Acid hydrolysis to partition plant material into decomposable and resistant fractions for use in the Rothamsted carbon model Yasuhito Shirato * , Masayuki Yokozawa National Institute for Agro-Environmetnal Sciences, Kannondai 3-1-3, Tsukuba, Ibraraki 305-8604, Japan Received 14 June 2005; received in revised form 15 July 2005; accepted 22 July 2005 Available online 24 August 2005 Abstract Using various plant materials, we identified two conceptual pools of plant litter, decomposable plant material (DPM) and resistant plant material (RPM), in the Rothamsted Carbon Model (RothC) by comparing the default proportions of DPM and RPM in the RothC and proportions in plant material fractions as determined by two-step acid hydrolysis with H 2 SO 4 . We collected 37 plant samples from 15 species at six sites on arable land, grassland, or forest in Japan. Carbon in the plant materials was divided into three pools by acid hydrolysis: (a) Labile Pool I (LP I), obtained by hydrolysis with 5 N H 2 SO 4 at 105 8C for 30 min; (b) Labile Pool II (LP II), obtained by hydrolysis with 26 N H 2 SO 4 at room temperature overnight, and then with 2 N H 2 SO 4 at 105 8C for 3 h; and (c) Recalcitrant Pool (RP), the unhydrolyzed residue. The average proportion of LP I in crops and grasses was 59%, which was the same as the proportion of DPM defined in the RothC as the default value for crops and grasses. The remaining 41% (23% LP IIC18% RP) was consequently the same as the RPM proportion defined in the RothC. Similarly, the average proportion of LP I in all tree leaves (19%) was very close to the proportion of DPM in the RothC (20%) for trees. These results indicate that DPM in the RothC can be identified as LP I from the acid hydrolysis analysis and RPM as LP IICRP. We conclude that, at least theoretically, the use of an independent DPM:RPM ratio, as determined by acid hydrolysis analysis for each plant material, should enable more reliable modeling of SOM dynamics than the use of default DPM:RPM values provided by the model, even though the practical advantages of this method require further evaluation. q 2005 Elsevier Ltd. All rights reserved. Keywords: DPM; Litter quality; RothC; RPM; Simulation model; Soil organic matter 1. Introduction Soil organic matter (SOM) turnover models are very effective at simulating changes in SOM associated with different agricultural management systems or with climatic changes. Many SOM turnover models have been developed (e.g. review by McGill, 1996). However, a limitation of such models—namely, the fact that most of the conceptual pools they contain do not correspond to experimentally measurable fractions—has been stressed (Christensen, 1996; Elliot et al., 1996). If these pools could be related to measurable fractions, it would be possible to initialize the model without the need to input historical data or run the model repeatedly assuming equilibrium conditions; it would also be possible to validate the model using the size of each pool as well as the total soil organic carbon (SOC). The Rothamsted Carbon Model (RothC: Coleman and Jenkinson, 1996) is one of the leading SOM turnover models and is widely used worldwide. It contains five compartments of SOC, including two plant litter compart- ments (decomposable plant material, DPM; and resistant plant material, RPM) and three other SOC pools (microbial biomass, BIO; humified organic matter, HUM; and inert organic matter, IOM). BIO is measurable by a fumigation– extraction method (Vance et al., 1987), but the other four pools are basically conceptual. Several studies (Balesdent, 1996; Ludwig et al., 2003; Skjemstad et al., 2004) have tried to match measurable fractions with some of the conceptual pools of the RothC. However, the two plant litter compartments (DPM and RPM) have not been matched with experimentally measurable fractions. Soil Biology & Biochemistry 38 (2006) 812–816 www.elsevier.com/locate/soilbio 0038-0717/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.soilbio.2005.07.008 * Corresponding author. Address: Department of Global Resources, National Institute for Agro-Environmetnal Sciences, Kannondai 3-1-3, Tsukuba, Ibraraki 305-8604, Japan. Tel.: C81 29 838 8235; fax: C81 29 838 8199. E-mail address: yshirato@affrc.go.jp (Y. Shirato).