Simultaneous effects of biochar and nitrogen fertilization on nitrous oxide and methane emissions from paddy rice

Synthetic fertilizers are major agents of gaseous emissions including nitrous oxide (N₂O), and rice cultivation is a primary source of methane (CH₄) emission. Biochar (BC) addition to agricultural soils is a potential approach to mitigate N₂O and CH₄ emissions. This greenhouse study was conducted to assess the simultaneous effects of BC and nitrogen (N) fertilization to reduce N₂O and CH₄ emissions along with higher biomass accumulation in rice under controlled conditions. Nine treatment combinations of BC amendments at 0, 2 and 4% by weight (weight of BC/weight of soil) mixed into 3500 g of unsterile soil with 0, 70 and 140 kg N ha^-1 were used in growing rice. Results show that BC-only treatments enhanced the volumetric water contents (VWC) by 9-14% and soil pH by 5-7% coupled with higher daily and cumulative seasonal CH₄-C fluxes by 85-95% and 48-51%, respectively, compared with control treatment. Under 2% and 4% BC amendments, N at 140 kg ha^-1 lowered the daily and cumulative seasonal CH₄-C fluxes by 24-42% and 20-30%, respectively as compared to 70 kg N ha^-1. Furthermore, BC amendments in N-treated soils reduced the daily and total seasonal N₂O-N emissions by 27-67% and 49-61%, respectively, relative to N-only treatments. However, N addition in BC-amended soils showed 10-16% decreased VWC compared with the BC-only treated soils. In terms of rice growth, BC-only reduced the above- and below-ground biomass accumulation, delayed the tillering phase, and resulted in fewer vegetative tillers except for BC-treated pots with 140 kg N ha^-1. Thus, this study suggests that the use of BC amendment at 2% with 140 kg N ha^-1 may be a beneficial strategy to reduce the net GHG emissions from paddy rice in an Alfisol.