Soil organic carbon sequestration: scoping study

11 The co-benefits of SOC

Soil carbon in important for keeping Scotland’s soils healthy and maintaining a good nutrient supply to growing crops, it promotes a good and stable soil structure that retains water for crop growth and improves infiltration. In cultivated soils, maintaining or increasing SOC concentration is therefore important for mitigating the effects of climate change.

Our cultivated topsoils can already hold around 3274 billion litres of water that crops need to grow but if we could increase the carbon content by the amount estimated by Lilly and Baggaley (2009) which was 174Mt C, then we have estimated that topsoils would retain an additional 109 billion litres of water, the equivalent of 6mm of rainfall over all of the cultivated land and helping make Scottish soils more resilient to drought.

Datasets like the NSIS 2007-9, Soilbio and some specific research projects have data on soil moisture retention and bulk density as well as SOC concentrations so the relationship between these properties and SOC can be examined (for example, see Appendix 3). Greater water retention will increase the water that is held in the soil after rainfall events, and a reduction in bulk densities will improve infiltration, both of which will reduce runoff. Meanwhile, increasing the stability of the soil aggregates will prevent smaller particles from being preferentially eroded.

There is a lack of data in Scotland on the relationship between SOC and erosion. One of the most widely used assessment of soil erosion is the Universal Soil Loss Equation first developed in the USA (Wischmeier and Smith, 1978). However, this model was developed in area with SOC concentrations much lower than is found in Scotland and therefore is relatively poor at modelling the erosion rates in Scottish soils. The model is also driven by rainfall intensity, however, in Scotland erosion has occurred due to prolonged low intensity rainfall, high intensity rainfall and snow melt (Lilly and Baggaley, 2015). While aggregate stability is important for limiting soil erosion, the ability of the soil to absorb rainfall or snowmelt before becoming saturated is perhaps more important in Scotland (Lilly and Baggaley (2015).