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.

Automated summary

Little information is available on soil nitrogen cycling in the grasslands of the Qinghai-Tibet plateau under different grazing regimes, which is required to evaluate management practices with respect to soil nitrogen availability and dynamics. In this study, it was found that continuous grazing significantly reduced the rates of nitrogen mineralization, microbial NH4+ immobilization, autotrophic nitrification, and NO3- consumption, while increasing the ratios of autotrophic nitrification to microbial NH4+ immobilization and the rates of total NO3- production to total NO3- consumption. On the other hand, grazing rest maintained a higher soil inorganic nitrogen supply capacity than continuous grazing and stimulated NO3- turnover.