CROP CARBON CONTRIBUTION TO THE SOIL WITH DIFFERENT CROPPING AND LIVESTOCK SYSTEMS

The level of organic matter or carbon in agricultural soils depends on additions from crop residues and manures and on losses from erosion a nd decomposition. The supply of C to soils is dependent on biomass yie lds, harvest index, and the proportion of feed C retained in the manur e with different crops. A simple mathematical procedure was used to es timate quantities of C supplied to the soil with different livestock s ystems. This procedure involved the partitioning of C into residue, ro ot, and animal components. There were uncertainties, however, about th e quantities of C originating from crop roots and C losses during manu re storage. Two typical livestock farm systems were compared; a swine system emphasizing grain crops, and a dairy system with grain and fora ge crops each in a typical rotation. This study revealed that manure c ontributes a relatively small proportion of the C to soil compared to crop roots and residue, especially in swine systems, unless straw bedd ing is included. The total quantity of C supplied to the soil was gene rally greater with the dairy system, primarily because of the inclusio n of a legume/grass crop in the rotation.