Increasing plant availability of selenium in rice soils under variable redox conditions.

This research focused on increasing plant availability of Se in Se-deficient paddy soils by agronomic biofortification. However, Se fertilization is complicated by the fact that the margin of safety between levels of Se compounds that will cause dietary deficiency and those that result in toxicity is small. The study was designed to understand the changes in Se availability in paddy rice soils under different moisture conditions and management practices. This knowledge was then used to develop an effective Se fertilization strategy. Because Se exists in several redox states and these can be transformed in soil by processes of oxidation or reduction, the study evaluated the effectiveness of different redox species of Se as fertilizers. In addition, methods of application and times of application were also evaluated. As the speciation of Se in rice grain also affects bioavailability to humans, Se speciation in rice grain was also evaluated. Isotopic dilution techniques were used to understand the potential availability of selenite (SeO₃⁻² ), selenate (SeO₄⁻² ) and elemental Se (Se (0)) applied to soils subjected to different water regimes – field capacity or submerged soil conditions. The availability of fertilizer Se (0), as measured by concentrations of labile Se species in soil, was low because of limited oxidation to SeO₃⁻² or SeO₄⁻². Elemental Se is therefore not suitable for pre-plant Se fertilization of lowland rice because it is not readily oxidized in paddy rice soils. In the submerged soils, concentrations of labile SeO₃⁻² and SeO₄⁻² were also low. More than, 80% of the Se added as either SeO₃⁻² or SeO₄⁻² was fixed into non-labile pools, likely through reduction to Se (0). Rates of oxidation of Se (0) will play a critical role either in determining whether reduced Se (0), which likely formed in submerged soils after fertilization, will contribute to plant Se uptake through oxidation during field drainage before harvest or in the rice rhizosphere. Kinetics of Se transformations occurring when a paddy soil is fertilized, flooded, and then re-oxidised were investigated. The results showed applied SeO₃⁻² was very quickly and completely transformed to non-labile pools under flooded soil conditions, with no detectable oxidation to SeO₄⁻² during the drainage period (7 days). Applied SeO₄⁻² was much more labile than SeO₃⁻², but the lability also decreased with time under submerged conditions and did not increase markedly during the drainage phase. These results indicate that SeO₃⁻² would not be an effective pre-plant fertilizer for rice production. Selenate is likely to be more effective, but losses to non-labile forms during the submerged phase of production also means that efficiency of pre-plant SeO₄⁻² fertilization is also compromised. The results of the first pot study indicated that most accumulation of Se in the grain occurred with SeO₄⁻² fertilizer when applied at heading; SeO₄⁻² -enriched urea applied at heading increased grain Se concentrations 5 to 6 fold (450 to 600 μg kg⁻ ¹) compared to the control (no Se fertilizer) in all three moisture treatments. Foliar SeO₃⁻² at heading and fluid SeO₄⁻² applied at heading in field capacity treatments increased grain Se 3.5- and 6-fold respectively compared to the control. The majority of Se in rice grains was identified in all treatments to be selenomethionine (SeM) which comprised over 90% of total grain Se. Selenate-enriched urea was the most effective Se fertilization strategy for paddy rice. A rice-growth experiment carried out with ⁷⁵Se radioisotope spiked fertilizer granule confirmed that Se-enriched urea granules applied at either tillering or heading produced significantly higher grain Se concentrations compared to any other Se application method. There were also higher concentrations of SeO₄⁻² in flood and pore water samples following the co-application of Se and urea compared to the other treatments. These results showed that urea applied in combination and co-located with SeO₄⁻² had a significant effect on the uptake and accumulation of Se in rice grains. Selenate-enriched urea granules applied to floodwater sank to the well-developed mat of adventitious roots on the soil surface. Oxygen release from the rice roots promotes production of NO₃⁻ from applied urea, which may have inhibited SeO₄⁻² reduction. The ability to maintain high concentration of SeO₄⁻² in soil solution with minimum speciation changes could be the reason for the higher uptake and accumulation of Se in rice grains in SeO₄⁻² -enriched urea treatment. Farmers well adapted to applying urea at tillering or heading are in a position to quickly and easily adopt these management practices. Future research should be conducted to verify these results under field conditions with other rice varieties and on different soil types.Thesis (Ph.D.) -- University of Adelaide, School of Agriculture, Food and Wine, 201

Selenate-enriched urea granules are a highly effective fertilizer for selenium biofortification of paddy rice grain

Citation: Premarathna, Lakmalie, Mike J. McLaughlin, Jason K. Kirby, Ganga M. Hettiarachchi, Samuel Stacey, and David J. Chittleborough. “Selenate-Enriched Urea Granules Are a Highly Effective Fertilizer for Selenium Biofortification of Paddy Rice Grain.” Journal of Agricultural and Food Chemistry 60, no. 23 (June 13, 2012): 6037–44. https://doi.org/10.1021/jf3005788.We examined the effects of applied selenium (Se) species, time of application, method of application and soil water management regime on accumulation of Se in rice plants. Plants were grown to maturity in a temperature- and humidity-controlled growth chamber using three water management methods: field capacity (FC), submerged until harvest, and submerged and drained two weeks before harvest; two Se species: selenate (SeO[subscript 4] ˉ²) and selenite (SeO[subscript 3]ˉ²) applied at a rate equivalent to 30 g haˉ¹; and four application methods: i) Se applied at soil preparation, ii) Se-enriched urea granules applied to floodwater at heading iii) foliar Se applied at heading and iv) fluid fertilizer Se applied to soil or floodwater at heading. Total Se concentrations in rice grains, husks, leaves, culms and roots were measured, as well as Se speciation in grains from the Se-enriched urea granule treatment. Highest Se concentrations in the grain occurred with SeO[subscript 4] ˉ² and with fertilizer applied at heading stage; SeO[subscript 4]ˉ²-enriched urea granules applied at heading increased grain Se concentrations 5 to 6 fold (by 450-600 μg kgˉ¹) compared to the control (no fertilizer Se applied) in all water treatments. Under paddy conditions other Se fertilization strategies were much less effective. Drainage before harvesting caused Se to accumulate in/on rice roots, possibly through adsorption onto iron plaque on roots. Rice grains contained Se mainly in the organic form as selenomethionine (SeM) which comprised over 90 % of the total grain Se in treatments fertilized with SeO[subscript 4]ˉ² -enriched urea granules. The results of this study clearly show of the fertilizer strategies tested that biofortification of Se in rice grains can best be achieved in lowland rice by broadcast application of SeO[subscript 4]ˉ² -enriched urea granules to floodwater at heading stage