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

Abstract Background and aims Soil organic carbon (SOC) dynamics are vital in the context of climate change and sustainable soil management. The ẟ 13 C signatures of SOC are powerful indicators and tracers of C fluxes through soils and of transformation processes within soils. Depth gradients of ẟ 13 C can be considered as their archive. However, many different drivers and processes impact ẟ 13 C signatures of SOC simultaneously, thus hampering their interpretation. Methods Here we summarize the current knowledge about drivers, processes and C sources determining the δ 13 C signatures of organic matter along soil profiles. Results The largest ẟ 13 C 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 δ 13 C signatures typically increase by 1–3‰ from topsoil to subsoil. Three main reasons for this are (i) the decreasing ẟ 13 C of atmospheric CO 2 (Suess effect) has led to a depletion in plant biomass by about 2.0‰ since 1850, (ii) increasing atmospheric CO 2 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 13 C-enriched hydrophilic dissolved organic C and other C input sources may impact ẟ 13 C gradients in soils. Conclusions External drivers, such as climatic and atmospheric changes, affect the ẟ 13 C signature of C inputs, and have stronger and increasing influence on ẟ 13 C gradients in soil profiles compared to soil internal processes.