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Fertilizer management can alter the availability and biological cycle of soil phosphorus (P). The objective of this study was to explore the relationship among soil P availability, organic phosphorus (Po) mineralization and soil bacteria characteristics under organic and inorganic fertilization in saline-alkali paddy soil. Four treatments were designed, namely no fertilizer (Control), mineral fertilizer (NPK), mineral fertilizer with rice straw (NPKS) or rice straw-derived biochar (NPKB). Compared with NPK treatment, straw and biochar addition greatly increased P uptake by rice while decreased soil organic P/total P ratio. The contents of soil available P (AP) and alkaline phosphatase (ALP) in NPKS treatment were higher than those in NPKB treatment. This was most likely due to the higher abundance of Streptomyces in NPKS treatment than that in NPKB treatment, which was known as a potential Po mineralizer. NPKS and NPKB treatments significantly increased diversity and abundance of phoD -harboring bacteria but have no significant effect on the total bacteria. Based on SEM analysis, the composition of phoD -harboring bacteria community was more important in ALP activity than the increased phoD -harboring bacterial abundance and diversity. Soil dissolved organic carbon (DOC) content and DOC: AP ratio were the key factors affecting the formation of total bacterial and phoD -harboring bacterial community, which were significantly increased in NPKS treatment. Compared with biochar application, straw was more efficient in increasing the abundance of some phoD -harboring bacteria, which preferred carbohydrate-rich environments, thereby improving the availability of P and its potential biological transformation in saline-alkali paddy soil. [ABSTRACT FROM AUTHOR]

Soil nitrogen deficiency and poor fertility have been widely concerned in saline-alkaline paddy soil of the Yellow River Delta (YRD). NH 4 +-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 NH 4 +-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 NH 4 +-N content showed no significant changes in N2 level at same straw returning rate. Under N2 level, compared with S0 and S2 rates, NH 4 +-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 NH 4 +-N content via feammox reaction. Therefore, half straw returning with the optimized N fertilization decreased NH 4 + 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. • Half-straw returning could maintain higher NH 4 +-N content than full-straw returning. • Straw and N additions could change soil organic carbon content and fractions. • Geobacter and Anaeromyxobacter related to iron reducing process, can be altered with straw returning rate. • Half-straw returning could slow down the feammox process. [ABSTRACT FROM AUTHOR]