[Effects of Long-term Biochar Addition on Denitrification N₂O Emissions from Bacteria and Fungi in Paddy Soil]

Denitrification driven by bacteria and fungi is the main source of nitrous oxide （N₂O） emissions from paddy soil. It is generally believed that biochar reduces N₂O emissions by influencing the bacterial denitrification process, but the relevant mechanism of its impact on fungal denitrification is still unclear. In this study, the long-term straw carbonization returning experimental field in Changshu Agricultural Ecological Experimental Base of the Chinese Academy of Sciences was taken as the object. Through indoor anaerobic culture and molecular biology technology, the relative contributions of bacteria and fungi to denitrifying N₂O production in paddy soil and the related microorganism mechanism were studied under different long-term biochar application amounts （blank, 2.25 t·hm^-2, and 22.5 t·hm^-2, respectively, expressed by BC0, BC1, and BC10）. The results showed that compared with that in BC0, biochar treatment significantly reduced N₂O emission rate, denitrification potential, and cumulative N₂O emissions, and the contribution of bacterial denitrification was greater than that of fungal denitrification in all three treatments. Among them, the relative contribution rate of bacterial denitrification in BC10 （62.9%） was significantly increased compared to BC0 （50.8%）, whereas the relative contribution rate of fungal denitrification in BC10 （37.1%） was significantly lower than that in BC0 （49.2%）. The application of biochar significantly increased the abundance of bacterial denitrification functional genes （nirK, nirS, and nosZ） but reduced the abundance of fungal nirK genes. The contribution rate of fungal denitrification was significantly positively correlated with the N₂O emission rate and negatively correlated with soil pH, TN, SOM, and DOC. Biochar may have inhibited the growth of denitrifying fungi by increasing pH and carbon and nitrogen content, reducing the abundance of related functional genes, thereby weakening the reduction ability of NO to N₂O during fungal denitrification process. This significantly reduces the contribution rate of N₂O production during the fungal denitrification process and the denitrification N₂O emissions from paddy soil. This study helps to broaden our understanding of the denitrification process in paddy soil and provides a theoretical basis for further regulating fungal denitrification N₂O emissions.