Grazing rest versus no grazing stimulates soil inorganic N turnover in the alpine grasslands of the Qinghai-Tibet plateau

Little information is available on soil N cycling in the grasslands of the Qinghai-Tibet plateau under different grazing regimes, which is required to evaluate management practices with respect to soil N availability and dynamics. In this study, a N-15-labeled ((NH4NO3)-N-15 and (NH4NO3)-N-15) incubation experiment was conducted to investigate gross N transformation rates in soils of the southeast region of the Qinghai-Tibet plateau (China) under no grazing, continuous grazing and grazing rest conditions. Compared with no grazing, continuous grazing significantly reduced the rates of mineralization, microbial NH4+ immobilization, autotrophic nitrification and NO3- consumption, but increased the ratio of autotrophic nitrification to microbial NH4+ immobilization and the ratio of the rates of total NO3- production to total NO3- consumption. This indicated that continuous grazing greatly decreased both the inorganic N supply capacity and N turnover but increased the NO3- production potential of the soil. By contrast, mineralization and microbial NH4+ immobilization rates did not significantly differ in the soils of the grazing rest and no grazing treatments, suggesting that a higher soil inorganic N supply capacity is maintained by grazing rest than by continuous grazing. In addition, grazing rest stimulated NO3- turnover, by increasing the rates of autotrophic nitrification, microbial NO3- immobilization and dissimilatory NO3- reduction to NH4+. Taken together, our study suggests that the most effective management practice for the sustainable utilization of local grasslands is a regime based on grazing rest, as it improved physicochemical and biological properties, which in turn increases N supply and N turnover.