Organic fertilizer amendment decreased N2O/(N2O+N2) ratio by enhancing the mutualism between bacterial and fungal denitrifiers in high nitrogen loading arable soils

Organic fertilizer can enhance soil health and multifunctionality in agroecosystems, but its impact on soil-borne greenhouse gas emissions needs mitigation. Fungal denitrification significantly contributes to N2O emissions in carbon-rich soils; yet, the interactions between bacterial and fungal denitrifers under organic fertilizer amendment, remain unclear. Here, we investigated the rates and proportions of N2O and N2 emissions, along with the interactions between fungal and bacterial denitrifiers in a high nitrogen (N) loading arable soil subjected to four treatments: ⅰ) Control, ⅱ) organic fertilizer (Manure), ⅲ) synthetic fertilizer (Urea), and ⅳ) synthetic plus organic fertilizer (Urea + Manure). Results showed that N2O and N2 fluxes increased by 35.4 and 7.7 folds, respectively, in the Manure treatment compared to Control treatment. And these fluxes increased by 62.9 and 37.0 folds, respectively, in the Manure + Urea treatment compared to Urea treatment. Meanwhile, the contribution of fungal denitrification to N2O emissions significantly increased in both Manure and Urea + Manure treatments, due to the significant enrichment of keystone fungal denitrifiers like Chaetomium among bacterial and fungal denitrifiers' co-occurrence networks. Additionally, N2O/(N2O + N2) ratio significantly decreased in the Manure and Urea + Manure treatments, which was primarily driven by significant enrichment of keystone bacterial denitrifiers carrying nosZ gene such as Achromobacter, Chelatococcus, and Shinella. These bacteria possess complete denitrification capability and can synergize with fungal denitrifiers, enhancing N2O reduction. Overall, our findings suggest that organic fertilizer amendment in high N loading arable soils decreases N2O/(N2O + N2) ratio mainly by enhancing fungal-bacterial denitrifier mutualism.