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
for indirect soil emissions following nitrate leaching the range is 0.05 to 2.5%. We have used these ranges
from the IPCC (2006) guidelines to show the uncertainty in the emissions of N2O per tonne of wheat.
The GHG emission assessments for multiple fields of wheat were disaggregated by the three main agricultural
GHGs (CO2, CH4 and N2O) and analysed using descriptive statistics to show the variability at the
farm level.
3. Results
The mean of GHG emissions from 220 wheat fields was 405 kg CO2e t -1 , with a range of 777 kg CO2e t -1 .
This range is large relative to the mean for both feed wheat and milling wheat (Figure 2).
Emissions per tonne wheat (kg CO 2e t -1 ) x
1000
900
800
700
600
500
400
300
200
100
Nitrous oxide (N fertiliser)
Nitrous oxide (other)
Methane
Carbon Dioxide
Figure 2. Greenhouse gas emissions (kg CO2e t -1 0
Feed Wheat Milling Wheat
) for individual fields of wheat, disaggregated to 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. Data are divided into feed wheat
(for animal feed) and milling wheat (for bread), and each of these groups is ranked by total GHG emissions
per tonne of wheat grain.
Analysis of the dataset indicates that the variation in GHG emissions was influenced by many factors.
There was a weak relationship between yield and GHG emissions (R 2 = 0.28), but the two fields with greatest
GHG emissions also had the lowest two yields and accounted for more than one quarter of the range in
total GHG emissions. Other factors affecting total GHG emissions included total N applied (R 2 = 0.21), and
deisel use (R 2 = 0.28).
On average, the GHG emissions per tonne of wheat grain were dominated by N2O (approximately 72% of
total global warming potential (GWP) expressed as kg CO2e), with important emissions of CO2 (approximately
27%), and less than 1% of the GWP attributable to CH4. The total of N2O emissions from N fertiliser
manufacture and from soil as a consequence of applying N fertiliser made up approximately 54% of the total
CO2e value. Other N2O emissions were from soil following organic N applications (animal manures), crop
residue incorporation, and release of N by organic soils. Table 1 shows the mean and variability for N2O and
CO2 emissions expressed as kg CO2e t -1 .
It can be seen from Figure 2 that fields with exceptionally large N2O (other) emissions (i.e. excluding
N2O from N fertiliser manufacture and N-fertiliser-related soil emissions) are in the upper quartile of the
total emissions. These fields had organic soils, leading to N2O emissions from N in mineralised soil organic
matter. The coefficients of variation (CVs) shown in Table 1 are greatest for N2O (other) emissions (i.e. excluding
N2O from N fertiliser manufacture and N-fertiliser-related soil emissions), and the CV was 75.4%
for all wheat crops, compared with 31.9% for fertiliser-related N2O. This reflects the presence of outlying
values in the N2O (other) category due to the fields with organic soils.
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