What factors determine the GHG savings of biofuels: cultivation emissions, part 1

The next two posts will constitute a limited literature review concerning cultivation emissions, a factor that may play an important role in determining biofuel GHG emissions savings relative to fossil fuels.

• Importance of cultivation emissions in total GHG emissions from biofuels:

This is the primary factor which results in the 7-77 % GHG emissions savings variation for wheat-derived ethanol, for example (DEFRA, 2007). However, the deemed importance of this factor varies between studies. For example, on the contrary to Schmidt et al. (2011), who concluded that agricultural emissions of biofuels are minor compared to the biofuel conversion process, Zamboni et al. (2011) suggest that cultivation conditions and management is responsible for ~45% of GHG emissions from biofuels. What is going on here – did one of the studies estimate this wrongly?

This is where an insight into the methodology employed, which I discussed in previous posts, is important to understand this. Zamboni et al. calculate biofuel emissions assuming previous agricultural land-use emissions were zero (i.e. land was not used for agriculture), while Schmidt et al. assume that land was previously used for agricultural purposes and thus only calculate the difference between biofuel production and other agricultural production. This means that both calculations are correct, depending on what the previous land-use was. In this post, the issue of land-use change will be put aside, meaning that Zamboni et al.’s findings are more relevant here.

• How do cultivation conditions vary:

They vary due to spatial and temporal climate, soil properties and cultivation management differences (Kim et al,, 2008). DFT (2010) suggested the importance of different cultivation factors towards the total emissions arising from cultivation. It found that nitrous oxide emissions from soil potentially accounts for 14-37% of total biofuel cultivation emissions; fertlizer synthesis may contribute 10-25% total emissions; the use of machinery may account for 13-34%; 7-34% of emissions arise during and after crop harvesting. These figures exhibit a lot of variation arising from the issues to be discussed below.