Tuesday, November 14, 2006 - 8:45 AM

Soil Carbon and Microbial Dynamics During Long-Term Incubation Involving 13C and 14C Measurements.

Eldor Paul, Colorado State University, 843 Rossum Dr., Loveland, CO 80537 and Ronald Follett, USDA-ARS, Soil Plant Nutrient Res., Fort Collins, CO 80526-8119.


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.