Assessing changes in carbon stocks of Scottish soils: Lessons learnt

Abstract

Between 1978 and 1988, the soils at 721 locations throughout Scotland were sampled at intervals 10 km apart. They were described, characterised and samples were taken from each of the main horizons. Material not used for analysis was stored in the National Soils Archive. Between 2007 and 2009, 183 of these locations were re-visited (20 km intervals) and fresh samples taken to identify changes in nutrient status, pH and, in particular, soil carbon concentrations and stocks over the 19-31 year period. The archived soil samples from this time were re-analysed alongside those from the recent sampling to determine carbon concentrations. Near Infrared (NIR) spectroscopy was used to estimate their bulk density so that carbon stocks could be calculated as this was not measured at the original sampling. The results showed no statistically significant change in soil carbon stocks to 1 m depth for the main, broad land use types in Scotland apart from a small but significant increase (P=0.035) in soils under woodland. There was approximately 11.5% difference in carbon concentration between the reanalysed, archived soil and values originally obtained, but this was attributed to an artefact of differences in analytical methods. Between the two sampling periods, a decrease in carbon concentration of 2.4 g kg-1 was detected in cultivated soils. However, a significant increase in topsoil thickness of 2.9 cm (P=0.024) was sufficient to compensate for these changes in arable soils such that there was no detectable change in carbon stocks. The work shows the value of soil archives and of measuring horizon thickness.