P addition weakens the positive effects of N addition on CH4 uptake and does not affect the effects of N addition on CO2 emission in alpine grasslands

Abstract Aims Anthropogenic activities have substantially elevated nitrogen (N) deposition globally and affect ecosystem processes, including soil carbon (C) storage potential. Phosphorus (P) can become a limiting factor for plant production in instances of N deposition, yet the responses of ecosystem C cycles to P enrichment are poorly understood, particularly in sensitive alpine ecosystems. Methods We conducted a short-term field study to appraise the effects of N and P addition on ecosystem CO₂ emissions and CH₄ uptake in three typical alpine grasslands, alpine meadow, alpine steppe, and cultivated grassland on the Qinghai-Tibet Plateau (QTP). The closed chamber technique was employed to monitor the fluxes of CO₂ and CH₄. Environmental factors, including plant biomass and diversity and soil nutrients, and the abundance of C-cycling genes were analyzed to investigate the factors regulating CO₂ and CH₄ fluxes. Results The results showed that: (i) N and P addition tended to increase CO₂ emissions and CH₄ uptake. Furthermore, P addition weakened the positive effects of N on CH₄ uptake across the three grasslands, but the interaction of N and P addition on CO₂ emissions varied across the three grasslands. (ii) N and P addition affected the fluxes of CO₂ and CH₄ both directly and indirectly through their impacts on soil and plant factors rather than C-cycling functional genes. Conclusions These results indicate that in the context of increasing N deposition in the QTP, short-term P addition is not an effective method for mitigating global warming potential and improving soil C sequestration in alpine grassland ecosystems.