Feammox is more important than anammox in anaerobic ammonium loss in farmland soils around Lake Taihu, China

Ammonium (NH4+) oxidation is a key step in nitrogen transformation in ecosystems. Prior to the recent discovery of Feammox (anaerobic NH4+ oxidation coupled with iron reduction), anammox (anaerobic NH4+ oxidation coupled with nitrite reduction) was thought of as the only pathway by which anaerobic NH4+ loss (NH4+ directly to N2) occurs in soils. Experimental evidence has confirmed that both anammox and Feammox contribute to anaerobic NH4+ loss; however, their relative contributions to this process in farmland soils are largely unknown. Therefore, in this study, we examined the seasonal activities of anammox and Feammox in conventional tillage (CT) and no-tillage (NT) soils around Lake Taihu, China. Isotopic tracing experiments showed higher anammox and Feammox rates in summer than in other seasons, and the contribution of Feammox to anaerobic NH4+ loss from the farmland soils (54.6%-69.3%) was higher than that of anammox. Further, the Feammox rates corresponding to the two soil tillage practices were significantly different, whereas their corresponding anammox rates showed no significant differences. Furthermore, molecular analysis showed that the abundance of Geobacteraceae differed significantly with season and tillage practice, whereas the abundance of anammox bacteria showed no significant differences between CT and NT practices. Structural equation modeling also revealed that the anammox rate was directly or indirectly driven by N availability and season, whereas the Feammox rate was driven by soil moisture content, Fe(III) concentration, Fe(III) reduction rates, tillage practice, and season. Overall, this study enhances understanding regarding anaerobic NH4+ oxidation in farmland soils and highlight the importance of Feammox in NH4+ loss in such an ecosystem.