Co-application of digestate and biochar reduced greenhouse gas emissions in paddy soil through enhanced denitrification and anaerobic methane oxidation.

Digestate from food waste (FW) has been identified as a promising nutrient resource for agriculture. However, applying digestate directly to soil often produces considerable greenhouse gas (GHG) emissions. As a soil amendment, biochar has demonstrated potential for mitigating GHG emissions. At present, the effect of biochar on GHG emissions and the associated regulatory mechanisms in paddy soils amended with digestate remains unclear. A 45-day soil incubation was conducted with different nitrogen substitution ratios of urea by digestate, coupled with biochar application: CK (100 % urea), DU (100 % urea + biochar), DU (50 % urea, 50 % digestate + biochar), and DU (100 % digestate + biochar). Results indicated that the co-application of biochar and digestate significantly reduced NO accumulation by 44.99 %-80.39 % compared to CK, primarily due to a decrease in soil NO-N content and an increase in soil pH, which together significantly improved the distribution of the nosZ gene involved in denitrification. The increase in the abundance of Conexibacter, Symbiobacterium, Anaerolinea, and Candidatus_Solibacter further contributed to NO reduction. Furthermore, the co-application led to a 21.68 %-38.15 % reduction in CH accumulation compared to CK. Biochar increased the abundance of methanotrophic bacteria, such as Methylococcaceae, Methyloligellaceae, and Methylomirabilaceae. Co-application increased the abundance of nitrate-reducing bacteria Symbiobacterium and Anaerolinea, thereafter facilitating nitrite-dependent anaerobic methane oxidation (AOM) dominated by Methylomirabilaceae. Additionally, sulfate-dependent and Iron(III)-dependent AOM likely further contributed to CH reduction. Overall, this study proposed a low-carbon management strategy for FW digestate and GHG emissions mitigation of paddy soil.