Abstract
Selenium plays an important, but vastly neglected role in human nutrition with a narrow gap between dietary deficiency and toxicity. For a potential biofortification of food with Se, as well as for toxicity-risk assessment in sites contaminated by Se, modelling of local and global Se cycling is essential. As bioavailability of Se for rice plants depends on the speciation of Se and the resulting interactions with mineral surfaces as well as the interaction with Se uptake mechanisms in plants, resulting plant Se content is complex to model. Unfortunately, simple experimental models to estimate Se uptake into plants from substrates have been lacking. Therefore, a mass balance of Se transfer between lithosphere (represented by kaolinite), hydrosphere (represented by a controlled nutrient solution), and biosphere (represented by rice plants) has been established. In a controlled, closed, lab-scale system, rice plants were grown hydroponically in nutrient solution supplemented with 0–10 000 μg L-1 Se of either selenate or selenite. Furthermore, in a series of batch experiments, adsorption and desorption were studied for selenate and selenite in competition with each of the major nutrient oxy-anions, nitrate, sulfate and phosphate. In a third step, the hydroponical plants experiments were coupled with sorption experiments to study synergy effects. These data were used to develop a mass balance fitting model of Se uptake and partitioning. Adsorption was well-described by Langmuir isotherms, despite competing anions, however, a certain percentage of Se always remained bio-unavailable to the plant. Uptake of selenate or selenite by transporters into the rice plant was fitted with the non-time differentiated Michaelis-Menten equation. Subsequent sequestration of Se to the shoot was better described using a substrate-inhibited variation of the Michaelis-Menten equation. These fitted parameters were then integrated into a mass balance model of Se transfer.
Highlights
It has been known for years that Se is both essential (< 55 μg/d [1]) and toxic (> 400 μg/d [2]) to humans
To calculate the mass transfer concentrations from the experimental closed-system model for a simple mass balance fitting model, the separately studied processes of adsorption, bioavailable exchange and plant uptake were combined. This approach was considered appropriate, as in practical application, the process of plant growth is slower than adsorption or desorption processes [43] and the growing rice plant, is placed in an already equilibrated system
As previously noted [45], Langmuir fitting itself allows for no information on mechanistic properties during the sorption process
Summary
It has been known for years that Se is both essential (< 55 μg/d [1]) and toxic (> 400 μg/d [2]) to humans. Se research faces two important, yet entirely diverse goals [3]: (1) securing Se nutrient resources for future generations, and (2), management of Se-enriched waste deposits to protect the environment and improve the quality of life in areas of contamination. For both issues, a quantitative understanding of selenium speciation and abundance on the path from the soil into the plant, and during the partitioning into different plant organs is crucial. While Se transfer has been studied in different models and observational scales, none of these approaches has allowed addressing both the combination of all three spheres while enabling standardization of parameters: 1. lab-scale modelling, i.e. surface complexation models [6, 7], sequential extraction procedures [8], plant-uptake and incorporation of nutrients which have culminated in the NST model 3.0 [9] has, so far, only addressed one of the spheres; 2. local-scale modelling, i.e. environmental and agronomical case studies, such as Kesterson Reservoir (USA), Punjab (India) [10, 11] or field experiments on phytoremediation [12] and biofortification, [13, 14, 15, 16] have to deal with the parameters present in the respective system and, do not allow for parameter control; only partial mass balance models or transfer equations can be derived from such studies
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