Soil depth gradients of organic carbon-13 – A review on drivers and processes

Abstract

Abstract Background and aims Soil organic carbon (SOC) dynamics are vital in the context of climate change and sustainable soil management. The ẟ13C signatures of SOC are powerful indicators and tracers of C fluxes through soils and of transformation processes within soils. Depth gradients of ẟ13C can be considered as their archive. However, many different drivers and processes impact ẟ13C signatures of SOC simultaneously, thus hampering their interpretation. Methods Here we summarize the current knowledge about drivers, processes and C sources determining the δ13C signatures of organic matter along soil profiles. Results The largest ẟ13C gradients within soil profiles (> 10‰) have been observed at sites where vegetation has shifted between C3 and C4 plants, thus changing the isotopic signatures of C inputs. In soil profiles without such vegetation changes, the δ13C signatures typically increase by 1–3‰ from topsoil to subsoil. Three main reasons for this are (i) the decreasing ẟ13C of atmospheric CO2 (Suess effect) has led to a depletion in plant biomass by about 2.0‰ since 1850, (ii) increasing atmospheric CO2 concentrations have also depleted plant biomass by about 1.8‰, and (iii) isotopic fractionation occurs during continuous microbial C recycling and necromass accumulation. Moreover, a greater mobility of 13C-enriched hydrophilic dissolved organic C and other C input sources may impact ẟ13C gradients in soils. Conclusions External drivers, such as climatic and atmospheric changes, affect the ẟ13C signature of C inputs, and have stronger and increasing influence on ẟ13C gradients in soil profiles compared to soil internal processes.