Radiocarbon based turnover rates of soil organic matter fractions along climatic and biogeochemical gradients across in Switzerland

Soil organic matter (SOM) is the largest organic carbon (OC) pool on Earth’s surface after sedimentary rocks. Soil carbon storage is a key process that can mitigate climate change through the sequestration of CO₂ from the atmosphere. However, numerous uncertainties persist concerning how SOM reacts to changing environments due to the challenges in disentangling the effects and interplay between different climatic and physico-chemical controls on SOM stabilization. Radiocarbon has proven to be a useful tool to identify SOM sources and turnover times, yet comprehensive investigations of ¹⁴C dynamics of SOM across climatic and environmental gradients remain scarce.

Our study aimed at better understanding the drivers of carbon dynamics across different ecoregions in a large suite of 54 Swiss soils (0-20 cm depth) that span a broad range of climate and geological conditions. We measure radiocarbon signatures of different SOM fractions separated on the basis of density and chemical reactivity from both recently sampled (2014) and archived soils (collected in the 1990s) in order to estimate the evolution of ¹⁴C in the different soil fractions over two decades. Results are interpreted in the context of a comprehensive soil database in order to assess the impact of different drivers, such as climatic conditions, bedrock, altitude, land-use, soil biogeochemical properties on ¹⁴C signatures and turnover times of different SOM pools.

First results show a strong contrast between particulate organic matter (POM) and mineral associated organic matter (MAOM) fractions of the soils. The particulate organic matter ¹⁴C signature decreased between the two soil inventories, on average from 113 ‰ to 78 ‰, following the decline of ¹⁴C bomb spike in the atmosphere. This shows that thePOM is a fast cycling reactive pool. In contrast, MAOM finer than 20 µm showed an increase in Δ¹⁴C from -35‰ in the 1990s’ samples to 0.8 ‰ in 2014, indicating substantial C fluxes through MAOM cycling at decadal time scales. Further oxidation of MAOM using hydrogen peroxide, removing about 80 to 90% of its C, revealed that MAOM is composed of very old SOM with Δ¹⁴C values as low as -104.9 ± 0.8 ‰ and thus millennia old. By contrast, the removed SOC had high Δ¹⁴C values around 40 ‰. This finding implies that MAOM consists of a continuum from rather stable SOM to rather rapidly cycling components. First results also indicate a strong influence of pH on turnover times, suggesting slower OM processing in acidic soils. By linking our ¹⁴C data to auxiliary data, we will explore the factors driving turnover rates of fast and slower cycling OC pools and pinpoint their vulnerability to climate change.