Article pubs.acs.org/JAFC
Determination of the Source of Bioavailable Sr Using Tracers: A Case Study of Hot Pepper and Rice
87
Sr/86Sr
Byeong-Yeol Song,†,‡ Jong-Sik Ryu,† Hyung Seon Shin,† and Kwang-Sik Lee*,†,‡ †
Division of Earth and Environmental Sciences, Korea Basic Science Institute, Chungbuk 363-883, Republic of Korea Graduate School of Analytical Science and Technology, Chungnam National University, Daejeon 305-764, Republic of Korea
‡
S Supporting Information *
ABSTRACT: The geographical origin of agricultural products has been intensively studied, but links between agricultural products and the environments are poorly established. Soils, water (streamwater and groundwater), and plants (hot pepper, Capsicum annuum; and rice, Oryza sativa) were collected from all regions of South Korea and measured Sr isotope ratios (87Sr/86Sr). Sequential leaching of soil showed that Sr in the exchangeable and carbonate fractions (bioavailable) had a lower 87 Sr/86Sr ratio than that in the silicate fraction, consistent with a low 87Sr/86Sr ratio in the plant. Although the bedrock−soil− water−plant system is closely linked, statistical analysis indicated that 87Sr/86Sr ratios of the plant showed the greatest agreement with those of water and the exchangeable fraction of soil. This study is the first report of 87Sr/86Sr isoscapes in South Korea and first demonstrates that the agricultural product is strongly linked with the exchangeable fraction of soil and water. KEYWORDS: bioavailable, plant, Sr isotopes, isoscapes, South Korea
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INTRODUCTION The geographic origin and authenticity of agricultural products have been intensively investigated,1−3 largely due to the expansion of global trade associated with, for example, the Free Trade Agreement (FTA). This is because false declarations, adulteration, and fraud can disrupt international trade in agricultural products. Numerous studies aiming to identify the origin of foodstuffs and their products have been performed,4−6 many of which have used trace element concentrations and stable isotope ratios because inorganic chemical patterns reflect the soil, water, and general environment in which the products were grown. For example, Rodrigues et al.3 used five different isotope systems (carbon, nitrogen, sulfur, oxygen, and strontium) to examine the relationship between coffee plants and their local environments in Hawaii, indicating they are closely related. With the same purpose, Ariyama et al.1 applied two radiogenic isotope systems (strontium and lead) and trace elements to rice in the Japanese market, highlighting clear variations depending on the country of origin. However, to date, most studies have focused on differences in the product itself, whereas only a few have examined the links between the product and the environmental components, such as soil, water, and rock.7,8 These recent studies applied three isotope systems (δ13C, δ15N, and 87 Sr/86 Sr) and multivariate statistical methods to agricultural products (wheat and meat) as well as soil and water and showed that the elemental and isotopic compositions of agricultural products are mainly related to those of soil and water. Nevertheless, it is still unclear how and to what extent the local soil and water affect agricultural products. Numerous studies have used strontium isotope ratios (87Sr/86Sr) to determine the geographic origin of agricultural products. This approach is based on three principles: (1) that 87 Sr/86Sr ratios are fractionated neither by low temperature nor © 2014 American Chemical Society
by biogenic processes, and their abundance in geological materials depends only upon the initial 87Sr/86Sr ratios, the age of the rock/mineral, and their 87Rb/86Sr ratio;9,10 (2) that the 87 Sr/86Sr ratios of agricultural products reflect the sources of strontium (Sr) available during their formation;11 and (3) that corrections for biological and instrumental mass-dependent fractionations are automatically made during measurement, in which the stable Sr isotopic ratio (88Sr/86Sr) was used for internal normalization, unlike with traditional stable isotopes. Indeed, many studies have used Sr from the bioavailable soil fraction, although there are three different Sr pools in soil: the exchangeable (generally considered bioavailable), carbonate, and residual (silicate) fractions.12 However, whether the agricultural product takes up only bioavailable Sr remains unclear. Therefore, it is necessary to examine the relationship between the isotopic composition of the agricultural product and each of the three different fractions in soil. On the other hand, the agricultural product also takes up Sr from water because it interacts and equilibrates with soils and rocks. Hence, the agricultural product will reflect the 87Sr/86Sr ratios of water inherited from soils and rocks, but how and to what extent water affects the agricultural product remains unclear. Here, we examine the sources controlling 87Sr/86Sr ratios of plants (hot pepper, Capsicum annuum; and rice, Oryza sativa) under various environments having different combinations of three soil fractions, bedrock, and water (streamwater and groundwater). This study allowed us to understand the different controls on 87Sr/86Sr ratios in the agricultural product and to elucidate the relationship between their 87Sr/86Sr ratios Received: Revised: Accepted: Published: 9232
April 15, 2014 August 13, 2014 September 4, 2014 September 4, 2014 dx.doi.org/10.1021/jf503498r | J. Agric. Food Chem. 2014, 62, 9232−9238
Journal of Agricultural and Food Chemistry
Article
Darien, IL, USA).15 Strontium isotope ratios (87Sr/86Sr) were measured using a Neptune MC-ICP-MS (Thermo Scientific) at KBSI. 87Sr/86Sr ratios were normalized to 86Sr/88Sr = 0.1194,9 and replicate analyses of NBS987 (U.S. National Bureau of Standards) gave 87Sr/86Sr = 0.710247 ± 0.000008 (2σ, n = 24) with a background value of
