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

ABSTRACT

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.