Pergamon Energy Convers. Mgmt Vol. 37, Nos 6-8, pp. 1303-1308, 1996

Copyright © 1996 Elsevier Science Ltd 0196-8904(95)00337-1 Printed in Great Britain. All rights reserved

0196-8904/96 $15.00 + 0.00

E S T I M A T E S O F N I T R O U S O X I D E E M I S S I O N S F R O M S O I L I N T H E U K

U.SKIBA It, I. P. McTAGGART 2, K, A. SMITH 3, K. J. HARGREAVES l and D. FOWLER I

llnstitute of Terrestrial Ecology, Bush Estate, Penicuik, EH26 0QB Scotland.

2Soils Dept., SAC and 3University of Edinburgh, School of Agriculture, West Mains Road, Edinburgh, EH9 3JG Scotland.

Abstract - Soil N20 emissions will be the largest single source of N20 in the UK, when industrial emissions as a result of adipic and nitric acid production will stop later this year. Total annual soil N20 emissions for the UK are estimated at 16 kt N, of which agricultural soils are responsible for 19% of the total soil emissions and grazed grasslands (16 kt N20-N yJ) are the single largest source. Deciduous and coniferous woodlands accounted for 15% and unmanaged grassland and moorlands for 16% of the total annual UK soil emission. Fertiliser N and atmospheric deposition of N are the most important variables controlling the N20 emission rate, followed by temperature and soil moisture. These three variables need to be incorporated in future more detailed estimates of regional scale soil N20 emissions.

1. INTRODUCTION On a global scale microbial processes in soils are the major source of N20 and account for 65% of the total source strength (5 to 15 Tg N20 yearl)[1]. In the UK, adipic and nitric acid production are currently the single largest source of N20, contributing to 50 kt N20-N year l [2]. Removal of N20 emissions from this source later this year will reduce the total annual N20 emission by around 50% to 50 - 65 kt N20 - N yearl.Then in the UK, too, soils will be the major source of atmospheric N20. Recent estimates suggest that soil emissions in the UK are 33 - 47 kt N20-N year 1 [2]. However, both global and national estimates of soil N20 emissions are vague and based on limited data sets, usually not accounting for the large spatial and temporal variability characteristic of N20 emissions from soils [3]. The main objectives of our research in the past few years has been to address these problems, in order to produce more accurate estimates of soil N20 emissions. Frequent seasonal flux measurements for several important agricultural [4] and semi-natural ecosystems were made in order to account for temporal variability. Micrometeorological t~hniques, which integrate the flux over areas larger than 10 ha, were employed to combat the spatial variability of N20 emissions for large emission sources (agricultural fields immediately after N fertiliser application) [5,6]. The way this information can be used to provide more detailed estimates for soil N20 emissions for the major land use classes in the UK is discussed below.

2. METHODS The conventional closed chamber technique was used to study temporal variability and the processes controlling the N20 emission from soil. Small cylindrical chambers (40 cm in diameter) were inserted into the soils to a depth of about 5 cm. Air samples were taken from the chambers in airtight, greased syringes (1 ml) or Tedlar bags (1 1) one hour after closure of the lids and were analysed for N20 by gas chromatography fitted with an electron capture detector. The N20 flux was calculated from the increase in N20 concentration inside the chambers during the closure period [3].

t To whom all correspondence should be addressed.

1303

1304 SKIBA et al.: N20 EMISSIONS FROM SOIL IN THE UK

Flux measurements were made at a range of agricultural and semi-natural soils in South and Central Scotland. On the agricultural soils sampling was carded out every few days during the growing season, with more frequent sampling immediately after fertiliser application. On the semi-natural soils sampling was carried out every 2 to 4 weeks. Soil samples were regularly collected at each site, and analysed for soil moisture content, soil pH and soil available NH4 ÷ and NO3" concentrations. Micrometeorological measurements of field scale N20 emissions were measured by eddy covariance. One such experiment in Denmark provided measurements over a wheat stubble and a carrot crop on an organic soil [6]. High fiVxluency measurements of the three components of turbulence were made by an ultrasonic anemometer, mounted at a height of 4.9 m and high frequency N20 concentrations were measured at the same height by tunable diode laser spectroscopy. Ten minute mean N20 and sensible heat fluxes were calculated on-line by eddy covariance on a near-continuous basis [6].

3. RESULTS AND DISCUSSION The measurements of N20 emissions on both agricultural and semi-natural soils suggested that soil nitrogen concentration, soil moisture and soil temperature were the main variables in determining the magnitude of fluxes. On agricultural soils the maximum N20 emissions were generally observed within 2 to 3 weeks of fertiliser application (Fig. 1). The magnitude of emissions depended on the rate and form of applied fertiliser, crop type, soil moisture and soil temperature. Emissions of N20 were greater from ungrazed grassland than from arable soils. Total N20 emissions from ungrazed grassland fertilised with NH4NO 3 were 1.7, 4 and 1.2 kg N20-N ha 1 in 1992, 1993 and 1994, respectively. Annual rainfall for each year was 868, 995 and 931 mm, respectively, with most of the rain in 1994 falling out with the growing season. The highest emissions were recorded in the wettest year, 1993, amounting to 1.1% of the applied NH4NO3-N, compared to 1.4% and 0.3% from urea and (NH4)2SO4, respectively [4]. These losses were comparable with recent studies in the Netherlands and Belgium [7].

1800

1600

~ 800 Z

M

Z

200

100 200 300 44 144 244 344 79 179 279

1992 1993 1994

Julian day

Fig. 1 Daily N2 O emissions from NH4NO 3 fertilised grassland over three growing seasons, starting April 1992. ( t)Time of fertiliser application.

The results from these long-term field studies of grassland and arable soils were used to estimate typical N20 losses for the major agricultural crops grown in the UK (Table 1). Grasslands were the major source of N20 from agricultural soils, especially grazed grassland which alone accounted for 70% of the total agricultural emissions. This was due to a greater percentage loss of applied fertiliser N, because of compaction and the additional unquantified input of N as urine and slurry [8], and also the high percentage of fertilised agricultural land under grazing. Emissions of N20 from potatoes were larger than from cereals; later application of fertiliser N (when the soil is wanner), root exudation during tuber development and more labile crop residues following harvest appeared to contribute to the greater emissions. For semi-natural land, of which forest and moorland soils have been mainly studied, fertiliser N inputs are rare and consequently rates of emission were much lower. Annual emissions ranged from 0.1 to 1.3 kg N20-N ha 1 y~ (Table 2) and were roughly comparable to emissions from agricultural soils once the fertiliser response had reduced to background levels (0.9 kg N20-N ha ~ y-i).

SKIBA et al.: N20 EMISSIONS FROM SOIL IN THE UK

Table 1. Estimated annual losses of N20 from UK crops and grassland. Crop areas and fertiliser N rates obtained from [9].

Crop

Grazed Grass Ungrazed grass Spring Barley Winter Wheat Winter Barley Oil Seed Rape Other Cereals Potato Sugar Beet Vegetables Fodder Crops

Crop Area ('000 ha)

4485 2284

516 1759 648 347 108 170 197 285 146

Applied Fertiliser N(kt)

515 267 49

322 88 62 10 31 22 43 10

Loss as N20 (%)

3.1 ~ 1.0 2 0.8 0.5 0.5 3 0.5 3 0.8 4 1.6 1.6 5

1.6 5 1.6 5

Total N20 Loss (kt N y~)

16.1 2.6 0.4 1.7 0.5 0.3 0.1 0.5 0.4 0.7 0.2

TOTAL 10945 1419 23.5 ~Data from grazed rassland sites in Belgium and the Netherlands in 1992 and 1993 [7].2Data from mown grassland sites in Scotland, Belgium and the Netherlands in 1992 and 1993 [7]. 3Assumed same value as for winter wheat, 4 as for spring barley, 5 as for potato

1305

Table 2. Nitrous oxide emissions from semi-natural soils in SE Scotland (Glencorse - GC, Dunslair Heights - DH, Auchencorth Moss -AM and North Berwick -NB), Central Scotland (Devilla -D) and North England (Great DunfeU -GDF).

Vegetation Location Altitude Soil type N20 emission ~ (m a.s.1.) kg N ha q y-i

Coniferous woodland

Sitka spruce GC 190 brown forest soil 0.35

" DH 380 brown forest soil 0.13

" DH 600 peaty podzol 0.31

Pine D 65 brown forest soil 0.36

Deciduous woodland

Alder GC 190 brown forest soil 1.32

Birch . . . . . . 0.6

Beech AM 270 drained peat 0.23

Sycamore NB 65 brown forest soil 0.66

Mowed grass GC 190 " 0.8 8

Moorland

upland grass GDF 400, 600, 800 peat 0.53

grass/heather DH 600 peaty podzol 0.26

grass/sphagnum AM 270 peat 0.1 1Mean fluxes for GC: Oct. 1992 - Oct. 1993, DH: April 1994 -June 1995, AMi Feb. 1995 - June 1995, NB: June 1995, D: May - Sept. 1993, June - Dec. 1994, GDF: May 1993, March 1995.

The highest emissions were recorded from a lowland alder plantation, where N fixation in the root nodules of the alder, ensured a constant supply of N. In 1993, mean annual N20 emissions from the alder plantation were twice as high as those from an adjacent birch plantation (1.3 kg N20-N ha q and 0.6 kg N20-N ha "~, respectively). The enhanced deposition of N by simulated acid rain to a Sitka spruce plantation [10] and occult deposition at high altitudes to coniferous woodlands increased the emission of N20. In both cases the N20 loss was equivalent to 1.3% of the additional atmospheric N input, and suggests that N deposition in the upland areas of theUK [11] is responsible for an annual emission of 1.1 kt N20-N, assuming an increase in wet deposited N of 20 kg haL This accounts for 4 % of the total soil N20 emission in the UK. The data in Table 1 and 2 provided a broad range of soil N20 emissions for the major land use classes in the UK

1306 SKIBA et al.: N20 EMISSIONS FROM SOIL IN THE UK

(Table 3). The total annual soil N20 emission was estimated at 26.2 kt N, which is lower than the UK Review Group's estimate of 33 - 47 kt N [2]. Agricultural land was responsible for 89% of the total annual soil N20 emission, grazed grassland alone contributed to61% of the total soil emissions. The agricultural emissions are very seasonal, limited to a few weeks after each fertiliser application. Therefore the bulk of the N20 emitted from soil will occur during the growing season between March and September. Emissions from woodland, moorland and unmanaged grasslands only contributed to 11% of the total annual soil emissions in the UK, of which deciduous woodlands appear to be a larger source of N20 than coniferous woodlands. The recent trend of a 10 fold increase in new plantings of deciduous species and a 3 fold decline in plantings of coniferous species since 1985 [12], suggests that emissions from deciduous woodlands are likely to increase. However, deciduous plantations have expanded mainly at the expense of managed grassland [11], causing a decrease in the overall soil N20 emissions. Compared to other European countries, forests only account for 10% of the total land area in the UK, whereas 77 % of the land is used for agriculture. At the other extreme, in Sweden 70% of the land area is used for forestry and only 9% for agriculture. In such countries N20 emissions from non - agricultural land, and elevated emission rates caused by high atmospheric N depositions, are of much greater importance than for the UK.

Table 3. Estimates of soil N20 emissions in the UK

Land cover class I

Managed grassland

Tilled land

Deciduous woodland

Coniferous woodland

Rough grass, moorland grass

Heath/bog/bracken 2

Others

Total

kin 2

67690

41760

12300

13700

32700

39300

26600

kt NzO-N y-t

18.6

4.7

0.9

0.4

0.6

1.0

not determined

26.2 as defined by [12] for agricultural soils, otherwise by [11]. 2Assume same flux rates as

for rough grass/moor grass.

The above estimates were based on long-term, frequent measurements of N20. However, regional differences in climatic conditions and the large spatial variability, important especially for high emission sources, were ignored. The long-term data on N20 emissions suggested, that when the N input is uniform, changes in soil temperature and soil moisture control the emissions of N20 from agricultural and semi-natural soils. For the agricultural soils large temperature effects occurred when soil moisture was at or near field capacity [13]. The low but uniform input of N to non-fertilised soils, however, offers an easier system to study the effect of seasonal temperature and soil moisture changes than in agricultural soils. Significant linear relationships between N20 emission and soil temperature were observed for most of our seasonal flux measurements from forest, grass and moorland soils (Table 4). Multiple regression analysis indicated, that seasonal changes in soil temperature exerted a stronger control over the N20 emission rate than seasonal changes in soil moisture content. Differences in rainfall and consequently soil moisture contents between years, had a significant effect on the total annual N20 emissions. For the agricultural grassland discussed above N20 emissions were almost twice as high in the wetter year 1993 than in the drier years of 1992 and 1994 (Fig. 1). For a coniferous forest soil at Devilla forest, Central Scotland, the average N20 emission rate was higher in the wetter year 1993 than in 1994 ( 0.47 kg N20 ha ~ with a mean soil moisture content of 34% in 1993 and 0.3 kg N20 ha ~ with a mean soil moisture content of 25% in 1994). Maximum N20 emission rates were measured in June/July when soil temperatures peaked (Fig. 2). Thus N20 emissions in the wanner and wetter SW of the UK are likely to be higher than for an equivalent soil in the drier, cooler NE, The next step will be to model the dependence of N20 emissions on soil temperature and soil moisture content, and to use this information together with geographical data bases for estimating regional differences in emission rates.

SKIBA et al.: N20 EMISSIONS FROM SOIL IN THE UK

Table 4. Covariance between N20 emission and soil temperature (5 cm depth) and soil moisture content (% dry weight) calculated from seasonal measurements in 1993/1994 in SE and Central Scotland.

1307

Vegetation type number of observations

N20 emission / soil temperature

Glencorse 1993

Alder

Birch

Sitka spruce + acid mist

Sitka spruce control

Mowed grass (unfertilised)

Devilla

Pine 1993

Pine 1994

Dunslair 1994

Sitka spruce at 380 m

Mixed conifer forest at 600 m

Moorland at 600 m level of significance: * 95 %, * * 99 %, * ** 99.9 %, otherwise not significant.

39 15.8

24 33.5

30 40.8

30 39.5

33 24.9

30 52.8 ***

36 0.7

13 1.1

13 42.3 *

12 85.0 ***

N20 emission/ soil moisture

r 2

0

12.2

3.4

3.7

1.8

13.2 *

12.2

9.6

19.1

13

12-

-~ I0 -

2:

% 2- z

o

Q 1993 • 1994

/ \

•

150 200 250 300 350 400 Jullall day

0 33 0 0

3 2 ~ ~ ' ~ ; ~ j ~40 Fluxes in ug N m'= h "1 310 50

300 60 2 9 0 ~ 0

280 80 ~'70 90

250 110 0

, o ~ / ~ r / i ~ 1 7 o 2201~0 ~ ~ 7 ~ ~ 5140 W|n. Direction ,degnm.,

180

Fig. 2 The dependence of N20 emissions from a pine forest at Devilla (Central Scotland) on soil temperature in 1993 and 1994.

Fig. 3 Sector dependence of eddy covariance N20 fluxes (B) at Lammefjord, Denmark, August 1993.

The spatial variability of soil N20 emissions is large. To improve current emission estimates it is particularly important to provide accurate field-scale emission measurements, particularly for the main sources such as agricultural soils, especially grasslands immediately after fertiliser application and when stocking densities are high (Table 1, 3). Large numbers of conventional small chambers are required to take into account the spatial variability. For a grassland field N:O emissions from 30 small chambers (40 cm diameter) ranged from 540 to 3000 lag N20-N m 2 h a [3]. Geostatistical interpretation of fluxes from many chambers offers some degree of accounting for large spatial variability [14], however this approach is very labour intensive. Recently micrometeorological measurements of N20 have become possible with the development of instrumentation capable of measuring ppb differences in N20 concentrations [6,15]. Comparison of several micrometeorological methods with conventional chambers at a rather untypical site in Denmark (unfertilised organic soil reclaimed

1308 SKIBA et al.: N20 EMISSIONS FROM SOIL IN THE UK

from the sea 100 years ago), where the N20 source was very uniform, provided good agreement between methods. Almost continuous measurements for a 10 day period by eddy covariance (integrating the flux over several hectare) (Fig. 3) and flux measurements by 47 chambers (each integrating the flux over a 0.13 m 2 area) for a 60 min period at noon each day ranged from 149 - 495 lag N20-N m 2 h t and 162 - 467 lag N20-N m -2 h "l, respectively. The continuous eddy covariance measurements clearly show the dependence of the N20 emission rate on the wind direction. Largest emissions were measured in the 270 ° sector (Fig. 3). To data only a few large scale micrometeorological N20 flux measurements have been made. More of these are required, particularly during periods immediately following fertiliser application. This is especially important for grazed grasslands where the inherent patchiness of the N20 source, both in time and space, is even greater than for non- grazed fields. Even larger scale flux measurements can be obtained by aircraft measurements [16]. A preliminary flight around the British coast in December 1994, showed that winter time N20 concentrations at 150 m height were very uniform at 313.2 ppb, but increased to 329.5 ppb downwind of an industrial source in NE England. Thus aircraft measurements can be used to provide regional scale flux measurements of N20. During peak soil N20 emission periods high emission regions can be identified and the modelled regional estimates can be verified.

4. CONCLUSIONS Agricultural fields are the largest source of biological N20 in the UK, of which grazed grassland accounted for 60 % of the total UK soil N20 emissions. These estimates were based on survey information on land use areas and fertiliser N use, on the total loss of fertiliser N as N2 O during the growing season and on long-term flux measurements over a range of soil ecosystems. For more accurate estimates the dependence of N20 emissions on soil temperature and soil water content needs to be modelled, to take into account regional differences in climatic conditions, in addition to further field measurements across a range of climatic conditions at both field and regional scales.

5. ACKNOWLEDGEMENTS We wish to thank the NERC TIGER programme and MAFF for financial support.

6. REFERENCES 1. Intergovemmental Panel on Climate Change, Climate Change 1995, (Ed. J.T. Houghton et al),

Cambridge University Press (1995). 2. United Kingdom Review Group, Impacts of Nitrogen Deposition on Terrestrial Ecosystems,

Department of the Environment, (1994). 3. H. Clayton, J. R. M. Arah and K. A. Smith, J Geophys Res D8 - 99, 16599 (1994). 4. I.P. McTaggart, H. Clayton and K. A. Smith, Non-CO 2 Greenhouse Gases (Ed. J. van Ham et al),

Kluwer Academic Publishers, 421 (1994). 5. K.A. Hargreaves, U. Skiba, D. Fowler et al, J Geophys Res D8 - 99, 16569 (1994). 6. K.A. Hargreaves, F. G. Wienhold, L. Klemedtsson, et al, Atmos Environ (in press). 7. G.L. Velthof, O. Oenema, I.P. McTaggart et al, Proceedings of the N Workshop, Ghent, Sept. 1994,

Kluwer Academic Publishers (in press). 8. J.C. Ryden, Developments Plant and Soil Sci - 23, 59 (1986). 9. Fertiliser Manufacturers Association, Fertiliser Review, Peterborough (1994). 10. U. Skiba, D. Fowler and K. A. Smith, Non-CO 2 Greenhouse Gases (Ed. J. van Ham et al), Kluwer

Academic Publishers, 153 (1994). 11. C. J. Barr, R. G. H. Bunce, R. T. Calrke et al, Countryside Survey 1990, Main Report, Department of the

Environment, (1993). 12. Department of the Environment, Digest of Environmental Statistics No. 17, HMSO (1995). 13. K. A. Smith, H. Calyton, I. P. McTaggart, Phil. Trans. R. Soc. Lond. A - 351, 1 (1995). 14. P. Ambus and S. Christensen, J. Geophys Res D8 - 99, 16549 (1994). 15. K. A. Smith, H. Clayton, J. R. M. Arah, et al, J. Geophys Res D8 - 99, 16541 (1994). 16. D. Fowler, K. J. Hargreaves, T. W. Choularton et al. Energy Conv. Managem., this issue.