Developing a methodology to improve soil C stock estimates: report

1. Executive Summary

Organic soils include true peats, and soils with a layer of organic material (less than 50 cm in depth) overlying mineral soil layers. Organic soils have a high organic matter content, holding huge quantities of carbon (C) and are particularly abundant in Scotland. When C is lost from these soils as carbon dioxide (the main greenhouse gas responsible for climate change) or as methane there may be implications for targets to reduce Scotland's emissions of greenhouse gases. Land use- and climate-change can both cause such losses from soils; in fact, land use change on organic soils has previously been estimated to be responsible for 15% of Scotland's total greenhouse gas emissions.

Because of the importance of organic soils, in a previous project funded by the Scottish Government and the Welsh Assembly (Smith et al., 2007), a computer model, ECOSSE, was developed to simulate greenhouse gas emissions from the organic soils of Scotland and Wales. This model has the capability to predict the impacts of changes in land use and future climate on greenhouse gas emissions from both mineral and organic soils. ECOSSE stands for Estimating Carbon in Organic Soils - Sequestration and Emissions.

The main objectives of the project were to explore approaches to improving data on soil C stocks, estimates and trends (in response to the lack of statistical confidence in our current estimates of total national soil C stocks), and to use ECOSSE to improve estimates of changes in soil C stock using data from the resampling of the National Soil Inventory of Scotland ( NSIS2), and information derived from the Scottish Soils Knowledge and Information Base ( SSKIB).

The main project outputs are:

1. An analysis of the minimum number of further samplings/analyses required to develop a more accurate estimate of soil C stock in Scotland.

• The results suggest 600 additional depth and density measurements and further details on the scales of variation in peats are needed to provide an estimate with a high degree of statistical confidence

2. An analysis of the timescales, logistics and deliverables for targeted resampling to measure total depth and bulk density.

• It is estimated that an adequate survey would have an estimated total cost of ca. £300k (at 2009 rates).

3. Retrospective use of archived dry bulk density data to determine C stocks for peat bogs.

• Using a new pedotransfer function derived from the archived data, mean bulk density values were found to be comparable to those used for C stock estimates and on average were not found to vary significantly with depth.

4. A comparison of the costs and benefits in the measurement of peat depth and changes in soil C stocks in the peatlands of Scotland by ground penetrating radar, light detection and ranging, and other technologies.

• Ground penetrating radar has the advantage of a continuous assessment of peat depth along a transect compared to the intermittent measurements achieved by probing, but is not very suitable for use on uneven terrain.
• Using light detection and ranging to measure the C content of peats has potential as a tool to monitor development of peat gullies in areas of peat erosion, although currently it can only achieve depth accuracies in the order of ± 0.15m and a horizontal accuracy of 1-2m.
• Airborne gamma radiation has potential to differentiate peat from non peat soils and may add more information on soil type distribution in complex environments

5. Use of data from the National Soil Inventory of Scotland ( NSIS1 & NSIS2) to evaluate and improve the accuracy of the ECOSSE model, and better define the uncertainty in national scale simulations.

• Simulated values of percentage change in soil C are within the experimental error of the measurements (11% simulation error, 53% measurement error), are highly correlated to the measurements and show only a small bias in the simulations compared to the measured values, suggesting that a small underestimate of the change in soil C should be expected in the national simulations (-1 to -2%).

6. Improved national estimates of changes in soil C due to land use and climate change.

• Increasing the area of land use change from arable to grass has the greatest potential to sequester soil C, and decreasing the area of grass to arable has the greatest potential to reduce losses of soil C.
• Climate change alone is predicted to result in a decline in the soil C stocks that are nearly 50 times smaller than the losses due to land use change. This illustrates the potential for C losses due to climate change to be mitigated by changing land use.
• Four mitigation options have been identified with high potential for achieving zero losses of C from Scottish soils:

1) Decrease in the rate of conversion of grassland to arable to 28% of the current rate;

2) Stop conversion of semi-natural land to arable or grassland and increase the conversion of grassland to semi-natural by 125% of the current rate;

3) Stop conversion of semi-natural land to arable or grassland and increase the conversion of arable to grassland by 63% of the current rate; and

4) Stop conversion of semi-natural land to arable or grassland and decrease the conversion of grassland to arable to 77% of the current rate.

• At this stage, forestry has not been included as a soil C mitigation option as there is a paucity of good quality data for all soils in Scotland within the Scottish Soils Database for modelling land use changes to forestry. This reflects the historic development of the soil survey of Scotland where early mapping and data collection was mainly concerned with cultivated, agricultural soils. The process employed to generate typical profiles and C contents for afforested soils is, therefore, not yet sufficiently robust. Further work on the C content of forested soils and the changes occurring in soil C on change of land use to forestry is needed.
• Note that when designing policies to reduce total greenhouse gas emissions, changes in C stocks in vegetation as well as in the soil should be considered. This project focuses on C losses from soils only; changes in greenhouse gas emissions associated with vegetation are beyond the scope of the project. Timber production can also bring additional emission reductions associated with substitution of high energy embedded materials and fossil fuels, which should also be included in any comprehensive analysis of greenhouse gas emissions from Scottish forests. The results of this project allow the design of policies to protect soil C stocks, but not to reduce total greenhouse gas emissions from the soil / plant system as a whole.