics, the TundraFlux Database enables the integration of field-scale ecological processes into large-scale models, offering new opportunities to refine global carbon budgets and test predictions of tundra ecosystem responses to warming. Open access to the TundraFlux Database will empower the research community to better quantify and predict how warming alters carbon cycling in Arctic and alpine tundra ecosystems. The data can be accessed on Zenodo (https://doi.org/10.5281/zenodo.17976235, Schwieger, 2026a).

Targeted removal experiments are a powerful tool to assess the effects of plant species or (functional) groups on ecosystem functions. However, removing plant biomass in itself can bias the observed responses. This bias is commonly addressed by waiting until ecosystem recovery, but this is inherently based on unverified proxies or anecdotal evidence. Statistical control methods are efficient, but restricted in scope by underlying assumptions.

We propose accounting for such biases within the experimental design, using a gradient of biomass removal controls. We demonstrate the relevance of this design by presenting (1) conceptual examples of suspected biases and (2) how to observe and control for these biases.

Using data from a mycorrhizal association-based removal experiment, we show that ignoring biomass removal biases (including by assuming ecosystem recovery) can lead to incorrect, or even contrary conclusions (e.g. false positive and false negative). Our gradient design can prevent such incorrect interpretations, regardless of whether aboveground biomass has fully recovered.

Our approach provides more objective and quantitative insights, independently assessed for each variable, than using a proxy to assume ecosystem recovery. Our approach circumvents the strict statistical assumptions of, for example, ANCOVA and thus offers greater flexibility in data analysis.

Vegetation changes in a warming Arctic may affect plant-associated soil microbial communities with possible consequences for the biogeochemical cycling of carbon (C) and nitrogen (N). In a sub-arctic tundra heath, we factorially removed plant species with ecto- and ericoid mycorrhizal associations. After two years, we explored how mycorrhizal type-specific plant removal influences microbial communities, soil and microbial C and N pools, and extracellular enzymatic activities. Removal of ecto- and ericoid mycorrhizal plants did not change the soil fungal or bacterial community composition or their extracellular enzyme activities. However, ericoid plant removal decreased microbial C:N ratio, suggesting a stoichiometric effect decoupled from microbial community composition. In other words, microbial communities appear to show initial plasticity in response to major changes in tundra vegetation. This highlights the importance of longer-term perspectives when investigating the effects of vegetation changes on biogeochemical processes in Arctic ecosystems.