Responses of soil bacterial functional group diversity to nitrogen enrichment in global grasslands

Abstract

Soil microbial functional groups are closely linked to ecosystem functioning and are highly sensitive to global environmental changes. However, compared to the species diversity, how soil microbial functional group diversity responds to environmental changes remains poorly understood. Here, we investigated the nitrogen (N) effect on soil bacterial functional group diversity and predicted its spatial pattern in global grasslands. By re-analyzing publicly available 16S rRNA gene amplicon data from 38 N-addition experiments including 96 paired measurements across five continents, we found that N enrichment significantly leads to a more similar β-diversity but had no pronounced effect on α-diversity of soil bacterial functional groups. Global mapping with a random forest model revealed a clear spatial pattern of the N effect: ∼78 % of the global grasslands have shifted towards a more similar bacterial functional group β-diversity under contemporary N deposition rates, and in the regions particularly affected by high N deposition and vulnerable to global changes (e.g. the Eurasian steppe), the magnitude of change was greater. These results make a step in understanding soil microbial functional group diversity in addition to species diversity in the face of environmental changes, and provide more knowledge and rationale for the conservation of soil microbial functional diversity hotspots in global grasslands.