Comparison of depth- and mass-based approaches for estimating changes in forest soil carbon stocks: A case study in young plantations and secondary forests in West Java, Indonesia Jumpei Toriyama a,⇑ , Tsuyoshi Kato b , Chairil Anwar Siregar c , Harris Herman Siringoringo c , Seiichi Ohta d , Yoshiyuki Kiyono a a Bureau of Climate Change, Forestry and Forest Products Research Institute (FFPRI), 1 Matsunosato Tsukuba, Ibaraki 305-8687, Japan b Former expert of Japan International Cooperation Agency (JICA), Nibancho Center Building 5-25, Niban-cho, Chiyoda-ku, Tokyo 102-8012, Japan c Forestry Research and Development Agency (FORDA), Jalan Gunung Batu 5, Bogor 16610, Indonesia d Graduate School of Agriculture, Kyoto University, Kyoto City 606-8502, Japan article info Article history: Received 31 May 2011 Received in revised form 20 July 2011 Accepted 23 July 2011 Available online 25 August 2011 Keywords: Soil carbon stock Mass-based approach Kyoto Protocol Acacia mangium Shorea leprosula abstract Soil carbon (C) stocks in forest ecosystems have been widely estimated to a fixed soil depth (i.e., 0–30 cm) to clarify temporal changes in the C pool. However, surface elevations change as a result of compaction or expansion of the soil under forest management and land use. On the other hand, the calculation of soil C stocks based on ‘‘equivalent soil mass’’ is not affected by compaction or expansion of forest soil. To con- tribute to the development of a forest C accounting methodology, we compared changes in soil C stocks over 4 years between depth- and mass-based approaches using original soil data collected at 0–30 cm depths in young plantations and secondary forests in West Java, Indonesia. Our methodology expanded on the mass-based approach; rather than using one representative value for the mass-based calculation of soil C stocks, we adjusted individual values, maintaining the coefficient of variance in soil mass. We also considered the effect of an increase or decrease in soil organic matter on equivalent soil mass. Both increasing and decreasing trends in soil C stocks became clearer when the mass-based approach was used rather than the depth-based approach. The trends in soil C stocks based on equivalent soil mass were par- ticularly evident in the surface soil layers (0–5 cm) and in plantation sites, compared with those for soil profiles including subsurface soil layers (0–30 cm) and in secondary forests. These trends in soil C stocks corresponded with temporal trends in litter stocks. We suggest that equivalent mass-basis soil C stock for the upper 30 cm of soil be calculated based on multiple soil layers to reduce estimation errors. Changes in soil organic matter mass had little effect on the estimation of soil C stock on an equivalent mass basis. For the development of a forest C accounting system, the mass-based approach should be used to character- ize temporal trends in soil C stocks and to improve C cycle models, rather than simpler methods of cal- culating soil C stocks. These improvements will help to increase the tier level of country-specific forest C accounting systems. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction Forest soil is an important component of the greenhouse gas balance at both national and global levels (Intergovernmental Panel on Climate Change (IPCC, 2006). Globally, more than 1500 Pg of soil carbon (C) is stored in the top 1 m of terrestrial eco- systems and approximately half of this is distributed in forested regions (Jobbágy and Jackson, 2000). The forest soil C pool is affected by changes in aboveground biomass, such as deforestation and afforestation. Afforestation of abandoned land often enhances soil C storage especially when long-term rotation, nitrogen-fixing species, or fertilizer is applied (Jandl et al., 2007; Johnson, 1992; Lal, 2005; Paul et al., 2002). Thus, over the last decade, soil C stocks have been measured under Afforestation/Reforestation (A/R) projects associated with the Clean Development Mechanism (CDM) (García-Oliva and Masera, 2004; Jandl et al., 2007; Ringius, 2002), one of the C sequestration strategies specified in the Kyoto Protocol, to determine whether soil C is eligible for credit under the CDM. In developing countries in tropical regions, the development of a C accounting system for Reducing Emissions from Deforesta- tion and Degradation (REDD) is also an important goal at present (GOFC-GOLD, 2010). To develop scientifically sound schemes for A/R CDM or REDD, the methodology used during the First Commit- ment Period of the Kyoto Protocol (2008–2012) should be re- examined in the transition to the Second Commitment Period 0378-1127/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2011.07.027 ⇑ Corresponding author. Tel.: +81 29 829 8330; fax: +81 29 874 3720. E-mail address: jtori@affrc.go.jp (J. Toriyama). Forest Ecology and Management 262 (2011) 1659–1667 Contents lists available at ScienceDirect Forest Ecology and Management journal homepage: www.elsevier.com/locate/foreco