Anaerobic primed CO2 and CH4 in paddy soil are driven by Fe reduction and stimulated by biochar

Abstract

Soil C inputs and its priming effect (PE) are important in regulating soil C accumulation and mitigating climate change; however, the factors that control the direction and intensity of PE remains unclear. Soil C accumulation is strongly affected by the reductive iron status in paddy fields, while the addition of organic substances increases the emission of certain gases (CO₂/CH₄) under the PE, contributing to climate change. Here, we elucidated the mechanism by which Fe reduction, measured by Fe(II) production, regulates PE for CO₂ and CH₄ in paddy soils. Specifically, we quantified PE induced by ¹³C-labeled straw in anaerobic paddy soil, augmented by ferrihydrite and/or biochar, over 150 days in a laboratory experiment. The PE of CO₂ was initially negative (-15.3 to -41.5 mg C kg^-1) before 20 days of incubation and subsequently became positive. PE intensity for both gases depended on ferrihydrite or biochar application. Straw+biochar had the highest PEs (CO₂, 116.5 mg C kg^-1; CH₄, 309.4 mg C kg^-1), while straw+ferrihydrite produced the lowest PEs (CO₂, 41.3 mg C kg^-1; CH₄, 107.8 mg C kg^-1). Fe reduction was approximately three times higher with straw+ferrihydrite than with straw alone and was further stimulated by additional biochar. Thus, biochar appeared to accelerate Fe reduction, destabilize mineral-bound organic C, and increase nutrient availability to microbes. Enhanced microbial C and N mining led to a positive PE for CO₂. Cumulative PE for CH₄ was 2-3 times higher than that for CO₂, indicating conversion via methanogenesis. Biochar acted as an electron shuttle, increasing Fe reduction and stimulating interspecies electron transfer, and increased CH₄ production. Therefore, Fe reduction and biochar jointly increased PE intensity for CH₄. In conclusion, water and fertilizer management of paddy soil could contribute toward mitigating climate change.

Keywords: CH(4) production; Electron shuttle; Fe reduction; Paddy field soil; Priming effect; Soil enzyme activity.