Organic carbon and carbon isotopes in modern and 100‐year‐old‐soil archives of the Russian steppe

Abstract

Archived soils can provide valuable information about changes in the carbon and carbon isotope content of soils during the past century. We characterized soil carbon dynamics in a Russian steppe preserve using a 100‐year‐old‐soil archive and modern samples collected from the same site. The site has been protected since 1885 to the present, during which time the region has experienced widespread conversion to cultivation, a decrease in fire frequency, and a trend of increasing precipitation. In the preserve, the amount of organic carbon did not change appreciably between the 1900 and 1997 sampling dates, with 32 kg C/m2 in the top meter and a third of that in the top 20 cm. Carbon and nitrogen stocks varied by less than 6% between two replicate modern soil pits or between the modern sites and the archive. Radiocarbon content decreased with depth in all sites and the modern SOM had positive Δ values near the surface due to nuclear weapons testing in the early 1960s. In the upper 10 cm, most of the SOM had a turnover time of 6–10 years, according to a model fit to the radiocarbon content. Below about 10 cm, the organic matter was almost all passive material with long (millennial) turnover times. Soil respiration Δ14CO2 on a summer day was 106–109‰, an isotopic disequilibrium of about 9‰ relative to atmospheric 14CO2. In both the modern and archive soil, the relative abundance of 13C in organic matter increased with depth by 2‰ in the upper meter from δ13C = ‐‐26‰ at 5 cm to ‐‐24‰ below a meter. In addition, the slope of δ13C vs. depth below 5 cm was the same for both soils. Given the age of the soil archive, these results give clear evidence that the depth gradients are not due to depletion of atmospheric 13CO2 by fossil fuel emissions but must instead be caused by isotopic fractionation between plant litter inputs and preservation of SOM. Overall, the data show that these soils have a large reservoir of recalcitrant C and stocks had not changed between sampling dates 100 years apart.

Comparison of soil organic matter dynamics at five temperate deciduous forests with physical fractionation and radiocarbon measurements

“…D 14 C also declined with depth and the rate of decline differed amongst sites. D 14 C values and turnover times in the subsurface at all of our sites were higher than those reported for similar depths from Russian steppe (Torn et al 2002) and tropical forest (Trumbore 1993) and may reflect bioturbation, disturbance or glaciation history, or simply faster cycling of deep soil carbon in these temperate forests.…”

“…Relatively high D 14 C values in Oe/Oa horizons, and 0-5 cm soil at MO-OZ where no Oe/ Oa is present, indicated the retention of bomb 14 C and resulted in estimated mean turnover times of 9-90 years (Table 3). Bulk mineral soil D 14 C values differed amongst sites and declined with depth, a pattern observed in other soil profiles where 14 C has been measured (Gaudinski and Trumbore 2003;Torn et al 2002;Trumbore 2000). D 14 C values in surface bulk soil were lowest at NH-BF and MA-HF and highest at MO-OZ (a = 0.01).…”

“…Reported turnover times are means of the five plots at each site and are for total soil carbon. To determine turnover times, we used the time-dependent steady-state model described below, which calculates the D 14 C of a given pool over time and varies turnover times to match measured D 14 C (Torn et al 2002;Gaudinski et al 2000). Samples with a significant (C5 %) contribution of inorganic carbon to total carbon were excluded because the model assumes all carbon is organic in origin.…”

Variation in the Molecular Structure and Radiocarbon Abundance of Mineral-Associated Organic Matter across a Lithosequence of Forest Soils

“…For each horizon, density/aggregate fractions and bulk soils from each site were composited for AMS analysis. Radiocarbon abundance measurements were converted to steady-state mean residence time (MRT) values following Trumbore [44] and Torn et al [45]. Estimated MRT values are not meant to represent absolute years of residence in the soil, but are used to allow for ease of comparison among samples (i.e., to allow for quick comparison of decadal versus millennial cycling rates).…”

Soil aggregate fraction-based 14C analysis and its application in the study of soil organic carbon turnover under forests of different ages

“…The SOC turnover times were estimated using a timedependent steady-state box model [46][47][48]. This model assumes that the decomposition of soil organic carbon follows a first-order kinetic law.…”