Iron Plaque Formation in Mollisols Following Irrigation with Groundwater: Effects on Organic Matter Biotransformation and Selenium Biogeochemistry

The widespread expansion and intensive irrigation of paddy wetlands exert substantial impacts on elemental cycling and health status of agroecosystems. In cold-region Mollisol paddy wetlands particularly, strong perturbations by periodic irrigation using groundwater with an elevated Fe(II) concentration may modify the speciation and mobility of essential micronutrients, e.g., selenium (Se), in the vadose zone. Nonetheless, the underlying mechanisms driving Se fate remain unclear. Combining field investigations and column experiments, this study reveals that periodic Fe(II)-rich groundwater irrigation drives widespread iron plaque formation in plow-layer Mollisols, which are distinct from pristine soils. The raised Fe(II) concentration in the irrigation water caused aggregation of high-molecular-weight aromatic compounds, thereby preventing their degradation under an anaerobic-aerobic cycle. Furthermore, the amorphous Fe oxide-organic matter (OM)-Se conglomerates formed during the aerated period of groundwater irrigation created a new Se pool, as opposed to organic-bound Se in pristine soils. Consequently, the iron barrier developed after a short-term anaerobic-aerobic cycle becomes a new hidden factor, decreasing Se bioavailability to food crops by temporarily depleting the highly mobile inorganic Se oxyanions. Ultimately, elucidating Se cycling driven by coupled Fe-OM biotransformation provides a scientific basis for managing Se bioavailability in Mollisol agroecosystems to mitigate dietary Se deficiency and safeguard human health.