Effects of biochar amendment on net greenhouse gas emissions and soil fertility in a double rice cropping system: A 4-year field experiment

•The practice of biochar amendment was proposed in a double rice cropping system.•Biochar addition decreased CH4 emissions by 20–51% in four years.•CH4 emission reduction and soil total C increase caused decreased net GHG emissions.•Biochar improved soil fertility via increased soil pH, total organic C, N and P. A 4-year field experiment was conducted to investigate the effects of biochar amendment on the net greenhouse gas emission (NGHGE), greenhouse gas intensity (GHGI), soil fertility and crop yield in a typical double rice cropping system in the central subtropics of China, from April 2012 to April 2016. Three biochar treatments were studied in this experiment, with application rates of 0, 24 and 48 t ha−1 (named CK, LB and HB, respectively) using straw-derived biochar applied once at the beginning of the experiment. In each treatment, the fluxes of methane (CH4), nitrous oxide (N2O) and soil heterotrophic respiration (Rh) were measured using a static chamber/gas chromatography method. Major soil fertility properties were also determined throughout the experimental period. Biochar amendment was found to persistently decrease annual total CH4 emissions by 20 to 51% in the four years, but increased the annual total N2O emissions and Rh by 150 to 190% and 2 to 19% in the first year and the following three years, respectively. On a 4-year average, biochar addition significantly reduced annual NGHGE and GHGI by 156 to 264% and 159 to 278%, respectively (p