This paper, Gilbert P, Thornley P and Riche A B (2011). The influence of organic and inorganic fertiliser application rates on UK biomass crop sustainability Biomass and Bioenergy, in press, highlights firstly, the GHG reduction potential of producing biomass (willow or miscanthus) as a substitute for natural gas and secondly, the need to consider environmental impacts other than just GHGs (in this case eutrophication and acidification).
The study finds that there are substantial GHG benefits to be had from growing biofuels as a substitute for gas.
This paper, Gilbert P, Thornley P and Riche A B (2011). The influence of organic and inorganic fertiliser application rates on UK biomass crop sustainability Biomass and Bioenergy, in press, highlights firstly, the GHG reduction potential of producing biomass (willow or miscanthus) as a substitute for natural gas and secondly, the need to consider environmental impacts other than just GHGs (in this case eutrophication and acidification).
The study finds that there are substantial GHG benefits to be had from growing biofuels as a substitute for gas.
It also finds, when growing the biomass crop, that if the willow or miscanthus is fertilised with sewage sludge rather than inorganic N, GHG emissions are slightly lower. However, sewage sludge applications lead to substantially higher acidification and eutrophication impacts. What is more, if the N fertiliser regime leads to an increase in biomass yields over
the sewage sludge regime, then measured in terms of output per unit of GHG emissions, the N fertiliser scenario emissions are lower than in the sewage scenario.
Abstract
Bioenergy and energy crops are an important part of the UK’s renewable energy strategy to reach its greenhouse gas reduction target of 80% by 2050.
Ensuring the sustainability of biomass feedstocks requires a greater understanding of all aspects of energy crop production, their ecological impacts and yields. This work compares the life-cycle environmental impact of natural gas and biomass from two energy crop systems grown under typical UK agronomic practice.
As reported in previous studies the energy crops provide significant reductions in global warming potential (GWP) compared to natural gas. Compared to no fertiliser application, applying inorganic fertiliser increases the GWP by 2% and applying sewage sludge increases the GWP by a lesser extent. In terms of an equivalent GWP savings per unit area of land, the emissions associated with fertiliser production and application can be offset by a yield increase of <0.2 t/ha. However, very large increases in eutrophication and acidification levels are incurred compared to the natural gas reference case when applying either fertiliser. For
sewage sludge the impact of varying the allocation factor between the
function of wastewater treatment and that of crop growth is also
illustrated.
Conclusions
SRC willow and miscanthus energy crops grown under typical UK conditions present substantial benefits in reduction of global warming potential compared to fossil fuel alternatives. There are marginal differences in the extent of the benefit between different crop nutrition regimes. The eutrophication and acidification potentials of unfertilised energy crops are marginally less than for fossil fuel alternatives. Applying inorganic fertiliser increases GHG emissions per unit of energy due to the manufacturing of the fertiliser and emissions to air and soil makes the acidification and eutrophication impacts comparable with those of the fossil fuel alternatives. Applying sewage sludge results in very large increases in acidification and eutrophication impacts compared to the natural gas reference system however, GHG emissions increased to a lesser extent compared to application of inorganic fertiliser.
However, when considering the GHG emissions per unit of area rather than per unit of energy, only a small yield increase is required for the application of fertiliser N to be beneficial in terms of GWP savings. Varying the assumption for the N2O emissions factor has only a marginal impact on the results, but the emission allocation split between wastewater treatment and crop growth when applying sewage sludge is very significant in affecting the acidification and eutrophication
potentials. Even a potential crop yield improvement of 25% would be
insufficient to offset the additional eutrophication and acidification impacts described above. There is a strong argument that development of sustainability criteria for solid biofuels should take into account ecological parameters such as eutrophication and acidification potentials. In order to do this accurately, consideration needs to be given to appropriate and
justifiable allocation procedures, which should be developed alongside any new legislative guidelines.
Two of the authors, Paul Gilbert and Patricia Thornley, are FCRN members
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