Soil carbon (C) cycling under anoxic conditions is mechanistically linked to dissimilatory iron (Fe) reduction, which could be regulated by exogenous dissolved organic matter (DOM). However, the impact of complex exogenous DOM on soil microbial activity and C-Fe coupling in paddy soils remains underexplored. With a 100-day microcosm experiment, we determined that the molecular composition of DOM derived from organic fertilizers and biochar on paddy soil affects soil microbial community, Fe reduction, and C emissions. Our results indicated that biochar-DOM significantly promoted Fe reduction and accelerated CH₄ and CO₂ emissions, and organic manure-DOM increased soil CO₂ emissions, which was closely related to the molecular composition of exogenous DOM. DOM molecules with high aromaticity (AI_mod) and high DBE, including lignins-vascular plant-derived polyphenols, lignins-polycyclic aromatics (PA), and condensed aromatics-PA, promoted soil Fe reduction and CH₄ emissions by enriching soil Fe-reducing bacteria (FeRB), reducing methanotrophs, and facilitating r-strategists at the early stage of incubation. Conversely, DOMs with low AI_mod, low DBE and high H/C, such as lignins-highly unsaturated and phenolic compounds and proteins-aliphatic compounds, enhanced CO₂ emissions by facilitating recalcitrant C degradation and CH₄ oxidation at the late stage of incubation. In conclusion, our study highlights the importance of the molecular composition of DOMs derived from organic amendments in regulating soil Fe reduction and C emissions. The findings offer novel insights into the effective utilization of agricultural resources and the mitigation of soil C emissions.

The cycling of soil phosphorus (P) is inherently linked with soil organic carbon-iron (C-Fe) cycling, yet empirical integration of these processes within paddy soils remains scarce. In this study, we conducted a microcosm experiment using paddy soils subjected to six distinct fertilization regimes involving varying P inputs for five years. In addition to evaluating P activation under reflooding conditions, we assessed the Fe reduction process and characterized the properties of dissolved organic matter (DOM) at the molecular level using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), alongside profiling the composition of soil microbial communities with high-throughput sequencing. Our findings revealed that after 25 days of reflooding, soil Olsen-P content increased by an average of 73% compared to its initial state, showing a strong correlation with the Fe reduction process. Specifically, treatments involving pig manure application exhibited higher Fe reduction rates and enhanced P activation, highlighting the role of organic matter in facilitating Fe reduction. Investigations on the relative abundance of typical iron-reducing microbes further supported their importance in P activation, but the rate of iron reduction is limited by soil organic matter content. Delving deeper into DOM properties, soil DOM composition profiling and network analysis suggested that high-molecular-weight DOM, particularly lignins, served as the primary resources driving Fe reduction by iron-reducing microbes, consequently promoting Fe reduction and P release. Taken together, our study assembled the C-Fe-P cycling dynamics in paddy soils, emphasizing the pivotal role of microbial-driven Fe reduction facilitated by soil DOM in P availability and subsequently sustainable agricultural practices.