Investigating the influence of Ca on soil organic carbon dynamics in a Swiss subalpine grassland with a natural variation in CaCO3 concentration.

Geochemical indicators are emerging as important predictors of soil organic carbon (SOC) accumulation and persistence¹, but the role of Ca has largely been overlooked². In this study we investigated the effects of Ca on SOC dynamics by comparing 6 profiles that had developed under similar conditions for soil formation, except that 3 contained CaCO₃ (CaCO₃-rich), and 3 did not (CaCO₃-poor). Our hypothesis was that SOC would predominantly be stabilised by occlusion in the CaCO₃-rich profiles, due to the positive effects of Ca²⁺ on aggregation.

To investigate the role of occlusion in our profiles, we fractionated samples by density (1.6 g cm^-1) into a free light fraction (f-LF), two occluded light fractions (o-LFs), and a heavy fraction (HF). Following carbonate removal, organic carbon content and δ¹³C values were quantified on the bulk soil and density fractions, while X-ray photoelectron spectroscopy was also performed on a subset of the fractions.

Bulk SOC concentration was approximately twice as high in the CaCO₃-rich profiles relative to the CaCO₃-poor. δ¹³C values of bulk SOC were lower at CaCO₃-rich profiles. Occluded SOC was an order of magnitude more abundant in CaCO₃-rich profiles, supporting the idea that Ca positively influences occluded SOC pools; but it remained a small pool overall, never accounting for more than 10 % of total SOC.

Instead, it was the HF that were of interest because, like the bulk soil, their SOC content was approximately twice as high in the CaCO₃-rich profiles relative to the CaCO₃-poor. The CaCO₃-poor profiles displayed the typical shift to higher δ¹³C values from LFs to HF. Contrastingly, at the CaCO₃-rich profiles, the δ¹³C values of the HF were more similar to the LFs. This implies that the LFs had undergone more oxidative transformation at the CaCO₃-rich profiles, but that SOC with similar δ¹³C values was also being abundantly stabilised in the HF. Thus, Ca-mediated stabilisation seems to predominantly affect the organo-mineral fractions, stabilising more SOC with a fresher signature.

¹Rasmussen et al., 2018. Beyond clay: towards an improved set of variables for predicting soil organic matter content. Biogeochemistry 137(3), 297-306.

²Rowley et al., 2018. Calcium-mediated stabilisation of soil organic carbon. Biogeochemistry 137(1), 27-49.