Effects of nitrogen enrichment on soil microbial characteristics: From biomass to enzyme activities

Soil microbes play an important role in ecosystem processes, including carbon (C) and nutrient cycling. Nitrogen (N) enrichment is known to affect soil microbes, but whether other factors affect the impact of N enrichment on soil microbial biomass and composition and extracellular enzyme activities (EEAs) remains unclear. In this study, to evaluate the responses of soil microbial characteristics, including microbial biomass, microbial community composition and EEAs to N enrichment, we conducted a meta-analysis using 1248 global data series from 120 published papers at 125 sites that cover five types of biomes worldwide. The results showed that N enrichment significantly decreased microbial biomass carbon (MBC) and arbuscular mycorrhizal fungi (AMF) across all studies. In addition, the responses of soil microbes depended on the N enrichment rate, and different thresholds (the N rate at which the microbial response changes) of MBC (64.85 kg N ha(-1) year(-1)), microbial biomass nitrogen (MBN, 57.00 kg N ha (-1) year (-1)), bacterial biomass (106.75 kg N ha(-1) year (-1)), fungal biomass (70.50 kg N ha(-1) year(-1)), beta-N-acetyl-glucosaminidase (NAG) (83.27 kg N ha(-1) year(-1)) and peroxidase activity (19.75 kg N ha(-1) year(-1)) were observed under N enrichment. Moreover, the responses of soil microbes to N enrichment were affected by biome type, N enrichment rate and type, experimental duration, precipitation and soil type. Furthermore, the results showed that N enrichment significantly altered soil physical and chemical properties, which may affect soil microbial biomass and composition under N enrichment. Our findings highlight that N enrichment decreased the soil microbial biomass and showed a significant effect on soil EEAs across all terrestrial ecosystems, with more pronounced effects observed with increasing N rate and duration.