## 2 Methodological Option for Including Additional Carbon Sinks into Ecological Footprint Methodology
### Grassland Sequestration  
In contrast to forestry, soil comprises the vast majority of 'labile' carbon in a grassland ecosystem, and therefore, the sequestration service of grassland is a service provided by the soil. [PERSON] and [PERSON] [11] provide an account of sequestration within temperate grassland areas. Soil organic matter (SOM) accrues within grassland soil through the decomposition of roots, senescent leaves and stems within the soil. Within most temperate grasslands, the majority of root biomass occurs within the top 30 cm of the soil. However, the process of root growth, death and decomposition occur simultaneously and at different rates, and are determined by species composition and climatic conditions. The death and decomposition of root material is not the only means by which carbon is added to soils: rhizodeposition is also important, resulting from mucilage production, root exudation, and fragmentation from living roots. Because of the relative quantity of root matter within the surface soil layers, it is difficult to differentiate between the processes of CO\({}_{2}\) production and carbon cycling that originates from plant matter, and those which are derived from microbial activity [12]. The SOC content is considered to reside within discrete pools. A considerable fraction of SOM is enclosed by clay minerals within soil aggregates; therefore, it remains physically protected through processes such as chemical binding with soil minerals. Structural protection is also provided by soil aggregates, which provide a physical impediment for microbes, enzymes and organic substrates [13]. Soil structure also plays an important role as organic matter may be located within pores that are too small to allow access for fungi and bacteria. Soil aggregates themselves are held together by fungal hyphae, roots, microbial debris and polysaccharides, such that an increase in any of these fractions may promote intermediate SOM stability. [PERSON] _et al._[14] summarize the processes that are relevant to carbon sequestration within a temperate European (intensively managed) grassland, and illustrate that the actual changes in SOC may be dwarfed by the size of the carbon fluxes entering and leaving the soil boundary. Expanding the carbon sequestration estimate away from using forestry as the reference land type has fundamental ramifications for footprint results and methodology. However, in some regards this alternative may be potentially more arbitrary as it poses a crucial methodological question. This is related to the discussion of single land use and how the energy footprint functions as a theoretical or notional measurement. The conventional energy footprint is treated as an additional area that must be maintained to offset current CO\({}_{2}\) emissions, commercial forest areas are not suitable as they are at best carbon neutral. Treating grassland in the same manner would mean incorporating net ecosystem exchange (NEE), which equates to net primary production minus heterotrophic respiration and represents the fluxes of carbon experienced by an area of grassland. By contrast, [PERSON] _et al._[15] calculates a farm-based sequestration estimate, which treats grassland as operating within a system and incorporates the lateral movement of carbon embodied in farm commodities across the farm boundary. However the estimate of NEE calculated in [15] will include the emissions due to cattle respiring while grazing (Figure 1). The fact that commercial and productive grassland can function as a carbon sink may seem to negate the presumption of a single land-use, however that is by no means certain. For that reason, two estimates of grassland sequestration were adopted in this study: one based on NEE and one where the movement of carbon is included. The integration of both grassland- and forest-based estimatesrepresents a further methodological decision. In this study, overall estimates are generated based on the relative area of grassland and forestry.