Rice paddy ecosystems receive huge amounts of synthetic nitrogen (N) fertilizers each year. However, the underlying mechanisms by which soil microbes regulate the nitrogen cycle in rice paddies and its response to elevated atmospheric CO2 remain elusive. Denitrification and ammonia volatilization have often been identified as two major pathways of ammonium-N losses in rice paddies, but could not fully explain the gap of the N balance estimation in field. In this study, we combine both 15N-labelling microcosm and field experiments to assess the potential for anaerobic oxidation of ammonium coupled to iron(III) reduction (AOA-IR) under elevated CO2 in a rice ecosystem.
We found that AOA-IR can produce dinitrogen (N2), nitrite (NO2-) or nitrate (NO3-) in rice paddies and elevated CO2 significantly facilitated this process. AOA-IR directly to N2 or AOA-IR-generated NO2- followed by denitrification were two main pathways of N losses under anaerobic conditions. Our results suggested that CO2-induced N loss through AOA-IR and/or denitrification in rice paddies may limit the sustainability of rice plants in response to higher atmospheric CO2, raising an important question about food production under future climate change scenarios.