420 E. Nemitz et al. Agricultural and Forest Meteorology 105 2000 405–425
impact of the uncertainty in the main parameters on i the stability of the prediction of the net exchange flux
F
t
and ii the partitioning of the net flux into com- ponent fluxes. Of interest are here the effects of errors
in the emission potentials Γ
sf
, Γ
sq
and Γ
l
and the resistances R
sf
, R
sq
and total in-canopy resistance, R
ac
+ R
b1
, and in particular in the parameters that have been derived from measurements χ
a
, F
t
and T z
′
. Sutton et al. 2000b estimated errors for the individual 10 min values of χ
a
and F
t
as 25 and 50, respectively, while the error of the average should be
much smaller. Parameters were varied over a range of typically ∓25 of their original values, and the
results were calculated i for a constant Γ
sq
= 2270
and ii by adjusting Γ
sq
for each modification to re- produce the measured net flux Table 3. Apart from
the response to changes in T z
′
, relative effects on F
t
and the ratio F
sq
F
t
were always much smaller than the relative change in the parameter investigated, in-
dicating that the model is very robust in predicting net emissions and partitioning the flux. Major uncertain-
ties in T z
′
can be ruled out as heatfluxes were mea- sured simultaneously with several instruments Sutton
et al., 2000b. The contribution of the litter emis- sion to the net emission increased most sensitively
with increasing Γ
l
as well as with decreasing χ
a
and R
ac
+ R
b1
, but variations were within 20. The mod- ification of a single parameter never led to a marked
change with implications for the mechanistic interpre- tation of the results, although it is evident that a 50
uncertainty is associated with the emission potentials Γ
l, max
and Γ
sq
that were derived through fit to the measurements.
7. Discussion
The χ
s
–R
w
model proposed by Sutton and Fowler 1993 has previously been successfully applied to
NH
3
exchange over wheat, barley, pasture, forest as well as moorland Sutton and Fowler, 1993; Sutton
et al., 1995, 1998; Plantaz, 1998; Wyers and Eris- man, 1998; Flechard et al., 1999. In all these studies
the apoplastic concentrations had to be fitted or taken from other studies. At North Berwick, independent
direct measurements of apoplastic [NH
4 +
] and pH were carried out by bioassay in the field to enable a
more rigorous model assessment. The NH
3
exchange over oilseed rape, however, proved to be more complex than over cereal crops.
In addition to the stomata, decomposing litter leaves at the ground surface formed a second major source,
which was not controlled by stomatal opening and cannot be dealt with in a 1-layer ‘big leaf’ model;
hence the poor model fit of Fig. 3. Emission from leaf stomata is regulated by Γ
s
, the canopy temperature dependent Henry and solubility equilibria Eq. 9,
as well as the light dependence of R
s
. In contrast, emission from the leaf litter depends on Γ
l
, the ground level temperature and the turbulence within
the canopy. Whilst Γ
s
was found to be reasonably stable, presumably due to the regulation by the living
plant, Γ
l
showed a large variability and is difficult to measure or predict reliably. The χ
s
–R
w
model fails to reproduce the measured flux, because for most of
the time the values of χ
s
are lower than χ
a
, leading to deposition to the leaves.
For oilseed rape the extraction procedure of the apoplast and the determination of [NH
4 +
] and pH are reasonably well established Husted and Schjoerring,
1995; Husted et al., 2000 and it is therefore unlikely that Γ
s
was considerably underestimated. Even when the χ
s
–R
w
model was applied with increased values of Γ
s
Fig. 3, it overestimated the emission of some days, while at other periods it could not account for a
great part of the emission, in particular when stomata were closed at night. Uncertainties in the parametriza-
tion of R
s
cannot account for flux reversal and can therefore be ruled out as a major uncertainty. The ef-
fects of three independent model amendments have been examined: a the inclusion of a litter layer Γ
l
, b the inclusion of a silique layer Γ
sq
and c the effect of a h-dependency of Γ
l
. 7.1. Leaf litter emission of NH
3
and the magnitude of Γ
l
Leaf litter appears to be the only possible sustained night-time source of NH
3
emission and thus has to be included in the modelling Fig. 5a. Using the mea-
sured values of Γ
s
and the three measured values of Γ
l
the model could still not reproduce the measured flux. Hence Γ
l
was fitted to the measurements for 24-h periods Fig. 7 but kept constant over as long periods
as possible and set to only one of three values. The model fit Fig. 8 and the overall correlation with the
E. Nemitz et al. Agricultural and Forest Meteorology 105 2000 405–425 421
measured flux Fig. 11 are encouraging. The value of Γ
1
= 13 000 necessary for 9–16 June is nevertheless
greater than the largest values estimated from initial bulk tissue measurements. A further uncertainty in
the parameterization of the leaf litter emission are the resistances R
ac
and R
b1
, both of which might be over- estimated in nights with low turbulence, when free
convection can be significant. As supported by the sensitivity analysis Table 3, a reduction of R
ac
+ R
b1
results in larger average fluxes. However, a systemati- cally different R
ac
+ R
b1
would have led to a different choice of Γ
l
, which was set to an arbitrary value that provided a good model fit to the measured values of
F
t
over a long time period. Therefore, uncertainties in R
ac
+ R
b
affect only the choice of Γ
l
and not the magnitude of the predicted litter emission flux.
The large value of Γ
l
required for the foliage–litter model results in a large litter flux, a considerable
part of which is taken up by the stomata of the foliage-layer Fig. 8, while the remainder is pre-
dicted to escape the canopy and contribute to the measured net emission. This is in contradiction to the
sourcesink analysis by Nemitz et al. 2000a, which indicated that all NH
3
emitted by the leaf litter during daytime was re-captured within the lower part of the
canopy, with all daytime emission originating from the top of the canopy.
7.2. Ammonia emission from siliques With the experimental leaf stomatal compensation
points χ
sf
being smaller than the ambient air con- centration χ
a
in the field, the NH
3
emission from the top layer of the canopy seen in the inverse Lagrangian
analysis must have originated from the siliques. The 3-layer model including the silique layer Fig. 5b
showed good agreement at daytime between measured and modelled flux for Γ
sq
= 1260 Table 1. Unfortu-
nately, apoplastic [NH
4 +
] and pH could not be mea- sured for the siliques, but bulk tissue [NH
4 +
] con- centrations measured for different rape plants showed
elevated values, typically 2–5 times larger than for rape leaves Husted et al., 2000. Assuming the same
ratio of bulk tissue to apoplastic Γ as observed for leaves, the value of Γ
sq
of 1000–1500 necessary to obtain the measured flux Table 1 is consistent with
the mean value for leaves of Γ
sf
= 390 derived from
the data of Husted et al. 2000. With a Γ
sq
of 1260 the foliage–litter–silique model using a constant value
of Γ
l
predicts half of the daytime net emission to orig- inate from the siliques, whereas the other half results
from leaf litter emission penetrating the canopy. Hence the results of the inverse Lagrangian sourcesink anal-
ysis are only partly reproduced by this 3-layer model.
7.3. Uncertainties and humidity dependence of rape leaf litter NH
3
emissions For live leaves the site of NH
3
exchange between air and plant tissue is well defined. Living leaves ex-
change NH
3
via the leaf apoplast in the sub-stomatal cavities, which, for oilseed rape, shows a [NH
4 +
] con- centration of typically
1 8
of the bulk tissue concentra- tion Husted and Schjoerring, 1996. As the structure
of the litter leaves breaks down during the decompo- sition process, it is difficult to identify the leaf com-
partments in contact with ambient air and their values of pH and [NH
4 +
]. These values could have differed significantly from the bulk tissue concentrations mea-
sured by Husted et al. 2000. Especially at onset of decomposition of organic matter pH values may be
locally elevated Freney et al., 1981 and more de- tailed pH measurements on leaf litter of oilseed rape
plants grown in growth cabinets have shown consid- erably higher pH values of 5.6–7 compared with the
bulk values of 5.1–5.4 in this study at the leaf sur- face Husted et al., 2000. The emission potential of
the leaf litter in terms of the solubility equilibrium could therefore have been significantly higher than es-
timated from bulk tissue measurements and values of Γ
1
= 13 000 as used for the foliage–litter model are
quite reasonable. In addition to the field measurements, controlled
chamber experiments have verified the increase of Γ
l
with h, almost certainly due to stimulated microbial ac- tivity. The results of supplementary laboratory studies
investigating this effect are shown in Fig. 13. Oilseed rape litter collected near Edinburgh was placed in a
cuvette 0.6 m×0.6 m×0.6 m and the flux was calcu- lated from the difference between the inlet and outlet
concentrations, determined by a 2-channel AMANDA denuder system based on the design by Wyers et al.
1993. During these experiments the temperature was kept constant at 21.5
◦
C, and after each step change in h
several hours were provided for the leaves to equili- brate. Although preliminary, the laboratory measure-
422 E. Nemitz et al. Agricultural and Forest Meteorology 105 2000 405–425
Fig. 13. NH
3
emission from decomposing B. napus leaves at different values of relative humidities as measured with a dynamic
chamber under controlled conditions. Preliminary results using a constant temperature of 21.5
◦
C. The linear regression solid line and 95 limits dotted lines are shown together with a best fit
line using the relative humidity h dependence of Eq. 15, with a constant a = 30 dashed line.
ments clearly show increased litter emissions at ele- vated h. Since at North Berwick measurements of Γ
l
were only carried out during daytime, it is likely that night-time values exceeded the measured values.
The fitted values for Γ
l
in Fig. 7 were sufficiently large to cause the night-time emissions observed, but
may present overestimates during daytime when Γ
l
decreased with decreasing h. An initial h-dependent formulation of Γ
l
Eq. 15 was successfully em- ployed resulting in the prediction of on an average
the correct night-time emission at the same time as allowing all daytime net emission to originate from
the silique stomata Fig. 9, Table 2. The validity of the h-dependence fitted to the field measurements
Eq. 15 may be assessed by the preliminary labora- tory results Fig. 13. The same h dependency may be
applied to the chamber flux as in Eq. 15. The h re- sponse curve according to Eq. 15 and a = 30 was
re-scaled for a value of Γ
l, max
matching the different type of oilseed rape and the chamber condition. The
close fit of the model to the measurement data shown in Fig. 13 provides independent support of the previ-
ously used value of a ≈ 30.
Nemitz et al. 2000a discuss the temporal vari- ability of Γ
l
and present first results obtained with a dynamic model that predicts Γ
l
as a result of the changing volume of the leaf tissue water as well as
the competing processes of nitrification, mineraliza- tion and NH
3
emission. The values of χ
l
that would be in equilibrium with this modelled estimate of Γ
l
agrees with the air concentrations measured 0.05 m above the ground much more closely than when using
a constant value of Γ
l
. Clearly, more controlled mea- surements are needed to develop a sound mechanis-
tic understanding of the processes governing Γ
l
, and eventually to provide parameterizations which could
be used in generalized models. 7.4. Chemical conversions within the plant canopy
Chemical conversions, such as the reaction of NH
3
with nitric acid HNO
3
and hydrochloric acid HCl or the evaporation of the associated ammonium salts
NH
4
NO
3
and NH
4
Cl, respectively, can potentially provide sources and sinks of NH
3
within the canopy, which are not accounted for in resistance modelling.
Nemitz et al. 2000b investigated the surface ex- change of these additional species and estimated
the importance of chemical conversions at North Berwick. It was concluded that, although the aerosol
concentration was estimated too low to cause signifi- cant gas-to-particle conversion above the canopy, the
much lower turbulence within the canopy may have provided enough time for chemical conversions to
occur. This would have provided an additional NH
3
sink within the canopy, which Nemitz et al. 2000b estimate to be small with an absolute upper limit of
25 of the NH
3
flux measured.
8. Conclusions
