Background and 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.