Storage and stability of organic carbon in soils as related to depth, occlusion within aggregates, and attachment to minerals

[摘要] Conceptual models suggest that stability of organic carbon (OC) in soildepends on the source of plant litter, occlusion within aggregates,incorporation in organo-mineral complexes, and location within the soilprofile. Density fractionation is a useful tool to study the relevance of OCstabilization in aggregates and in association with minerals, but it hasrarely been applied to full soil profiles. We aim to determine factorsshaping the depth profiles of physically unprotected and mineral associatedOC and test their relevance for OC stability across a range of European soilsthat vary in vegetation, soil types, parent material, and land use. At eachof the 12 study sites, 10 soil cores were sampled to 60 cm depth andsubjected to density separation. Bulk soil samples and density fractions(free light fractions – fLF, occluded light fractions – oLF, heavyfractions – HF) were analysed for OC, total nitrogen (TN), δ¹⁴C,and Δ¹⁴C. Bulk samples were also incubated to determine CO₂evolution per g OC in the samples (specific mineralization rates) as anindicator for OC stability.

Depth profiles of OC in the light fraction (LF-OC) matched those of rootsfor undisturbed grassland and forest sites, suggesting that roots areshaping the depth distribution of LF-OC. Organic C in the HF declined lesswith soil depth than LF-OC and roots, especially at grassland sites. Thedecrease in Δ¹⁴C (increase in age) of HF-OC with soil depth wasrelated to soil pH as well as to dissolved OC fluxes. This indicates thatdissolved OC translocation contributes to the formation of subsoil HF-OC andshapes the Δ¹⁴C profiles.

The LF at three sites were rather depleted in ¹⁴C, indicating thepresence of fossil material such as coal and lignite, probably inheritedfrom the parent material. At the other sites, modern Δ¹⁴Csignatures and positive correlations between specific mineralization ratesand fLF-OC indicate the fLF is a potentially available energy and nutrientsource for subsurface microorganisms throughout the profile. Decliningspecific mineralization rates with soil depth confirm greater stability ofOC in subsoils across sites. The overall importance of OC stabilization bybinding to minerals was demonstrated by declining specific mineralizationrates with increasing contributions of HF-OC to bulk soil OC, and the lowΔ¹⁴C values of HF-OC. The stability of HF-OC was greater insubsoils than in topsoils; nevertheless, a portion of HF-OC was activethroughout the profile. While quantitatively less important than OC in theHF, consistent older ages of oLF-OC than fLF-OC suggest that occlusion ofLF-OC in aggregates also contributes to OC stability in subsoils. Overall,our results indicate that association with minerals is the most importantfactor in stabilization of OC in soils, irrespective of vegetation, soiltype, and land use.

[学科分类] 地球化学与岩石