The measurement of the decreases in SOC and SMB-C during an 853 d incubation were used to determine SOC pool sizes and fluxes and the role of microbial biomass as a source of mineralizable C in cultivated and native grassland soils. The SMB-C decreased ~90% during incubation without added substrate. It accounted for 29% of the CO₂ during the first 79 days of incubation and 16% during the last 599 days. Incubator malfunction between days 500 to 550 resulting  in a 3- to 6.5ēC temperature increase in which microorganism respiration rate increased ~50%. This raised questions of whether biochemical recalcitrance of the SOC was limiting its decomposition. Measurement of the ¹³CO₂ allowed us to determine whether it came from the more recent plant residues or from original, native grassland SOC.  Soils under conventional-till (with lower SOC) lost ~ 15% of their SOC during incubation.  Crop rotation and sod plots with higher SOC lost 25%. This resulted in an increase of the MRT, as determined by carbon dating, of 120 yr for the SOC after incubation. The nonhydrolyzable C representing 62% of the SOC, increased from 3057 yr-MRT to 4967yr. The active pool representing 1 to 6% of the SOC depending on treatment, had an MRT of 100 to 200 days in the laboratory. The slow pool representing 32 to 35% of the SOC had MRTs of 12 to 14 yr for the sod and rotational plots but 21 to 28 yr in the conventional till. Acid hydrolysis- incubation together with ¹³C and ¹⁴C measurements allowed us to measure the SOC dynamics of different management of grassland soils. The pool sizes tended to stay constant as a percent of the SOC, which changed significantly with management. The MRT of all the SOC pools were, however, very sensitive to management.