Land-use change (LUC) is probably the most controversial issue associated with biofuels (Fargione et al., 2008). The main concern is related to possible additional GHG emissions when carbon stored in the soil is disturbed and released as CO2 due to the LUC. Two types of LUC are considered: direct and indirect. Direct LUC involves the conversion of existing land from a current use to the cultivation, in this instance, of biomass feedstocks for biofuel production. As shown in Table 3.2, direct LUC is considered in most international approaches and the IPCC factors are used for these purposes (IPCC, 2007). These are summarised for selected countries in Table 3.3.

Indirect LUC is associated with the displacement of existing agricultural activity (Searchinger et al, 2008). This is often difficult to assess due to the uncertainties involved, particularly at the international level. Currently, only the US approach considers indirect land-use change (US EPA, 2009).

An illustration of the influence of direct LUC for biodiesel from rapeseed is given in Table 3.4. Based on the assumptions used in this example (Fehrenbach et al., 2007), biodiesel from rapeseed can provide GHG savings of 48% compared to diesel. However, if direct LUC occurs, the saving drops to below ten per cent. Given that most countries require significantly higher GHG savings (see Table 3.2), it is important to ensure that biofuels can still meet these requirements if LUC is involved.

1Assumes worst case — conversion of land with high carbon content. Source: Fehrenbach et al. (2007).

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Therefore, 91.6% of GHG emissions are allocated to RME and 8.4% to glycerine so that:

GHG emissions allocated to RME: 0.307 kg CO2 eq. x 0.916

= 0.28 kg CO2 eq.

GHG emissions allocated to glycerine: 0.307 kg CO2 eq. x 0.084

= 0.027 kg CO2 eq.

2. Energy-content based allocation

Total energy content of RME and glycerine based on their respective LHVs:

37 MJ/kg x 1 kg RME + 17 MJ/kg x 0.092 kg glycerine = 38.76 MJ

Allocation factor:

RME: (37 x 1)/38.76 x 100 = 96%

Glycerine: (17/0.0.092)/38.76 x 100 = 4%

Therefore, 96% of GHG emissions are allocated to RME and 4% to glycerine so that:

GHG emissions allocated to RME: 0.307 kg CO2 eq./38.76 MJ x 1000 GJ x 0.96

= 7.6 kg CO2 eq./GJ RME

GHG emissions allocated to glycerine: 0.307 kg CO2 eq./38.76 MJ x 1000 GJ x 0.04

= 0.32 kg CO2 eq./GJ glycerine

Converting these back to per mass output from the process, the GHG emissions allocated to:

RME: 7.6 kg CO2 eq./GJ RME x 38.76 MJ/1000 GJ = 0.29 kg CO2 eq.

Glycerine: 0.32 kg CO2 eq./GJ RME x 38.76 MJ/1000 GJ = 0.012 kg CO2 eq.

Comparing these to the mass allocation, the results for RME are similar but by a factor of two different for glycerine. Although arguably in the example presented here, the differences in the results due to the different allocation methods are small, in many cases they can be much larger and can affect the LCA results significantly. It is therefore important that sensitivity analysis is carried out to determine the influence on the results of different allocation methods.

In addition to LUC, different crop management practices can also influence emissions of carbon from soil. However, there is still considerable uncertainty and lack of knowledge regarding the loss from or sequestration of carbon in soils due to this. It is also unclear how temperature increase due to climate change might alter farm management practices (and other activities) and what effect that will have on the change of carbon in soils (Baker et al, 2007; Bellamy et al., 2005; Davidson and Janssens, 2006).

A recent study in the UK found that the Soil Organic Carbon (SOC) stocks are being depleted at an alarming rate due to a combined effect of these factors. The measurements of SOC on 6000 sites across all types of land use over the past 25 years have shown that the estimated annual losses of carbon are equal to 13 million tonnes. This is equivalent to eight per cent of the UK emissions of CO2 in 1990 and as much as the entire UK reduction in CO2 emissions achieved between 1990 and 2002 (Bellamy et al, 2005).

It is widely believed that soil disturbance by tillage was a primary cause of the historical loss of SOC in North America. It is also believed that substantial SOC sequestration can be accomplished by changing from conventional ploughing to less intensive methods known as conservation tillage (Baker et al., 2007). However, some studies have demonstrated that conservation tillage leads to higher concentrations of S OC near the surface while conventional tillage accrues more S OC in deeper soil layers. Long-term measurements have also been unable to detect carbon gain in soil due to reduced tillage. Overall, although there are other good reasons to use conservation tillage, there is no proven evidence that it promotes carbon sequestration in soil (Baker et al., 2007).

Similarly, adding fresh organic matter (e. g. crop residues, compost, livestock manure and green manure) is widely practised as a way of increasing the nutrient levels in soils. However, there is also evidence that this may stimulate microbiological activity in the soil and can lead to the decomposition of ancient carbon buried in deep soil layers (Davidson and Janssens, 2006).