Nitrogen content and C/N ratio in straw are the key to affect biological nitrogen fixation in a paddy field

Abstract

Background and aimsStraw amendment can increase nitrogen xation in paddy eld, however, the e ciency of carbon sources with different biochemical properties to enhance N 2 xation and nitrogen xation activity is still unclear. MethodsA 15 N 2 -labelling system was used in the eld environment to determine biological nitrogen xation (BNF) under the addition of three different carbon substrates. The nifH gene (DNA) and nifH RNA gene (cDNA) of soil were ampli ed by real-time uorescent quantitative PCR to study the number of nitrogen xing microorganisms and nifH gene expression. The diversity and community composition of nitrogen xing microorganisms were studied by high-throughput sequencing of nifH DNA gene. The study is expected to reveal how different carbon sources impact biological nitrogen xation and its mechanism in the paddy eld system. ResultsResults 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 signi cant changes. Straw amendment had no impact on cyanobacteria abundance. The proportion of N 2 xation increased by MWS was 2.07 times, but diazotrophic nitrogen xation activity decreased. The enhanced N 2 xation by MWS compared with JWS and PB was due to its higher C/N ratio. ConclusionsLow nitrogen content and high C/N ratio are vital to increase N 2 xation, but the e ciency of straw amendment on N 2 xation might not be as high as expected.

How does water management impact the effects of straw returning on biological nitrogen fixation and diazotrophs?

“…Under continuous ooding, straw addition mainly increased the absolute abundance of Gammaproteobacteria and delta/epsilon subdivisions classes (Table 1), but it didn't signi cantly change the absolute abundance of cyanobacteria (Nostocales, Oscillatoriophycideae and Stigonematales), which was a main contributor of BNF in rice eld (Ma et al, 2019;Wang et al, 2020). Straw returning may be more bene cial to anaerobic heterotrophic diazotrophs than cyanobacteria (Roper and Ladha, 1995;Rahav et al, 2016;Zhang et al, 2022), since heterotrophic diazotrophs can't x carbon source through photosynthesis like cyanobacteria. At genus level, the absolute abundance of Desulfovibrionaceae, Azonexus and Azotobacter were increased by straw addition under continuous ooding and they are signi cantly positively correlated with xed nitrogen (Table 2).…”

“…1969). Straw returning, as an important and widespread agricultural management practice in rice eld (Huang et al, 2013;Huang et al 2019), can bring new available carbon sources to soil, thus it may stimulate BNF (Charyulu et al 1981 Zhang et al, 2022). But it also found that straw returning didn't increase the number of diazotrophs using the soil from eld experiment (Liao et al 2017;Yang et al 2019).…”

Comparison of enrichment methods for efficient nitrogen fixation on a biocathode

“…The fixed N is then used for the synthesis of proteins and in particular for the synthesis of L-glutamate and L-glutamine which are inhibitors of nitrogenase synthesis (Moreno-Vivián et al, 1989;Y. Zhang et al, 2022).…”