This document summarizes research measuring emissions of nitrogen oxides (NO and N2O) from a grazed pasture ecosystem in Switzerland using eddy covariance methods. Key findings include: 1) The pasture was found to be a continuous source of the greenhouse gas N2O throughout the grazing season, while being a source of NO only under very dry soil conditions. 2) Optimum soil moisture levels for N2O emissions were higher than for NO emissions. 3) Measured NO fluxes were affected by chemical reactions with ozone in the atmosphere, depending on temperature and ozone concentrations. 4) On a seasonal scale, NO emissions were counterbalanced by NO2 deposition, making

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)