Using radiocarbon to identify the impact of climate and mineralogy on soil organic matter turnover

Soils are the largest carbon (C) reservoir in terrestrial ecosystems. There are still numerous uncertainties concerning the fate of soil organic carbon and its feedback on climate change. Radiocarbon is a useful approach to better understanding the carbon cycle. The nuclear weapon testing in the 1960^s induced a peak in ¹⁴C atmospheric concentration – a signal that can be used to trace the incorporation and turnover of C in soil. By separating the soil in different fractions and measuring the ¹⁴C in them, we can quantify how much C and for how long is stored in soils, and where soil organic carbon is stabilised.

Our study aimed at identifying the impact of climate and mineralogy on soil organic matter turnover on a regional scale. We analysed C pools and ¹⁴C contents in the organic layer, mineral soil (0-20cm) and its fractions from 54 sites across Switzerland. These 54 sites are systematically spread across natural climatic and geological gradients and were repeatedly sampled in the 1990s and 2014. The mineral soil was incubated for 181 days and ¹⁴C was measured in the respired CO₂. The mineral soil was fractionated according to density into particulate organic matter (POM) and mineral-associated organic matter (MAOM). We then oxidised the mineral-associated organic matter with hydrogen peroxide to remove its labile fraction of carbon. Our ¹⁴C dataset was analysed together with ancillary data comprising soil properties and climatic variables from the studied sites.

Our radiocarbon dataset showed that the carbon that was respired from the mineral soil originated predominantly from particulate organic matter. The bomb spike signal was incorporated in the organic layer and in the particulate organic matter, while the mineral-associated organic matter had turnover times on centennial to millennial time scales (from 94 to 3060 years). Further chemical oxidation of MAOM using hydrogen peroxide revealed a stronger depletion in radiocarbon of the residual fraction with Δ¹⁴C values ranging between -173 ‰ and -47 ‰. This indicates that the MAOM is a mixture of ¹⁴C-enriched organic matter and very old material.

With respect to the controlling factors of soil organic matter turnover time, the radiocarbon signature of the POM is most strongly affected by climatic variables such as mean annual temperatures. In contrast to POM, the mineral-associated organic matter, comprising the greatest pool of soil organic carbon is driven by chemical soil properties. For instance, older ¹⁴C ages are found in acidic soils with low pH values ranging between 3 and 4. In these soils, Al and Fe oxides concentrations are high. We showed that the concentrations of pedogenic oxides in the soil correlate with soil organic carbon concentrations in the mineral-associated organic matter. In soils with higher pH (>7), we can also find old ¹⁴C ages. In these soils, C is stabilised by interactions with calcium ions and carbonates.

Overall, our regional scale dataset shows that the net accumulation of labile soil organic matter seems to be climate sensitive, while mineralogy and weathering contribute most significantly to the stabilisation of organic carbon in the soil.