Contrasting Reaction of Dissolved Organic Matter with Birnessite Induced by Humic and Fulvic Acids in Flooded Paddy Soil

Manganese (Mn) oxides exhibit significant potential to either stabilize or destabilize soil organic carbon (SOC) through the polymerization and/or oxidation of organic molecules via organo-mineral interactions. Birnessite (MnO₂) is known to strongly interact with soil dissolved organic matter (DOM), which is DOM composition-dependent. Humic acid (HA) and fulvic acid (FA) are commonly used as organic fertilizers in soils. In this study, the contrasting reaction of DOM with birnessite in flooded paddy soil with HA and FA amendment was investigated at a molecular level. The results demonstrated that HA amendment enhanced the reaction of phenolic compounds in soil DOM with birnessite, leading to the formation of condensed aromatic compounds and polymeric products (PP) with higher molecular weights and aromaticity. This suggests that HA amendment enhances the birnessite-induced polymerization of soil DOM. In contrast, FA facilitated the birnessite-induced oxidation of soil DOM, yielding dicarboxylic acids (DA), monocarboxylic acids (MA), and quinones products (QP). These findings demonstrate that the reactivity of soil DOM with birnessite is significantly influenced by the composition of DOM exogenously added. This study provides comprehensive understandings of the interactions among Mn and C and helps to predict behaviors of DOM molecules in flooded paddy soil, which is critical for optimizing sustainable soil management.