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
Table 1. Means and coefficients of variation, for grain yield (t ha -1 ) and GHG emissions (kg CO2e t -1 ; total,
and CO2 and N2O components), and for all wheat crops, and subsets of feed wheat and milling wheat.
Statistic Variable All wheat Feed wheat Milling wheat
Mean Yield (t ha -1 ) 8.7 8.9 8.6
GHG emissions (kg CO2e t -1 ) 405 378 427
CO2 (kg CO2e t -1 ) 109 98 119
Fertiliser-related N2O (kg CO2e t -1 ) 218 197 235
170
Coefficient of
variation (%)
Other N2O (kg CO2e t -1 ) 74 80 69
Yield (t ha -1 ) 15.5 15.8 15.1
GHG emissions (kg CO2e t -1 ) 25.5 30.3 20.7
CO2 (kg CO2e t -1 ) 32.3 33.5 29.3
Fertiliser-related N2O (kg CO2e t -1 ) 31.9 35.0 28.0
Other N2O (kg CO2e t -1 ) 75.4 80.7 68.6
A source of uncertainty in estimating GHG emissions of farm products is the choice of emission factors
for N2O emissions. Figure 3 uses an example of GHG emission assessment for one UK wheat crop to compare
use of the default IPCC emissions factor for N2O emissions with the upper and lower limits of the uncertainty
ranges for N2O emissions factors given by IPCC (IPCC, 2006). The range between the total emissions
using either the upper or the lower limits was 959 kg CO2e t -1 , more than twice the total GHG emissions
using the IPCC default emissions factors.
Emissions per tonne wheat (kg CO2e t -1 ) x
1,400
1,200
1,000
800
600
400
200
-
IPCC 2006 - default
EF
Nitrous oxide (N fertiliser)
Nitrous oxide (other)
Methane
Carbon dioxide
IPCC 2006 -
minimum EF
IPCC 2006 -
maximum EF
Figure 3. Greenhouse gas emissions (kg CO2e t -1 ) for one tonne of wheat at the farm gate, assessed using
alternative emissions factors (EFs) from IPCC (2006). The emissions are disaggregated to show CO2e values
for nitrous oxide from N fertiliser (manufacture and soil emissions), nitrous oxide from other sources (including
from soil following incorporation of crop residues), methane and carbon dioxide.
4. Discussion
Estimates of wheat GHG emissions indicate that N2O is the dominant GHG, accounting for 72% of CO2e
per tonne of grain. This supports IPCC conclusions (De Klein et al., 2007) that GHG emissions associated
with most crop products are dominated by N2O. Because of this, variability and uncertainty in N2O emissions
have a large influence on variability and uncertainty in the total emissions for crop products at the farm gate,
and for retailed products made from crops, such as bread.
Understanding the sources of variation in GHG emissions is useful to guide strategies for mitigation of
emissions, by adoption of on-going improvements in efficiency of energy and resource use. Collection of
more data and further data analysis in this project will provide more detail of the activities that lead to GHG
emissions on farms. The analysis to date indicates that there are multiple sources of variation between crops,
including yield, quantity of applied N fertiliser, soil type, and energy use. A small minority of fields had
organic soils with high soil organic matter content, where mineralisation of organic matter led to release of
plant-available N, and emission of N2O, contributing to emissions variation between fields. The marked ef-