Se Fertilization: An Agro-Ecosystem Approach
Selenium (Se) is an essential micronutrient for humans, animals and certain lower plants, and its supply in global food systems is highly variable. The variation of Se status in humans largely depends on their diet, which is strongly related to the geographical variation in soil’s Se level. Selenium deficiency is regarded as a major health problem for 0.5 to 1 billion people worldwide. Whereas the global importance of selenium deficiency has been recognized for decades, strategic micronutrient interventions to overcome this deficiency are still limited. Basically, there are two groups of fortification strategies available to increase Se intake worldwide. First of all, human Se intake may be increased by supplementation of livestock, direct food fortification or supplementation with Se pills. Alternatively, agronomic strategies like plant breeding and fertilization can be used to increase Se uptake of staple food crops. We argue that the best strategy depends on the natural, societal and economic properties of local agro-ecosystems. Adapting the fortification strategy to the local properties of an agro-ecosystem is the way forward to solve Se deficiencies worldwide without resource exhaustion of the worlds’ scarce Se resources and potentially harmful environmental side-effects. An essential part of such an agro-ecosystem approach will be a robust and reliable fertilizer strategy that takes the spatial and temporal variability in climatic conditions, soil properties and cropping systems into consideration. Selecting the proper fertilizer strategy requires a mechanistic understanding of Se plant-soil-atmosphere cycling and insights in plant availability of added Se fertilizers. The research presented in this report aims to identify when applying Se fertilizer is effective in specific agroecosystems based on an inventory of specific production-ecological causes for its deficiency in relation to fertilizer application. Important factors controlling Se availability and uptake are identified using meta-analysis and are integrated in a framework for a decision support tool that guides users in the selection of effective fortification strategies. This research primarily focuses on fertilization as a fortification strategy, but other strategies are briefly introduced and evaluated. The review and meta-analysis indicate that fertilizer doses need to match crop demand with Se supply, given the capacity of soils to supply or retain Se during the growing season. Main soil properties controlling crop uptake efficiency of applied Se include acidity, redox potential, texture and organic matter. Agronomic practices such as liming, irrigation and basic fertilization (nitrogen, phosphorus and sulfur) additionally affect the crop uptake efficiency. Adapting fertilizer strategies to the local agronomic situation and soil properties can increase the crop uptake efficiency from 10% (common situation) up to 50%. Important fertilizer strategies include: The use of a site specific fertilizer dose: Se fertilizer use should account for the Se supply and availability in the soil and any residual effects of former Se fertilizer applications. The choice for a specific Se fertilizer: Selenate is about 8 times more effective on the short term than selenite and has smaller residual effects. Application technique: Both foliar- and soil-applied fertilizers are able to enhance Se uptake, but foliar application is more resource efficient. Seed coating can be an alternative, but the crop uptake efficiency is usually less than 10%. Application timing: Fertilizer application during the growing season results in higher Se levels in the crop in comparison with fertilizer applications before the growing season. By far, the most resource-efficient way to increase the Se intake in the world’s population appears to be by adding Se to food products along the production chain. The positive effects of food processing is however limited by the fact that a limited number of people have access to processed foods, particularly in developing countries. Fortification through agronomic practices can therefore be an efficient and effective approach to increase human (and animal) Se intake through simple techniques that can be integrated in current farm management. Plant breeding for enhanced Se uptake efficiency and Se fertilization are currently the most promising agronomic strategies to increase Se status of human populations as they can deliver increased Se to a whole population safely, effectively, efficiently and in the most suitable chemical forms. These strategies might also be complementary to fortification strategies like food processing. Social and economic factors such as the availability of Se-enriched fertilizers and governmental incentives and regulations are needed to increase farmers and public acceptance of fortification programs and Se-enriched food products. The developed decision support tool integrates all these aspects in such a way that it can be applied to any agro-ecosystem. In summary, agro-ecosystem-dependent fortification strategies are necessary to increase human Se intake without exhaustion of the worlds’ scarce Se resources. The use of Se fertilizers is currently one of the most promising strategies, in particular when the fertilizer strategy (dose, formulation, application and timing) is adapted to the local properties of an agro-ecosystem.
Selenium, Micronutrients, Bioavailability, Decision Support Systems
G.H. Ros, A.M.D. van Rotterdam, G.D. Doppenberg, D.W. Bussink and P.S. Bindraban, 2014. Se Fertilization: An Agro-Ecosystem Approach. VFRC Report 2014/3. Virtual Fertilizer Research Center, Washington, D.C. 62 pp.; 1 table; 21 figs.; 282 ref.