Selenium Dynamics in Cereal Biofortification: Optimising Fertiliser Strategies and Assessing Residual Fate

Selenium (Se) is an essential micronutrient for humans and animals and hence, a low intake of Se in the diet can lead to health problems. The application of Se fertilisers to staple crops, a process called agronomic biofortification, can effectively improve humans’ Se intake levels. The overarching aim of this study was to develop improved strategies for Se biofortification through an enhanced understanding of Se dynamics in arable systems. A pot trial was set up to investigate whether the application of 3.33 μg kg⁻¹ of Se (equivalent to 10 g ha⁻¹) to wheat can be made more efficient by its co-application with macronutrient carriers, either to the soil or to the leaves. In the soil, Se was applied either on its own (selenate only) or as a granular, Se-enriched macronutrient fertiliser supplying nitrogen, phosphorus, potassium or sulphur. Selenium was also applied to leaves at head emergence with, or without, 2% w/v N fertilisers. With grain Se concentrations varying from 0.13–0.84 mg kg⁻¹, soil application of selenate-only was 2–15 times more effective than granular Se-enriched macronutrient fertilisers in raising grain Se concentrations. Foliar Se application was superior to soil-applied Se treatments in increasing grain Se levels, especially when foliar Se was co-applied with an N carrier. Under foliar Se+N treatments, grains accumulated twice as much Se as those fertilised with foliar Se only, the majority of which was highly bioavailable (selenomethionine). This study was perhaps the first to show the efficiency of co-applying foliar Se with N in improving Se uptake and recovery in wheat. Such findings support the hypothesis that the efficacy of existing agronomic practices for Se biofortification can be improved through the use of macronutrient carriers, which could potentially reduce costs associated with fertiliser application and management. The second experiment shed light on the residual fate of Se in different soils over a 300-day period, using both chemical and biological assays to estimate Se availability. Eight soils varying in physicochemical properties were spiked with 0.5 mg kg⁻¹ Se in the form of sodium selenate and incubated at 25°C for different periods (1, 123 30, 60, 90 and 300 d). At the end of the ageing period, soil Se was fractionated by sequential extraction procedures into soluble, adsorbed and organically-bound Se fractions. Simultaneously, a pot trial was set up where wheat was grown in the Se-aged soils for six weeks. A rapid decline in Se solubility (> 50% within 24 h) was observed in the Oxisol, probably due to its high mineral oxides and clay contents. Over time, calcareous soils showed more pronounced Se ageing than non-calcareous soils as solubility reached 0 at 300 d, probably due to the fixation of Se onto calcite surfaces. In highly calcareous soils, plant Se concentrations decreased from 37 mg kg⁻¹ to 100 days in plants grown in non-calcareous soils. The soluble Se fraction at specific ageing times was best represented by a reversible first order model, and was primarily influenced by soil pH. Understanding how added Se behaves in soils over time could be used to make more informed decisions about the rate and frequency of Se fertiliser application in agronomic biofortification programs. The third experiment was undertaken to investigate time-dependent changes in the uptake and partitioning of Se in wheat. It also investigated whether the uptake efficiency of Se in wheat was influenced by timing of fertiliser application. In a pot trial, 3.33 μg kg⁻¹ Se was as ⁷⁷Se-enriched sodium selenate (Sefert) to wheat at two growth stages – stem elongation (GS1) and heading stage (GS2), by two methods – soil and foliar (foliar Se on its own and foliar Se + 2% urea-N). Wheat was harvested 3, 10 and 17 d and 3, 10, and 34 d after Se application at GS1 and GS2, respectively. Only foliar treatments were effective in raising grain Se concentrations (> 0.25 mg kg⁻¹) above the target level of 0.1 mg kg⁻¹ for biofortification. However, the poor efficiency of the soil-applied Se fertiliser was speculated to be predominantly caused by accidental leaching of the applied Se from the free-draining pots. This study showed that, when applied at an early growth stage, foliar Se with N improved the uptake of Se into wheat, compared to foliar application of Se on its own. At the later growth stage, N inclusion to foliar Se fertilisers significantly increased grain Se concentration in the grain (0.32 mg kg⁻¹) compared to foliar Se on its own (0.26 mg kg⁻¹), the majority of which was highly bioavailable. Speciation analysis data of the foliar-treated leaves suggested that the presence of N in foliar solutions improved the assimilation and translocation of organic Se compounds. Practical knowledge gained about the optimisation of Se fertiliser formulation, method and timing of application will be of importance in refining biofortification programs across different soil and climatic regimes.Thesis (Ph.D.) -- University of Adelaide, School of Agriculture, Food and Wine, 202