Long term grazing impacts on carbon and nitrogen soil particle size fractions in South American grasslands

PIÑEIRO, G.; PARUELO, J.M.; JOBBAGY, E.G.; OESTERHELD, M.; JACKSON, R.B.

Savannah, Georgia, USA

Simposio; 89th Meeting of the Ecological Society of America; 2003

Ecological Society of America

We evaluated the effects of grazing on grasslands of South America using CENTURY, a process based biogeochemical model and field experiments. Our simulations suggested that grazing by domestic herbivores accelerate nitrogen cycling and increases nitrogen losses, mainly due to increases in nitrogen volatilization and leaching from urine and feces patches. The lower nitrogen availability under grazing limited soil organic matter formation and resulted in a mean reduction of 22% in soil organic carbon (SOC) content after 400 years of grazing (since introduction of cattle by European’s in the region to the present). The model suggested that these losses should be higher, and hence detectable, in the more labile SOC pools (33%) and less (5%) in the large passive SOC pools. To test this hypothesis, we selected 7 grazing-exclosure sites in the “Rio de la Plata” grasslands of Uruguay. We sampled soil and roots and measured carbon and nitrogen contents at six depths in two different soil size fractions: the particulate organic matter (POM) of rapid turnover and the mineral associated organic matter (MAOM) of low turnover. Unlike other Northern-hemisphere experiments (with increased nitrogen depositions form industry), our results show that for the first 30 cm grazing reduces total SOC and alters vertical distribution of POM, increasing the more labile SOC fractions towards the surface. Vertical changes of POM can be associated with the higher root contents in the first 5 cm of depth under grazing.