Effects of selenite and selenate application on distribution and transformation of selenium fractions in soil and its bioavailability for wheat (Triticum aestivum L.)

This study investigated the effect of selenate and selenite application on the distribution, transformation of selenium (Se) fractions in soil, as well as the accumulation and availability of Se in each part of wheat plants. A pot experiment was conducted using different concentrations of exogenous selenite or selenate (0.5, 1, 2.5, 5, and 10 mg Se kg⁻¹ soil). Sequential extraction was used to determine the Se fractions in soil, and different models were used to study the behavior of Se in soil and its availability to wheat. Results showed that the distribution and availability of Se in soil and its accumulation in wheat affected both by Se concentrations and forms of exogenous Se. In selenite-treated soil, the proportion of exchangeable and carbonate-bound Se (EXC–Se) (21–42%) fraction increased compared to that in control (12%), while organic matter-bound Se (OM–Se) (23–33%) and Fe–Mn oxide-bound Se (FMO–Se) (11–15%) fractions decreased compare with those in control (37 and 32%, respectively). In selenate-treated soil, soluble-Se (SOL–Se) fraction (30–54%) increased and the OM–Se (9.8–20%) and FMO–Se (4.7–14.2%) fractions decreased compared with those in control. Residual Se (RES–Se) fraction was increased for selenite (7.4–13.4%) and selenate (12–20%) treatments compared with that in control (6.5%). In comparison with control, the available Se (SOL−Se + EXC−Se) fraction increased for both selenite (32–47%) or selenate (54–72%) treatments. Moreover, at the same rate of Se application, Se availability was higher in wheat grown in selenate-treated soils than that in selenite-treated soils. The redistribution index (U ₜₛ) of Se increased from 1 (in control) to 1.2–1.9 and 1.5–2 for selenite and selenate treatments, respectively; additionally, the mobility factor (MF) in selenate-treated soil was 40–90% higher than that in selenite-treated soil. Furthermore, relative bonding intensity (I R) for both selenite (0.38–0.45) and selenate treatment (0.33–0.41) decreased compared with that in control (0.55). These differences indicated that selenite and selenate varied in terms of fixation capacities in soil, in transformation and distribution of Se in soil fractions, and in their availability to plants. The results of Michaelis–Menten equation demonstrated the high affinity of leaf to selenate, and the high affinity of roots and grains to selenite. Selenate was dominant in nearly all parts of wheat plants and in each application level. However, the affinity of selenite to wheat grains suggests that selenite is a useful Se fertilizer that must be considered in biofortification programs. In-depth studies at the pot and field scales by using different wheat varieties and application methods of Se in different ecological zones must be conducted to elucidate the mechanism and biochemical properties of Se in soil-plant system and ultimately produce Se-rich staple foods.