Five-year assessment of multiple inter-tillage weeding on greenhouse gas emissions, rice yield, and carbon balance in organic rice farming in Hokkaido, Japan

Traditional multiple inter-tillage weeding (MIW) is believed to improve rice yield and anaerobic soil conditions, reducing methane (CH₄) emissions and enhancing carbon sequestration. However, as global climate change accelerates and necessitates sustainable organic rice farming, the effects of MIW on greenhouse gas (GHG) emissions and carbon balance remain unclear. This study investigated whether MIW decreases CH₄ emissions, lowers global warming potential (GWP), reduces GHG intensity (GHGI) through higher yields, and enhances carbon sequestration. Field experiments were conducted in Hokkaido, Japan (2019–2023), performing MIW 0, 2, or 5 times (T0/T2/T5) without fertilizers or agrochemicals. Subsequently, we determined the GHG balance (GHGB) and net ecosystem carbon balance (NECB), measured soil temperature, redox potential (Eh), and physicochemical properties, and GHG emissions, using the closed chamber method. GHGB values were positive, indicating emissions, in the order T0 < T5 < T2 (30.2, 44.9, 48.0 Mg CO₂-eq ha⁻¹). NECB values were negative, indicating a loss, in the order T2 < T5 < T0 (-10.3, −7.96, −5.42 Mg C ha⁻¹). Contrary to our hypothesis, MIW increased CO₂ (heterotrophic respiration (RH)) and CH₄ emissions, elevating GHG emissions and carbon loss risk. However, T5 showed the lowest GHGI, suggesting improved yield efficiency. To mitigate MIW-induced carbon loss and GHG risks, the adoption of slow-decomposing organic amendments, such as biochar, or adjusting water management practices is recommended. These strategies could enhance the potential of MIW for sustainable organic rice farming. Our findings highlight the urgent need for improved MIW practices to balance yield gains and mitigate climate change.