Soil carbon dynamics in corn-based agroecosystems: Results from carbon-13 natural abundance

We used natural C-13 abundance in soils to calculate the fate of C-4-C inpu ts in fields cropped to continuous corn (Zea mays L.). Soil samples were co llected from eight cultivated and six adjacent, noncultivated sites: of the Corn Belt region of the central USA, The amount of organic C in cultivated soils declined an average of 68%, compared with adjacent, noncultivated si tes. The delta C-13 Of cultivated soil profiles that had been under continu ous corn for 8 to 35 yr increased in all depth increments above that of the noncultivated profiles. The percentage of soil organic C (SOC) derived fro m corn residues and roots ranged from 22 to 40% of the total C. The proport ion of corn-derived C, as determined by this technique, decreased with soil depth and was minimal iri the 50- to 100-cm depth increments of fine-fextu red soils. The mean residence time of the non-corn C (C-3) ranged from 36 t o 108 yr at the surface, and np to 769 yr at the subsoil depth, The longer turnover times were associated with soils high in clay. Prairie-derived soi ls have a higher potential to sequester C than those derived from forests. The significant loss of total C at all sites and the slow turnover times of the incorporated C lead us to conclude that there is a substantial potenti al for soils to serve as a C sink and as a significant nutrient reserve in sustainable agriculture.