406 E. Nemitz et al. Agricultural and Forest Meteorology 105 2000 405–425
costs of continuous measurements of NH
3
exchange with ecosystems permits measurements to be made at
only a few sites and for limited periods, it is neces- sary to parameterize the surface interaction of NH
3
. A key goal of model development is the derivation
of net fluxes from single height concentration fields. Over the past 20 years, many parameterizations of the
deposition of atmospheric pollutants have been de- veloped e.g. Hicks et al., 1987; Fowler et al., 1989;
Wesley, 1989; Erisman et al., 1994; Walmsley and Wesley, 1996, which are now frequently used, in at
least simplified form, in atmospheric transport and de- position models Asman and Van Jaarsveld, 1992; Bar-
rett and Berge, 1996; Sorteberg and Hov, 1996.
Unlike some other gases, such as ozone, sul- phur dioxide or nitric acid, NH
3
can be both emit- ted by plants and soils as well as deposited. This
bi-directional behaviour is not accounted for in most resistance models, the applicability of which for
NH
3
is therefore limited to certain conditions. More advanced models are necessary i to estimate cor-
rectly the magnitude of NH
3
emission from fertilized agricultural surfaces, ii to estimate the net surface
interaction in atmospheric transport models and iii for the assessment of the exceedance of critical loads
of nitrogen, especially for sensitive ecosystems. Am- monia emission from plants has often been identified
to originate from ammonium in the leaf apoplast [NH
4 +
], leading to a compensation point acting through stomata Farquhar et al., 1980. A single
layer resistance model that accounts for simultaneous stomatal emission of NH
3
and recapture by leaf sur- faces was presented by Sutton and Fowler 1993. The
application of this model has so far been restricted by the limited availability of direct measurements of
apoplastic [NH
4 +
] and pH. As part of the EU ‘EXAMINE’ project, a field
campaign was conducted over oilseed rape near North Berwick, southeast Scotland, during which both the
net flux of NH
3
with the atmosphere and stomatal compensation points were measured independently
Sutton et al., 2000a. This dataset therefore provides the unique possibility for the assessment of existing
bi-directional models, as well as for improvement of the mechanistic description of NH
3
exchange. The analysis of the NH
3
sources and sinks in this rape canopy demonstrated that decomposing plant litter at
the soil surface provided a second major source in this canopy, in additional to stomatal emission Nemitz
et al., 2000a. While this emission was recaptured by the plant foliage during daytime, it appears to have
escaped the canopy during some nights.
The single-layer canopy compensation point model for NH
3
has been shown to work well for agricul- tural and forest vegetation in which adsorption and
desorption processes take place at a common height, and where soil processes can be ignored Sutton and
Fowler, 1993; Sutton et al., 1995, 1998. However, the oilseed rape canopy provides an example showing
that multiple sources and sinks of NH
3
can be found in certain canopies. These sources and sinks can be
allocated to certain heights within the canopy and dif- fer in their controlling physiological and meteorologi-
cal parameters. As with the evaporation from a sparse crop, which may originate from both the foliage and
the soil Shuttleworth and Wallace, 1985, such pro- cesses can only be dealt with in multi-layer modelling
approaches.
For the oilseed rape canopy, novel multi-layer ap- proaches and new parameterizations of NH
3
exchange are developed here i to reproduce emission that orig-
inates from different heights at different times of the day, ii to quantify component fluxes for the different
plant parts and iii to develop and test a mechanistic understanding of the exchange process of ammonia
with oilseed rape.
In parameterizing the surfaceatmosphere exchange of NH
3
, two opposing interests are distinguishable. On one hand, a deeper mechanistic understanding can re-
sult in increasingly complex scientific models, which require a large number of input parameters. Against
this may be set the principle that parametrizations de- veloped for use in operational regional scale atmo-
spheric transport and deposition models should be easy to calculate and based on as few variables as possible.
This paper presents models of increasing complexity, including NH
3
from fallen leaf litter and siliques rape seed cases, but also considers the gain in accuracy vs.
simplicity and applicability.
2. Theory: existing single-layer resistance models of NH
3 3
3
exchange
By analogy to electrical resistances, a flux F
χ
of a tracer is driven by the concentration χ difference
E. Nemitz et al. Agricultural and Forest Meteorology 105 2000 405–425 407
between two heights z
1
and z
2
and impeded by the atmospheric resistances between these heights Rz
1
, z
2
; e.g. Monteith and Unsworth, 1990: F
χ
= − χ z
2
− χ z
1
Rz
1
, z
2
. 1
In this paper, an uppercase R is used for resistances de- fined on a canopy area basis, and a lowercase r refers
to leaf area based resistances. While two atmospheric resistances, i.e. the aerodynamic resistance, R
a
z, and the quasi-laminar bulk resistance, R
b
e.g. Sutton et al., 1993, always contribute to the total exchange re-
sistance R
t
, the parameterization of the canopy in- teraction depends on the tracer under consideration.
The canopy exchange may be modelled as a network of parallel and serial resistances and in a single- or
multi-layer approach e.g. Baldocchi et al., 1987.
In general, plant foliage offers at least two par- allel pathways of NH
3
exchange: in addition to bi-directional exchange through stomata, there can be
adsorption of NH
3
by dew and thin water films on leaf cuticles, which accounts for night-time deposi-
tion that is often observed despite stomatal closure Sutton and Fowler, 1993; Sutton et al., 1995. The
analysis of Sutton and Fowler 1993, accounting for these two pathways Fig. 1, represents an approach
referred to here as a single-layer canopy compensa- tion point model χ
s
–R
w
model. The stomatal exchange flux F
s
is governed by the relative magnitude of the canopy concentration χ
c
and the gas concentration in the sub-stomatal cavity χ
s
, which is expected to be in equilibrium with the [NH
4 +
] in the leaf apoplast governed by temperature T and pH Farquhar et al., 1980. Here χ
s
represents the stomatal compensation point, leading to stomatal
emission for χ
c
χ
s
and deposition for χ
c
χ
s
. Stomatal exchange is the dominant pathway under
warm and dry conditions, such as in controlled envi- ronment studies, where conditions are kept constant
over long periods so that the loadings of leaf cuticles equilibrate with air concentrations. In these conditions
the χ
s
–R
w
model is reduced to a stomatal compensa- tion point model, often used to describe bi-directional
fluxes of CO
2
and evapotranspiration with vegeta- tion. As with CO
2
transfer and evapotranspiration, the stomatal resistance R
s
depends on leaf morphology, leaf area index L
s
, photosynthetically active radiation PAR, temperature T, and can also be influenced by
Fig. 1. The single-layer canopy compensation point χ
c
resis- tance model χ
s
–R
w
model allows for simultaneous bi-directional ammonia transfer through leaf stomata controlled by the stom-
atal resistance, R
s
, and the stomatal compensation point, χ
s
and deposition to leaf cuticles controlled by the cuticular resistance
R
w
. Also shown are the net flux F
t
as well as the component fluxes through stomata F
s
and to the cuticle F
w
. R
a
and R
b
are the atmospheric and the boundary-layer resistance, respectively.
relative humidity h and drought closure due to water stress e.g. Jarvis, 1976.
The cuticular desorption flux F
w
is governed by the cuticular resistance R
w
and χ
c
. At night-time, R
s
is large and the model is reduced to a model similar to the R
c
-model used for tracers which are only deposited and for which the surface concentration is thought to
be zero, such as HNO
3
and O
3
. Usually the NH
3
gas concentration at the surface of the cuticle is assumed
to be negligible Wesley, 1989. R
w
is expected to change with leaf wetness, which in return depends
on h. The magnitude of χ
c
is the result of the two competing pathways and may be calculated as Sutton
and Fowler, 1993; Sutton et al., 1998: χ
c
= χ
a
R
a
z + R
b −
1
+ χ
s
R
s −
1
R
a
z + R
b −
1
+ R
s −
1
+ R
w −
1
. 2
The net flux F
t
is given by F
t
= − χ
a
− χ
c
R
a
z + R
b
, 3
408 E. Nemitz et al. Agricultural and Forest Meteorology 105 2000 405–425
Fig. 2. The dependence of the canopy compensation point χ
c
and the net flux F
t
on the air concentration χ
a
for constant values of the total atmospheric resistance R
a
+ R
b
= 30 s m
− 1
, cuticular resistance R
w
= 100 s m
− 1
, stomatal resistance R
s
= 50 s m
− 1
and stomatal compensation point χ
s
= 1.1 mg m
− 3
. The true canopy compensation point χ
c
is the value for χ
c
at F
t
= 0.
where χ
c
is here termed a ‘canopy compensation point’, although unlike the stomatal compensation
point, it is not only a function of the physiological state of the canopy but depends also on χ
a
Fig. 2. Strictly, the true canopy compensation point χ
c
denotes the mean concentrations when all emission and deposition processes within the canopy balance
in such a way that the net flux vanishes Sutton et al., 1998. For the model of Fig. 1 this is the case when
stomatal emission is balanced by cuticular adsorption. Nevertheless, χ
c
remains the central term in resolving the net effect of within-canopy component emission
and deposition fluxes Eq. 2. As a further modification of this single-layer
model, Sutton et al. 1998 treated cuticular adsorp- tiondesorption fluxes of NH
3
in a dynamic approach as an electric capacitor with a surface charge χ
d
which may be released when water layers evaporate in the morning. Flechard et al. 1999 predicted χ
d
by modelling the full leaf surface chemistry. The present
study, however, concentrates on multi-layer modelling and is restricted to resistance calculations that change
only with micrometeorological conditions, but not according to the emissiondeposition history of the
canopy.
3. Summary of the measurement results at North Berwick
