Roles of plant biomass and vegetational heterogeneity, and energy-matter cycling in grassland sustainability

A long-term survey of energy, carbon and nitrogen budgets in a grassland has been carried out since 1974 at the National Grassland Research Institute, Japan. Based on the survey, a system model describing energy and matter flows was constructed. Sustainability of grassland ecosystems in relation to productivity was evaluated by simulations. The results indicated that at low and high levels of animal productivity the grassland ecosystems were able to accumulate carbon in soil year by year, and there was no appreciable indication of decrease in the nitrogen content in soil provided that a given amount of fertilizer was applied to the grassland. The heterogeneity in plant biomass can be described by a gamma distribution. Grasslands grazed by domestic animals such as cattle and sheep display a strong spatial heterogeneity. This characteristic is caused by the uneven deposition of excreta by grazing animals, uneven use of the grassland by grazing animals, and uneven dispersal of plant seeds by grazing animals over the grassland. It was suggested that such unevenness may accelerate energy and matter cycling, and then contribute to grassland sustainability.