RADIOCARBON SIGNATURES OF TURNOVER RATES IN ACTIVE SOIL ORGANIC-CARBON POOLS

Identifying turnover rates in active soil organic carbon (SOC) pools i s a critical step toward the understanding of soil nutrient cycling an d global C cycles. The mean residence times (MRT) of the dominating ac tive SOC pools in surface soils are usually less than a few decades. T his range of MRT is difficult to determine because it is too short for radiocarbon dating and too long for most tracer techniques. I develop ed a procedure to construct radiocarbon signatures for identifying MRT in active SOC pools based on the residue bomb C-14 in the atmosphere. The radiocarbon signature is specific to the year when a soil sample is taken. The sensitivity of the radiocarbon signatures is a function of MRT and the time when a soil sample is taken. This method is sensit ive to MRT of active SOC pools in a range from 1 to 80 yr at the prese nt time. Using those radiocarbon signatures, the MRT of active SOC poo ls in the Morrow Plots (Urbana, IL) and Sanborn Field (Columbia, MO) w ere estimated at 67 and 34 yr, respectively.