Dynamics of resistant soil carbon of midwestern agricultural soils measured by naturally occurring ¹⁴C abundance

Information on the mean residence time (MRT) of soil organic carbon (SOC) on different soil types and management regimes is required for pedo-geological, agronomic, ecological and global change interpretations. This is best determined by carbon dating the total soil together with acid hydrolysis and carbon dating of the non-hydrolyzable residue (NHC). Midwestern US soils in a transect from Lamberton, MN to Kutztown, PA were found to contain from 33% to 65% of their SOC in the non-hydrolyzable fraction. Soils on lacustrine deposits had the most NHC; glacial till and shale soils, the least. The MRTs of the SOC of surface horizons of soil ranged from modern to 1100 years with an average of 560 years. The MRT increased to an average of 1700 years in the 25-50-cm depth increment and 2757 years at 50-100 cm. The NHC was 1340 years greater at the surface and 5584 years at depth. The MRTs of the total SOC were inversely correlated to sand and directly related to clay content. Silt did not have a significant effect on the MRT of total SOC, but was significantly correlated with the MRT of the NHC. A four-parameter model described the relationship between the SOC content and MRT. The complexity of this equation reflected the strong effect of depth, which greatly decreased SOC while increasing the MRT. The MRT of these soils, as determined with carbon dating of the naturally occurring ¹⁴C, was compared to that measured with the ¹³C signal produced by approximately 30 years of continuous corn (Zea mays L.) (C₄) on soils with a known plant history of C₃-C₄ cropping. The equation of ¹⁴C MRT = 176(¹³CMRT)^0.54 with an R² of 0.70 showed that although short-term ¹³C studies correlate well with the total MRT, they reflect the dynamics of the active and slow pools, not the total SOC.