European Journal of Soil Science, December 2009, 60, 924–934 doi: 10.1111/j.1365-2389.2009.01185.x A simplified modelling approach for quantifying tillage effects on soil carbon stocks D. CHATSKIKH a,b , S. HANSEN a , J.E. OLESEN b & B.M. PETERSEN b a Department of Basic Sciences and Environment, Faculty of Life Sciences, University of Copenhagen, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark, and b Department of Agroecology and Environment, Faculty of Agricultural Sciences, University of Aarhus, PO Box 50, DK-8830 Tjele, Denmark Summary Soil tillage has been shown to affect long-term changes in soil organic carbon (SOC) content in a number of field experiments. This paper presents a simplified approach for including effects of tillage in models of soil C turnover in the tilled-soil layer. We used an existing soil organic matter (SOM) model (CN-SIM) with standard SOC data for a homogeneous tilled layer from four long-term field experiments with conventionally tilled (CT) and no-till (NT) treatments. The SOM model was tested on data from long-term (>10 years) field trials differing in climatic conditions, soil properties, residue management and crop rotations in Australia, Brazil, the USA and Switzerland. The C input for the treatments was estimated using data on crop rotation and residue management. The SOM model was applied for both CT and NT trials without recalibration, but incorporated a ‘tillage factor’ (TF ) to scale all decomposition and maintenance parameters in the model. An initial value of TF = 0.57 (parameter uncertainty, PU = 0.15) for NT (with TF set to 1.0 for CT) was used on the basis of a previous study with observations of soil CO 2 respiration. The simulated and observed changes in SOC were then compared using slopes of linear regressions of SOC changes over time. Results showed that the SOM model captured observed changes in SOC content from differences in rotations, N application and crop residue management for conventional tillage. On the basis of SOC change data a mean TF of 0.48 (standard deviation, SD = 0.12) was estimated for NT. The results indicate that (i) the estimated uncertainty of tillage effects on SOC turnover may be smaller than previously thought and (ii) simple scaling of SOM model parameters may be sufficient to capture the effects of soil tillage on SOM turnover in the tilled layer. Scenario analyses showed that the average extra C input needed to compensate for soil tillage was 762 (SD = 351) kg C ha −1 year −1 . Climatic conditions (temperature and precipitation) also affected how much extra C was needed, with substantially larger inputs being required for wetter and warmer climates. Introduction Soil tillage intensity, along with crop residue management, influences soil organic matter (SOM) turnover and is considered to be one of the key factors in regulating soil carbon (C) balance (Lal, 2002), and thereby determining the whole net greenhouse gas (GHG) budget of an agroecosystem (Grant et al., 2004). Throughout the history of human civilisation soil tillage has been integral to crop production; it is possibly one of the man- agement factors that most severely affects soil quality and environ- ment (Holland, 2004). Often, intensive land cultivation has caused soil degradation, for whole regions in some cases, decreased qual- ity of groundwater and surface waters, and contributed to air pollution, including emissions of greenhouse gases (Lal, 2002). Correspondence: D. Chatskikh. E-mail: dmitri.chatskikh@gmail.com Received 29 October 2008; accepted after revision 2 July 2009 The availability of modern soil and crop management practices enables tillage intensity to be reduced considerably. The latest reform of the EU CAP (Common Agricultural Policy) dealt with reduction of agricultural inputs and outputs. In future, there is a need to stimulate support for CAP efforts to give greater weight to environmental issues in Europe (EEA, 2005). There is therefore a need to quantify better the environmental effects of changes in soil tillage intensity. It is well known that reduction in tillage intensity can cause changes in physical, chemical and biological properties within the soil (Carter, 1986, 1994) which cannot be simultaneously measured in situ, but which affect crop yields (Rydberg, 1992) and the turnover of soil organic C (SOC) (Six et al., 2002). Many factors such as climate, soil properties and agricultural management influence SOC changes over time (West & Post, 2002). Determination of effects of tillage on SOM turnover therefore has to rely largely on observed changes in SOC under  2009 The Authors 924 Journal compilation  2009 British Society of Soil Science