Evaluating the effect of different biochar application sizes on methane emission reduction from rice cultivation

Application of biochar to the soil has been reported as one of the mitigation technologies of CH4 emission from rice cultivation due to its unique characteristics of high porosity and surface area. The application of small particle size of biochar is rich in surface area that may enhance the mitigation potential. Rice cultivation and soil incubation experiments were conducted to evaluate the effect of two groups of biochar particle size on CH4 emission and production in order to show the mitigation potential. This experiment consists of three treatments including no biochar (CT), small particle size (0.5-2 mm) biochar (SB), and large particle size (2-4 mm) biochar (LB). Both biochar sizes were amended at 10 t ha−1 equivalent rate and all treatments were applied chemical fertilizer at 100 kg N ha−1 equivalent rate. The results demonstrated that SB and LB reduced cumulative CH4 emission by 24.0% and 17.1% and cumulative CH4 production by 24.6% and 15.0% as compared to CT, respectively. Our results showed that SB achieved higher mitigation potential than LB by an average of 8.47%, although it was not significant. The mitigation of both biochar sizes was supported by the significant change of soil methanogens and methanotrophs abundances. The suppression of methanogens abundance and the stimulation of methanotrophs abundance indicated in the ratio of mcrA to pmoA was significantly reduced in SB (68.0%) which higher than in LB (56.3%) as compared to CT. Both application sizes also increased soil oxidation capacity through soil Eh increase which no difference between SB and LB. In term of grain yield, SB and LB were not different and both did not show the significant change as relative to CT. The application of small size biochar in this study affected more mitigation potential of CH4 emission as compared to larger size, therefore there is a need of further study on typical size of biochar in order to recommend the most mitigation potential of biochar application.