Sustainability and long-term dynamics of soil organic matter and nutrients under alternative management strategies

Sustainable agricultural production systems should meet the requirements of the farm household in terms of food, income and leisure, without endangering the productive capacity of the natural resource base. Land use systems are characterized by the crops grown and their order, the production techniques applied and soil and crop management. To judge their degree of sustainability, the processes determining production and soil characteristics and their interactions, as influenced by environmental conditions and management practices should be understood quantitatively. An experimental approach to this problem is problematic: it requires long-term experimentation, which is laborious and time-consuming and takes too long to yield relevant results within the timeframe of the current priorities. Moreover, the erratic and unpredictable nature of environmental conditions makes it difficult to use time- and location-specific information for extrapolation and prediction. A modelling approach to the exploration of the dynamics of sustainability indicators seems promising for formulating criteria for different management strategies. In this paper a model is presented that harnasses existing knowledge into quantitative, descriptive relations. An example is given of calibration of this model on the basis of results of a long-term agronomic experiment. The performance of the model in terms of organic matter dynamics appears ‘reasonable’, and reproduces effects of soil and crop management in a recognizable form. The apparent discrepancy between organic nitrogen and organic carbon behaviour must be analysed in more detail, to establish its possible causes. It may well be that contrary to model predictions, additional losses of nitrogen through volatilization, denitrification or leaching have taken place. Further testing of the model is necessary, before confidence can be placed in its predictive capacity.