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United States Department of Agriculture

Agricultural Research Service

Title: Estimates of Soil Organic Matter Decomposition Rates and Pool Size from Laboratory Incubation Experiments and Radiocarbon Measurements

item Fries, T - USGS
item Harden, J - USGS
item Huntington, T - USGS
item Kramer, Larry

Submitted to: Trans American Geophysical Union
Publication Type: Abstract Only
Publication Acceptance Date: June 4, 1999
Publication Date: N/A

Technical Abstract: Soil samples from depths of 0-20 cm and 20-40 cm at ridge, upper slope, and lower slope positions were taken at cultivated and control sites in Mississippi and Iowa. The samples were taken with a 1 inch i.d. stainless steel tube and were placed in sealed glass jars, equipped with sampling ports. Incubations were carried out at +21 deg C, and CO2 concentrations were measured using gas chromatography. When CO2 concentrations exceeded 2.5%, the jars were flushed with hyrdocarbon and CO2 free air and resealed. The CO2 concentrations were monitored for 6-12 months. Similar experiments were also carried out at -2 deg C, +5 deg C, and +10 deg C to establish temperature response functions. Equations of the form som1* (1-(exp(-k1*t))+som2*(1-exp(-k2*t))+som3*(1-exp(-k3*t)), where som1,2,3 are carbon pool sizes and k1,2,3 are rate constants for the pools, were fitted to the time versus carbon evolved data. Turnover times for each pool fell within ranges of 0.1 to 0.6 years for the fast pool, 14 to 29 years for th slow pool, and 600 to 1500 years for the passive pool. Pool size estimates based on these solutions covered a much wider range. Using bulk **14C measurements for the same depth increments as a further constraint on pool size estimates narrowed the range for pool sizes to 2 to 8% of total carbon in the fast pool, 60-70% of total carbon in the slow pool, and 20-30% of total carbon in the passive pool. Also, **14C measurements were taken on samples of CO2 generated during these experiments. Measurements made early in the incubation were consistent with both bulk **14C measurements, turnover time estimates, and pool size estimates. However, **14C data for samples taken late in the incubation showed **14C depletion that was not consistent with turnover time or pool size estimates.

Last Modified: 4/22/2015
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