Loss of Selenium from Mollisol Paddy Wetlands of Cold Regions: Insights from Flow-through Reactor Experiments and Process-Based Modeling

Abstract

Mollisols are critical agricultural resources for securing global food supply. Due to its health importance, selenium (Se) fate in the Mollisols attracts growing concerns. Land use change from conventional drylands to paddy wetlands impacts Se bioavailability in the vulnerable Mollisol agroecosystems. The underlying processes and mechanisms however remain elusive. Here, results of flow-through reactor experiments with paddy Mollisols from northern cold-region sites indicate that continuous flooding with surface water for 48 d induced redox zonation that facilitated the loss of Mollisol Se by up to 51%. Further process-based biogeochemical modeling suggests largest degradation rates of dissolved organic matter (DOM) in 30 cm deep Mollisols that contained the highest-level labile DOM and organicbound Se. Electron shunting from degradation of Se-bearing DOM coupled to reductive dissolution of Se-adsorbed Fe oxides accounts mainly for Se(IV) release into the pore water. Consequent changes in DOM molecular composition make the reservoir of organic-bound Se vulnerable to flooding-induced redox zonation and likely enhance Se loss through destruction of thiolated Se and emission of gaseous Se from the Mollisol layer. This study highlights a neglected scenario where the speciation-driven loss of bioavailable Se from the paddy wetlands can be a significant consequence in the cold-region Mollisol agroecosystems.

Mechanistic Insight into the Abiotic Interactions of Selenate and Selenite with Natural Organic Matter

“…Currently, in the literature, the decreased mobility and, therefore, the reduced bioavailability of Se in organic-rich soil and sediment has been attributed to the microbially catalyzed reductive precipitation of elemental Se and selenide coupled with the oxidation of organic matter and the assimilation of the reduced Se species into different biomolecules and subsequent enrichment into the soil with living and dead biomass. ,,, However, the current study by macroscopic sorption experiments and molecular-level characterization of the sorption mechanism demonstrates that NOM can abiotically reduce highly mobile selenate into less mobile selenite and considerably sorbs selenate and selenite through direct and metal cation bridged ternary complexation; these processes can also reduce Se mobility in organic-rich soil and sediment. Therefore, the current study explains the earlier field observations, such as selenite and selenate to dominate the speciation of organic matter-associated Se in the soil and sediment. − Sorption of Se species onto particulate organic matter (POM) can be a potential Se sequestration mechanism in soil and sediment, such as organic-rich soil, ,,, wetland sediment, peatlands, etc. In such systems, in addition to ternary and binary complexation with the oxygen-containing functional groups, Se species can be sorbed onto the NOM via binary complexation with the reduced −SH groups (thiol coordination), as has been observed for the arsenic species complexation with NOM. , The possibility of the thiol coordination of Se species with NOM should be investigated in future research.…”

“…Therefore, the current study explains the earlier field observations, such as selenite and selenate to dominate the speciation of organic matter-associated Se in the soil and sediment. 13−15 Sorption of Se species onto particulate organic matter (POM) can be a potential Se sequestration mechanism in soil and sediment, such as organic-rich soil, 6,26,58,59 wetland sediment, 17 peatlands, 60 etc. In such systems, in addition to ternary and binary complexation with the oxygen-containing functional groups, Se species can be sorbed onto the NOM via binary complexation with the reduced −SH groups (thiol coordination), as has been observed for the arsenic species complexation with NOM.…”

Groundwater redox dynamics across the terrestrial-aquatic interface of Lake Erie coastal ecosystems

“…For instance, flooded soils are often reported with lower redox than dry soils in floodplains, (Aeppli et al 2022;Jia et al 2020;Megonigal et al 1993), coastal marshes (Noyce et al 2023), and agricultural systems (Honma et al 2016;Wang et al 2020), highlighting the impact of dynamic hydrological changes. Experiments also confirm the influence of flooding and draining on the transition between reducing and oxidizing phases (Pi et al 2023;Rezanezhad et al 2014;Zhao et al 2023). These findings support the recently proposed biogeobattery concept, based on alternating charge and discharge of electrons, providing energy for microbial processes in environments under varying hydrological regimes (Kappler et al 2021;Peiffer et al 2021;Zhao et al 2023).…”

“…Coastal groundwater has been increasingly impacted by water table fluctuations in response to the intensification of the hydrological cycle, which may alter groundwater biogeochemistry (Burri et al 2019). This motivates increasing demand for understanding and modeling the effects of water table fluctuations on groundwater biogeochemical processes (Pi et al 2023;Wang and Chen 2021). Fluctuations of the groundwater can occur over multiple spatial and temporal scales due to patterns of infiltration and evapotranspiration that may vary according to precipitation patterns, temperature, hydraulic conductivity, freezing, and thawing (Condon et al 2021;Gómez-Gener et al 2021).…”

Mollisol Erosion-Driven Efflux of Energetic Organic Carbon and Microflora Increases Greenhouse Gas Emissions from Cold-Region Rivers

“…The relative abundances of eight categories of organic compounds were compared to shed further light on the DOM quality change (Figure F–H). It was considered that lipid, protein, aminosugar, and carbohydrate (H/C molar ratio >1.25) were more biodegradable than unsaturated hydrocarbon, lignin, tannin, and condensed aromatics (H/C ≤ 1.25). , Compound-specific parameters NOSC, DBEw, and AI mod w were systematically lower in the former four categories than in the latter four categories of compounds (Figure S7). Consequently, these three molecular parameters of the former four categories were generally smaller than the bulk DOM (solid color lines in Figure S7), whereas those of the latter four categories were above the bulk DOM.…”