Comparison of methods for the estimation of inert carbon suitable for initialisation of the CANDY model Martina Puhlmann 1, *, Katrin Kuka 2 and Uwe Franko 2 1 Leibniz-Centre for Agricultural Landscape Research (ZALF) e. V., Institute of Landscape Systems Analysis, Eberswalder Straße 84, 15374, Mu ¨ncheberg, Germany; 2 Department of Soil Science, UFZ – Centre for Environmental Research Leipzig-Halle, Theodor-Lieser-Str. 4, 06120, Halle, Germany; *Author for correspondence (e-mail: martina.puhlmann@zalf.de; fax: +49-334-3282334) Received 25 August 2005; accepted in revised form 23 January 2006 Key words: CANDY, Carbon and nitrogen dynamics, Inert carbon, Model initialisation, Modelling Abstract Almost all soil organic carbon turnover models rely on a partitioning of total organic carbon into an inert and a decomposable pool. The quantification of these pools has a large impact on modelling results. In this study several methods to estimate inert carbon in soils, based either on total soil organic matter or physical protection, were assessed with the objectives of (1) minimising errors in carbon and nitrogen dynamics and (2) ensuring usability for sites with marked differences in site conditions. CANDY simulations were carried out by varying solely the method for calculating the size of the inert carbon pool used to initialise the model. Experimental data from Bad Lauchsta¨dt and Mu¨ncheberg were used for the simulation. The data were made available for modellers at a workshop held at Mu¨ncheberg (Germany) in 2004. The results concerning not only carbon but also nitrogen dynamics were analysed by applying selected statistical methods. It was shown that even in short-term simulations model initialisation procedure may influence the simulation results considerably. Three methods of estimating inert carbon were identified as being the most appropriate. These methods are either based on soil texture or pore-space classes and therefore account for the physical protection of soil organic matter. Thus, physical protection seems to be of major importance. By extending the scope of the investigation into nitrogen dynamics, additional support for the applicability of a selected method was obtained. Introduction The simulation model CANDY (Carbon and Nitrogen Dynamics, Franko et al. 1995, 1997; Franko 1996, 1997) has been developed in order to provide information about carbon stocks in soils, organic matter turnover, N uptake by crops, leaching and water quality. A comparison of soil organic carbon models in 1997 showed CANDY to be in the group with the best performance (Smith et al. 1997). In most long-term organic matter models (Hansen et al. 1991; Franko et al. 1995; Coleman and Jenkinson 1996; Parton 1996, etc.), soil or- ganic matter is partitioned in several pools with different turnover rates. In CANDY the decom- posable soil organic matter can be subdivided into biologically active and stabilised soil organic Nutrient Cycling in Agroecosystems (2006) 74: 295 –304 Ó Springer 2006 DOI 10.1007/s10705-006-9005-2