A grassland ecosystem is a field where energy flows and various materials, such as nitrogen and potassium, circulate from component to component. This ecosystem usually depends on a large number of environmental, biological and, often, agricultural factors, which control the productivity in the ecosystem and the interactions between them are intricately entangled. Moreover, studies of a grassland ecosystem require such a large area of land, even in an experiment, that it is difficult, in most cases, to replicate the experiment. For these reasons, this problem should be best solved by a system approach instead of a series of field experiments.
The system approaches to grassland ecosystems have been considered for the following purposes:
1) To analyse various factors and their interactions which affect grassland productivity,
2) To forecast short-term grassland productivity for a year or one season, and to make decisions for short-term grassland management, such as grazing intensity and amount of fertilizer to apply.
3) To analyse the conditions under which high and stable grassland productivity can be maintained on a long-term basis i.e., 10 years or more, since grassland plants are perennial and since theoretically, land should be utilized for many years continuously.
4) To seek optimal grassland conditions and to evaluate the conversion efficiencies of solar energy, including both solar radiation and fossil-fuel, to biomass including plant and animal products.
5) To compare efficiencies among various livestock production systems from the viewpoints of energy and economics, and to decide which system or which combination of systems should be adopted. For solving these problems, in this study, a system model of grassland ecosystem, with emphasis placed on energy flow and nitrogen cycle, will be proposed and several results of system simulation will be reported.