Plant and Soil 181: 139-144, 1996. 139 © 1996 KluwerAcademic Publishers. Printed in the Netherlands.

Measurement of field scale NzO emission fluxes from a wheat crop using micrometeorological techniques

U. Sk iba I , K.J. H a r g r e a v e s I , I.J. B e v e r l a n d 2, D .H. ONei l l 2, D. F o w l e r 2 and J.B. M o n c r i e f f 2 I lnstitute of Terrestrial Ecology, Bush Estate, Penicuik, Midlothian, EH26 OQB, Scotland * and 2 Institute of Ecology and Resource Management, University of Edinburgh, Mayfield Road, Edinburgh, EH9 3JU, Scotland

Key words: eddy covariance, eddy accumulation, micrometeorology, nitrous oxide

Abstract

Measurements of N20 emission fluxes from a 3 ha field of winter wheat were measured using eddy covariance and relaxed eddy accumulation continuously over 10 days during April 1994. The measurements averaged fluxes over approximately 105 m2of the field, which was fertilised with NH4NO3 at a rate of 43 kg N ha-I at the beginning of the measurements. The emission fluxes became detectable after the first heavy rainfall, which occured 4 days after fertiliser application. Emissions of N20 increased rapidly during the day following the rain to a maximum of 280 ng N m-2s - l and declined over the following week. During the period of significant emission fluxes, a clear diurnal cycle in N20 emission was observed, with the daytime maximum coinciding with the soil temperature maximum at 12 cm depth. The temperature dependence of the N20 emission was equivalent to an activation energy for N20 production of 108 kJ mol - l . The N20 fluxes measured using relaxed eddy accumulation, averaged over 30 to 270 rain, were in agreement with those of the eddy covariance system within 60%. The total emission of N20 over the period of continuous measurement (10 days) was equivalent to about 10 kg NzO-N, or 0.77% of the N fertiliser applied.

Introduction

Emissions of N20 from soil dominates the global N20 budget, but for all regions the estimated fluxes are uncertain and this is partly due to the difficulty of extrapolating from the conventional emission measure- ments by chambers at the 0.1 to 1 m 2 scale to the field scale (Folorunso and Rolston, 1984; Smith et al., 1994). Spatial variability problems for scales smaller than 1 km 2 can be overcome by micrometeorologi- cal techniques, which average the fluxes typically over 100 m z to I km 2. The most robust micrometeorological method, being free of much of the empiricism of other techniques is eddy covariance. The flux F is provided by the product of w', the instantaneous deviation of vertical wind speed from the mean (m s - j ) and X', the instantaneous deviation of the gas concentration from the mean (#g m -3) (Fowler and Duyzer, 1989):

F x = W x,

* FAX No: + 4411314453943

However, eddy covariance measurements require instrumentation capable of sampling at rates of typ- ically 10 Hz, and in order to adequately sample the spectrum of flux carrying eddies, it is also necessary to detect concentration differences of the order of 0.1 to 1 ppbv. Ultrasonic anemometers have been the instru- ment of choice for some time to measure the three components of the turbulence, but few instruments are available for rapid measurements of trace gas concen- trations other than CO2 and water vapour. For N20 these conditions were only very recently met, mainly by the development of a tunable diode laser spectro- meter (TDL) (Zahniser et al., 1995).

An alternate approach, not involving the use of fast response sensors, is provided by the 'relaxed' eddy accumulation or conditional sampling technique (Businger and Oncley, 1990). The vertical wind veloc- ity sensed using an ultrasonic anemometer is used to switch continuously sampled air into one of two col- lecting resevoirs, i.e. one to hold air from updrafts and one to hold air from downdrafts. The flux is given by

140

V = 9 ( L ) o w p . ( X u - (1)

where fl is a dimensionless empirical constant, aw is the standard deviation of the vertical windspeed (m s - 1), pais the mean dry air density over the half hour integration period (g m -3) and Xu-Xd are the mixing ratios of the trace gas (ppbv). The conditional sampling system requires a vertical wind sensor and sampling valve assembly with a time resolution of at least 0.1 s and gas sensors capable of detecting the small con- centration differences between updraft and downdraft air.

Here we report measurements of N20 fluxes over a field of winter wheat by eddy covariance and relaxed eddy accumulation, using a TDL, which provided hourly average fluxes for an area of ,~105m 2 for a 10 day period after the application of N fertiliser.

Methods

Site description

Measurements of N20 were made over a winter wheat crop at Howmuir Farm in SE Scotland (National Grid Reference NT 613767) in April 1994, starting imme- diately after the second application of NHaNO3 (43 kg N ha- l).The soil was a sandy clay loam of the Winton series. The site had a gentle slope of 15 m, running perpendicular to the prevailing wind direction with a fetch of over 0.5 km to the South and West. Simulta- neous flux measurements by chambers were made by colleagues from the School of Agriculture, Edinburgh, these results will not be discussed here.

Micrometeorological measurements

Eddy covariance measurements of N20 were made on a near continuous 24 hour basis from 20th to 29th April 1994, using a TDL. Air was sampled from the top of a 4 m high mast and was pumped through nylon tub- ing (3/16" internal diameter, 25 m long) at a flowrate of 10 L min -1 into the TDL, which was housed in a mobile laboratory at the edge of the field. Power was provided by a generator located about 200 m to the NE, generally downwind of the mast.The three compo- nents of turbulence were measured using an ultrasonic anemometer (Gill Instruments), which was mounted on top of the 4 m mast. The gas inlet for the eddy covariance measurements was attached as close to the

measuring point of the ultrasonic anemometer as pos- sible, without obstructing the path of the anemometer. The instrumentation for eddy covariance and the TDL are described in detail by Hargreaves et al. (1996) and Zahniser et al. (1995), respectively. Eddy covari- ance fluxes of N20 were calculated using the software Eddysol, providing raw data and mean fluxes over 10 min sample periods (Moncrieff et al., 1995). For con- ditional sampling two miniature solenoid valves with a response time of approximately 10 ms (Clippard Mi- nimatic) were mounted onto the mast and were located 10 cm from the centre point of the anemometer path. Computers were used to monitor the N20 concentra- tion, the vertical wind speed signal and to control the switching of the conditional sampling valves. Updraft and downdraft air was collected into Tedlar bags (5 or 20 L ) housed in an aluminium case 25 m from the base of the mast. The bags were filled over 30-270 minute periods and were analysed by TDL spectroscopy at a later stage. Updraught and downdraught pairs of bags were analysed together by switching between each at 30 second intervals. The average of 1-2 updraught (or downdraught) measurement blocks and the average of the surrounding 3-5 downdraught (or updraught) blocks were calculated in order to determine a concen- tration difference between the pair. This 'chopping' technique was employed to remove any systematic bias in the gas analysis on the assumption that this bias was linear over the relatively short measurement period, e.g. drift caused by relatively linear tempera- ture change.

Corrections to the fluxes of N20 for the effect of latent heat on air density were made as described by Hargreaves et al. (1994). The energy balance for this site were provided by a Bowen - Ratio system (Camp- bell Scientific Ltd.).

Results and discussion

The eddy covariance measurements of N20 for the first 10 days after the second fertiliser application are shown in Figure 1. Although measurements were made almost continuously, there are gaps in the data set, caused by unsuitable wind directions from the NE,N or NW, espe- cially in the first 3 days of the experiment, and by the daily disruptions, e.g. refilling the TDL with liquid N2. Eddy covariance fluxes were corrected for the effect of simultaneous latent heat fluxes as described by Webb et al. (1980). Corrections for the air density effects of the sensible heat were not required, as the sampled air was

141

300

200

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100

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- 1 O0

g 0

o u

0 0

O o

Oo o o © • 0

0 0

0 0 0 0 0 O 0

m~o o o 0 0 0 o

oo o~oOO 0 o 0 0 ~I~

°o0° 0 0 ° ~ 0 ° 0 ° o o

CO 0 0 O n O N 0 0

O0 0 0 0 0 o o6b ° o o o o °

o 0

I I I i I I I I

3 4 5 6 7 8 9 10

Days after fertiliser application

Figure 1. Nitrous oxide emissions measured by Eddy covariance (hourly averages) (o) and conditonal sampling (30 to 270 min periods) J ) from a winter wheat crop at Howmuir farm, starting on 20 April 1994, 1 day after fertiliser application.

brought to a uniform temperature before analysis. The very small fluxes imply uptake of N20. However soils have never been shown to be a sink for N20 at the rates suggested by the negative fluxes in Figure 1. The small fluxes were within the noise of the measuring system, which in these conditions had a detection threshold of about 10 ng N20-N m-2s -1. It is also possible that the apparent N20 uptake rates were an artefact caused by the storage and advection problems outlined by Fowler et al. (1995). The horizontal and vertical flux divergence during periods when N20 concentra- tions are changing systematically during the measure- ment appears to be a greater problem for very patchy sites and was not observed in a previous experiment of N20 measurements by eddy covariance over a very homogeneous agricultural site in an area in Denmark reclaimed from the sea (Hargreaves et al., 1996). There is evidence, that at the Howmuir site spatial variability was present, as simultaneous chamber measurements by the School of Agriculture, Edinburgh (SAC), within the same field, but not within the prevalent fetch of the tower showed much smaller fluxes, with a maximum emission rate of 30 ng N20-N m -2 s -1 (I Mc Taggart, pers. comm.).

With the conditional sampling technique, a total of 12 pairs of bag samples were collected on days when the eddy covariance and static chamber (I Mc Taggart, pers. comm.) N20 flux measurements suggested that the expected differences in the up- and downdraft air should be measureable on the TDL. Of these, the analy- sis of 5 pairs of sample bags provided significant differ- ences between up- and downdraft and the N20 fluxes obtained were in good agreement, within 60% of fluxes

measured simultaneously by eddy covariance (Fig. 1). Instrument drift on the TDL and insufficent sample for the repeated analysis necessary for statistical eval- uation made it impossible to analyse the remaining 7 sample pairs. The validity of the conditional sam- piing method was confirmed by the comparison of the sensible heat flux measured by eddy covariance and a conditional sampling simulation (Fig. 2). Both calcu- lations were made using the virtual temperature output by the sonic anemometer at 20.5 Hz resolution. The conditional sampling flux was calculated using a sim- ple binning technique depending on the sign of the vertical velocity and Businger and Oncley's original estimate of/3 =0.6 (Businger and Oncley, 1990).

The fertiliser induced response in N20 emission was not detected until rainfall 3.5 days after fertiliser application (7.6 mm of rain in 22 hours) had wetted the soil sufficiently for the fertiliser to be transported into the soil and to increase the number of anaerobic microsites necessary for N20 production (Arah et al., 1988) (Fig. 3). The maximum N20 emission (280 ng N20-N m -2 s - l , hourly mean) was measured 1 day after the first heavy rainfall since fertiliser application (5 days after fertiliser application) (Fig. 1). From then on the daily average NzO emission declined steadily to a value of 15 ng N20-N m -2 s - I , 10 days after fertiliser application. Diurnal variations in the N20 emission were observed for several days, with emis- sion rates peaking around noon at 284 ng N20-N m -2 s -1 and with a minimum of about 25 ng N20-N m -2 s- l during the night (5 days after fertiliser application) (Fig. 4). The diurnal cycle appears to be closely linked with soil temperature, and appeared linear over the

142

150

100

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N o

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-50

-100

1

b

2 3 4 5 7 8

Days after fertitiser application

Figure 2. A comparison of the sensible heat flux measured by eddy

r

10

covariance (A) and conditional sampling (0).

small temperature range measured (r2= 0.406, d.f.=28, p< 0.001). An activation energy of 108 kJ mo1-1 was calculated from the Arrhenius plot in Figure 5. These data are consistent with the range of activation energies observed for microbial N20 evolution by denitrifica- tion and nitrification (28 to 166 kJ mol -J) (Conrad et al., 1983).

The delayed response in N20 flux to fertiliser appli- cation caused by lack of rainfall and the response to changes in soil temperature are typical of the observa- tions for N20 emission measurements using chambers (Goodroad and Keeney, 1984; Skiba et al., 1992), but in the case of the micrometeorological measurements the averaging area is larger by many orders of magni- tude. The measurements and analysis show, that eddy covariance measurements of N20 can be used to study processes as well as obtaining fluxes. Not enough con- ditional sampling measurements were made to observe the response in fluxes to rainfall or diurnal temper- ature variation. However, conditional sampling mea- surements were larger 5 days after fertiliser application ( 161 ng N20-N m-2 s - 1 ), when maximum N20 emis- sions by eddy correlation were measured than 7 and 8 days after fertiliser application (71 and 32 ng N20-N m -2 s - j , respectively) (Fig. 1).

Enhanced emissions of N20 are usually only observed for 2 to 3 week periods following the appli- cation of N fertiliser, and although micrometeorologi- cal measurements of N20 were stopped 10 days after fertiliser application, measurements of N20 by cham-

bers (SAC), showed, that in the following 2 weeks the N20 emissons continued to decline (I McTaggart, pers. comm.). The N20 emissions measured by 12 chambers (0.13 m 2) were much smaller than those measured by eddy covariance and conditional sampling. The peak chamber measurement made late in the morning, also 5 days after fertiliser application, was around 30 ng N20-N m -2 s - l , about an order of magnitude smaller than the 280 ng N20-N m -2 s - l measured by eddy covariance and 220 ng N20-N m -2 s -1 measured by conditional sampling. The discrepency between cham- ber and micrometeorological measurements was also observed for a grassland site in Central Scotland (Smith et al., 1994). Such differences are not surprising, con- sidering the spatial variabilities generally observed for N20 emissions and that the 12 chambers were integrat- ing a flux over a total area of less than 2 m 2, in a part of the field, which was not sampled by the eddy covari- ance and conditional sampling systems, and that the eddy covariance and conditional sampling techniques were integrating fluxes over an area of 0.3 km 2.

The total loss of N20 during the first 10 days after fertiliser application averaged over the field was 10 kg N20-N, and accounted for 0.77% of the total N fertiliser added. Very similar figures for the percentage of NH4NO3-N fertiliser lost as N20 from Scottish soils have been reported by Mc Taggart et al. (1994).

~ 150 t

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E

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Days after fertiliser application

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Figure 3. Daily mean N20 fluxes measured by eddy covariance (open bar) and rainfall (hourly total) (solid bar). For comparison the short period conditional sampling fluxes are included (shaded bar).

Acknowledgements

T h e au tho r s w i s h to t h a n k the Natura l E n v i r o n m e n t

R e s e a r c h C o u n c i l , T ige r p r o g r a m m e , for f inancia l sup-

por t and M r C G S p e n c e for p r o v i d i n g the field site.

References

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Z

,.U. c

143

300 18

250 • ~ • • 16

200 • 14

• "o

150 12 o E o

E A "~ o

"~ 100 10

50 8 E

0 6

- 5 0 • 4 i i i i i i i i i

0 6 12 18 24 6 12 18 24

Time (gmt)

Figure 4. Diurnal variations of the N20 flux (mean hourly flux) (0) 5 and 6 days after fertiliser application. The soil temperature was measured at a depth of 5 cm (A).

• O O •

"E • 0 •

v 0 • • •

E • •

© •

_c 3

Activation energy = 108 ld mo1-1

2 I [ I I I J I

3.46 3.48 3.50 3.52 3,54 3.56 3.58 3.60 3.62

1/T * 0.001 (o K-I)

Figure 5. Temperature dependenceof the N20 emission 5 and 6 days after fertiliser application.

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