## 5 Discussion
### Methodological and Conceptual Options for Estimating Grassland Sequestration  
The inclusion of two alternative grassland sequestration estimates within this study is related to the discussion of single land use and the energy footprint as a theoretical or notional measurement. For that reason, two estimates of grassland sequestration were adopted in Tables 3 and 4, and reflect different choices of study boundaries. The study, which provided the data seen in Table 2, viewed the farm gate as the boundary between the technosphere and the ecosphere. While this is at odds with a strict definition of what constitutes an 'ecosystem', it is based on the perception that an area of productive farm land is not a natural system and its role in the movement of fixed carbon should be acknowledged. In other words, the natural processes within a grazed area cannot be viewed in isolation, and so inclusion of lateral carbon movement from the farm provides a more realistic appraisal of the potential sequestration services provided by agricultural grassland. By contrast, if the boundary is simply placed around the grassland (above and below ground), then NEE estimated by eddy covariance (while removing emissions due to respiration, _etc._) will be suitable. This is an attempt to only account for the natural process in the soil which adds to the carbon stock. A footprint calculated using a farm-based estimate provides the farmed area needed to sequester a given quantity of carbon, whereas applying an estimate of NEE is intended to represent an evaluation of the sequestration of grassland in a more natural state. Footprint practitioners may well suggest that since farm activities can reduce the natural carbon sequestration rate, a productive grazing area will not function as the optimal grassland sink and therefore should be not be included within ecological footprinting. Therefore, the rationale for adopting NEE is based on the fact that it corresponds to current practice as applied to forest areas. Whereas a farm-based footprint represents an area that may currently offset carbon emissions, a footprint calculated using NEE represents both the additional land area that would be required and also the optimal grassland type for long term carbon sequestration. The question of whether a farm based sequestration estimate is suitable within ecological footprinting is more complex than simply a discussion of the theoretical nature of the energy footprint would suggest. The application of NEE assumes that the grassland area will not be disturbed, and maintained specifically for the purpose of carbon sequestration. However, it may be argued that farm production and grassland sequestration are closely linked [41]. As grassland sequestration is related to productivity, the estimates of NEE are related to farm practices. For example, the application of fertilizers, both organic and inorganic, may increase grass production and subsequently increase NEE. Given the intensive nature of modern Irish grassland management, it may be the case that an individual estimate of NEE is inextricably linked to the grazing herd and farm management structure regardless of whether it is calculated as part of a farm carbon balance or not. If this is accepted, then some commentators may view the concept of 'natural' or 'unimproved' NEE as unrealistic within an Irish context. However, that does not mean that the lateral movement of carbon should be accepted without question. The discussion of farm products in relation to carbon sequestration is related to the issue of double counting and single land use. Any removal of carbon in farm products is carbon that has been removed from the grassland area and has effectively lost the opportunity to enter that carbon pool. While farm products would already be included in a national footprint (increasing the footprint as they represent a demand placed on bio-capacity), their inclusion within the lateral movement of a farm carbon balance will reduce the sequestration rate and therefore increase the energy footprint.