Quantifying terrestrial carbon stocks: examining the spatial variation in two upland areas in the UK and a comparison to mapped estimates of soil carbon

Intensive field surveys were undertaken in two upland catchments in the UK, Plynlimon in mid-Wales and Glensaugh in North East Scotland. The survey was to examine the spatial variation across the area and to assess the accuracy of the database underpinning the soil carbon map for the UK. In each area three 1-km(2) squares were sampled on a 200-m grid, with samples taken from both the organic and mineral horizons. Carbon stock was estimated, from the sample data, for each 1-km(2) square and compared with values from the UK database for that square. The results showed large differences between some squares, particularly for Plynlimon. In this area, the overall discrepancy between field and database values was 45%, compared with 8% for Glensaugh. Various sources of uncertainty were examined, including bulk density, organic horizon depth, and the proportion of different soil types within a square. The value for bulk density, assumed to determine carbon stock, had a significant effect on the estimates. In both catchments the organic layer showed a gradual decrease in bulk density with depth, resulting in a large proportion of the carbon being stored in the top part of the profile. The soil types, mapped during the survey, also showed large differences from those previously identified for each 1-km(2) square. This would have a considerable effect on the estimates of carbon stock within the UK database. It highlights that caution needs to be used when interpreting the UK soil map at this spatial scale.