Fate and transport of substances associated with aerobic and anaerobic SOM decomposition in paddy soils

Abstract

We developed a reactive transport model that describes aerobic and anaerobic soil organic matter (SOM) decomposition, carbon and nitrogen cycling, ion exchange, and mineral dissolution and precipitation in paddy soils using the HYDRUS1D-PHREEQC (HP1) program. The first-order organic matter decomposition model based on LEACHM was modified to incorporate oxidation of organic carbon, sequential reduction of electron acceptors, and pH buffering of variable charges. These modifications enable the model to simulate aerobic and anaerobic SOM decomposition processes depending on redox potential (Eh) and pH. A numerical experiment was conducted, assuming a one-dimensional soil layer with organic matter application under saturated water flow conditions. The results successfully replicated the formation of about 1 cm surface oxidized layer and a reduced layer where reduction has progressed to the methane generation, and evaluated the concentration distribution changes of each component, as well as reactive transport, including ion exchange, depending on Eh and pH. Furthermore, the dissolution and precipitation of minerals such as manganite and goethite had significant impacts on the composition of the soil solution and solid-phase minerals. The high concentration of FeS in the reduced layer, formed through precipitation, played a critical role in mitigating H2S formation, while also influencing the increase in H2S due to a depletion in Fe2+ concentration.