Long Term Measured and Simulated Soil Organic Carbon in Crop-Pasture Rotations.

Favorable grain prices in the last decade have lead to a pronounced increase in the cropped area of Uruguay, which expanded to soils with lower land use capability. In addition, production systems based on crop-pasture rotations shifted to a longer annual cropping phase with a shorter pasture phase, or to continuous annual cropping. The objective of this study is to assess the impact of the pasture phase and cropping intensity on an Oxyaquic Argiudoll soil of eastern Uruguay using field data and a cropping systems simulation model (CropSyst). Treatments compare a sorghum-soybean-pasture sequence as follows: 1) Continuous annual cropping (CC) with a winter cover of ryegrass (Lolium multiflorum Lam.) for grazing; 2) Short rotation (SR) of two years pasture of Trifolium pratense L. and ryegrass and two years identical to CC and; 3) Long rotation (LR) with four years of a Dactylis glomerata, Trifolium repens L. and Lotus corniculatus L. pasture, followed by two years identical to CC. After 12 years, the frequency of highly fertilized annual crops led to 30% more P in the topsoil in CC than in SR or LR. In contrast, cropping systems including perennial pastures had grater soil organic carbon content than CC (14.2, 15.9, and 17.8 g kg^-1 for CC, SR and LR, respectively), or 3 to 6 Mg C ha^-1 more in the top 0.15 m of soil. These results suggest that for Uruguayan Arguidolls, perennial pastures rotating with crops are necessary to preserve soil organic C even under no-till. However, grain yield of sorghum was unrelated to soil organic carbon, with sharp interannual variations reflecting water availability during the growing season. The long-term trend in soil organic carbon and its eventual effects on annual crops yields are discussed based on CropSyst simulations.