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

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.