Effects of N-enriched biochar on ecosystem greenhouse gas emissions, rice yield, and bacterial community diversity in subtropical rice paddy soils

Nitrogen (N)-enriched biochar is a novel type of biochar prepared by mixing biomass with N-rich sources at a higher temperature via pyrolysis. Although the agronomic benefits of N-enriched biochar are reportedly positive, its effects on GHG emissions, crop yields, and soil bacterial communities in rice paddy fields remain undefined. This study examines the short-time effects of adding different rates of N-enriched biochar (0, 4, and 8 t ha−1) to subtropical early and late rice cropping soils on GHG emissions – namely carbon dioxide (CO2) and methane (CH4), rice yields, and soil bacteria communities. Across the two seasons, cumulative CO2 emissions increased under both biochar treatments while cumulative CH4 emissions declined with the application of biochar at 8 t ha−1. Early rice's global warming potential increased by 86 and 21% following application of biochar at 4 and 8 t ha−1, respectively, and by 43 and 41%, respectively, in late rice. Rice yields significantly (P < 0.05) increased with the application of 4 t N-enriched biochar ha−1 while rates at 8 t ha−1 had no effect. Application of N-enriched biochar altered the relative abundance of dominant bacterial taxa related to N and C cycling, particularly of denitrifying Rhodanobacter and Sphingomona in early and late rice soils, respectively. Furthermore, both rates of N-enriched biochar raised the relative abundance of Thiobacillus and Lysobacter in early and late rice-the latter associated with rice yields-while reducing Streptomyces and H16 taxa. Overall, findings suggest that the application of 4t ha−1 nitrogen-enriched biochar can increase rice yield and reduce CH4 emissions while increased soil cumulative C emissions GWP caused by CO2 emission increase, in part via biochar-induced changes on the plant growth characteristics and soil's bacterial community. This study reflects that the application of moderate doses of biochar can become an important sustainable soil management method for rice fields in subtropical regions, and the application of exogenous organic nutrients can promote crop yields and reduce GHC emissions. Now, a long-term experiment would be required to assess what is the optimal yearly frequency of application of these moderate doses of biochar.