Warming and nitrogen-addition alter soil fungal communities and rates of organic matter decomposition

Background/Question/Methods

Global changes – such as warming, increasing atmospheric CO₂ concentrations, and nitrogen (N) deposition – are altering ecosystem processes. The balance between soil carbon (C) accumulation and decomposition is determined in large part by the activity and biomass of detrital organisms, particularly soil fungi, and yet their sensitivity to global changes remains unresolved. Warming and N deposition create physiologically stressful environments that select for fungi capable of tolerating stress. Conversely, CO₂ enrichment can stimulate mycorrhizal fungi as plants assimilate C and allocate it belowground. The functional fungal community functional may switch from domination by species with traits associated with decomposition via oxidative enzymes to traits associated with stress tolerance if global changes push fungal physiological limits. The effects of global changes on soil fungi have been investigated in several ecosystems globally. We synthesized results from the literature to answer (1) how do fungal communities and their function respond to global changes, and (2) are fungal responses magnified in some ecosystems? We present results from a meta-analysis of 200+ manipulative field studies to quantify fungal responses to global change.

Results/Conclusions

Global changes generally reduced fungal abundance and decomposer activity. Simulated N-deposition reduced fungal abundance by 12% and enzyme activity by 19%, and these effects were stronger in deciduous forests than other ecosystems. N-addition over 12 g N m^-2 y^-1 reduced fungal abundance by 27%, whereas lower amounts had no effect on fungal abundance indicating a potential N-addition threshold. Warming decreased fungal abundance by up to 26% in forests and grasslands, but increased fungal abundance by up to 29% in steppe and alpine meadow ecosystems. Across all ecosystems, warming reduced fungal enzyme activity by 45%. Surprisingly, we found no effect of increased CO₂ on fungal abundance or enzyme activity. While site-specific studies have examined the role of soil fungi in ecosystem responses to global change, we document general patterns of global change impacts on soil fungal communities, biomass, and activity. In sum, we provide evidence that soil microbial community shifts and activity plays a large part in ecosystem responses to global changes and have the potential to alter the magnitude of the C-climate feedback.