The influence of soil processes on carbon isotope distribution and turnover in the British uplands

Understanding the natural variation of carbon within the soil, and between soil types, is crucial to improve predictive models of carbon cycling in hi gh and mid-latitude ecosystems in response to global warming. We measured t he carbon isotope distributions (C-12, C-13 and C-14) in soil organic matte r (SOM) from Podzols, Brown Podzolic soils and Stagnohumic Gleysols from th e British uplands, which were then compared with the total amounts and turn over of carbon in these soils. We did so by sampling at 2-cm intervals down six profiles of each soil type. The average amount of carbon stored in the top 28 cm of the Stagnohumic Gleysols is twice that of the other two soils . The C-13 content and C-14 age show a general increase with depth in all s oils, and there is also a significant correlation between isotopic variatio n and the main pedogenic features. The latter suggests that soil-forming pr ocesses are significant in determining the carbon isotope signatures retain ed in SOM. Organic matter formed since 1960 is not found below 5 cm in any of the soils. Evidently organic detritus in the surface layers (LF and Oh) is rapidly mineralized. This accords with our modelled net annual C fluxes which show that more than 80% of the CO2 emanating from these soils is deri ved from the top 5 cm of each profile. Although these soils contain much ca rbon, they do not appear to assimilate and retain SOM rapidly. The mean res idence time of most of their carbon is in the 2-50 years range, so the soil s are fairly ineffective sinks for excess CO2 in the atmosphere. Under the predicted future 'greenhouse' climate, likely to favour more rapid microbia l decomposition of organic materials, these soils are a potential source of CO2 and are therefore likely to accelerate global warming.