Soil Carbon and Nitrogen Contents Under Long Term Cropping Systems in Uruguay.

Soil use intensity controls organic carbon dynamics through different practices, such as fertilizer use, tillage and crop rotations. We evaluated selected treatments from two long-term experiments in Uruguay (Typic Argiudols). Experiment I was established in 1963 at INIA La Estanzuela with conventional tillage. Selected treatments were: Continuous cropping with no fertilizer additions (CS-1); Continuous cropping with N-P fertilization (CS-2); and a six-year crop–pasture rotation with 3 years as CS-2 and 3 years with a grass-legume pasture (CS-5). Experiment II was established in 1995 at INIA Treinta y Tres under no tillage. Treatments were: Continuous cropping with annual forage crops for forage (CC); a short rotation (SR) of 2 years as CC and 2 years with a biannual grass-legume pasture; a long rotation (LR) of 2 years as CC and 4 years with a grass-legume pasture; and a permanent pasture (PP) overseeded every four years. Soil organic carbon (SOC) and organic C in particulate organic matter (C-POM) were determined. After 45 years SOC stocks (0-20 cm depth) in experiment I declined 45 and 29% in CS-1 and CS-2 compared to CS-5. Inclusion of pastures (CS-5) resulted in significantly higher values of C-POM than continuous cropping systems and the relative magnitude of these differences were higher than those observed with SOC. A higher C input through residue return from crops and pastures, as well as less soil erosion may explain these findings. In Experiment II, after 11 years, SOC (0-15 cm depth) declined 23% in CC compared to PP due to a higher biomass removal. Moreover, C-POM was more sensitive to detect differences between CC and PP. Crop-pasture rotations (SR and LR) reduced SOC (11 %) compared to PP. Regardless of the type of tillage, the inclusion of perennial pastures is necessary to maintain or increase SOC in the long term.