Nitrogen additions increase N2O emissions but reduce soil respiration and CH4 uptake during freeze-thaw cycles in an alpine meadow

Freeze-thaw cycles (FTCs) and increasing nitrogen (N) availability may affect soil carbon (C) and N turnover and thus stimulate greenhouse gas (GHG) emissions in cold regions. However, the combined effects of FTCs and increased N availability on GHG fluxes remain unexplored, especially in high-altitude alpine meadows. We conducted an incubation study to investigate the effects of different forms and levels of N additions on soil trace gas fluxes during three FTCs in an alpine meadow on the Qinghai-Tibetan Plateau. Our results showed that the N2O and CO2 emissions as well as CH4 uptake substantially increased during FTCs. N additions generally enhanced the freeze-thaw-related soil N2O emissions but inhibited soil respiration and CH4 oxidation. NO3--N additions induced significantly higher cumulative N2O emissions during FTCs than NH4+-N additions. The soil respiration rates were significantly reduced with increasing levels of N additions and were positively correlated with the soil DOC and MBC contents. Soil CH4 uptake was substantially inhibited by increasing levels of NH4+-N additions, but was significantly reduced only by high levels of NO3--N additions. Our results indicate that N addition plays an important role in affecting soil GHG fluxes during FTCs. The effects of different forms and levels of N additions on soil GHG fluxes should be considered in future estimations of GHG budget in alpine meadows under a changing climate.