Several process-based models exist for the assessment of soil, water and air nutrient dynamics. These models have been developed after several years of continuous monitoring, testing, and re-validation. Two of the most widely used models, the CENTURY 4.0 and the RothC models have been used extensively. Modelling helps to understand the principal mechanisms affecting ecosystem functioning, and the causes of disturbances to them. They are essential for long term predictions and in making recommendations aimed at reducing harmful effects and preventing environmental disturbances. Many authors have demonstrated the benefits of using computer models in agriculture . The CENTURY model version 4.0 embodies the best understanding to date of the biogeochemistry of C, N, P, and S. The primary purposes of the model are to provide a tool for ecosystem analysis, to test the consistency of data, and to evaluate the effects of changes in management and climate on ecosystems. The CENTURY Agroecosystem Version 4.0 was developed to deal with a wide range of cropping system rotations and tillage practices, for the systematic analysis of the effects of management, and global change on productivity, and sustainability of agroecosystems. Version 4.0 integrates the effects of climate and soil driven variables including agricultural management to simulate C, N, and H2O dynamics in the soil-plant system. Simulation of complex agricultural management systems including crop rotations, tillage practices, fertilization, irrigation, grazing, and harvest methodologies are now possible in this enhanced release of the model. The CENTURY model is a general FORTRAN model of the plant-soil ecosystem that has been used to represent C and nutrient dynamics for different types of ecosystems (grasslands, forest, crops, and savannahs). Aspects of the current version are discussed in Metherell (1992) while a more detailed description of the earlier development of the CENTURY model can be found in Parton et al. (1983), Parton et al. (1987), and Sanford et al. (1991). SOM changes slowly in temperate regions, following changes in land use and management, and it often takes over 20 years to observe a significant change in soil content. In a series of experiments based at Rothamsted different land use and management treatments including arable crops, grasslands, ley periods and woodland, have been studied. Samples of soil have been taken over the last 150 years and stored in the Rothamsted Archive. By measuring the C content of these archived soils, long-term changes in soil C are recorded. The winter wheat experiment at Broadbank has been a useful example of how arable soil C content can be changed over decades by different land management practices. SOM on plots where no applications of mineral or organic fertilisers have been made (nil plots) has remained at a low level reaching equilibrium between inputs from crop debris and losses through decomposition. The addition of NPK fertilisers increased crop growth and hence crop residue inputs to soil, slightly increasing SOM content. However the greatest increase in SOM content has been achieved by adding 35 t ha-1 of FYM. This has not only increased crop growth and residue inputs but provided a huge input of OM to the soil in its own right (Falloon and Poulton, 2005). Data from the long term experiments at Rothamsted has been used to develop the Rothamsted C model known as "RothC model" which is used all over the world and in natural CO2 inventory calculations. Data from the Broadbank experiment also contributes to the Global Soil Organic Matter Network (known as SOMNET) which aid calculations of C-sequestration potential for different land management scenarios in Europe and UK (Fallon and Poulton, 2005). SOMNET was established during 1995 to help predict the effects of changes in land use, agricultural practice and climate on SOM. The book is outcome of predicting carbon sequestration up to 2055 for English agriculture.