Effects of Biochar Particle Size on Methane Emissions from Rice Cultivation

Biochar amendment is generally recognized as an effective mitigation option of methane (CH4) emissions from rice cultivation. Although its mitigation mechanisms are not well understood, the potential relevance of surface area and porosity of biochar has been discussed. This study aimed to evaluate the application of different biochar particle sizes on CH4 production, oxidation, and emissions from rice cultivation in a clay loam soil, based on the assumption that porosity and surface area of biochar are directly related to its mitigation effects. Rice was grown under greenhouse conditions for two growing seasons, either with 0.5-2 mm (small, SB) or with 2-4 mm (large, LB) biochar. The results show that both sizes of biochar increased soil pH and redox potential (Eh) during rice growth. Soil dissolved organic carbon (DOC), nitrate (NO3-), and sulfate (SO42-) also increased under both biochar amendments, but size effects were not observed. SB and LB suppressed the abundance of CH4 producers (methanogens) but stimulated the abundance of CH4 consumers (methanotrophs). The increase of soil Eh and electron acceptors (NO3 and SO42-) indicated the increase in soil oxidation capacity is a barrier to CH4 production by methanogens in both biochar treatments. Laboratory incubation experiments showed that CH4 production activity was significantly (p <= 0.05) reduced by 18.5% using SB and by 11.3% using LB compared to the control. In contrast, the stimulation of methanotrophs promoted greater CH4 oxidation activity by 15.0% in SB and 18.7% in LB compared to the control. It shows that CH4 production was reduced more by larger surface area biochar (SB), while a greater increase in CH4 oxidation was found using larger pore volume biochar (LB). The effects on CH4 production were more pronounced than those on CH4 oxidation, resulting in a greater reduction of cumulative CH4 emissions by SB than LB (by 26.6% and 19.9% compared to control, respectively).