Alteration of gaseous nitrogen losses via anaerobic ammonium oxidation coupled with ferric reduction from paddy soils in Southern China

Abstract

The bacteria-mediated anaerobic ammonium oxidation under iron reducing conditions, termed feammox, represents a process for alleviating N accumulation in anoxic soils. Fertilization, as an important agricultural strategy, needs to be investigated in order to determine its effects on nitrogen (N) removal via the feammox process in paddy soils. In this study, a slurry incubation experiment was conducted in fertilized paddy soils with a gradient of microbial reducible Fe(III) levels obtained from Southern China using 15N-isotope tracing techniques. Four fertilizer treatments were examined: an un-fertilized control (NF), chemical fertilizers (CF), chemical fertilizers plus manure (CMF) and chemical fertilizers plus crop straw (CSF). It was estimated that the potential N losses linked with feammox were 3.6–24.9 kg N ha−1 y−1 in all the examined soils. Compared to the unfertilized soil (NF), fertilization stimulated feammox and led to higher (3.4–5.8 times) N losses. We postulate that the variations in the extent and rate of feammox between the unfertilized and fertilized soils were most likely due to differences in the abundance of the Acidimicrobiaceae bacterium A6 and the amounts of microbial reducible Fe(III). Further, the variations between soil treated with fertilizer (CF) only and soils coupled organic-chemical fertilizer (CMF and CSF) were due to the differences in the electron transfer mechanism mediated by electron shuttles from bacteria to Fe(III) minerals arising from the organic carbon applied. Overall, this study clearly illustrated the stimulatory effects of fertilization on feammox that resulted in higher N losses and suggested that feammox could be a crucial N removal pathway in paddy soils.