Phosphorus addition promotes Nitrogen retention in alpine grassland plants while increasing N deposition

Nitrogen (N) and phosphorus (P) are common limiting nutrients affecting plant growth in alpine ecosystems. Anthropogenic activities significantly increase N input in ecosystems, thereby increasing P limitation when in excess. The increase of N input and P limitation may affect N retention, which is a critical ecosystem function. To understand how ecosystem N retention and N loss respond to N and P addition, this study investigated the effects of N and P addition on NH3 volatilization, N2O emissions, and N pools of plants and soil in two alpine grasslands (alpine meadow and alpine steppe). The results showed that P addition (with or without N) increased N retention, mainly by promoting plant N uptake, underlining the importance of P availability in N retention in the QTP alpine grassland ecosystems while increasing N deposition. N addition without P showed neutral effects of N retention of plant, soil and microbes, and N addition (with or without P) showed positive effects on gaseous losses of N. N loss through NH3 volatilization and N2O emissions was quite low (less than 1%), suggesting that N gaseous loss was not a significant pathway of N loss from the alpine grassland ecosystems. These results were meaningful for estimating the impacts of N retention and N loss under future N deposition scenarios in alpine grassland ecosystems in the QTP.