Soil carbon sequestration increment and carbon-negative emissions in alternate wetting and drying paddy ecosystems through biochar incorporation

Biochar has been recognized as one of the most important negative emission technologies due to its indirect removal of atmospheric carbon dioxide. However, how biochar incorporation converts rice paddy into carbon negative and enhances soil carbon sequestration (SOCS) remain largely unexplored, especially under alternate wetting and drying irrigation (IAWD). A 3-yr field experiment was conducted utilizing a split-plot design with continuous flooding irrigation (ICF) and IAWD as main plots and two biochar incorporations at the rate of 0 t ha−1 (B0) and 20 t ha−1 (B20) as sub-plots. Results indicated that IAWD reduced CH4 emissions by 73 %, GWP by 69 %, and greenhouse gas emission intensity (GHGI) by 69 % on average, but increased N2O emissions by 109 % over the three years. Biochar inhibited N2O emissions by 31 % over the three years, increased grain yield by 5 % and 11 %, and reduced CH4 emissions by 22 % and 38 % in the 2nd and 3rd year, although a 6 % lower grain yield was observed in the 1st year. Biochar can increase SOCS and SOCS rate more remarkably under IAWD than ICF. Biochar coupled with IAWD obtained the highest SOCS (56.9 t C ha−1) and lowest net GWP (-23.0 t CO2-eq ha−1) and efficiently inhibited the negative effect of IAWD on the increase in N2O emissions. Moreover, compared with ICFB0, ICFB20 and IAWDB20 decreased net GWP by nearly 4 and 8 times, which was mainly attributed to a substantial increase in SOCS (48.3 and 56.9 t C ha−1) through biochar incorporation. The further decline in net GWP comparing IAWDB20 with ICFB20 was caused by decreased GWP under IAWD. Therefore, biochar combined IAWD offers significant benefits in mitigating net greenhouse gases emissions and promoting soil C sequestration in paddy ecosystems.