Evaluating timescales of carbon turnover in temperate forest soils with radiocarbon data

Timescales of soil organic carbon (SOC) turnover in forests were investigat ed with soil radiocarbon data. The C-12/C-14 ratios were measured by accele rated mass spectroscopy on soil sampled from a deciduous temperate forest i n Switzerland during 1969-1995. The resulting Delta(14)C values (125-174 pa rts per thousand) were in line with previously published C-14 soil data. We applied FORCLIM-D, a model of nonliving organic matter decomposition inclu ding nine litter and two soil compartments to estimate SOC turnover times f or this forest type. Carbon 14 aging in woody vegetation was explicitly acc ounted for. Parameters were calibrated to match radiocarbon ratios observed for forest soils at Meathop Wood, United Kingdom [Harkness et al., 1986]. We estimated that roughly 50-94% (best estimate, 49%) of foliar litter carb on and 11-74% (73%) of fine root litter carbon are eventually respired as C O2 at Meathop Wood; the rest is transferred to soil humus, where it undergo es further decomposition. Turnover times for the 0-20 cm mineral soil layer ranged from 9-50 years (25 years) for a fast overturning soil compartment comprising 38-74% (68%) of bulk SOC and 155-10,018 years (3,570 years) for a slowly overturning compartment. For the Swiss site, SOC turnover times we re in the same range. Parameter uncertainties were correlated and induced b y uncertainties in C-14 observations from small-scale spatial inhomogeneiti es, sample preparation and by lack of reliable C-14 observations for the "p rebomb" test period. Model-based estimates of soil organic C turnover deriv ed from C-14 data must be used cautiously since they depend on the underlyi ng model structure: bypassing litter in FORCLIM-D overestimated SOC turnove r by a factor of 2.5. Such an error might remain undetected in studies lack ing samples from the late 1960s and early 1970s. Thus litter C turnover sho uld be included when estimating SOC turnover in temperate forests from C-14 data.