Modelling spatial variation and environmental impacts of land use change in the exploitation of land-based renewable bioenergy crops
Spatial factors are of particular importance to the sustainability of land based energy crops, due both to the need to minimise feedstock transport, and to the importance of cultivation site attributes in determining key environmental impacts. This study uses geographical information system (GIS) mapping to identify sites suitable for the cultivation of Miscanthus or short rotation coppiced (SRC) SRC willow for co-firing with coal or generation of combined heat and power (CHP). Modelling using an adapted version of DayCent was performed for typical sites to assess variation in yield, nitrous oxide (N2O) emissions, evapotranspiration (ET) and change in soil organic carbon (SOC) according to soil properties, hydrologic regime and previous land use. Development of the DayCent model as part of this research gave improved simulation of the impacts of tillage on soil porosity, and resultant N2O emissions from soil, and improved simulation of growth of SRC willow following coppicing management, leading to improved yield predictions. For land use change from arable to perennial cultivation, increased SOC was simulated, along with reduced N2O emissions, particularly on soils prone to anoxia. However, in general, benefits of cultivation of Miscanthus and SRC willow for energy are maximised when the crops are grown at sites where high yields are achieved, and used to generate CHP, since this minimises the land area required per unit energy generated. Further model development work and additional field data for model verification are necessary for firmer conclusions on the change in net greenhouse gas (GHG) emissions following land use change. Additionally, indirect land use change may negate perceived benefits, and locations are difficult to predict or identify in a complex global system. Given the magnitude of identified variations in yields and changes in N2O emissions, spatial variation in benefits of bioenergy cultivation should be a factor in decisions to provide economic support for cultivation. However, calculations suggest that emissions offset by replacing energy generation from fossil fuels may have greater impact on GHG savings per gigajoule (GJ) than cultivation site attributes. Since total energy conversion efficiency may be in the region of 30% for electricity-only generation and up to 90% for CHP generation, planning feedstock supply chains to maximise efficiency of feedstock end use is therefore beneficial.
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