Abstract
Radiocarbon data from soil organic matter and soil respiration provide powerful constraints for determining carbon dynamics and thereby the magnitude and timing of soil carbon response to global change. In this paper, data from three sites representing well-drained soils in boreal, temperate, and tropical forests are used to illustrate the methods for using radiocarbon to determine the turnover times of soil organic matter and to partition soil respiration. For these sites, the average age of bulk carbon in detrital and Oh/A-horizon organic carbon ranges from 200 to 1200 yr. In each case, this mass-weighted average includes components such as relatively undecomposed leaf, root, and moss litter with much shorter turnover times, and humified or mineral-associated organic matter with much longer turnover times. The average age of carbon in organic matter is greater than the average age predicted for CO2 produced by its decomposition (30, 8, and 3 yr for boreal, temperate, and tropical soil), or measured in total soil respiration (16, 3, and 1 yr). Most of the CO2 produced during decomposition is derived from relatively short-lived soil organic matter (SOM) components that do not represent a large component of the standing stock of soil organic matter. Estimates of soil carbon turnover obtained by dividing C stocks by heterotrophic respiration fluxes, or from radiocarbon measurements of bulk SOM, are biased to longer time scales of C cycling. Failure to account for the heterogeneity of soil organic matter will result in underestimation of the short-term response and overestimation of the long-term response of soil C storage to future changes in inputs or decomposition. Comparison of the 14C in soil respiration with soil organic matter in temperate and boreal forest sites indicates a significant contribution from decomposition of organic matter fixed >2 yr but <30 yr ago. Tropical soil respiration is dominated by C fixed <1 yr ago. Monitoring the 14C signature of CO2 emitted from soils give clues as to the causes of seasonal and interannual variability in soil respiration in these systems.
Highlights
Interest in the storage and cycling of organic matter in soils has increased recently because of its importance to the global carbon cycle
Radiocarbon data in organic matter may give a reasonable estimate of the mean residence time for organic matter and the rate at which it will respond over decades, they are not useful for estimating C fluxes from the soil on an annual basis, in mineral soil horizons
Soil organic matter is heterogeneous with respect to decomposition, as evidenced by the different 14C signatures measured in physically and chemically fractionated soil organic matter (SOM) components, and in comparisons of 14C in SOM with respired 14CO2
Summary
Interest in the storage and cycling of organic matter in soils has increased recently because of its importance to the global carbon cycle. We must infer the presence of a SOM fraction at depth that has mean residence times of years to decades, which has ⌬14C values greater than atmospheric 14CO2 The source of this deep, rapidly cycling C is either decomposition of fine roots (which have been identified as having high 14C at depth in Harvard Forest; Gaudinski et al, in press) or soluble organic matter transported down from surface layers. Soil respiration must be considered to be derived from three components: (1) decomposition of SOM fractions with turnover times of several years to decades, as deduced from 14C in organic matter substrates (Table 1); (2). Gaudinski et al, (in press) describe the method used for measurement of 14C in soil respiration Comparison of these values with those predicted from decomposition of SOM fractions with Ͼ1 yr turnover times allows partitioning of the total annual CO2 flux (Table 2). With a record of 14C in soil respiration, it is possible to assess what fraction of the year-to-year variability in soil respiration is due to enhanced decomposition vs. metabolic respiration
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