Grazing effects on belowground C and N stocks along a network of cattle exclosures in temperate and subtropical grasslands of South America

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

GLOBAL BIOGEOCHEMICAL CYCLES

AMER GEOPHYSICAL UNION

Año: 2009 vol. 23 p. 1 - 14

0886-6236

We evaluated the effects of grazing on C and N belowground pools by comparing15 grazing-exclosure pairs across the Rı ´o de la Plata grasslands of Uruguay andArgentina. We measured C and N pools of belowground biomass, particulate organicmatter (POM), and the mineral associated organic matter (MAOM) in the top meter of thesoil. Grazing exclusion in the Rı ´o de la Plata grasslands promoted (1) decreasedbelowground biomass stocks across all sites, (2) increased soil organic carbon (SOC) andsoil organic nitrogen (SON) stocks in upland soils, and (3) decreased stocks in shallowand lowland soils. In all cases, SOC and SON variations were largely derived bychanges in MAOM stocks that maintained their C:N ratios unchanged. In contrast, stocksof the labile POM fractions changed little, but C:N ratios of these fractions decreased aftergrazing removal. We hypothesize that changes in soil organic matter (SOM) contentsbetween grazed and ungrazed stands result from the balance between changes inbelowground N allocation patterns (root N retention hypothesis) and the ability of the soilto retain the extra N available after the exclusion of herbivores and the cessation ofvolatilization and leaching from urine and dung patches (N loss hypothesis). On the basisof our results we suggest that the relative importance of these two cooccurringmechanisms will shape grazing effects on SOM stocks, depending on soil properties,including texture, pH and soil depth, and vegetation type, particularly allocation patternsand C:N ratios of different plant species.