Recycling annual self-straw as biochar reduces long-term greenhouse gas emissions from paddy fields economically

Abstract Paddy fields contribute significantly to agricultural greenhouse gas (GHG) emissions, and their mitigation is imperative for achieving global carbon neutrality. The commonly adopted practice of a one-time application of approximately 1% topsoil weight biochar (high single, HS) has proven effective in reducing GHG emissions from paddy fields. However, the long-term efficacy of this approach remains uncertain. Our comprehensive 8-year field experiments reveal that the HS method substantially reduces 125 tonnes of CO2-eq per hectare, accompanied by a net financial benefit of US$ 3,732 per hectare. Nevertheless, the HS method exhibits diminishing returns as biochar aged. This decline is attributed to the higher soil ammonium concentration, which hinders methanotrophic activity and reduces soil total carbon content. To counteract this ageing effect, a more sustainable alternative known as the annual low (AL) method involves the yearly recycling of biochar. The AL method surpasses the HS approach, recording a reduction of 197 tonnes of CO2-eq per hectare and a net financial benefit of US$ 4,568 per hectare. This represents a 58% increase in CO2 reduction and a 16% increase in financial benefit compared to the HS method. If the AL method were implemented globally in paddy fields, it could remarkably reduce 4 billion tonnes of CO2-eq GHG, yielding an annual financial benefit of US$ 90 billion. This underscores the viability and economic attractiveness of adopting sustainable practices in paddy field management aligned with the Sustainable Development Goals.