Effects of soil moisture on aging of exogenous selenate in three different soils and mechanisms

Soil moisture influences the aging of metals/metalloids in soil by affecting soil properties and metal/metalloid speciation transformation. However, the effect of soil moisture on selenium (Se) aging in soil and its mechanism remain unclear. This study is the first to investigate the effects of water-deficient (50% WFC), -sufficient (75% WFC), and -saturated (100% WFC) conditions on Se aging in three distinct soils (loess soil, black soil, and krasnozem) through incubation experiment (140 d) with selenate addition. The results indicated that hexavalent Se (Se(VI)) was the predominant species in the soluble Se fractions in soils regardless of soil moisture condition, while Se species in the exchangeable Se fractions were affected by soil moisture and type. The soluble Se fractions were gradually transformed into less available Se fractions during aging. Among the three soil moisture conditions, the decrease in the soluble Se fractions was largest under the water-saturated condition (94%–98%). Combined with the results of the mobility factor (MF) of soil Se and the Elovich model simulation, the water-saturated condition accelerated Se aging in soils compared with water-deficient and -sufficient conditions. The water-saturated condition decreased soil Eh and increased soil pH and DOC content, resulting in the reduction of Se(VI) to the low-valence Se states such as Se(IV), Se(0), or Se(–II). Se aging was fastest for the loess soil among the three tested soils due to its high pH and low Eh which are conducive to the reduction of Se(VI). 70 d was the equilibrium time of selenate aging in the three tested soils regardless of soil moisture condition. The findings provide a guideline for water management in Se biofortification practice.