Fluvial Deposition and Land Use Change Control Selenium Occurrence in Mollisols of Cold Region Agroecosystems

Abstract

Mollisols support the most productive agroecosystems in the world. Despite their critical links to food quality and human health, the varying distributions of selenium (Se) species and factors governing Se mobility in the mollisol vadose zone remain elusive. This research reveals that, in northern mollisol agroecosystems, Se hotspots (≥0.32 mg/kg) prevail along the regional river systems draining the Lesser Khingan Mountains, where piedmont Se-rich oil shales are the most probable source of regional Se. While selenate and selenite dominate Se species in the water-soluble and absorbed pools, mollisol organic matter is the major host for Se. Poorly crystalline and crystalline Fe oxides are subordinate in Se retention, hosting inorganic and organic Se at levels comparable to those in the adsorbed pool. The depth-dependent distributions of mollisol Se species for the non-cropland and cropland sites imply a predominance of reduced forms of Se under the mildly acidic and reducing conditions that, in turn, are variably impacted by agricultural land use. These findings therefore highlight that fluvial deposition and land use change together are the main drivers of the spatial variability and speciation of mollisol Se.

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

“…18,25 Accumulation of organic matter in the Mollisols can fix Se, which consequently becomes less available than the other three pools for crops. 11,25 The content of Se(VI) either in the water-soluble or in the adsorbed pool generally decreased with depth in the Mollisol layer (Figure 2A,B). In comparison, the contents of Se(IV), SeMet, and SeCys in the water-soluble and adsorbed pools, as well as those of organic-bound and elemental Se, increased vertically to the 30 cm depth (Figure 2).…”

“…The content of organic-bound Se averaging at 0.14 ± 0.04 mg/kg was significantly higher than that of Se present in the water-soluble (0.03 ± 0.01 mg/kg), adsorbed (0.05 ± 0.01 mg/kg), or elemental pool (0.01 ± 0.004 mg/kg) (Figure ). The binding to soil organic matter has been proposed to be a key process of Se immobilization in most organic-rich soils. , Accumulation of organic matter in the Mollisols can fix Se, which consequently becomes less available than the other three pools for crops. , …”

“…(A) Mollisol regions in northeastern China, and (B) locations of Mollisol sampling sites within the northern part of Hailun watershed where the river water is widely used for paddy irrigation. See Pi et al for the generation method of the regional distribution map of topsoil (<25 cm depth) total selenium in (B) (data source indicated in Text S2). …”

“…A prolonged flooding of the Mollisol paddy wetlands results in the saturated vadose zone with evolving reducing conditions . This enabled reactive-transport modeling of Se migration from Mollisols in the FTRs with the public code PHREEQC-3 .…”

“…Hydro­(bio)­geochemical dynamics in the vadose zone form the basis of Se mobility and thus bioavailability through the food chain. , Depending upon the pH condition, inorganic Se­(VI) (mainly SeO 4 2– ) forming outer-sphere complexes on Fe/Mn-oxide minerals (e.g., goethite, ferrihydrite, birnessite, and hydrous Fe/Mn oxides) adsorbs less strongly than Se­(IV) (mainly HSeO 3 – and SeO 3 2– ) forming inner-sphere complexes. − Likewise, adsorption of SeO 4 2– on Al (hydr)­oxides (e.g., gibbsite and allophane) and clay minerals (e.g., montmorillonite and kaolinite) is generally weaker than that of SeO 3 2– . , This engenders SeO 4 2– the most mobile and bioavailable species under most soil conditions . In comparison, reduced Se species (e.g., Se 0 and selenides), owing to their poor solubility, are much less mobile and hardly bioavailable. , In organic-rich soils (e.g., Mollisols), Se binding to organic matter, in particular under acidic conditions (pH 4.9–6.9), can reduce Se mobilization, making such soils a significant reservoir of Se. ,, Partial degradation of the organic-bound Se yields organic Se species including selenocystine and selenomethionine, whereas complete oxidation coupled to the reductive dissolution of Fe oxides releases inorganic Se into the pore water. − Therefore, oxidation of the reduced/organic-bound Se species to Se­(IV)/Se­(VI) may strengthen Se mobility and bioavailability in the vadose zone.…”