Contribution of Soil Diazotrophs to Crop Nitrogen Utilization in an Acidic Soil As Affected by Organic and Inorganic Amendments

Biological nitrogen (N) fixation (BNF) driven by diazotrophs plays an essential role in reducing N fertilizer input in acidic soils. Amendments improve the acidic soil environment and increase crop N utilization; however, whether their effects on soil BNF and diazotrophs contribute to crop N utilization remains unclear. A 5-year field trial was implemented to assess the effects of the sole or joint application of organic and inorganic amendments (straw, manure and alkaline slag) on soil BNF rates, canola N utilization, and diazotrophic communities of acidic bulk and rhizosphere soils. Amendments, especially joint application of organic and inorganic amendments, significantly increased canola biomass, N utilization, soil BNF rates, and diazotrophic abundance. There were positive correlations among BNF rates, N uptake and physiological N use efficiency of canola. Both amendments and canola planting significantly affected the structure of diazotrophic communities, with the stronger effects in the joint application of organic and inorganic amendments. Influences of soil C/N, SOC and diazotrophic α-diversity on BNF rates in rhizosphere soils were greater than in bulk soils. The dominant genus affecting BNF rates differed between bulk and rhizosphere soils. Rhizosphere BNF rates were positively correlated with the number of nodes, total edges, and positive correlations of edges of rhizosphere diazotrophic network, but not in bulk soils. The improved soil BNF by organic and inorganic amendments in acidic soils have positive effects on crop N utilization, and the mechanisms underlying the contribution of diazotrophs to BNF rates differ in bulk and rhizosphere soils.