Integration of Microbial Metabolic Regulation and Abiotic Oxidation: Mechanisms of Artificial Humic Acid-Enhanced Hydroxyl Radical Generation in Paddy Soil

Hydroxyl radicals (•OH) generated under redox fluctuations in paddy soils are crucial for biogeochemical processes. Artificial humic acid (A-HA), as an emerging soil amendment, has been widely applied to improve soil quality. However, the impacts of A-HA on •OH generation and biogeochemical consequences remain under-explored. This study investigated the effects of A-HA on •OH generation, revealing that A-HA enhanced •OH accumulation from 43.93 to 142.30 μM upon oxygenation. The 0.5 M HCl-extracted Fe(II) was the dominant driver for •OH production via H 2 O 2 as an intermediate. Multi-omics analyses demonstrated that A-HA reshaped the microbial community from a sulfate-reducing state to a copiotrophic state dominated by iron-reducing and fermentative bacteria. By acting as an electron shuttle and accelerating intermediate metabolism, A-HA overcame Fe(III) reduction kinetic barriers, providing a thermodynamic advantage that competitively suppressed sulfate-reducing bacteria. Additionally, •OH-induced mineralization and adsorption/fractionation by Fe(III)(oxyhydr)oxides decreased the content of dissolved organic matter (DOM). Fourier transform ion cyclotron resonance mass spectrometry confirmed that “saturated and oxidized” compounds were enriched during anaerobic incubation and subsequently preferentially degraded by •OH through dealkylation, decarboxylation, and oxygen addition, revealing a complementary pattern between anaerobic enrichment and oxidative degradation. Machine learning identified N/C, S/C, m/z, and O/C as key predictors of DOM transformation susceptibility, aligning with the metabolic enrichment of nitrogenous pathways. Furthermore, the enhanced •OH degraded propanil with a removal rate of up to 56.54%, suggesting that contaminants adsorbed on mineral surfaces were more susceptible to degradation. This study revealed a coupled mechanism in which microbial metabolic regulation during reduction enhances iron reduction for subsequent abiotic oxidation, providing new insights into microbial iron reduction, organic matter, and contaminant transformation in paddy ecosystems. Overall, these results further emphasized that A-HA substantially impacts paddy environments far beyond improving soil fertility.