Agricultural and Forest Meteorology 105 2000 405–425
Resistance modelling of ammonia exchange over oilseed rape
Eiko Nemitz
a,b,∗
, Mark A. Sutton
a
, Jan K. Schjoerring
c
, Søren Husted
c
, G. Paul Wyers
d
a
Centre for Ecology and Hydrology CEH, Edinburgh Research Station, Bush Estate, Penicuik, Midlothian EH26 0QB, UK
b
University of Manchester Institute of Science and Technology UMIST, PO Box 88, Manchester M60 1QD, UK
c
Department of Agricultural Sciences, Plant Nutrition Laboratory and Centre for Ecology and Environment, Royal Veterinary and Agricultural University RVAU, Thorvaldsensvej 40, 1871 Frederiksberg C, Copenhagen, Denmark
d
Netherlands Energy Foundation ECN, Petten ZG1755, Netherlands Received 1 February 1999; received in revised form 19 May 2000; accepted 20 June 2000
Abstract
Ammonia NH
3
surfaceatmosphere exchange is bi-directional and as such resistance models must include canopy concen- trations. An existing single layer model that describes the exchange in terms of adsorption to leaf cuticles and bi-directional
transport through leaf stomata, which is governed by a stomatal compensation point χ
s
, is applied here to NH
3
exchange over oilseed rape and compared with measured fluxes. For the first time the model is tested using values of χ
s
based on the apoplastic ratio [NH
4 +
]pH Γ
s
measured directly in the field. Strong NH
3
emission from decomposing leaf litter at the ground and the likelihood of high [NH
4 +
] in the siliques complicate the exchange pattern with oilseed rape and limit the application of the original model. This is therefore extended by: a the inclusion of a litter layer 2-layer model, with an
emission potential Γ
l
, b additionally dividing the plant canopy into a foliage- and a silique-layer 3-layer model and c considering the relative humidity h dependency of Γ
l
. The 2-layer model is able to predict night-time emission, but daytime emission is estimated to originate from the litter layer, which is in contradiction to the NH
3
sources and sinks derived for this canopy. The 3-layer model using a constant value of Γ
l
requires an emission potential for the siliques of about 1300, which is consistent with bioassay estimates. Together with a parameterization of Γ
l
that increases with h this model indicates that during daytime emission originates from the siliques, in agreement with the sourcesink analysis. It is concluded that
the leaf stomata were an effective NH
3
sink, whereas the leaf litter dominates night-time emissions and the silique-layer probably daytime emissions. Although the 2-layer model reproduces the net exchange, the 3-layer model appears to be the
mechanistically more accurate description. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Ammonia fluxes; Brassica napus; Resistance analogue; Multi-layer models; Compensation point
∗
Corresponding author. Present address: Centre for Ecology and Hydrology CEH, Edinburgh Research Station, Bush Estate,
Penicuik, Midlothian EH26 0QB, UK. Tel.: +44-131-445-4343; fax: +44-131-445-3943.
E-mail address: enceh.ac.uk E. Nemitz.
1. Introduction
With the recognition of the adverse effects of atmo- spheric ammonia NH
3
as a contributor to acidifica- tion and ecosystem eutrophication e.g. Van Breemen
and Van Dijk, 1988; Pearson and Stewart, 1993, it has become important to estimate the magnitude of
the surfaceatmosphere exchange of NH
3
. Since the
0168-192300 – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 1 9 2 3 0 0 0 0 2 0 6 - 9
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
