Soil carbon pools and fluxes in long-term corn belt agroecosystems

The dynamics of soil organic carbon (SOC) play an important role in long-term ecosystem productivity and the global C cycle. We used extended laboratory incubation and acid hydrolysis to analytically determine SOC pool sizes and fluxes in US Corn Belt soils derived from both forest and prairie vegetation. Measurement of the natural abundance of 13C made it possible to follow the influence of continuous corn on SOC accumulation. The active pools (Ca) comprised 3 to 8% of the SOC with an average field mean residence time (MRT) of 100 d. The slow pools (Cs) comprised 50% of SOC in the surface and up to 65% in subsoils. They had field MRTs from 12–28 y for C4-C and 40–80 y for C3-derived C depending on soil type and location. No-till management increased the MRT of the C3-C by 10–15 y above conventional tillage. The resistant pool (Cr) decreased from an average of 50% at the surface to 30% at depth. The isotopic composition of SOC mineralized during the early stages of incubation reflected accumulations of labile C from the incorporation of corn residues. The CO2 released later reflected 13C characteristic of the Cs pool. The 13C of the CO2 did not equal that of the whole soil until after 1000 d of incubation. The SOC dynamics determined by acid hydrolysis, incubation and 13C content were dependent on soil heritage (prairie vs. forest), texture, cultivation and parent material, depositional characteristics. Two independent methods of determining C3 pool sizes derived from C3 vegetation gave highly correlated values.