Mycorrhizal fungi mitigate nitrogen losses of an experimental grassland by facilitating plant uptake and soil microbial immobilization

Nitrogen (N) is one of the most limited nutrients of terrestrial ecosystems, whose losses are prevented in tightly coupled cycles in finely tuned systems. Global change-induced N enrichment through atmospheric deposition and application of vast amounts of fertilizer nowadays challenge the terrestrial N cycle. Arbuscular mycorrhizal fungi (AMF) are known drivers of plant-soil nutrient fluxes, yet a comprehensive assessment of AMF involvement in N cycling under global change is still lacking. Here we simulated N enrichment by fertilization in experimental grassland microcosms grown under greenhouse conditions in the presence or absence of AMF and steadily monitored different N pathways over nine months. We found that N enrichment by fertilization decreased the relative abundance of legumes, and the plant species dominating the plant community changed from grasses to forbs in the presence of AMF based on aboveground biomass. AMF always maintained plant N: phosphorus (P) ratio between 14 and 16, no matter changes in the soil N availability. Shifting plant N:P ratios due to the increased plant N and P uptake thus might be a primary pathway of AMF altering plant community composition. Furthermore, we constructed a comprehensive picture of AMFs' role in N cycling, highlighting that AMF reduced N losses primarily by mitigating N leaching, while N2O emissions played a marginal role. AMF reduced N2O emissions directly through the promotion of N2O-consuming denitrifiers. The underlying mechanism for reducing N leaching is mainly the AMF-mediated improved nutrient uptake and AMF-associated microbial immobilization. Our results indicate that synergies between AMF and other soil microorganisms cannot be ignored in N cycling, and the integral roles of AMF in N cycle terrestrial ecosystems can buffer the upcoming global changes.