Mechanisms of changing speciation and bioavailability of selenium in agricultural mollisols of northern cold regions

The distribution, speciation, and bioavailability of selenium (Se) - an essential micronutrient for human beings - in agricultural soils influence the resource recovery of agricultural benefits and the sustainable use of Se in agroecosystems. Quantitative understanding in this regard however remains limited in the world's mollisol agroecosystems, despite their critical importance in securing global food supply. Herein, a systematic investigation of Se in the river sediment-irrigation water-mollisols-rhizosphere-rice seeds continuum, at the core zone of the northern mollisol regions, was conducted to elucidate the hydrological-hydrogeochemical processes and mechanisms responsible for the distribution and bioavailability of Se. The content of total Se in the mollisols ranged between 0.12 and 0.54 mg/kg with an average of 0.31 mg/kg. At the riverside flood plains, humic-acid bound Se accounted on average for 39 % of total Se. This pool of Se can be transformed to water-soluble and ion-exchangeable Se(VI), supporting a higher potential of Se bioavailability at riparian agricultural mollisols. For mollisol lands far from the river channels, the topography affects the speciation and partitioning of Se presumably through regulating water retention and organic matter transport. Moreover, altering pH and redox conditions in response to irrigation with the river water may boost Se bioavailability in weakly acidic and high Eh mollisols. It can be in part ascribed to the transformation of organic-bound Se along with infiltrated oxygenated water that leads to the increase of water-soluble and ion-exchangeable Se. These findings reinforce that hydrological-hydrogeochemical perturbations due to irrigation with surface water need to be assessed carefully in the management of Se resources in the mollisol agroecosystems.