Strategies for agronomic biofortification with selenium and iodine in sweet potato grown in tropical soil

Abstract

Sweet potato (Ipomoea batatas L.) is an important crop for assuring food security in many developing countries. Considering that tropical soils are poor in many human-essential elements, such as selenium (Se), and that the concentration of an element in the plant is directly related to its availability in the soil, agronomic biofortification is one of the main alternatives to increase nutrient availability in staple foods. In this context, this study evaluated different strategies for agronomic biofortification of sweet potatoes with Se and iodine (I), aiming to: i) assess the biofortification potential of Se in the presence or absence of I addition, in response to increasing Se rates applied as a foliar spray; ii) evaluate Se(VI) adsorption capacity in a tropical soil and its further desorption by phosphorus and nitrogen; and iii) assess biofortification of Se by applying increasing Se rates in the soil using Seenriched fertilizers (MAP and/or urea). To achieve these goals, two field trials were conducted using the following Se rates (0, 50, 100, and 200 g ha-1). The first experiment evaluated the effects of increasing Se(VI) foliar application rates in the presence or absence of I (500 g ha-1), also applied as foliar spray), as a factorial design 4x2+1, with an additional treatment consisting of the higher rates of Se+I and 2.5 kg ha-1 of zinc (Zn). The second assessed applications of the same Se rates (i.e., 0, 50, 100, and 200 g ha-1), but the application was carried out in the soil using Se-enriched fertilizers (Se-monoammonium phosphate) - (Se- MAP), Se-urea, and Se-MAP+urea. Also, a Se adsorption test was conducted on soil samples collected from the field experiment, with Se desorption being evaluated by adding increasing rates of P2O5 and N (equivalents to 0, 100, 250, 500, and 1000 kg ha–1) using both, desorption solutions prepared with pure salts or MAP/urea. Results demonstrated that, in both studies, Se contents in sweet potato shoots and roots increased upon increasing Se rates, with these contents being higher when Se was added as a foliar spray. The foliar addition of I did not affect Se contents accumulated in sweet potato, yet the I content found in the plant was higher when Se was added at 50 g ha-1. The simultaneous application of Se, I, and Zn (as a foliar spray) resulted in the highest Se contents determined in sweet potatoes. The second study showed that Se adsorption increased with increasing Se concentrations added in solution, while Se desorption was greater following the addition of P (as a pure reagent) and N (urea) in solution. Sweet potato plants responded positively to applications of Se in the soil, but the highest biofortification potential was achieved with Se-enriched urea, agreeing with the desorption test. Agronomic biofortification of sweet potatoes with Se is promising in tropical soils, however, Se rates to be applied should be different depending on the strategy used (foliar or via soil).