Estimating the pre-harvest greenhouse gas costs of energy crop production

Abstract

Full greenhouse gas (GHG) life-cycle analysis of bio-energy production chains is often constrained by a lack of information on pre-harvest GHG costs and emissions during production of the energy crop. In this paper, we assessed pre-harvest GHG costs of production of short rotation coppice (SRC), Miscanthus and oil seed rape (OSR: for liquid bio-fuel production) when compared to a range of former land-use baselines. It was found that GHG costs are very low for Miscanthus and SRC but higher for OSR production, determined mainly by the need for nitrogen fertilisation. Compared to baseline land uses, SRC and Miscanthus have much lower GHG costs than arable cropping or intensively managed grasslands, with OSR production having similar GHG costs to arable cropping. Establishing broadleaved forests have low GHG costs, but 5-year GHG costs of Miscanthus and SRC are similar to forest. We show that former land use is of critical importance when determining if energy crops are a net source or sink of GHGs. Converting to SRC and Miscanthus are the most favourable energy crops in terms of GHG savings. Converting to OSR from arable cropping results in either small increases or decreases in GHG emissions, depending upon the former tillage practice on the arable land, but replacing either broadleaved woodland with OSR (mainly due to soil carbon loss and increased fertiliser-related N2O emissions), or grassland with OSR (mainly due to loss of soil carbon), greatly increases emissions. Policies to maximise GHG benefits from energy crops should encourage that they be established preferentially on croplands.