Quality of future N2O emission estimates: how might we improve

1
Quality of future N2O emission estimates how
might we improve?

• Keith Smith
• School of GeoSciences
• University of Edinburgh, UK

2
Possible areas of focus

• 1. Reviewing/revising default EFs for use when
  better data not available
• 2. Linked with (1), restructuring of some
  aspects of the methodology
• 3. Doing what we can to follow the Good Practice
  approach of using country- or regional-EFs where
  data can be obtained (Tier 2)
• Not just using the default as the easy way out.
• Including pooling information for neighbouring
  countries/ countries with similar
  climates/soils/farming systems.

3
Default EFs

• Currently 1.25 of all N applied is assumed to be
  emitted as N2O.
• As later slides indicate, much evidence that this
  is not so, and disaggregation of various sorts is
  desirable, e.g
• By crop
• By climatic zone
• By the type of N applied

4
Some effects of the type of N

• Evidence is accumulating that mineral N
  fertilisers containing nitrate (e.g. ammonium
  nitrate, or CAN) added to wet soils, e.g. wet
  grassland in early spring, can give rise to much
  bigger emissions than those that are ever
  reported from urea or ammonium-N fertilisers.
• Manure spread evenly over fields yields less N2O
  than mineral N forms.
• In contrast, urine spots from grazing animals can
  give higher emissions, influenced by the state
  of the soil.

5
Possible restructuring of the present methodology

• The calculation of N2O emission from grazed land
  could be dealt with under Direct emissions from
  soils, where it more logically belongs.
• Specific EFs could be adopted, if a consensus can
  be found, for (a) the manure N deposited in situ,
  taking into account the state of the soil under
  the grazing regime and (b) the manure from
  animal housing etc spread on the fields (see
  previous slide).

6
Possible restructuring of the present methodology
(cont.)

• Consideration might be given to replacing the
  complicated separate calculations concerning N
  input from legumes and from other crop residues,
  with a combined system for grain legumes and all
  non-leguminous crops
• Estimating the N remaining in the field from the
  above-ground parts, but crucially (and as a new
  development), including the root N.
• This would have to be complemented by separate
  calculations for forage legumes such as alfalfa,
  where all the shoot is carried off (see Robertson
  et al., Science, 2000), and clover-grass mixtures
  .
• The role of rotational renewal of grass/clover
  leys by ploughing and reseeding every few years
  also needs attention.

7
Comparison of some UK research findings with
general assumptions used by Intergovernmental
Panel on Climate Change (IPCC) in existing
methodology
8
Here, results of work in the UK (Dobbie et al,
1999 Dobbie Smith, 2003) show values peaking
well above the default EF, for grass, but well
below for cereal crops.
Contrasting approx. log- normal distributions of
annual EFs top all crops centre cut
grassland bottom arable crops.
9
Inter-seasonal variation in annual emissions of
N2O from cut grassland, site near Edinburgh,
Scotland, fertilised 3 times per season with
ammonium nitrate -- consequence of variations
in amount of rainfall around the times of
fertilisation
10
Some other European studies strongly indicate EFs
with average values greater than the IPCC default
value of 1.25, even though Bouwman et al. (2002)
have produced a new global average value of
0.9. In the figure below, the regression line
indicates a mean EF of 2.5 twice the IPCC
default value.
Relationship between total N input (by synthetic
fertiliser, cattle waste and crop residues) and
annual N2O emission measured on differently
managed fields at Scheyern research station,
South Germany. The regression excludes one data
point (). (From Flessa et al., 2002).
11
Freeze-thaw events large peaks in winter, can
contribute 50 of annual emissions, but no
allowance for strong continental winters in the
methodology. Recent work in Japan (Koga et al.,
2004) gives very similar results to European
experience.
N2O emissions (continuous curve), soil nitrate
contents, 0-30 cm (?), N fertiliser inputs, crops
grown in the rotation and N2O-N emissions, in a
field experiment near Braunschweig, Lower Saxony,
Germany (from Kaiser et al., 1998).
Peaks following winter freeze- thaw events
12
The CN ratio of crop residues appears to be a
key variable in determining the amount of N2O
produced during winter -- could this also be
allowed for, or is this too complicated??
Relationship between the ratio of dry matter to N
content of incorporated crop residues and N2O
emissions during winter, in a field experiment
near Braunschweig, Lower Saxony, Germany (from
Kaiser et al., 1998).