Continuous biochar amendment to achieve long-term CH4 mitigation in paddy fields under water-saving irrigation: a 5-year experiment

Biochar amendment is an effective strategy for mitigating methane (CH 4 ) emissions in paddy fields. However, the long-term efficacy of different biochar amendment strategies for mitigating CH 4 emissions remains unclear, particularly under different water management regimes. To address this knowledge gap, a 5-year (2018–2022) field experiment was conducted to determine CH 4 production potential, CH 4 oxidation potential, and CH 4 emissions in paddy fields under six treatments: flooded irrigation (F) without biochar amendment (FB0), F with once biochar amendment (12.5 t ha −1 , FB1), F with continuous biochar amendment (2.5 t ha −1 year −1 , FB5), controlled irrigation (C) without biochar amendment (CB0), C with once biochar amendment (12.5 t ha −1 , CB1), and C with continuous biochar amendment (2.5 t ha −1 year −1 , CB5). Additionally, random forest analysis and structural equation modeling (SEM) were used to elucidate interaction pathways among biochar amendment, water management, CH 4 production and oxidation potentials, and key soil properties affecting CH 4 emissions. In the first year, once biochar amendment demonstrated optimal CH 4 mitigation efficacy under different water management regimes, reducing cumulative CH 4 emissions by 18.87–36.32% compared to other treatments. However, this mitigation effect diminished progressively over 5 years under different water management regimes, with the most rapid decline occurring under C. Consequently, there was no significant difference in the 5-year cumulative CH 4 emissions between FB1 and FB5, while CB5 achieved a significant 29.32% reduction in 5-year cumulative CH 4 emissions compared to CB1. Random forest analysis and SEM identified soil redox potential (Eh), dissolved organic carbon (DOC), and ammonium nitrogen ( NH⁺₄-N NH 4 + -N ) as key soil properties driving CH 4 emissions. CB5 maintained higher soil Eh and NH⁺₄-N NH 4 + -N levels with lower DOC, leading to the lowest CH 4 production potential and highest CH 4 oxidation potential over 5 years, ultimately achieving the greatest CH 4 mitigation. Furthermore, CB5 achieved the lowest CH 4 -attributed greenhouse gas intensity and net greenhouse gas emissions over 5 years. These findings highlight the necessity of long-term monitoring to accurately assess the effects of biochar amendments on CH 4 mitigation and offer technical guidance for implementation of biochar strategies in paddy fields under different water management regimes. Highlights Water-saving irrigation rapidly diminished the CH 4 mitigation effect of the once biochar amendment. Continuous biochar amendment combined with water-saving irrigation was the most effective strategy for CH 4 mitigation over 5 years. Soil Eh, DOC, and NH⁺₄-N NH 4 + -N were key drivers of CH 4 emissions over 5 years.