LCA Food 2012 in Saint Malo, France! - Manifestations et colloques ...

PARALLEL SESSION 2B: EMISSIONS MODELLING 8 th Int. Conference on LCA in the
Agri-Food Sector, 1-4 Oct 2012
fect on GHG intensity of describing a soil as ‘organic’ implies that more finesse may be required at the field
scale than at national scale in defining a soil’s organic matter status; some low emission intensities were
associated with low fertiliser N use on fields which might almost have been deemed ‘organic’. However, it
is interesting to note that the fields with largest emissions per tonne of grain, for feed wheat and milling
wheat, did not have organic soils. The fields with highest emissions per tonne of grain tended to have low
yields and/or high applications of N fertiliser.
Fig. 2 shows that the range of GHG emissions values was greater for feed wheat (486 kg CO2e t -1 ) than
for milling wheat (777 kg CO2e t -1 ). Reasons for this are not clear, but we speculate that milling wheat crops
are managed to meet a tighter product specification (especially for grain protein content), leading to less
variation in agronomic practices.
Because it is technically difficult to measure emissions of N2O from soil over the life-span of a crop, it is
not practical to measure N2O emission directly at an individual field scale, for multiple fields. Thus, an estimation
method is necessary for field scale GHG accounting of crop products. The internationally accepted
IPCC tier 1 method has a large uncertainty range (the default value for applied N lost directly as N2O is 1%
with an uncertainty range of 0.3–3%). This reflects uncertainty in the relationship between applied N and
N2O emission from soil, which is mediated by microbial processes that are influenced by soil conditions,
especially temperature, moisture, organic carbon and available N. These soil factors are very variable, and
their effects interact strongly (Brown et al., 2000) causing emissions to be spatially and temporally episodic
(Dobbie and Smith 2001). Other work in the MIN-NO project is exploring improved N2O emission factors
for use in GHG accounting for crop products.
The GHG emissions for milling wheat grain represent a large component of the GHG emissions of a loaf
of bread at the retail stage. Other work has shown that a standard 800 g white loaf (typical for UK consumption)
has GHG emissions of 0.6 kg CO2e, of which 0.2 kg CO2e was for processing, packaging (Wiltshire et
al., 2009). In that study, also using PAS 2050 as the LCA method, an emissions total for wheat of 640 kg
CO2e t -1 was assumed. This indicates that, correcting for a lower emissions total for milling wheat as assessed
in this work (427 kg CO2e t -1 ) more than half of the GHG emissions for a loaf of white bread are from
on-farm wheat production. Therefore, the emissions during crop production, and the variability in these
emissions, strongly influence the emissions of a retailed loaf of bread.
5. Conclusion
Emissions of N2O during wheat crop production are important, in both scale and variability, for the GHG
emissions associated with retailed bread. There is an urgent need for (a) better precision and certainty in estimation
of N2O emissions, and (b) mitigation of GHG emissions on farms.
6. Acknowledgements
We acknowledge the UK Department for Environment, Food and Rural Affairs and the Scottish Government
in sponsoring Sustainable Arable LINK Project LK09128, and the contributions of ADAS, Agricultural
Industries Confederation, Bayer CropScience, British Sugar, Country Land and Business Association, The
Co-operative, Frontier, GrowHow, HGCA – the cereals and oilseeds division of the Agriculture and Horticulture
Development Board (AHDB), Hill Court Farm Research, National Farmers Union (NFU), North
Energy Associates, Rothamsted Research-North Wyke, The Processors and Growers Research Organisation
(PGRO), Renewable Energy Association, Scottish Agricultural College (SAC), Scotch Whisky Research
Institute, Soil Essentials, Vivergo fuels, Warburtons and Yara.
7. References
Brown, L., Armstrong Brown, S., Jarvis, S.C., Syed, B., Goulding, K.W.T., Phillips, V.R., Sneath, R.W., Pain, B. 2000. An inventory
of nitrous oxide emissions from agriculture in the UK using the IPCC methodology: emission estimate, uncertainty and sensitivity
analysis. Atmospheric Environment 35, 1439-1449.
BSI, 2011. PAS 2050:2011, Specification for the assessment of the life cycle greenhouse gas emissions of goods and services. Published:
BSI, UK.
De Klein, C., Novoa, R.S.A., Ogle, S., Smith, K.A., Rochette, P., Wirth T.C., McConkey, B.G., Mosier, A., Rypdal, K. 2007. N2O
emissions from managed soils, and CO2 emissions from lime and urea application. Chapter 11 in 2006 IPCC Guidelines for National
Greenhouse Gas Inventories Volume 4: Agriculture, Forestry and Other Land Use. 54 pp.
Dobbie, K.E., Smith, K.A. 2001. The effects of temperature, water-filled pore space and land use on N2O emissions from an imperfectly
drained gleysol. European Journal of Soil Science 52, 667-673.
Hoben J.P., Gehl R.J., Millar N., Grace P. R., Robertson G. P., 2011. Nonlinear nitrous oxide (N2O) response to nitrogen fertilizer in
on-farm corn crops of the US Midwest. Global Change Biology 17, 1140-1152.
IPCC, 2006. 2006 IPCC Guidelines for National Greenhouse Gas Inventories, Prepared by the National Greenhouse Gas Inventories
Programme, Eggleston H.S., Buendia L., Miwa K., Ngara T. and Tanabe K. (eds). Published: IGES, Japan.
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