Biochar interacted with organic compounds from digestate in controlling N₂O emissions

Treating biochar with digestate can form a C- and N-enriched biochar fertilizer, but its role in controlling N₂O emission from soil with different pH is unclear. This study assessed N₂O emission from rhizosphere soil after growing ryegrass with urea, urea plus biochar, solid digestate, and digestate-incorporated biochar, with and without liming. The abundances of bacteria, fungi, two nitrification genes (bacterial amoA; archaeal amoA), and four denitrification genes (nirK, nirS, nosZ for clade I and nosZII for clade II) were quantified using quantitative PCR. Bacterial community composition was characterized using amplicon sequencing. Solid digestate and urea plus biochar decreased N₂O emission by 48% and 56%, respectively, relative to urea under non-liming. This corresponded to the increased bacterial abundance and greater increases in N₂O-consuming (nosZ and nosZII) than N₂O-producing (archaeal amoA, nirK, nirS) gene abundances. Digestate-incorporated biochar decreased N₂O emission by 75% compared to solid digestate, with decreased nirK gene abundance and increased prevalence of the denitrifier Dokdonella. Liming resulted in the lowest N₂O emissions and highest nosZII gene abundance among all treatments. This study demonstrated the value of incorporating biochar in digestate in reducing N₂O emission while enhancing plant nutrition.