Straw and Biochar Amendments Over a Decade Differently Modulates Denitrification Gas Products

Abstract

Straw return is a sustainable agricultural strategy aimed at raising soil organic carbon (SOC), but tends to stimulate nitrous oxide (N₂O) emissions, potentially counteracting gains in SOC sequestration. Nevertheless, knowledge gaps remain on how long-term different forms of straw incorporation (direct straw return or pyrolyzed to biochar) affect N₂O production and reduction, and interactions with associated key nitrogen (N)-cycling microbial communities. Here, the emission rates and proportions of N₂O and N₂ emissions were quantified using a 13-year field trial with sequential incorporation of straw or straw-derived biochar, and interactions with key functional genes were assessed by metagenomic sequencing. Results revealed that incorporation of straw and biochar increased N₂O emission rates by 2.55 and 0.54 folds, while that of N₂ by 6.41 and 9.77 folds, respectively, compared with conventional fertilization. Correspondingly, the N₂O/(N₂O + N₂) ratios were reduced by 10.75% and 39.74% with straw and biochar treatments. Higher N₂O emissions with straw incorporation were primarily driven by concurrent increase in labile C and N sources with nitrate and nitrite reducers (narG, narH, nirK, nirS, norB) outweighing the N₂O reducer (nosZ). In contrast, biochar incorporation decreased nitrate levels, increased electron conductivity and the N₂O reducer (nosZ), which accelerated N₂ emissions and reduced the N₂O/(N₂O + N₂) ratio. Moreover, reduced N₂O/(N₂O + N₂) ratios were closely associated with altered denitrifier communities, with genera belonging to Acidobacteriota being the key contributors to biochar incorporation, and Pseudomonadota being the dominant contributors to straw. Overall, biochar incorporation was more efficient in reducing global warming potential and increasing SOC sequestration, as evidenced by lower N₂O/(N₂O + N₂) ratios and higher SOC levels. This work provides valuable insights designing net-zero C strategies towards sustainable agricultural C sequestration and greenhouse gas mitigation to address the challenges posed by global climate change.