LAUSR

Grazing and arbuscular mycorrhizal fungi (AMF) influence soil nitrogen (N) cycling in grassland ecosystems. However, it remains unclear whether AMF mediate grazing effects on soil N cycling. We investigated the influence of benomyl (a broad-range fungicide) application to suppress AMF on soil N fluxes under different levels of grazing intensity in a steppe ecosystem on the Mongolia plateau. In situ soil core incubation method was used during both growing and non-growing seasons. Benomyl application to suppress AMF remarkably stimulated the net nitrification rate across all grazing intensities during the growing season. Benomyl application exerted a negative effect on soil N fluxes and/or N pools under no to moderate levels of grazing and a positive effect under heavy grazing during the growing season. The responses of amino acid and inorganic N stocks to grazing differed substantially between growing and non-growing seasons. The accumulation of amino acid in grazed plots (especially heavily grazed plots) during the non-growing season enhances the substrate availability for microbes in the early growing season of the following year. Our study provides field evidence that N losses are controlled to some extent by AMF and suggests a mechanism that grazing may affect soil N cycling through changing the AMF-plant symbiosis and therefore the interactions between AMF and other microbes. The findings improve our understanding of the effects of grazing and AMF on seasonal dynamics of different N forms and N pools in semiarid grassland.