Convergent and divergent responses of topsoil nitrogen and phosphorus content to nutrient addition in natural and restored alpine grasslands around the Qinghai Lake Basin

Abstract

The availability and variation of soil nutrients are especially important for plant growth and soil carbon accumulation in the ongoing climate change. As a climate-sensitive region, the Qinghai Lake Basin (QLB) has experienced extensive land-use transitions. A three-year manipulative experiment was conducted in two grassland types around the QLB, including a natural alpine grassland (NG) and its adjacent restored grassland (RG), to examine the responses of soil nitrogen (N) and phosphorus (P) content to nutrient application with land-use. N addition continuously increased soil NO3−-N (SAN) content of NG in the whole experiment period, while displaying no detectable effect on SAN content of RG in the first experiment year. Nutrient addition presented no direct effects on soil total N (STN) content. However, N addition potentially promoted STN content indirectly by increasing SAN content, both in NG and RG. P addition significantly increased soil available P (SAP) content during the whole study period for both grasslands. Nevertheless, P addition significantly increased soil total P (STP) content of NG in the last two experiment years, while only elevating STP content of RG in the last experiment year. The results revealed that soil N and P content presented convergent and divergent responses to nutrient addition between NG and RG. STN and STP content were both positively related to growing season precipitation (GSP). SAN content was positively associated with growing season temperature in NG, and RG in N addition treatment. SAP content was consistently positively related to GSP in P addition treatment, but negatively related to GSP in the treatment without P addition, both in NG and RG. These findings highlight the importance of precipitation for soil nutrient accumulation, and imply that warming and increasing precipitation will be beneficial for the mineralization of soil N and P, respectively, in this semi-arid grassland ecosystem.