Warming and grazing increase mineralization of organic P in an alpine meadow ecosystem of Qinghai-Tibet Plateau, China

Little is known about the soil phosphorus (P) biogeochemical cycling in response to combined warming and grazing, especially in the alpine meadow ecosystem of the Qinghai-Tibet Plateau. Here, we used a free-air temperature enhancement system in a controlled warming-grazing experiment to test the hypothesis that combined warming and grazing would significantly accelerate mineralization of soil organic P. A two factorial design of warming (1.2-1.7A degrees C temperature increase) and moderate grazing was utilized. A fractionation method was applied to investigate the sizes of different soil inorganic and organic P fractions. Results showed that both warming and grazing significantly decreased the quantity of organic P extracted by first NaOH (N(I)Po), as well as the total extractable organic P (TPo) at the 0-10 cm depth. Warming also decreased the total P of soil at 0-10 cm. The combined warming and grazing treatment (WG) led to the reduction of major soil organic P fractions (N(I)Po, TPo) by 40-48% and 28-32%, respectively compared with other treatments at 0-10 cm. The activities of acid and alkaline phosphomonoesterase (AcPME and AlPME) were both enhanced by warming and grazing, and their interaction. Decreased concentrations of soil N(I)Po and TPo were accompanied by increased AcPME activity (P < 0.01) and soil temperature (P < 0.05), indicating the enhanced mineralization of organic P under rising temperature. Meanwhile, leaf biomass P of two major species (Potentilla anserine and Gentiana straminea) within these plots were significantly enhanced by either grazing or warming. The microbial mineralization of soil organic P could be strongly increased under combined warming and grazing conditions as driven by increasing plant demand for P and enhanced microbial activities.