Paddy fields are major contributors to agricultural greenhouse gas emissions. Applying ~1% biochar by topsoil weight (high single, HS) effectively reduces greenhouse gas emissions from paddy fields, but long-term impacts are unclear. Here we present 8-year field experiments showing HS reduces CO2 equivalent per hectare by 59% and yields a net benefit of US$1,810 per hectare. However, its effectiveness declines over time due to the decreased soil carbon content and methanotrophic activity triggered by higher soil ammonium concentrations. To counteract this, the annual-low method, involving yearly biochar recycling, surpasses the HS approach with a 52% CO2 reduction and yields a net benefit of US$2,801 (35%) per hectare—highlighting the economic and environmental viability of annual-low biochar use in sustainable paddy field management practices.