Responses of Methane and Nitrous Oxide Emissions and Soil Methanogenic Community Composition to Biochar and Nitrogen Fertilizer in Rice Field

To enhance the crop production and soil fertility, the application of biochar (B) and nitrogen (N) has been practiced for many years. However, the effects of combined biochar (B) and nitrogen (N) treatments on the soil methanogenic community, methane (CH4) emissions, and nitrous oxide (N2O) emissions in paddy fields are not well known. Herein, we investigated the effects of four different B rates (0, 10, 20, and 30 ton ha -1) combined with two N levels (low: 135 kg ha−1; high: 180 kg ha−1) on CH4 and N2O emissions from rice fields, as well as the composition of the soil methanogenic community. The study was conducted at two locations - Guangxi University in Nanning (GXU) and Yulin City - across different years. Results showed that in both locations, B (30 ton ha-1) combined with both N (135 and 80 kg ha-1) improved soil pH (23 ~ 29%), soil organic carbon (25 ~ 32%), total N (24 ~ 28%), available phosphorus (16 ~ 18%), available potassium (78 ~ 85%), soil microbial biomass carbon (122 ~ 129%), microbial biomass N (110 ~ 133%), and the grain yield of rice (20 ~ 30%) compared with sole N-treated plots. Furthermore, increases in the B rate (> 20 ton ha-1) under both low and high N reduced N2O emissions in both locations and all seasons. However, CH4 emissions and the abundance of methanogenic archaea were increased in high B rate (30 ton ha−1) treatments under both N levels. CH4 emissions were 15 ~ 17% higher in GXU and Yulin City in the 20 ~ 30 ton B ha−1 treatments compared with the sole N treatments. Rice grain yields were highest, CH4 and N2O emissions were lowest, and soil characteristics were enhanced in the 10 ton B ha−1 with low N fertilizer treatment. The results of our study suggested that the use of 10 ton ha−1 combined with 135 kg N ha−1 is the most efficient method for enhancing soil fertility and rice yields while maintaining environmental sustainability.