G04_05

Insight in high alpine soil carbon dynamics from compound-specific and soil fraction radiocarbon analysis on a glacier forefield chronosequence

Smittenberg R^1,2, Schwab V³, Gierga M², Bernasconi S², Hajdas I⁴, Wacker L⁴, Trumbore S^3,5, Xu X⁵

¹Department of Geological Sciences, Stockholm University, Stockholm, Sweden, ²ETH Zurich, Geological Inst., Zurich, Switzerland, ³Department Biogeochemical Processes, Max-Planck-Institute for Biogeochemistry, Jena, Germany, ⁴Laboratory of Ion Beam Physics, ETH Zurich, Zurich, Switzerland, ⁵Department of Earth System Science, University of California, Irvine, USA

The ecosystem carbon balance of high latitude and high altitude ecosystems are particularly sensitive to climate change, where increasing temperatures generally lead to a rise of the ecosystem carbon balance, but also increasing carbon turnover times. In this study, we investigated the carbon dynamics of the 150 year-long Damma Glacier forefield chronosequence, Switzerland. Specifically, we performed radiocarbon analysis of total soil carbon, supposedly 'stable' carbon pools (fine mineral-bound, and peroxide-resistant carbon), respired CO₂, dissolved soil organic carbon (DOC), hydrophobic leaf wax-derived alkanes, and microbial-derived fatty acids. Comparison of our results with the penetration of the radiocarbon bomb spike and the increase of soil and ecosystem carbon over the chronosequence allowed us to make the following inferences: (i) A small but persistent contribution of ancient carbon is present in forefield, which is particularly visible in the hydrophobic leaf wax ¹⁴C data. From this we conclude that this old carbon pool is at least in part a remnant of ancient soil carbon from a previous warm and glacier-free period, besides a potential contribution of fossil-fuel derived black carbon deposition. (ii) There is a significant portion of soil carbon with a decadal-scale carbon turnover rate, and (iii) mineral-bound carbon clearly has a lower turnover time. (iv) Microbial lipids, soil CO₂ and DOC ¹⁴C content reflect different carbon sources: in younger soils, relatively low ¹⁴C contents indicate a higher relative contribution of ancient carbon decomposition, while in older soils this signal is swamped by decomposition freshly photosynthesized organic matter.