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
Modelling N2O emissions from organic fertilisers for LCA inventories
Matthias S. Meier * , Christian Schader, Alfred Berner, Andreas Gattinger
FiBL - Research Institute of Organic Agriculture, Ackerstrasse, CH-5070 Frick, Switzerland
Corresponding author e-mail: matthias.meier@fibl.org
ABSTRACT
Apart from mineral fertilisers, organic fertilisers act mainly via the soil C-N pool in order to provide available nitrogen to plants. This
different mode of action is not reflected so far in the current IPCC N2O emission model. Here we propose a simple model to calculate
N2O emissions from organic fertilisers and plant residues. It considers the long-term immobilisation of N within stable organic matter
in the soil as well as the mineralisation of additional N from the soil pool. A first test with field data showed reliable simulations of
measured N2O emissions. By comparing values generated by our model with values generated by the IPCC model we show that the
IPCC model may overestimate emissions from organic fertilisers. Therefore, within LCA inventories modelling of soil borne N2O
emissions from organic fertilisers and crop residues should consider the different dynamics of N via the C-N pool in the soil.
Keywords: soil borne nitrous oxide emissions, agricultural inventory data, organic fertilisers, crop residues
1. Introduction
The IPCC model for determining nitrous oxide (N2O) emissions from soils (IPCC, 2006) – originally developed
for the reporting of national GHG inventories – is widely used within life cycle assessment (LCA) inventories
to calculate soil N2O emissions from agricultural products and processes. This emission factor
based model considers the total N input by fertilisation and plant residues to estimate cumulative direct and
indirect N2O emissions from soils. Regarding direct N2O emissions, the model does not differentiate between
different fertiliser types, i.e., nitrogen from mineral vs. organic sources including plant residues.
However, there is growing evidence that N2O emissions from organic fertilisers may be different from
emissions from mineral fertilisers. First, from nitrogen-use efficiency studies it is known that mineral and
organic fertilisers differ in their mode of action through the way nutrients are transformed in the soil and
utilised by plants (Gutser et al., 2005). In organic fertilisers only a fraction of the total N is readily available
for plants (as NH4 + -N and NO3 - -N). This fraction ranges from up to 85% of total N in poultry slurry down to
0% in compost. The rest of the total N in organic fertilisers is organically bound entering the C-N-pool of the
soil where it is released in the mid- and long-term by microbial degradation. By calculating N2O emissions
based on total N in the IPCC model the different mode of action of organic fertilisers is ignored.
Second, 15 N tracer studies with monitoring periods of up to 9 years indicate that N losses from mineral
fertiliser are higher than from (organic) crop residue input (Delgado et al., 2010). Again, the higher immobilisation
of N from crop residues within the C-N-pool is made responsible for the lower N losses during the
monitoring period. Using model simulations Delgado et al., (2008) showed that in support with 15 N tracer
studies N losses through leaching and N2O emissions from crop residue sources are lower compared to mineral
fertiliser. Based on these simulation results they argue that the IPCC N2O emission model (IPCC, 2006)
overestimates N2O emissions from crop residues.
Third, Alluvione et al., (2010) measured significantly lower N2O emissions from compost compared to
urea in corn fields. In this study, N2O emissions were only measured during the vegetation period of the crop
and therefore, no conclusions can be drawn regarding the long-term emissions resulting from the different
fertilisers. Nevertheless, the study by Alluvione et al., (2010) showed that there are measurable differences in
N2O emissions between mineral and organic fertilisers as well as within different organic fertilisers.
Due to the different dynamics of N from organic input sources, different N2O emissions from organic fertilisers
and crop residues compared to mineral fertiliser can be expected. Therefore, N2O emissions from
organic fertilisers and plant residues should be modelled differently from mineral fertiliser. This is of special
relevance when the GWP of crops fertilised mainly or exclusively with organic fertilisers (e.g. from organic
farming) are to be compared with the GWP of crops fertilised with mineral fertilisers.
We developed a simple model taking into account the different mode of action of organic fertilisers. The
accuracy of the model was tested using measured N2O emissions from a 3-year study. A comparison of the
model calculations from the IPCC N2O emission model (IPCC, 2006) was made using data from a long-term
field trial at the Research Institute of Organic Agriculture (FiBL), Frick, Switzerland.
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