Long term grazing impacts on carbon and nitrogen cycling in South American grasslands

PIÑEIRO, GERVASIO.; PARUELO, JOSÉ M.; OESTERHELD, MARTÍN.

Porto Alegre, Brasil

Congreso; Congress of the International Association of Vegetation Science (IAVS); 2002

International Asociation of vegetation Science

Only short term information is available on the impact of grazing in South American grasslands. We calibrated CENTURY, a process based biogeochemical model, for 11 sites across the "Río de la Plata Grasslands". We analyzed the impact of grazing, since the introduction of cattle by Europeans and for the last 400 years. Our simulations shows that grazing by domestic herbivores increases nitrogen (N) losses in all study sites and N cycling in N limited sites. These changes reduced total N in soil organic matter pools and led to a consistent decrease in soil organic carbon (SOC). Changes in N uptake, resulting from the balance between increases in N cycling and decreases in the soil N pool, modified both the magnitude and sign of annual net primary production (ANPP). Factors controlling ANPP changes and SOC losses were mainly local variables affecting resource availability. Regional factors as precipitation and temperature had secondary importance in explaining the impact of grazing on biogeochemical cycles. Our results pointed out the shortcomings of short term experimental studies to understand the effects of grazing on sub-humid regions and suggests that grazing could have altered nutrient cycling on these grasslands.