Threshold response of arbuscular mycorrhizal mycelial respiration to a nitrogen addition gradient in an alpine grassland

Abstract Arbuscular mycorrhizal fungi (AMF) are crucial symbiotic partners for plant nitrogen (N) acquisition and are sensitive to exogenous N inputs. However, the AMF mycelial respiration ( R AMF ) has always been overlooked in previous studies, and the extent to which different levels of N input can induce changes in R AMF remains elusive. Based on a long‐term field experiment with a gradient of N addition rates (0, 2, 4, 8, 16, 32 g N m −2 year −1 ) in an alpine meadow on the Qinghai–Tibetan Plateau, we investigated the response of R AMF to N enrichment and explored the underlying mechanisms. R AMF exhibited a threshold response to N enrichment, with no changes observed at low N addition rates, but a significant decrease above the threshold of 8 g N m −2 year −1 . R AMF accounted for 33% of soil respiration across different treatments. N‐induced changes in plant (richness, coverage and root biomass), soil (available N, pH and moisture) and AMF attributes (biomass, Shannon diversity and relative abundance of Glomeraceae ) collectively explained 64% of the variation in R AMF , and soil factors contributed more to the variation of R AMF than plant and AMF factors. Our findings confirm the significant contribution of R AMF to soil carbon emissions, and incorporating its threshold response into carbon cycling models is imperative to inform soil carbon dynamics under climate change. Read the free Plain Language Summary for this article on the Journal blog.