GHG balance and carbon mitigation potential of bioenergy crops

Abstract

SRC willow for bioenergy is a promising source of renewable energy for temperate climates such as the UK. Mass deployment of this technology will require substantial land use change, which will have consequential effects on ecosystem services. In order to avoid competition with food, bioenergy production has been proposed for cultivation on degraded or marginal land. In the UK, this land will likely come in the form of ex set-aside grasslands. The aim of this work was to quantify the greenhouse gas (GHG) balance of land use change (LUC) to 2G bioenergy, with a particular focus on SRC willow.

A systematic review and meta-analysis revealed that a conversion from arable cropping to second generation bioenergy results in an increase in soil carbon, whereas a conversion from forest results in a loss of soil carbon. Transitions from grasslands to SRC were shown to be broadly neutral, whereas a transition to perennial grasses such as Miscanthus there was a loss in soil carbon. This work highlighted the limited long-term empirical data available surrounding the effects of LUC to bioenergy, with particular uncertainty surrounding grasslands. A field site was established at commercial scale in the south of England to conduct a side-by-side comparison of the net ecosystem exchange (NEE), soil GHG fluxes and soil properties of an ex-set aside grassland and SRC willow plantation. There was also the opportunity to capture the effects of a commercial harvest of SRC willow on net gas exchange.

After three years of measurements, net ecosystem exchange (NEE) indicated that SRC willow was a net sink for carbon and grassland was a net source. Soil respiration was lower in SRC willow than in grassland. Soil fluxes of nitrous oxide and methane were low at both sites and did not contribute a significant portion of the GHG balance of these land uses. However, there was net emission of methane from grassland and a net uptake from the SRC willow over the measurement period. Soil carbon in the upper 30 cm portion was higher in grassland than in SRC willow, however for the whole 100 cm profile there was no significant difference between land uses. The effects of a commercial harvest were detected in the SRC willow where there was an increase in NEE and ecosystem respiration (Reco). Despite these increases in NEE and Reco, the site became a net sink of carbon again as soon as 3 weeks post-disturbance. Soil chemistry analysis revealed that a conversion from grassland to SRC willow results in increased bulk density and a lower soil moisture.

Overall, these results suggest that a transition from grassland to SRC willow could result in GHG savings, though this is likely to be highly site and management specific. SRC willow is able to act as a sink for carbon which could have positive implications for climate change mitigation. Soil carbon differences between sites indicate that the SRC willow is still in the recovery phase for soil carbon, and these differences are likely due to the larger input of organic material in the grassland where it is mowed and left on site. Management plays a large part in determining the whole GHG balance of the grassland ecosystem and this will need to be considered for future land use change scenarios. Grasslands remain one of the most viable options for land use change to bioenergy to avoid large loss of soil carbon (such as those observed from forest transitions) and competition with food crops, though decisions to convert land will require a site by site evaluation.

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