Fe(II)-OM Complexes Formed by Straw Returning Combined with Optimized Nitrogen Fertilizer Could Be Beneficial to Nitrogen Storage in Saline-Alkaline Paddy Soils

Abstract

Soil organic carbon (SOC) plays a crucial role in controlling the nitrate-dependent Fe(II) oxidation (NDFO) process, especially for saline-alkaline soils. The effects of straw returning combined with Nitrogen (N) fertilizer application on soil NO3−-N content, Fe(II) form and nirK genes in saline-alkaline soil were studied in a five-year field experiment to explore the regulatory mechanism of SOC on NDFO process. Six treatments were designed with two factors (1) three straw returning rates (C0, C1 and C2, which was 0, 4500 and 9000 kg C ha−1, respectively) and (2) two N fertilization rates (N1 and N2, which was 255 and 400 kg N ha−1, respectively). Under both N levels, compared with C0 and C2 rates, NO3−-N content was increased by 65% and 50% in C1 rate, respectively. NirK genes were decreased with straw returning, in which they were 42.9–58.8% lower in C1 and C2 treatments than that in C0 treatment, respectively. In the N1C1 treatment, the increase of SOC reduced the denitrification by converting aqueous Fe(II) (Fe(II)aq) into Fe(II)-OM complexes and reducing the abundance of nirK genes. Overall, appropriate straw returning (C1) under optimal N fertilization rate (N1) could reduce N loss by decreasing the NDFO process in saline-alkaline paddy soils.