pubs.acs.org/JAFC Published on Web 10/21/2010 © 2010 American Chemical Society J. Agric. Food Chem. 2010, 58, 11837–11843 11837 DOI:10.1021/jf1026185 Selenium Speciation in Soil and Rice: Influence of Water Management and Se Fertilization HUA-FEN LI, †,§ ENZO LOMBI, #,^ JACQUELINE L. STROUD, § STEVE P. MCGRATH, § AND FANG-JIE ZHAO* ,§ † College of Resources and Environmental Sciences, China Agricultural University, Beijing 100193, China, § Rothamsted Research, Harpenden, Hertfordshire AL5 2JQ, United Kingdom, # Centre for Environmental Risk Assessment and Remediation, Building X, Mawson Lakes Campus, University of South Australia, Mawson Lakes SA-5095, Australia, and ^ CRC CARE, P.O. Box 486, Salisbury, South Australia 5106, Australia Rice (Oryza sativa) is the staple food for half of the world’s population, but the selenium (Se) concen- trations in rice grain are low in many rice-growing regions. This study investigated the effects of water management on the Se speciation dynamics in the soil solution and Se uptake and speciation in rice in a pot experiment. A control containing no Se or 0.5 mg kg -1 of soil of selenite or selenate was added to the soil, and plants were grown under aerobic or flooded conditions. Flooding soil increased soluble Se concentration when no Se or selenite was added to the soil, but decreased it markedly when selenate was added. Selenate was the main species in the þselenate treatment, whereas selenite and seleno- methionine selenium oxide were detected in the flooded soil solutions of the control and þselenite treatments. Grain Se concentration was 49% higher in the flooded than in the aerobic treatments without Se addition. In contrast, when selenate or selenite was added, the aerobically grown rice contained 25- and 2-fold, respectively, more Se in grain than the anaerobically grown rice. Analysis of Se in rice grain using enzymatic hydrolysis followed by HPLC-ICP-MS and in situ X-ray absorption near-edge structure (XANES) showed selenomethionine to be the predominant Se species. The study showed that selenate addition to aerobic soil was the most effective way to increase Se concentration in rice grain. KEYWORDS: Selenium; selenium speciation; Oryza sativa; soil pore water; HPLC-ICP-MS; XANES INTRODUCTION Selenium (Se) is an essential micronutrient for humans. Sub- optimal intake of Se is associated with a range of health effects such as oxidative stress, impaired immune function, reduced fertility, and increased risk of some cancers ( 1 ). Globally, between 0.5 and 1 billion people are estimated to have an insufficient intake of Se ( 2 ). A recent global survey of rice (Oryza sativa), the staple food for more than half of the world’s population ( 3 ), showed that 75% of the samples had a Se concentration insuffi- cient for human requirements ( 4 ). Because Se enters the food chain through plants, biofortification of food crops through Se fertili- zation or genetic improvement is considered to be an effective way of raising Se intake in a target population ( 1 , 5 , 6 ). This approach has the merit of using plants as effective buffers to prevent accidental overdose of Se. Moreover, the assimilation of inorganic Se into organic forms by plants also enhances its nutritional efficacy ( 7 ). Selenium biofortification of crops through fertilization has been successfully practiced in Finland since the mid-1980s ( 8 ). The accumulation of Se by plants is determined by the ability of a plant to take up Se, which varies widely among plants species ( 1 , 9 , 10 ), and the bioavailability of Se in the soil. Plant-available Se in soil is strongly influenced by Se speciation, with the redox potential and pH playing a central role ( 11 , 12 ). Selenate tends to be the predominant species in aerobic and neutral to alkaline environments; selenite becomes the dominant species in the envi- ronment with an intermediate redox potential, especially with an acidic pH, whereas elemental Se and selenide may be produced in anaerobic environments ( 13 ). The various chemical forms of Se differ widely in their water solubility and sorption to soil. Selenate and selenite are highly water-soluble. However, selenate behaves mainly as a nonsorbing solute, whereas selenite can be adsorbed strongly by the soil solid phase (e.g., iron oxides/hydroxides), resulting in a lower solubility in the soil solution ( 14 -16 ). Thus, selenate is generally more available for plant uptake than selenite ( 11 , 17 -19 ). A number of studies have investigated the effects of redox potential, pH, and the addition of organic matter on the transformation of Se species and Se mobility ( 20 -22 ); however, these studies have focused on Se-contaminated soils or sediments with the aim of reducing Se mobility in the environment and Se toxicity to the organisms. Moreover, the forms of Se were quantified by subtraction of different chemically reactive pools, which may not be specific to individual Se species. Much less is known about Se speciation, transformation, and bioavailability in low Se soils, which produce low Se crops in many regions of the world. *Corresponding author (e-mail Fangjie.Zhao@bbsrc.ac.uk); phone (þ44) 01582 763133; fax (þ44) 01582 760981).