Redox controls on anaerobic ammonium oxidation coupled to reduction of natural organic matter in paddy ecosystems

Microbially mediated anaerobic ammonium oxidation can be coupled to the reduction of various terminal electron acceptors. Redox-active natural organic matter (NOM) can serve as an extracellular electron acceptor or donor in microbial redox reactions. The coupling of the anaerobic ammonium oxidation process to the reduction of NOM (herein termed NOM-ammox) is thermodynamically feasible and evidence has been provided demonstrating its environmental occurrence. Ecosystems with fluctuating redox conditions such as paddy fields are favorable for maintaining the redox reactivity of NOM, and thus may promote NOM-ammox. Here, we synthesize recent literature on NOM-ammox and discuss how redox conditions could control this process at different spatial scales in paddy ecosystems. We conclude that the inherent redox properties of NOM and environmental redox fluctuations are key for fueling and regulating NOM-ammox in paddy ecosystems.