Carbon balance of an intensively grazed permanent grassland in southern Belgium

Grasslands are an important component of the global carbon balance but their carbon storage potential is still highly uncertain. Especially, the impact of weather variability and management practices on grassland carbon budgets need to be assessed. This study investigates the carbon balance of an intensively managed permanent grassland in southern Belgium and its uncertainties by combining 5-years of eddy covariance measurements and other organic carbon exchanges estimates. The specificities of this study lie in: (i) the age of the pasture, which has probably been established since more than one century; (ii) the intensive character of the management with a mean grazing pressure larger than 2 livestock unit ha-1 and stocking cycle including stocking and rest periods, (iii) the livestock production system, typical of Wallonia, farming intensively Belgian Blue breed of cattle in order to produce meat.
The results showed that, despite the high stocking rate and the old age of the pasture, the site acted as a relatively stable carbon sink from year to year with a 5-year average Net Biome Productivity of ‒161 g C m-2 yr-1. Lateral organic carbon fluxes were found to increase the carbon sink because of high carbon imports (organic fertilization, feed complements) and low carbon exports compared to dairy pastures.
Cattle stocking density was adapted to grass production, which itself depends on weather conditions and leaf area index, in order to maintain a steady meat production. This resulted in a coupling between grazing management and weather conditions. As a consequence, both weather and grazing impacts on net ecosystem exchange were difficult to distinguish. Indeed, no correlation was found between weather variables anomalies and net ecosystem exchange anomalies.