Modelling the carbon cycle of grassland in the Netherlands under various management strategies and environmental conditions.

A simulation model of the grassland carbon cycle (CCGRASS) was developed to evaluate the long-term effects of different management strategies and various environmental conditions on carbon sequestration in a loam soil under permanent grassland in the Netherlands. The model predicted that the rate of increase in the amount of soil organic carbon will be greatest at low to moderate application rates of nitrogen (100-250 kg N/ha per year). This is because the annual gross photosynthetic uptake of CO2 in permanent grassland is hardly influenced by the level of N supply. Since N shortage stimulates the growth of the unharvested plant parts (roots and stubble) the carbon supply to the soil is highest at low to moderate N application rates. The rate of increase in soil organic carbon will be greater under grazing than under mowing as a result of a greater amount of carbon added to the soil. Increase of atmospheric CO2 concn may induce an increase in decomposition rate of soil organic matter due to simultaneously increased temperatures. At the same time, plant productivity and thus carbon supply to the soil will be stimulated due to the CO2-fertilization effect. Assuming a temperature increase of 3 degrees C if the present atmospheric CO2 concn doubles, the model predicted that the combined effect of elevated CO2 and temperature will slightly reduce the rate of increase in the amount of organic carbon in grassland soils compared to that under unchanged environmental conditions. There was 2% less carbon sequestration by grassland at the end of a 100 year period as a result of these changes in environmental conditions. The separate effects of increased temperature or elevated CO2 were 10% less and 10% more carbon storage after 100 years, resp.