Seasonal and snowmelt-driven changes in the water-extractable organic carbon dynamics in a cool-temperate Japanese forest soil, estimated using the bomb- 14 C tracer Takahiro Nakanishi a, * , Mariko Atarashi-Andoh a , Jun Koarashi a , Yoko Saito-Kokubu b , Keizo Hirai c,1 a Research Group for Environmental Science, Nuclear Science and Engineering Directorate, Japan Atomic Energy Agency, Ibaraki 319-1195, Japan b Neotectonics Research Group, Geological Isolation Research and Development Directorate, Japan Atomic Energy Agency, Gifu 509-5102, Japan c Tohoku Research Center, Forestry and Forest Products Research Institute, Iwate 020-0123, Japan article info Article history: Received 28 June 2013 Received in revised form 30 October 2013 Accepted 31 October 2013 Available online Keywords: 14 C 13 C WEOC Carbon dynamics Snowmelt Forest soil abstract Water-extractable organic carbon (WEOC) in soil consists of a mobile and bioavailable portion of the dissolved organic carbon (DOC) pool. WEOC plays an important role in dynamics of soil organic carbon (SOC) and transport of radionuclides in forest soils. Although considerable research has been conducted on the importance of recent litter versus older soil organic matter as WEOC sources in forest soil, a more thorough evaluation of the temporal pattern of WEOC is necessary. We investigated the seasonal vari- ation in WEOC in a Japanese cool-temperate beech forest soil by using the carbon isotopic composition ( 14 C and 13 C) of WEOC as a tracer for the carbon sources. Our observations demonstrated that fresh leaf litter DOC significantly contributed to WEOC in May (35e52%) when the spring snowmelt occurred because of the high water flux and low temperature. In the rainy season, increases in the concentration of WEOC and the proportion of hydrophobic compounds were caused by high microbial activity under wetter conditions. From summer to autumn, the WEOC in the mineral soil horizons was also dominated by microbial release from SOC (>90%). These results indicate that the origin and dynamics of WEOC are strongly controlled by seasonal events such as the spring snowmelt and the rainy season’s intense rainfall. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction In forest soils, dissolved organic carbon (DOC) is the main form in which organic carbon is transported downward into the subsoil to be mineralized, stabilized, or further leached into groundwater (Kalbitz et al., 2000; Qualls et al., 2000). The leaching of DOC ac- counts for significant carbon fluxes in terrestrial ecosystems (Siemens, 2003). It is also well known that DOC plays an important role as a transporter of radionuclides in soils. For example, down- ward flux of 137 Cs accompanied by DOC flux has been observed in a forest soil (Tegen and Dörr, 1996). In addition, DOC itself comprises a radioactive isotope of carbon ( 14 C), which is one of the most radiologically important radionuclides released from the nuclear industry (e.g., Koarashi et al. 2011) because of its relatively long half-life of 5730 y and the role of carbon in the metabolism of all life forms. Although several studies have obtained information on the evolution, stability, and dynamics of DOC in soils, further research is needed for better understanding of the role of DOC in the trans- location and mineralization of soil organic carbon (SOC) and in the migration of radionuclides through the soil profile. Water-extractable organic carbon in soil (WEOC) is commonly used to study the processes involved in DOC formation, and pro- vides information for identifying the relevant DOC fraction type such as hydrophobicity. Hagedorn et al. (2002) reported that WEOC contains more recent C than DOC collected by lysimetry. The water- soluble “new” organic components with short turnover rates are most likely not captured by standard sampling techniques for DOC (Küsel and Drake, 1999), or they vanish in the bulk DOC, where refractory compounds dominate (Qualls and Haines, 1992). Thus, * Corresponding author. Tel.: þ81 29 282 6433; fax: þ81 29 282 6760. E-mail address: nakanishi.takahiro@jaea.go.jp (T. Nakanishi). 1 Present address: Department of Forest Site Environment Forestry and Forest Products Research Institute, Forestry and Forest Products Research Institute, Ibaraki 305-8687, Japan. Contents lists available at ScienceDirect Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad 0265-931X/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jenvrad.2013.10.028 Journal of Environmental Radioactivity 128 (2014) 27e32