Soil organic carbon sequestration: scoping study

12 Conclusions

Scotland has a wealth of soil data, due its long mapping and soil characterisation programme and soil research, primarily Government funded. However, most of the data were collected for purposes other than evaluating SOC contents and have been adapted to estimate SOC stocks and to measure change over time. Some of the datasets were collected using the same or similar protocols while others used different sampling protocols. A thorough, statistical examination of the various datasets may be able to determine their potential to further assess change over time, to assess SOC contents by land use or to guide any future monitoring of SOC.

Scottish soils are relative rich in SOC which potentially makes sequestering more SOC more difficult. Work by Lilly and Baggaley (2013) also showed that there is also the potential to lose a considerable amount of SOC from Scottish topsoils. Therefore, as well putting measures in place to increase SOC, there should be measures to protect existing stocks. This is especially important in the organic and organo-mineral soils of the hills an uplands which hold 66% of the SOC stocks in the upper 100 cm.

Some of the datasets identified have data spanning a number of years and consequently have SOC concentrations measured by different analytical techniques and equipment. Correction factors have been developed to standardise these SOC concentrations though this has not yet been applied to all the stock estimates.

Although maps provide a useful overview of the spatial distribution of soils with different SOC concentrations, they can be limited by the scale they were produced at. Online tools are valuable as they can allow the user to view information on variability that cannot be represented on an individual map. Online tools with the same underlying data are more useful in that a user can zoom in to specific fields or locations to get an estimate of the SOC concentrations, however, zooming-in does not change the resolution of the map.

There are some datasets available to assess change in SOC contents or stocks over time though few were specifically designed for this purpose. These data sets can seem to give conflicting results on the nature of change. However, this is partly due to differences in sampling protocols (fixed depth vs whole soil) but also to the limited sample size for some land uses. To be statistically robust large sample sizes are often needed, especially when trying to identify statistically significant small changes. This is important as SOC changes over long periods of time.

Field-based Infra-Red spectroscopy techniques are not yet able to provide robust and consistent measurements of SOC in the field but once operational this method could be used both for the soil analysis/ monitoring of SOC but also as a management tool – deployed on farms to assess how current practices are impacting on SOC. Work in ongoing to develop these methods and reduce the uncertainties in predictions.

Finally, there are many co-benefits to increasing SOC in soils besides mitigating climate change. SOC promotes the development and maintenance of soil structure and porosity which leading to improved water retention to maintain the growing crop and infiltration to reduce runoff thereby helping to reducing the risk of erosion and down-stream flooding. SOC is also important for maintaining soil biodiversity including organisms such as bacteria, fungi, insects and worms. These organisms help maintain a healthy functioning soil and have a role to play in nutrient cycling and pest control. Achieving these goals will also help mitigate the effects of a changing climate.