Surface age, ecosystem development, and C isotope signatures of respired CO2 in an alpine environment, north Iceland

Abstract

We studied the late Holocene foreland and adjacent unglaciated terrain of a small cirque glacier system in north Iceland to explore the relationship between soil/ surface age, vegetation and soil evolution, and C isotope signatures of respired CO2 Field-based sampling of respired CO2 from vegetation/soil monoliths across the chronosequence was used as the basis for an analysis of the 12C:13C:14C atom ratios of CO2 using Isotope Ratio Mass Spectrometry (IRMS) and Accelerator Mass Spectrometry (AMS). Residual soil organic matter (SOM) fractions (humic and humin) were also 14C-dated from each of the surfaces, vegetation described, and soil C and N contents analyzed. Our major conclusions are (1) that ecosystem respiration in this mid-alpine environment is strongly dominated by "young" C and is not related to the 14C age of residual SOM fractions; (2) δ13C values of respired CO2, by contrast, do vary both with age of surface and with absolute respiration rate, but there is no clear indication of any effects mediated by plant species and functional type and/or the degree of reworking of SOM by decomposer organisms; and (3) the 14C dating of residual SOM fractions, together with the soil profile characteristics (including tephra deposits) and vegetation cover, both suggest some radical disturbance in soil development and SOM formation at Site 1 (the oldest surface studied here), and no clear signs of classical succession when comparing Sites 1 to 3. Finally, in the light of these observations, the familiar concept of chronosequences, and the predictable processes of ecosystem development that they often imply, are challenged in a mid-alpine tundra setting where recent climate change and anthropogenic influences (e.g., grazing pressure) are superimposed upon time as an ecological factor.