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 and decomposition. The supply of C to soils is dependent on biomass yields, harvest index, and the proportion of feed C retained in the manure with different crops. A simple mathematical procedure was used to estimate quantities of C supplied to the soil with different livestock systems. This procedure involved the partitioning of C into residue, root, and animal components. Mere were uncertainties, however, about the quantities of C originating from crop roots and C losses during manure storage. Two typical livestock farm systems were compared; a swine system emphasizing grain crops, and a dairy system with grain and forage crops each in a typical rotation. This study revealed that manure contributes a relatively small proportion of the C to soil compared to crop roots and residue, especially in swine systems, unless straw bedding is included. The total quantity of C supplied to the soil was generally greater with the dairy system, primarily because of the inclusion of a legume/grass crop in the rotation.