Presented at the 3rd ICOS Science Conference in Prague, Czech Republic, 11-13 Sep 2018.

1. Agroscope
Emissions of NO and N2O
from a pasture ecosystem measured
by eddy covariance
Christof Ammann
Federal Research Station Agroscope, Zürich, Switzerland

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Hole-in-the-pipe model
[Davidson et al., 1991; 2000]
Emission processes
 Soil NO and N2O emissions are an integral part of the reactive nitrogen cycle
of the ecosystems (microbial nitrification and denitrification processes)

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Characteristics of NO and N2O in the atmosphere
 Different reactivity (lifetime) in the troposphere
 different background concentration  importance of N2O as GHG
 Fast reversible conversion between NO and NO2
(dynamical photochemical equilibrium)
 it is often meaningful to consider the sum NOX = NO+NO2
agriculturalsemi-natural
ecosystems
HNO3
NO3
–
industry, traffic,
households
NONO2
h
O3
RO2

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Motivation
 NO (+NO2) and N2O have different characteristics and are usually measured
by different methods
 NO and N2O emission have been
measured mostly by chamber systems
in the past decades
 For grazed pastures, a high spatial
(and temporal) variability has to be
expected, which is difficult to cover
by chambers
 This study : eddy covariance measurements of all three trace gases
over a grazed pasture field
Fast-box survey at
Lelystad (NL) pasture site
[Flechard et al., 2007]

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Site and Management
 Swiss central plateau,
near Posieux
 3.7 ha pasture field
with rotational grazing
by 20 dairy cows
(April – Oct.)
 EC flux mast in the centre
of the field
 main wind directions:
NE & SW
 assumption: influence of
rot. grazing on emissions is
“smeared out” over the season
 Two fertiliser applications per year
(urea / slurry) on entire field
12:00 – 16:00

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EC Instrumentation and setup
 Sonic anemometer: Gill-HS at 2 m height
 CO2 / CH4: Licor LI-7500 and Los Gatos FGGA
[details see Felber et al., 2015; 2016]
 N2O: Aerodyne QCL
 NO: EcoPhysics chemiluminescence detector
CLD899 (1st channel) [e.g. Karl et al., 2017]
 NO2: CLD899 detector (2nd channel) with photolytic converter Air Quality
Design BLC [e.g. Stella et al., 2013]
in air-conditioned
trailer
2-channel
NO analyserNO2 photolytic
converter
20 m

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EC setup and data processing
 Flux data filtering for low turbulence: moderate filter u* < 0.06 m/s
 Stationarity filter (using sub-interval covariances): not very strict filter
because non-stationarity is generally high for NO due to regional point and
line sources  use of 4h running mean in the plots
 Correction for high-frequency damping in (long) intake lines and converters
based on observed covariance spectra / ogives
 Correction for chemical reaction of soil emitted NO with ozone between
surface and measurement height

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Effect of management and soil moisture
urea
rotational grazing

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Soil moisture dependence of NO and N2O emission
 half-hourly fluxes and 95% envelope
 excluding fertilizer events and low temperatures

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Temperature dependence of NO and N2O emission
 selection of suitable soil moisture range:
N2O: optimum SWC range (32-42%)
NO: intermediate SWC range (23-33%)

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Effect of chemistry on NOX and O3 fluxes
 Soil emitted NO partly reacted to NO2 before reaching the flux meas. height
 Simple first order estimation of chemical conversion effect
depending mainly on aerodynamic resistance and ozone concentration
 Considerable effect of chemical reaction in summer
summer
(avg. Tsoil = 21°C; [O3] = 40 ppb)
autumn
(avg. Tsoil = 14°C ; [O3] = 21 ppb)

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Seasonal gas exchange
 Simple upscaling of mean diurnal cycles to seasonal emissions
 Significant NO emission only in summer (low SWC)
but highest NO2 deposition in autumn (high concentration, active vegetation)
 Continuously high N2O emissions
NOy* = NOy – NO (NOy fluxes measured in different year at the same site)
n.a.

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Conclusions
 The investigated pasture is a (continuous) source of N2O and NO during the
grazing season
 Emission of N2O was considerably higher than for NO, except for very dry
soil conditions
 Distinct difference between optimum SWC for emission of NO (17 vol.%) and
N2O (vol.37%)
 NO2 measurement setup: important to place the converter on the
measurement tower close to the sample inlet (not inside the analyser)
 NO fluxes at measurement height (2 m) are affected by fast chemical
reaction under high-ozone conditions
 In terms of NOx, seasonal NO emission is counterbalanced by NO2
deposition  pasture is net NOx sink

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Thank you
for your attention
We acknowledge the financial support by the
Swiss National Science Foundation (Projects GEOGS and NICEGRAS)