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

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. 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. We found that the effects of N
enrichment on soil microbial biomass strongly varied 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. The short-term of N enrichment
(experimental duration ≤ 5 years) could reduce microbial biomass via
decreasing nematode abundance across diverse ecosystems, whereas the
long-term of N enrichment (experimental duration > 5 years) mainly
promoted microbial biomass via increasing plant biomass. 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.