Iron forms regulate methane production and oxidation potentials in paddy soils.

Paddy fields serve as significant sources of methane (CH) emissions. The periodic flooding and draining in paddy soils induce alternating redox processes, leading to iron transformations and further influencing the production and oxidation of CH. However, the relationships between CH production/oxidation and the concentrations/forms of iron oxides in rice paddies across different regions are largely unknown. Here we collected 26 paddy soil samples from various regions spanning from North to South China. We show that the CH production potential varies from 0.005 to 0.618 mg kg d, which exhibits an overall trend of higher values in the south and lower values in the north. Moreover, the CH oxidation potential spans from 0.888 to 57.384 mg kg d, showing no significant latitudinal trend. Highly weathered soils exhibit higher CH production potentials, mainly due to the high content of free iron oxides and the low reactivity of aged iron minerals. This hinders the protection of organic carbon (OC) by iron minerals, therefore increasing substrate availability for methanogenesis. In addition to the direct effect, iron forms also indirectly influence CH production and oxidation potentials by affecting soil pH, OC availability, and CH-related microbial abundances. The coefficients of the indirect effect of iron forms on CH production and oxidation potential are 0.44 and 0.26, respectively, which are larger than that of the direct effects. Our research reveals the pivotal role of various iron forms in controlling CH production and oxidation processes in paddy soils, helping to expand the understanding of the effect of iron biogeochemistry on CH emissions in paddy soils and offering new perspectives for mitigating agricultural greenhouse gas emissions.