MODELING THE CARBON-CYCLE OF GRASSLAND IN THE NETHERLANDS UNDER VARIOUS MANAGEMENT STRATEGIES AND ENVIRONMENTAL-CONDITIONS

A simulation model of the carbon cycle of grassland (CCGRASS) was deve loped to evaluate the long-term effects of different management strate gies and various environmental conditions on carbon sequestration in t he soil. The results presented here refer to permanent grassland on a young sedimentary loam soil in the Netherlands. The model predicted th at the rate of increase in the amount of soil organic carbon will be h ighest at low to moderate application rates of nitrogen (100 - 250 kg N ha(-1) yr(-1)). This is due to the fact that the annual gross photos ynthetic uptake of CO2 in permanent grassland is hardly influenced by the level of N supply. Since N shortage stimulates the growth of the u nharvested plant parts (roots and stubble) the carbon supply to the so il is highest at low to moderate N application rates. The rate of incr ease in the amount of soil organic carbon will be higher under grazing than under mowing as a result of a greater amount of carbon added to the soil. Increase of atmospheric CO2 concentration may induce an incr ease in decomposition rate of soil organic matter due to simultaneousl y increased temperatures. At the same time: plant productivity and thu s carbon supply to the soil will be stimulated due to the CO2-fertiliz ation effect. Under the assumption of a temperature increase of 3 degr ees C if the present atmospheric CO2 concentration doubles, the model predicted that the combined effect of elevated CO2 and temperature wil l slightly reduce the rate of increase in the amount of organic carbon in grassland soils compared to that under unchanged environmental con ditions. There was 2% less carbon sequestration by grassland at the en d of a period of 100 years as a result of these changes in environment al conditions. The separate effects of increased temperature or elevat ed CO2 were 10% less and 10% more carbon storage at the end of a perio d of 100 years, respectively.