Chemical speciation and availability of molybdenum in soils to wheat uptake.

Molybdenum (Mo) is an essential micronutrient for plants, yet it also poses potential environmental risks when present in excess. This study investigated the Mo speciation in soils with varying properties and their influences on Mo uptake by wheat (Triticum aestivum L.), a staple crop with significant implications for global food security. Mo K-edge X-ray absorption spectroscopy (XAS), combined with a sequential extraction method, was employed to analyze the chemical speciation and fractionation of Mo in the soils before and after wheat cultivation. The predominant Mo species identified were sorbed molybdate (Mo(VI)) and Ca- and Fe-Mo(VI) precipitates. After wheat cultivation, sorbed Mo(VI) and Ca-Mo(VI) decreased while Fe-Mo(VI) increased, with the most notable changes observed in the alkaline soil. These changes indicated that the desorption of sorbed Mo(VI) and dissolution of Ca-Mo(VI) contributed to soil Mo availability, while Fe-Mo(VI) precipitation restricted it. Bioaccumulation and translocation factors revealed efficient Mo uptake and transport within wheat plants, with shoots being the primary site of Mo accumulation. Elevated Mo concentrations in wheat grains raise potential human health concerns due to dietary exposure. These findings underscore the critical role of soil Mo speciation in controlling Mo dynamics in soil-wheat systems, providing valuable insights for managing Mo in agricultural soils to balance its nutritional benefits with the risks of excessive crop accumulation.