Short-term P addition may improve the stimulating effects of N deposition on N2O emissions in alpine grasslands on the Qinghai- Tibetan Plateau

The response of N2O emissions to nitrogen (N) addition is usually positive, but its response to phosphorus (P) addition varies, and the underlying mechanisms for the changes in N2O emissions remain unclear. We conducted field studies to examine the response of N2O emissions to N and P addition over two years in three typical alpine grasslands, alpine meadow (AM), alpine steppe (AS), and alpine cultivated grassland (CG) on the Qinghai-Tibet Plateau (QTP). Our results showed consistent increases in N2O emissions under N addition alone or with P addition, and insignificant change in N2O emissions under P addition alone in all three grasslands. N addition increased N2O emissions directly in AM, by lowering soil pH in AS, and by lowering abundance of denitrification genes in CG. N and P co-addition increased N2O emissions in AM and AS but only showed an interactive effect in AM. P addition enhanced the increase in N2O emissions caused by N addition mainly by promoting plant growth in AM. Overall, our results illustrate that short-term P addition cannot alleviate the stimulation of N2O emissions by N deposition in alpine grassland ecosystems, and may even further stimulate N2O emissions.