Effects of winter grazing and N addition on soil phosphorus fractions in an alpine grassland on the Qinghai-Tibet Plateau

Nutrient cycling in alpine grasslands is susceptible to climate change and anthropogenic activities, which can affect soil phosphorus (P) availability. Despite the crucial role of soil P availability in maintaining stability and productivity of grassland ecosystems, limited research has been conducted on the effects of nitrogen (N) addition and winter grazing on P transformation on the Qinghai-Tibet Plateau. In an 8-year experiment, we applied four different N addition rates (0, 25, 50, and 100 kg urea ha ? 1 year- 1) in combination with winter grazing to investigate the effects of N addition and winter grazing on the soil P fractions. The results reveal that increasing the N addition gradually reduced the resin-Pi and NaOH-Pi contents in the soil by increasing the plant P uptake and promoting the release of carboxylates in the rhizosheath, regardless of grazing. Winter grazing decreased the NaHCO3-Pi and NaOH-Pi contents compared with the no-grazing treatment by increasing the P uptake of the plants. In contrast, neither grazing nor N addition affected the HClconc.-P or residual-P content. In the no-grazing plots, the soil NaHCO3-Po content exhibited a gradual increase in response to N addition, whereas N addition had no discernible effect on the NaOH-Po content. In the grazing plots, the NaHCO3- and NaOH-Po contents gradually decreased with N addition, which was associated with the increased acid phosphatase activity in the rhizosheath and the export of forage. Thus, we conclude that N addition promotes the dissolution of NaOH-Pi to more available inorganic P forms. Under winter grazing conditions only, the transformation of P from inorganic to organic forms gradually decreased with increasing N additions.