Nitrogen Content and C/N Ratio in Straw are the Key to Affect Biological Nitrogen Fixation in a Paddy Field

Abstract Background and aims Straw amendment can increase nitrogen fixation in paddy field, however, the efficiency of carbon sources with different biochemical properties to enhance N2 fixation and nitrogen fixation activity is still unclear. Methods A 15N2-labelling system was used in the field environment to determine biological nitrogen fixation (BNF) under the addition of three different carbon substrates. The nifH gene (DNA) and nifH RNA gene (cDNA) of soil were amplified by real-time fluorescent quantitative PCR to study the number of nitrogen fixing microorganisms and nifH gene expression. The diversity and community composition of nitrogen fixing microorganisms were studied by high-throughput sequencing of nifH DNA gene. The study is expected to reveal how different carbon sources impact biological nitrogen fixation and its mechanism in the paddy field system. Results Results showed that the absolute abundance (nifH DNA gene copy numbers) of heterotrophic diazotrophs in the treatment of mature stage wheat straw (MWS) was 4.88 times as high as that in the CK treatment, but jointing stage wheat straw (JWS) and poplar branch (PB) did not induce any significant changes. Straw amendment had no impact on cyanobacteria abundance. The proportion of N2 fixation increased by MWS was 2.07 times, but diazotrophic nitrogen fixation activity decreased. The enhanced N2 fixation by MWS compared with JWS and PB was due to its higher C/N ratio. Conclusions Low nitrogen content and high C/N ratio are vital to increase N2 fixation, but the efficiency of straw amendment on N2 fixation might not be as high as expected.