Grazing Horse Effects on Desert Grassland Soil Gross Nitrification and Denitrification Rates in Northern China

The aim of this study was to investigate the effects of grazing on soil gross nitrification (GN) and denitrification (DN) rates and soil environmental factors on GN and DN in the desert grassland of northern China. Soil samples were collected from July to November in 2017 and March to June in 2018, with 5-5 soil samples taken from three enclosures (CK) vs. three heavy-grazing (G) randomized treatment blocks. We determined: (1) the soil moisture (SM), pH, bulk density (BD), total nitrogen (TN), soil organic carbon (SOC), and inorganic nitrogen (IN, NH4+-N, and NO3−-N) content, and (2) GN and DN. The relationship between the changes in GN, DN, and the soil environment was analyzed using stepwise multiple-regression analysis. Gross nitrification, DN, pH, BD, C/N, SM, IN, and NO3−-N varied significantly by month. Grazing induced significant increases in SM and NO3− only. GN in the CK treatment was related to NH4+-N and NO3−-N, while GN in the G treatment was related to NH4+-N and SM. DN in the CK treatment was related to NH4+-N, while DN in the G treatment was related to C/N. Additionally, GN and DN had obvious seasonal variations and reached a maximum in July. This highlights the different underlying mechanisms that affect soil GN and DN and the dynamics, particularly in the desert grassland system.