Trophic regulation of soil microbial biomass under nitrogen enrichment: A global meta-analysis

1. Eutrophication, including nitrogen (N) enrichment, can affect soil microbial communities through changes in trophic interactions. However, a knowledge gap still exists about how plant resources (‘bottom-up effects’) and microbial predators (‘top-down effects’) regulate the impacts of N enrichment on microbial biomass at the global scale.
2. To address this knowledge gap, we conducted a global meta-analysis using 2885 paired observations from 217 publications to evaluate the regulatory effects of plant biomass and soil nematodes on soil microbial biomass under N enrichment across terrestrial ecosystems.
3. We found that the effects of N enrichment on soil microbial biomass varied strongly across ecosystems. N enrichment decreased the soil microbial biomass of natural grasslands and forests due to soil acidification and the subsequent losses of predatory and microbivorous nematodes stimulating microbial growth. By contrast, N enrichment increased the microbial biomass of managed croplands
mainly via increasing plant biomass production. Across diverse ecosystems, the short-term
N enrichment (experimental duration ≤5 years) could reduce microbial biomass via decreasing nematode abundance, whereas the long-term N enrichment (experimental duration >5 years) mainly promoted microbial biomass via increasing plant biomass.
4. These findings highlight the critical roles of microbial predators and plant input in shaping microbial responses to N enrichment, which are highly dependent on ecosystem type and the period of N enrichment. Earth system models that predict soil microbial biomass and their linkages to soil functioning should consider the variations in plant biomass and soil nematodes under future scenarios of N deposition.