Reclaimed saline-alkali paddy field may be a hotspot of methane and ammonia emissions

Salinization and alkalization are global environmental issues, and a growing area of saline-alkali land has been developed as paddy fields. However, the information on the characteristics and driving mechanisms of greenhouse gas (i.e., methane (CH4), carbon dioxide (CO2) and nitrous oxide (N2O)) and ammonia (NH3) emissions from paddy fields with different saline-alkali levels is limited. We conducted a 137-day mesocosm experiment to investigate these issues using light (L), moderate (M), and heavy (H) saline-alkali soils with a history of reclamation of 20, 4 and 2 years, respectively. The results demonstrated that both the cumulative CH4 and NH3 fluxes in H treatment were significantly (p < 0.05) higher than L. While, the increasing saline-alkali levels reduced the cumulative CO2 and N2O fluxes, respectively. With the increasing saline-alkali levels, the species richness and diversity of microbial communities decreased. High saline-alkali level inhibited the growth of ammonia-oxidizing archaea, resulting in less N2O produced by nitrification, thus reducing N2O emission. Cumulative CH4 flux and the mcrA gene copy numbers showed a significant (p < 0.05) negative correlation. The gene copy number in H treatment was lower than M and L, respectively. The highest global warming potential and greenhouse gas intensity were observed in H treatment. Overall, recently reclaimed saline-alkali paddy field with an initial heavy saline-alkali level may be a hotspot of farmland CH4 and NH3 emissions, highlighting the necessity of optimizing water and fertilizer management for controlling these gas emissions at the initial stage of developing saline-alkali lands into paddy fields.