Biochar-based urea increases soil methane uptake in a subtropical forest

Novel biochar-based fertilizers, produced by combining biochar particles with chemical fertilizers, have strong potential to enrich soil with carbon (C) and nitrogen (N), and to increase plant productivity. Application of biochar-based fertilizers modifies soil microbial community compositions, thereby influencing greenhouse gas emissions, including methane (CH4) fluxes. Due to the improved aeration in soils amended with biochar particles, we hypothesized that biochar-based urea would decrease CH4 production and increase CH4 oxidation, leading to increase of total CH4 uptake by soil. A three-year field experiment was conducted in a subtropical Moso bamboo forest to compare the impacts of biochar-based urea and traditional urea on seasonal CH4 uptake by soil, as well as on soil physicochemical and microbial attributes. Urea application lowered the annual soil CH4 uptake by 6 % and 16% at 100 and 300 kg N/ha, respectively, within the first year. Biochar-based urea application at 300 kg N/ha increased the annual CH4 uptake by 12 % in both the first and second years, whereas the effects weakened over time. Soil CH4 uptake was positively correlated with CH4 oxidation rate but negatively with CH4 production rate. The urea-induced decrease in CH4 uptake was attributed to the increased NH4+ and NO3- contents and mcrA gene abundance as well as decreased pmoA/mcrA ratio, thereby increasing CH4 production rate. The biocharbased urea increased CH4 uptake by enhanced soil aeration and labile C supply, which created favorable conditions for methanotrophs. This resulted in increased pmoA gene abundance and the pmoA/mcrA ratio, thereby accelerating CH4 oxidation. Overall, these findings underscore the potential of biochar-based fertilizers in augmenting CH4 uptake within subtropical forest soils. Notably, the transition from traditional urea to biochar-based urea in China Moso bamboo forests alone has the potential to lift annual soil CH4 uptake by an estimated 4450 t.