Page 123 - Materials Chemistry, Second Edition

Life Cycle Assessment: Principles, Practice and Prospects
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This three-scenario approach, with an estimated average and two ‘extremes’, provides a
useful indication of variability.
The second problem is that many of the dominant environmental impacts from cropping
systems are associated with dynamic processes in agricultural soils. This is a challenge for LCA
due to the general lack of LCA inventory data relating to soil dynamics, and it indicates the
importance of integrating agricultural soil modelling data with LCA. Specifically, Renouf
(2006) derives emission rates for nitrogen through modelling, using the agricultural simula-
tion model APSIM-Sugar cane, a specific sugar cane model designed to be able to be used to
estimate nitrogen losses through soil denitrification and leaching under various soil and
climatic conditions and for different trash management regimes. For this study, nitrogen (N)
denitrification and leaching rates were derived from work undertaken by Thorburn et al.
(2004) with further information from Brentrup et al. (2000), and this is supplemented with
other studies (including Denmead et al. 2005; IPCC 1997; AGO 2003; Thorburn et al. 2005) to
derive partitioning of soil nitrogen losses between nitrous oxide (N O), nitrogen gas (N ) and
2 2
nitrogen oxides (NO ). Nitrogen emissions are expected to vary across the three scenarios,
x
according to climatic conditions, soil type and agricultural practice (particularly trash blan-
keting, as practiced in the Wet Tropics). Moreover, all N O emission rates derived for this
2
study are considerably higher than the generic 1.25% figure for arable cropping provided by
the Intergovernmental Panel on Climate Change (IPCC), in keeping with other work on irri-
gated maize (Beer et al. 2005).
Allocation of inventory flows is another common issue for agriculture-related LCA, where
co-products and/or by-products are produced from the same crop, husbandry or other agricul-
tural practice. The co-production of raw sugar and molasses was taken into account using both
economic allocation and system expansion. Using economic allocation, the impacts were allo-
cated 96% to sugar and 4% to molasses based on production rates of 143 kg raw sugar and 26 kg
molasses per tonne of cane, and average market values of A$300/tonne for raw sugar and $70/
tonne for molasses. Using system expansion, the difficulty lies in the equivalency of the substi-
tute co-products or by-products, and the efficacy of the data and subsequent values used in the
analysis. In this study, substitution of molasses was assumed to be 40% by barley (pasture supple-
ment), 20% by wheat (for fermentation for ethanol production) and 40% by nothing – as a sig-
nificant proportion of molasses is used as an attractant in feed and is considered non-essential.
The results were almost identical for each allocation approach, suggesting that choice of
allocation method is not an important issue for this system. Agricultural activities dominate
the energy profile, in particular fertiliser production, on-farm fuel use for tractors and har-
vesters, and electricity for pumping irrigation water (where applicable). The milling of cane to
produce raw sugar requires very little input of fossil fuel energy since the steam and power
used in the mills is generated from bagasse combustion. The combined contribution of on-farm
and cane railway capital goods was significant (between 5% and 10% of total energy input),
and in regions where road transport is also necessary (as in the wet tropics), this aspect becomes
significant too. However, the difference in energy input between the scenarios is not great con-
sidering the uncertainties. Cane production in the Burdekin has higher inputs per hectare
(fuels, fertilisers and water), but also higher cane and sugar yields. Conversely, the wet tropics
has lower inputs per hectare, but lower cane and sugar yields. Hence, on the important per-
formance measure of ratio of inputs to yield, the energy results are of a similar magnitude.
Regarding eutrophication potential, emissions of nutrients to air (ammonia NH ; nitrogen
3
–
oxides, NO ) and water (nitrate, NO , phosphate, PO ) from sugar cane fields provide the
x 3 4
most significant contribution, with differences in the results for each scenario attributed to
different environmental conditions (climate, soil type), which influence field emissions; wetter
areas (Wet Tropics) tend to have higher losses of N via denitrification, leaching, and ammonia

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