Much of the work in understanding and modeling soil C is the study of relative decomposition rates for various organic substrates and organic matter byproducts. While organic matter may actually be a continuum, best represented by a distribution of turnover times (39), continuous distributions are difficult to constrain with field measurements. There is general agreement that SOM contains at least three identifiable C pools: root exudates and rapidly decomposed components of fresh plant litter (“active” pool); stabilized organic matter that persists in soils over several thousands of years (“passive” pool); and a poorly defined “intermediate” or “slow” C pool that has turnover times in the range of years to centuries (Fig. 1). I will use these terms in this paper, and in addition will refer to fast-cycling C as the combined active and intermediate pools (all nonpassive C).
The mechanisms through which climate or land-use change may influence soil carbon cycling include a combination of biotic controls that affect the amount and resistance to decay of C added to soils by plants, and physical controls, including the area and chemistry of surfaces for stabilization of organic matter, and the availability of oxidants to decomposers. These mechanisms operate on vastly different time scales. Plant productivity and soil microbial communities respond to shifting climate conditions within hours (41), whereas changes in production and respiration on an annual basis are linked to interrannual variability in climate (42, 43). Persistent changes in climate can lead to species changes over time scales of decades to centuries (44), with concomitant shifts in plant production and litter quality (45). Soil minerals, forming over millennial time scales, control how much of the soil carbon is chemically/physically protected from decomposition (46, 47, 48 and 49). Finally, ecosystem disturbance caused by fire, flooding, tree blow-down, etc. will cause drastic changes in plant production and soil conditions for decomposition that far exceed those associated with interrannual variability. Thus the general design of experiments to understand soil C dynamics should depend on sampling existing environmental and disturbance gradients as well as manipulations to test soil forming factor effects on the amount and turnover times of C in active, intermediate, and passive C pools.
No single, satisfactory method yet exists to separate soil C into components with different turnover times. Instead, the dynamic makeup of soil C is deduced using many constraints, including: physical and chemical fractionation of organic matter, field and laboratory decomposition studies, measures of C fluxes into and out of the soil, measurements of 14C in soils sampled at various times before and after the peak of atmospheric nuclear weapons testing, changes in the 13C content of SOM after a vegetation change from plants with C3 to C4 photosynthetic pathway, and measurements of changes in the total amount of C in soils of different age or after disturbance. Each of these tools is suitable for determining different time scales of soil C dynamics, and much confusion may result from mixing of terms related to turnover times (active, intermediate, or passive) with those derived from measured soil parameters. For example, the increase in bomb 14C, or changes in 13C in SOM during the first several decades after conversion of a C3-dominated forest vegetation to C4-dominated pasture vegetation may be used to determine the amount and average turnover time of C that is fast-cycling (nonpassive). Fast-cycling C may be further comprised of active and intermediate pools with different turnover times, but isotopic techniques are not suitable for distinguishing this split.
Active Carbon Pools. Field decomposition studies involve both observation of loss of native plant litter and differences in decomposition of a common litter substrate at different sites. Isotope labeling studies usually follow specific compounds or compound classes (such as amino acids or carbohydrates), but also may follow the fate of below-ground C allocation, as in pulse-labeling studies. Although some incubations have been followed over periods of more than 10 years (26, 50), most are designed for shorter time periods. These studies are used to provide multiple rate constants for decomposition models like those of Jenkinson and Raynor (15).
Soil-respired CO2 is produced either by metabolic root respiration or by decomposition of fast-cycling SOM pools (Fig. 1). In addition, CO2 fluxes measured at the soil surface are most likely dominated by decomposition in near-surface layers (including surface detritus). With these caveats, CO2 flux measurements provide an estimate of total decomposition C flux from the soil, if correction for root-respiration can be made (34). Radiocarbon measurements of respired CO2 may help distinguish sources of respiration experimentally. Root respiration and CO2 derived from decomposition of C with very rapid turnover should have ∆14C close to atmospheric values for the year of sampling, whereas CO2 derived from slow C pools will have higher or lower ∆14C values, depending on the average turnover time.
Field measurements of decomposition and CO2 flux often are complicated by seasonal differences in the quality and quantity of detrital material (for example, in temperate zones, deciduous leaves are shed in autumn, but may be responsible for a pulse of decomposition the next spring). Laboratory incubations, where soil is separated from roots, allow observation of the CO2 evolved from decomposition alone (51). Long-term incubations (>1 year) allow estimation of the relative contributions of active and slow pools to soil CO2 flux (5). However, the magnitude of laboratory fluxes of C may not be easily mapped to field conditions.
Experiments manipulating soil temperature to determine the net response of soils (19) or soils plus vegetation (52) to increased temperature are now in progress in several places. These treatments have been in place in most cases for <10 years, and preliminary results are reflective of adjustments in the fast-cycling pools.
Passive Soil Carbon and the Depth Distribution of 14C. The existence of passive SOM is deduced from radiocarbon measurements and the fact that some C3-plant-derived organic matter persists in disturbed soils even after a century or more of cultivation with C4 plants. In general, the amount and 14C content of SOM decrease with depth in soil profiles, indicating the increasing importance of passive organic matter with depth in the soil (37, 53).