THE INFLUENCE OF ATMOSPHERIC N DEPOSITION ON NITROUSOXIDE AND NITRIC OXIDE FLUXES AND SOIL AMMONIUM AND

NITRATE CONCENTRATIONS

U. SKIBA1,∗, C. PITCAIRN1, L. SHEPPARD1, V. KENNEDY2 and D. FOWLER1

1Centre for Ecology and Hydrology, Edinburgh, EH26 0QB, UK; 2Centre for Ecology andHydrology, Lancaster, LA1 4AP, UK

(∗author for correspondence, e-mail: ums@ceh.ac.uk; fax: (44) 131 445 3943)

Abstract. The deposition of atmospheric N to soils provides sources of available N to the nitrifyingand denitrifying microbial community and subsequently influences the rate of NO and N2O emissionsfrom soil. We have investigated the influence of three different sources of enhanced N deposition onNO and N2O emissions 1) elevated NH3 deposition to woodlands downwind of poultry and pig farms,2) increased wet cloud and occult N deposition to upland forest and moorland and 3) enhanced Ndeposition to trees as NO−

3 and NH+4 aerosol. Flux measurements of NO and N2O were made using

static chambers in the field or intact and repacked soil cores in the laboratory and determination ofN2O by gas chromatography and of NO by chemiluminescence analysis. Rates of N deposition toour study sites were derived from modelled estimates of N deposition, NH3 concentrations measuredby passive diffusion and inference from measurements of the 210Pb inventory of soils under treecanopies compared with open grassland. NO and N2O emissions and KCl-extractable soil NH+

4 andNO−

3 concentrations all increased with increasing N deposition rate. The extent of increase did notappear to be influenced by the chemical form of the N deposited. Systems dominated by dry-depositedNH3 downwind of intensive livestock farms or wet-deposited NH+

4 and NO−3 in the upland regions

of Britain resulted in approximately the same linear response. Emissions of NO and N2O from thesesoils increased with both N deposition and KCl extractable NH+

4 , but the relationship between NH+4

and N deposition (ln NH+4 = 0.62 ln Ndeposition + 0.21, r 2 = 0.33, n = 43) was more robust than the

relationship between N deposition and soil NO and N2O fluxes.

Keywords: ammonia deposition, forests, grasslands, linear regression, NO, N2O, peat, soil ammo-nium, soil nitrate, wet deposition

1. Introduction

Nitrogen deposition in the UK and other developed countries has increased over thelast century and currently averages 17 kg N ha−1 per year, with a range of 5–80 kgN ha−1 per year. The split in N between the reduced NH3 and NH+

4 deposition andoxidised NO2, HNO3 and NO−

3 deposition in the UK is 2:1 (218 kt Nred per yearand 117 kt Nox per year) (Fowler, 2001). To a seminatural soil, not receiving any Nfertilisers, an increase in N deposition provides a substantial increase in mineral Ninput to the soil, which can be taken up by the plant and microbial biomass, storedas soil N or nitrified and denitrified to gaseous products of NO, N2O and N2. Inthis paper, we explore whether the rate and form of N deposition influences the

Water, Air, and Soil Pollution: Focus 4: 37–43, 2004.C© 2004 Kluwer Academic Publishers. Printed in the Netherlands.

38 U. SKIBA ET AL.

rate of soil mineral N accumulation and emissions of the atmospheric pollutantsNO and N2O. Nitric oxide is involved in reactions leading to ozone production inthe troposphere and is a precursor of acid rain, and N2O is a long-lived greenhousegas (Williams et al., 1992). Measurements were made at a wide range of sites,some dominated by local sources of NH3 from livestock, others dominated bywet deposited N in the high rainfall uplands or sites close to urban areas, whereN deposition was dominated by gaseous and particular dry deposition.

2. Site Description and Methods

The sites dominated by point sources of N deposition were deciduous woodlandsdownwind of a poultry manure fired power station in East Anglia (TL585287)and poultry and pig farms in Scotland and East Anglia (Skiba et al., 1998). Thelocations dominated by wet deposition included 1) five sites of extensively grazedrough grassland sites on peat along a transect from the West Coast of Cumbria(SD146896) over Kirkston Pass into the Eden Valley to the foot and summit ofGreat Dunfell (NY708321) to Moorhouse (NY751334); 2) coniferous forests andadjacent grass moorland in the Transtrand Mountains, Sweden (Pitcairn et al.,2001); 3) Dunslair Heights, Scotland (NT288436, MacDonald et al., 1997); 4)Rough grass and wooded areas on three golf courses along an increasing rainfalland altitude transect in Edinburgh (Site B = NT194760, Site L = NT240668) andWest Linton (NT314652); and 5) a site subjected to experimental application ofwet N to mature Sitka spruce at rates equivalent to 48 and 96 kg N ha1 per year atDeepsyke forest near West Linton (NT185545) (Sheppard et al., 1999).

The relationships between N deposition rates, KCl extractable mineral N andN trace gas fluxes were studied in the field and in the laboratory using small fluxchambers and intact soil cores, respectively. Field measurements ranged from 12monthly measurements at Dunslair Heights (MacDonald et al., 1997), 31 measure-ments at Deepsyke over 5 years (monthly during the fifth treatment year) (Crossleyet al., 2001), to five measurements during the growing season downwind of theScottish poultry and pig farms (Skiba et al., 1998). Laboratory experiments wereconducted at 15 ˚C, with soil moisture maintained constant throughout the ex-periment. The headspace was sampled after 60 and 15 min enclosure for field andlaboratory studies, respectively, and analysed for N2O by electron capture gas chro-matography (MacDonald et al., 1997). NO was measured only from the soil coresin the laboratory. Air was passed over the soil surface into the chemiluminescenceanalyser; flow rates were recorded and monitored by mass flow controllers (Dicket al., 2001). Soils were analysed for NH+

4 and NO−3 by colorimetric methods after

extraction with 1 M KCl solution (MacDonald et al., 1997). NH3deposition wasestimated from monthly average concentrations of NH3, measured using passivediffusion samplers for more than 1 year downwind of the poultry and pig farms(Pitcairn et al., 1998; Sutton et al., 1998). Wet deposition of N was taken from UK

THE INFLUENCE OF ATMOSPHERIC N DEPOSITION 39

Figure 1. The influence of NH3 deposition downwind of a poultry-manure-fired power station in EastAnglia.

N deposition maps (Fowler, 2001). Long-term N deposition under trees was estab-lished by surrogate measurements of 210Pb in soil undisturbed for at least 20 yearsunderneath the tree canopy and adjacent open areas (Fowler et al., 2003).

3. Results and Discussion

The influence of elevated deposition of N downwind of a poultry-manure-firedpower station in East Anglia is shown in Figure 1. The dominant wind directionis south-westerly and occurs at a frequency of about 60%. The storage of poultrymanure at the power station provides a point source of NH3 release similar to thatfrom intensive poultry and pig farms. The deposition of NH3 declined within the first400 m downwind of the power station. Similarly, N2O and NO effluxes and NH+

4 andNO−

3 concentrations were measured at the three woodland sites. Emissions of N2Owere much smaller than NO emissions, but unlike in our previous studies (Skibaet al., 1998) there was no correlation between N2O emission and N deposition.The very sandy texture of the soil and the high ratio of NO/N2O suggest thatNO emission by nitrification dominated gaseous N loss at this site. These resultsconfirm the linear relationships established between NH3 deposition and N tracegas emissions and mineral N concentrations measured downwind of poultry andpig farms on Scottish woodland soils (Skiba et al., 1998).

Elevated N deposition arising from orographic cloud and rainfall at high altitudecan be seen in the transect from the west coast of Cumbria across to the summitof Great Dunfell in the Eden Valley. The N deposition gradient ranged from 17 to40 kg N ha−1 per year. The soil cores taken from the same type of grassland on peatextensively grazed by sheep at the five transect sites provide a clear linear relation-ship between N deposition and NH+

4 concentrations and NO and N2O emissions(Figure 2). Similar to those reported by MacDonald et al. (1997, 1998).

40 U. SKIBA ET AL.

Figure 2. Linear relationships between wet deposition of N, soil emissions of NO and N2O and KClextractable NH+

4 and NO−3 along a wet deposition gradient in Cumbria.

In contrast, in the experimental situation very high rates of N deposition had to besimulated before a measurable response was obtained. Five years of treatment withsimulated acid mist at 48 and 96 kg N ha−1 per year significantly increased NH+

4concentrations by 43 and 90% above background, but only significantly increasedN2O emissions above background for the 96 kg N application rate. Average N2Oemissions and standard errors (in brackets) for the 10 measurement dates on fourreplica plots during the fifth year of application were 1.78 kg N2O-N ha−1 per year(0.45), 0.67 kg N2O-N ha−1 per year (0.31), 0.21 kg N2O-N ha−1 per year (0.10)for the 96 and 48 kg N ha−1 per year applications and control plots, respectively.

Deposition of reactive gases such as HNO3 or NH3 to trees is substantiallylarger than to adjacent shorter vegetation (Fowler et al., 1989). This was reflectedby increased NH+

4 and NO−3 concentrations under the tree canopy compared to the

open moorland or grassland in the Transtrand Mountains in Sweden (unpublisheddata) and on golf courses near Edinburgh (Figure 3). Differences, however, weresmall and not always significant, probably due to large spatial variability between

THE INFLUENCE OF ATMOSPHERIC N DEPOSITION 41

Figure 3. The influence of increased N deposition underneath groups of trees on two golf courses(B & L) in Edinburgh. White and shaded bars refer to data from rough grassland, black and greybars refer to data from adjacent groups of trees at sites B and L, respectively. Error bars are standarddeviations from three soil samples.

the soil cores collected and inevitable differences in the chemical and physicalproperties of soils under different vegetation.

Responses described earlier suggest that an elevated rate of N deposition in-creases soil emissions of NO and N2O and rates of NH+

4 and NO−3 when soil type

and land use are the same. But, can this relationship be generalised; does the formof N deposited affect it and can gaseous N emission rates be used as indicators of Ndeposition? To answer these questions, we pooled the above data, and showed thatgaseous emission rates were not linearly related to N deposition. For N depositionranging from 2 kg N ha−1 per year (in Sweden) to 96 kg N ha−1 per year (experi-mental application of N at Deepsyke) the NO and N2O emission rate, expressed as apercentage of atmospheric N deposition, varied from 0 to 10% for NO and 0 to 16%for N2O in a fairly random manner. The absence of linearity questions the validity ofthe oversimplified IPCC methodology for calculating N deposition-induced N2Oemissions, in which 1% of the N deposited is emitted as N2O (Houghton et al.,2001) and similar methodologies to calculate NO emissions (Webb et al., 2001).Furthermore, there was no evidence that the form of N deposition, dry depositionof NH3 or the wet deposition of NH+

4 and NO−3 influenced the rate of NO and N2O

emissions. Emission rates of NO and N2O are therefore unsuitable indicators oflong term elevated N deposition rates.

However, for the same data set a significant linear relationship between soilextractable NH+

4 and total N deposition was shown, in which ln NH+4 = 0.62

(ln Ndep) + 0.21(r2 = 0.33, n = 43) (Figure 4). NH+4 therefore appears to be a

more robust indicator of N deposition than measurements of gaseous emissions, andto some extent seems capable of overriding constraints of soil type and vegetation.The relationship was also insensitive to the form of N deposited.

42 U. SKIBA ET AL.

Figure 4. The relationship between atmospheric N deposition and KCl extractable NH+4 concentration.

ln NH+4 = 0.62 (ln Ndep)+0.21(r 2 = 0.33, n = 43). Filled symbols represent sites dominated by wet

deposition from field studies at Dunslair Heights and Great Dunfell (�) and laboratory studies on soilsfrom Sweden, Edinburgh golf courses and Cumbria (�). Open symbols represent sites dominated byNH3deposition from field studies in Scotland (♦) and laboratory studies on soil from East Anglia ( �).The grey squares represent NH+

4 concentrations collected during the fifth year of acid mist treatmentat Deepsyke.

4. Conclusions

NO and N2O emissions from soils and KCl extractable NH+4 concentrations increase

with increasing rates of N deposition. There is no evidence from this study that theform of N deposited, wet deposition of NH+

4 and NO−3 or dry deposition of NH3

influences this relationship. The linear relationship between N deposition and NH+4

concentrations is much more robust than that between N deposition and N2O orNO efflux and even holds over a broad range of soil types and land uses.

Acknowledgements

The authors wish to thank landowners for access to their land and NERC – GANE,for financial support for this work under grant (NER\TIS\2000\00195), and ref-erees for valuable comments.

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