Air warming and CO₂ enrichment cause more ammonia volatilization from rice paddies: An OTC field study

Ammonia (NH₃) volatilization in rice paddies may be affected by elevated atmospheric CO₂ concentration ([CO₂]) and temperature due to changes in plant and soil nitrogen (N) metabolism. At present, little is known about the individual and combined effects of CO₂ enrichment and warming on NH₃ volatilization under field conditions. An experiment was conducted in a rice paddy in Central China, after 4 years of warming and CO₂ enrichment using open-top chamber (OTC) devices. Compared with ambient conditions, elevated [CO₂] had no significant effects on NH₃ volatilization, although increases in soil pH and urease activity were observed. The stimulation on plant N assimilation under CO₂ enrichment might offset the possible enhancement on NH₃ volatilization, as more soil N was absorbed by plant thus reducing NH₃ loss potential. Elevated temperature increased NH₃ volatilization significantly, which could be attributed to increased soil ammonium nitrogen (NH₄⁺-N) concentration, pH, and urease activity. Combination of CO₂ enrichment and warming caused the highest cumulative NH₃ loss, which increased by 26.5% compared with ambient conditions, but the interaction was not significant. Higher plant N uptake, soil NH₄⁺-N concentration, pH and urease activity were also observed with co-elevation of [CO₂] and temperature, but the combined effects were variable and not synergistic. Our findings confirm that field warming and CO₂ enrichment cause more NH₃ volatilization in rice paddies, among which warming effects are dominant, and suggest that improved N management or field practices are required to reduce NH₃ losses under future climate change.