Pastoral dairy farming systems in Uruguay exhibit an accelerated process of intensification with major changes in soil management during the last four decades. However, the production systems were always based on the same concept of sustainable rotations of annual forage crops and perennial grasses sown in association with legumes. The soil organic carbon (OC) in the plots of the Dairy Experimental Farm at La Estanzuela was monitored since 1974 up to 2010. Those records quantify the effect on OC dynamics of four production systems with different soil management carried out in long periods. This information is extremely relevant considering that survey studies showed that commercial dairy farming systems followed the same general pathway of technical changes. The extreme modification in tillage, animal stocking rate and feed imported to the farm determined two contrasting periods in the OC dynamics. Heavy losses of OC were estimated at an average of 0.89 Mg ha -1 year -1 during a first period of 17 years. This tendency was reversed for the following 18 years when OC increased at 0.94 Mg ha -1 year -1. Three major management factors are discussed to explain the fast build up of OC: the progressive tillage reduction, improvements in pasture and forage crop productivity and imported feed coming into the system. The large OC and N gains in semi-confinement paddocks are discussed as an opportunity to capitalize the high nutrient availability in the soil for crop production.
