Arbuscular mycorrhizal fungi decrease soil ammonium availability and nitrous oxide emissions under nitrogen input

Arbuscular mycorrhizal fungi (AMF) form a mutualistic symbiosis with the roots of most land plants and assist the host plants in taking up soil phosphorus and nitrogen (N) in exchange for carbon. Thus, AMF plays an important role in regulating the soil N cycle and mitigation of nitrous oxide (N2O). However, how AMF mediates the effect of N addition on soil N2O emissions remains unclear. To address this knowledge gap, we manipulated three independent soil environments in the field under four N addition rates (0, 50, 100, and 200 kg N ha-1 yr-1) in a subtropical Chinese fir (Cunninghamia lanceolata (Lamb.) Hook) plantation using split-plot design, which included (i) both roots and AMF hyphae (R+AMF) having access to N; (ii) only AMF hyphae having access to N (-R+AMF); and (iii) neither roots nor AMF hyphae (-R-AMF) having access to N. We found that increased N addition consistently increased soil N2O emissions, and that AMF without (-R+AMF) or with root (R+AMF) significantly decreased soil N2O emissions by 62% and 79%, respectively, as compared with the no AMF and root treatment (-R-AMF). A negative correlation between AMF hyphal length density and soil NH4+-N concentration suggested that AMF could effectively decrease soil N2O emissions through hyphal N uptake that reduces soil NH4+-N. Further, we found that AMF increased the abundance of nitrification genes (archaeal amoA), but did not affect the key genes responsible for denitrification (nirS, nosZ and nosZ/(nirK+nirS)). However, AMF and N addition did not interactively affect soil N2O emissions or N2O-producing microbes. Due to the limitation of methods, we were unable to control the species of AMF and quantify the effect of AMF-induced soil moisture changes on N2O emissions. Our findings indicated that the presence of AMF could effectively prevent the increase of soil N2O emissions under increased N deposition to some extent, and therefore mycorrhizae should be highlighted for their role in mitigating global warming.