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

We evaluated the effects of grazing on C and N belowground pools by comparing 15 grazing‐exclosure pairs across the Río de la Plata grasslands of Uruguay and Argentina. We measured C and N pools of belowground biomass, particulate organic matter (POM), and the mineral associated organic matter (MAOM) in the top meter of the soil. Grazing exclusion in the Río de la Plata grasslands promoted (1) decreased belowground biomass stocks across all sites, (2) increased soil organic carbon (SOC) and soil organic nitrogen (SON) stocks in upland soils, and (3) decreased stocks in shallow and lowland soils. In all cases, SOC and SON variations were largely derived by changes in MAOM stocks that maintained their C:N ratios unchanged. In contrast, stocks of the labile POM fractions changed little, but C:N ratios of these fractions decreased after grazing removal. We hypothesize that changes in soil organic matter (SOM) contents between grazed and ungrazed stands result from the balance between changes in belowground N allocation patterns (root N retention hypothesis) and the ability of the soil to retain the extra N available after the exclusion of herbivores and the cessation of volatilization and leaching from urine and dung patches (N loss hypothesis). On the basis of our results we suggest that the relative importance of these two cooccurring mechanisms will shape grazing effects on SOM stocks, depending on soil properties, including texture, pH and soil depth, and vegetation type, particularly allocation patterns and C:N ratios of different plant species.