Impacts of straw returning and N application on NH4+-N loss, microbially reducible Fe(III) and bacterial community composition in saline-alkaline paddy soils

Soil nitrogen deficiency and poor fertility have been widely concerned in saline-alkaline paddy soil of the Yellow River Delta (YRD). NH₄⁺-N content regulated by soil organic carbon (SOC) and the rate of feammox reaction which provides a worth understanding in nitrogen (N) transformation. A 5-year field experiment was carried out to investigate the effect of straw returning and N fertilization rates on the NH₄⁺-N, microbially reducible Fe(III) (MR-Fe) and bacterial community composition in YRD. Six treatments were designed, including three straw returning rates (S0, S1 and S2, which was 0, 4500 and 9000 kg C ha⁻¹, respectively) under two N fertilization levels (N1 and N2, which was 400 and 255 kg N ha⁻¹, respectively). Our results showed that, compared with N1 level, soil NH₄⁺-N content showed no significant changes in N2 level at same straw returning rate. Under N2 level, compared with S0 and S2 rates, NH₄⁺-N content was increased by 64.8% and 50.3% in S1 rate, respectively. SOC content showed no difference between S1 and S2 treatments, which were higher than that in S0 treatment. While the labile carbon (i.e., readily oxidized organic carbon and dissolved organic carbon) was increased in S2 treatment, which promoted the dissolution of MR-Fe and increased the relative abundance of Geobacter and Anaeromyxobacter. Additionally, the structural equation models further indicated that more soil labile carbon could improve MR-Fe content and Geobacter and Anaeromyxobacter abundance, which decreased soil NH₄⁺-N content via feammox reaction. Therefore, half straw returning with the optimized N fertilization decreased NH₄⁺ oxidation and Fe reduction by supporting a more stable bacteria community structure, which played a key role in N availability and promoted the sustainable development of modern agriculture in saline-alkaline paddy soils.