Arbuscular mycorrhizal type increases the negative feedback of soil microbial biomass to nitrogen deposition

Soil microbes are crucial for ecosystem health and functioning, playing key roles in decomposing organic matter, nutrient cycling, and carbon sequestration. Mycorrhizal fungi, a vital group of soil microbes, establish symbiotic relationships with plant roots, enhancing plant nutrient uptake and improving soil structure. Globally nitrogen (N) enrichment is recognized as a significant regulator of soil microbial communities. However, whether and how mycorrhiza mediate the effects of N deposition on soil microbial biomass remains unclear. Here, we conducted a global meta-analysis using 1945 paired observations (1309 AM type and 636 NonAM type) from 113 independent studies to assess the mycorrhiza-mediated responses of soil microbial biomass and respiration to N deposition. The results showed that N deposition reduced total, bacterial and fungal biomass, as well as fungi to bacteria ratio (F:B ratio), and the negative impact was more pronounced under AM type compared to NonAM type. Notably, the adverse effects intensified with increasing N application rate under AM type. Moreover, root respiration exhibited a greater increase with N deposition in AM type than in NonAM type, whereas microbial and soil respiration displayed a more significant decrease in AM type compared to NonAM type. The structural equation modeling revealed that the effects of N deposition on microbes were primarily driven by mean annual temperature (MAT) for AM type, whereas for NonAM type, it was mean annual precipitation (MAP) that played a significant role. Overall, our results indicated that soil microbes of the AM type were more susceptible to N deposition compared to those of the NonAM type. The observed patterns indicated that mycorrhizal type could effect the responses of plants and soil to nitrogen deposition, which has implications for ecosystem nutrient cycling and sustainable agriculture.