PARENT SESSION
Symposium 14: Respiratory Control of the Global C Cycle in a Changing Environment
Organized by: M Gonzalez-Meler and W Schlesinger
Wednesday, August 6. 1:30 PM to 5:00 PM, SITCC Chatham Ballroom B.
Mean residence time of soil carbon pools: controlling factors and implications for soil carbon cycling.
Jastrow, Julie*,1, 1 Argonne National Laboratory, Argonne, IL
ABSTRACT- Soil organic matter (SOM) is a heterogeneous mixture of plant, animal, and microbial materials in all stages of decay combined with a variety of decomposition products of differing ages and complexity. Because the turnover of these components varies continuously, any estimate of the mean residence time (MRT) for SOM as a whole simply represents an overall average. The use of whole soil MRT for predicting responses to management practices or environmental perturbations can be misleading because soils with similar average MRTs can have very different distributions of C among pools with varying turnover rates. Recent advances in approaches to fractionating SOM have improved our ability to isolate functionally meaningful pools tied to the mechanisms that control soil C cycling, and isotopic techniques enable the MRTs of soil fractions to be estimated and compared. Even so, predictions of responses to perturbations are problematic because (1) estimates of MRT are made under the assumption of steady state conditions and (2) the MRT of C in physically or chemically protected pools is not controlled by its inherent decomposability. Thus, under aggrading conditions relatively new C can accumulate in pools with long MRTs, and disturbance can accelerate the loss of old C pools by exposing protected but otherwise labile C to decomposers. Similarly, responses to environmental changes that influence decomposer activity, such as temperature or moisture, cannot be directly predicted from estimates of pool MRTs without accounting for effects on the transfer between pools and knowledge of the saturation levels for protective mechanisms. Hence, simulation models that account for variations in turnover rates for different soil C pools and the transfers between pools are necessary to generate realistic predictions of soil C dynamics.
Key words: soil carbon, mean residence times, turnover, soil fractions