Nitrogen fertilization weakens the linkage between soil carbon and microbial diversity: A global meta‐analysis

Soil microbes make up a significant portion of the genetic diversity and play a critical role in belowground carbon (C) cycling in terrestrial ecosystems. Soil microbial diversity and organic C are often tightly coupled in C cycling processes; however, this coupling can be weakened or broken by rapid global change. A global meta‐analysis was performed with 1148 paired comparisons extracted from 229 articles published between January 1998 and December 2021 to determine how nitrogen (N) fertilization affects the relationship between soil C content and microbial diversity in terrestrial ecosystems. We found that N fertilization decreased soil bacterial (−11%) and fungal diversity (−17%), but increased soil organic C (SOC) (+19%), microbial biomass C (MBC) (+17%), and dissolved organic C (DOC) (+25%) across different ecosystems. Organic N (urea) fertilization had a greater effect on SOC, MBC, DOC, and bacterial and fungal diversity than inorganic N fertilization. Most importantly, soil microbial diversity decreased with increasing SOC, MBC, and DOC, and the absolute values of the correlation coefficients decreased with increasing N fertilization rate and duration, suggesting that N fertilization weakened the linkage between soil C and microbial diversity. The weakened linkage might negatively impact essential ecosystem services under high rates of N fertilization; this understanding is important for mitigating the negative impact of global N enrichment on soil C cycling. Microbial diversity plays a vital role in soil C cycling as microbes control soil biochemical processes. However, in this meta‐analysis we found that soil microbial diversity is strongly negatively related to soil C content (mainly SOC, MBC and DOC), and their correlation coefficients decreased with increasing N fertilization rate and experimental duration, suggesting that the linkage between microbial diversity and soil C is weakened by N fertilization rate and duration. This global meta‐analysis presents evidence that long‐term N fertilization led to the decoupling between microbial diversity and soil C.