Agricultural and Forest Meteorology 105 2000 351–369
Micrometeorological measurements of net ammonia fluxes over oilseed rape during two vegetation periods
M.A. Sutton
a,∗
, E. Nemitz
a,b
, C. Milford
a
, D. Fowler
a
, J. Moreno
c
, R. San José
c
, G.P. Wyers
d
, R.P. Otjes
d
, R. Harrison
e
, S. Husted
f
, J.K. Schjoerring
f
a
Centre for Ecology and Hydrology CEH, Edinburgh Research Station, Bush Estate, Penicuik, Midlothian, EH26 0QB, UK
b
Department of Physics, UMIST, Sackville Street, Manchester, UK
c
Department of Computer Science, Technical University of Madrid UPM, Madrid, Spain
d
Netherlands Energy Foundation ECN, Petten ZG1755, Netherlands
e
ADAS Boxworth, Cambridge, UK
f
Plant Nutrition Laboratory and Centre for Ecology and Environment, Royal Veterinary and Agricultural University RVAU, Thorvaldsenvej 40, Frederiksberg C, Copenhagen, Denmark
Received 1 February 1999; received in revised form 19 May 2000; accepted 20 June 2000
Abstract
Ammonia fluxes were measured semi-continuously over oilseed rape Brassica napus as part of the EXAMINE North Berwick experiment during ripening of the growing plants and following cutting prior to harvest. The first period was
investigated intensively, including flux measurements by continuous wet denuders at six heights, as well as by filter packs and passive flux samplers.
The aerodynamic gradient method and eddy covariance were applied to estimate friction velocity u
∗
, sensible heat fluxes H and latent heat fluxes λE, with best estimates provided by inter-comparisons for each. The measurements represent a
major exercise in data acquisition and processing, and provide approximately 30 days of semi-continuous fluxes. The largest errors in estimating the NH
3
fluxes arise from the concentration profile measurements. Fluxes from the ripening canopy were bi-directional, varying in the range −150 to 180 ng m
− 2
s
− 1
, with the largest emission fluxes during the day, and both emission and deposition occurring at night. Larger net emissions were measured following cutting of the rape up to 620 ng m
− 2
s
− 1
, with an overall mean of 57 ng m
− 2
s
− 1
compared with 16 ng m
− 2
s
− 1
prior to cutting. The patterns may be explained in relation to the expected major driving forces controlling exchange cuticular fluxes, foliar compensation point, leaf litter emissions,
the link to temperaturewetness, and the potential for overlying leaves to recapture emissions from leaf litter. The comparison with the passive estimates indicates that continuous measurements of several months duration are required to make a reliable
assessment of the passive approach. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Aerodynamic gradient method; Eddy covariance; Continuous ammonia denuders; Bi-directional fluxes; Passive flux samplers
∗
Corresponding author. Fax: +44-131-445-3943. E-mail address:
msceh.ac.uk M.A. Sutton.
1. Introduction
Work on the quantification of atmospheric am- monia NH
3
exchange with crop canopies has been motivated both by the need to address the potential for
nitrogen N losses from farming systems, as well as
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 3 - 3
352 M.A. Sutton et al. Agricultural and Forest Meteorology 105 2000 351–369
net atmospheric budgets and atmospheric transport of NH
3
e.g. Denmead et al., 1978; Harper et al., 1987; Schjoerring, 1991; Sutton et al., 1993b,c. The ear-
lier studies were primarily driven by the agronomic interest, particularly since indirect measurements had
often suggested large NH
3
emissions e.g. 10 up to 20 kg N ha
− 1
yr
− 1
from the tops of plants e.g. Wetselaar and Farquhar, 1980; Schjoerring et al.,
1989. More recently it has become apparent that net fluxes of NH
3
with fertilized crop canopies are gener- ally smaller than 10 kg N ha
− 1
yr
− 1
and therefore of less agronomic concern Sutton et al., 1994; Schjo-
erring, 1997. The result is that the primary current interest in quantifying plant–atmosphere NH
3
fluxes with arable crops is to determine atmospheric bud-
gets. From an atmospheric perspective, even where emissions are small, it is necessary to quantify net
fluxes with agricultural surfaces since the amount either added to the atmosphere or removed by dry
deposition affects the amounts available for transport further afield to sensitive ecosystems.
Although direct measurements of NH
3
fluxes have often shown increased emissions from senescing fo-
liage compared with growing leaves e.g. Farquhar et al., 1979; Harper et al., 1987; Husted et al., 1996,
these tend to be smaller than estimated by the indi- rect approaches. One reason to explain this applies to
studies estimating NH
3
fluxes from changes in
15
N. It is now clear that NH
3
fluxes with crops are generally bi-directional, both at different times for net fluxes, as
well as for a given time, being the balance of upward and downward component fluxes e.g. Sutton et al.,
1995a,b, 1998. As a result, studies inferring NH
3
losses using
15
N, may experience an isotopic substitu- tion of
14
NH
3
for
15
NH
3
e.g. Janzen and Gilbertson, 1994; Harper and Sharpe, 1998, providing the com-
ponent
15
NH
3
flux rather than net fluxes. Direct field measurements of NH
3
fluxes with vege- tation have often been limited by the difficulty to cover
extended time periods. Measurements using simpler batch NH
3
sampling methods, such as filter packs or bubblers impingers have been limited to sampling
every few hours either covering a few consecutive days e.g. Sutton et al., 1993b, or sampling occasional days
through the growing season e.g. Harper et al., 1987. More recent developments of continuous NH
3
anal- ysers, such as the AMANDA system Wyers et al.,
1993, have permitted a more detailed temporal anal- ysis of NH
3
fluxes with crops, e.g. considering fluxes over diurnal periods with averages being calculated for
10 min periods e.g. Sutton et al., 1995a. While this approach has provided data with a temporal structure
sufficiently detailed for model development, it has of- ten been limited to measurements for a few key days
during short campaigns. The extensive resources re- quired for longer term continuous flux measurements
e.g. Plantaz et al., 1996; Erisman et al., 1998; Fowler et al., 1998; Wyers and Erisman, 1998 have so far not
been applied to croplands.
Each of these sampling methods apply classical mi- crometeorological techniques to determine the NH
3
fluxes from vertical concentration gradients measured above the canopy. The most usual approach is the
aerodynamic gradient method, which requires deter- mination of the stability corrected turbulent diffusion
coefficient for entrained properties such as heat and trace gases K
H
e.g. Fowler and Unsworth, 1979; Sutton et al., 1993a. One of the constraints is that
the individual sample runs should not include periods of significantly changing stability or K
H
, which effec- tively limits runs to a few hours at most, thereby multi-
plying the effort required in long-term measurements. Recently, however, a new ‘passive flux sampling’ ap-
proach has been proposed that permits sampling inte- gration with runs of several days or weeks Schjoer-
ring, 1995. The objective of this approach is to pro- vide a long-term, low-cost method, making compar-
ison of fluxes between several different fields possi- ble. This method is recognized as an approximation,
since effects of stability are only partly treated. Cur- rently, there is a need to test the performance of this
approach against the classical aerodynamic gradient method.
There have been few micrometeorological NH
3
flux measurements over oilseed rape Brassica napus
. Estimates using passive flux sampling over three consecutive growing seasons 1992–1994;
Schjoerring, 1997 suggest that net emission amounts to 2–5 kg NH
3
-N ha
− 1
per season and is the same order of magnitude as over cereal crops. In the context
of the North Berwick experiment Sutton et al., 2000, this paper provides an analysis of NH
3
fluxes over oilseed rape measured semi-continuously for a total
of 30 days. The NH
3
fluxes are calculated following inter-comparison of both the key micrometeorolog-
ical exchange parameters and NH
3
concentrations
M.A. Sutton et al. Agricultural and Forest Meteorology 105 2000 351–369 353
from different sampling techniques. Fluxes measured before and after cutting are compared, and an initial
inter-comparison is also made between the classical flux estimates and those from the passive flux sam-
plers. The objective of this inter-comparison was to see whether the passive approach could reproduce
the net flux over the campaign as measured by the classical aerodynamic gradient method. The microm-
eteorological parameters and continuous NH
3
fluxes presented here form the basis for further analysis and
modelling as reported by Nemitz et al. 2000a–c.
2. Micrometeorological theory
