Agriculture, Ecosystems and Environment 133 (2009) 139–149

Biosphere–atmosphere exchange of reactive nitrogen and greenhouse gasesat the NitroEurope core flux measurement sites: Measurement strategyand first data sets

U. Skiba a,*, J. Drewer a, Y.S. Tang a, N. van Dijk a, C. Helfter a, E. Nemitz a, D. Famulari a, J.N. Cape a,S.K. Jones a,b, M. Twigg a,c, M. Pihlatie d, T. Vesala d, K.S. Larsen e, M.S. Carter e, P. Ambus e, A. Ibrom e,C. Beier e, A. Hensen f, A. Frumau f, J.W. Erisman f, N. Bruggemann g, R. Gasche g, K. Butterbach-Bahl g,A. Neftel h, C. Spirig h, L. Horvath i, A. Freibauer j, P. Cellier k, P. Laville k, B. Loubet k, E. Magliulo l,T. Bertolini l, G. Seufert m, M. Andersson m, G. Manca m, T. Laurila n, M. Aurela n, A. Lohila n,S. Zechmeister-Boltenstern o, B. Kitzler o, G. Schaufler o, J. Siemens p, R. Kindler p,C. Flechard q, M.A. Sutton a

a Centre for Ecology and Hydrology, Edinburgh, UKb Scottish Agricultural College, Edinburgh, UKc University of Edinburgh, UKd University of Helsinki, Finlande Risø National Laboratory for Sustainable Energy, Technical University of Denmark, Roskilde, Denmarkf Energy Research Centre of the Netherlands, Petten, Netherlandsg Forschungszentrum Karlsruhe, IMK-IFU, Garmisch-Partenkirchen, Germanyh Agroscopy Reckenholz-Tanikon ART, Zurich, Switzerlandi Hungarian Meteorological Service, Budapest, Hungaryj Max-Planck Institute, Jena, Germany1

k Institut National de la Recherche Agronomique, Grignon, Francel Institute of Agronomy and Irrigation, Naples, Italym JRC, EC Joint Research Centre, Ispra, Italyn Finnish Meteorological Institute, Finlando Federal Office and Research Centre for Forests, Wien, Austriap Technische Universitat, Berlin, Germany2

q Institut National de la Recherche Agronomique, Rennes, France

A R T I C L E I N F O

Article history:

Received 1 September 2008

Received in revised form 12 May 2009

Accepted 26 May 2009

Available online 2 July 2009

Keywords:

N input

N loss

N turnover

Deposition

Denitrification

Nitrous oxide

Carbon dioxide

Methane

A B S T R A C T

The NitroEurope project aims to improve understanding of the nitrogen (N) cycle at the continental scale

and quantify the major fluxes of reactive N by a combination of reactive N measurements and modelling

activities. As part of the overall measurement strategy, a network of 13 flux ‘super sites’ (Level-3) has

been established, covering European forest, arable, grassland and wetland sites, with the objective of

quantifying the N budget at a high spatial resolution and temporal frequency for 4.5 years, and to

estimate greenhouse gas budgets (N2O, CH4 and CO2). These sites are supported by a network of low-cost

flux measurements (Level-2, 9 sites) and a network to infer reactive N fluxes at 58 sites (Level-1), for

comparison with carbon (C) flux measurements.

Measurements at the Level-3 sites include high resolution N2O, NO (also CH4, CO2) fluxes, wet and dry

N deposition, leaching of N and C and N transformations in plant, litter and soil. Results for the first 11

months (1.8.2006 to 30.6.2007) suggest that the grasslands are the largest source of N2O, that forests are

the largest source of NO and sink of CH4 and that N deposition rates influence NO and N2O fluxes in non-

agricultural ecosystems. The NO and N2O emission ratio is influenced by soil type and precipitation. First

budgets of reactive N entering and leaving the ecosystem and of net greenhouse gas exchange are

outlined. Further information on rates of denitrification to N2 and biological N2 fixation is required to

Contents lists available at ScienceDirect

Agriculture, Ecosystems and Environment

journal homepage: www.e lsev ier .com/ locate /agee

* Corresponding author at: CEH, Bush Estate, Penicuik, Midlothian EH26 0QB, UK. Tel.: +44 (0)1314458532; fax: +44 (0)1314453943.

E-mail address: ums@ceh.ac.uk (U. Skiba).1 Present address: Institut fur Agrarrelevante Klimaforschung, Braunschweig, Germany.2 Present address: Universitat Bonn, Germany.

0167-8809/$ – see front matter . Crown Copyright � 2009 Published by Elsevier B.V. All rights reserved.

doi:10.1016/j.agee.2009.05.018

U. Skiba et al. / Agriculture, Ecosystems and Environment 133 (2009) 139–149140

complete the N budgets for some sites. The quantitative roles played by CO2, N2O and CH4 in defining net

greenhouse gas exchange differ widely between ecosystems depending on the interactions of climate, soil type,

land use and management.

Crown Copyright � 2009 Published by Elsevier B.V. All rights reserved.

1. Introduction

Since the industrial revolution the production of reactive N hasrisen steadily, increasing by an order of magnitude from 15 Tg N y�1

in 1890 to 156 Tg N y�1 in 1990 (Galloway et al., 2004). Todayreactive N from fossil fuel combustion, industrial fixation by theHaber–Bosch process and biological N fixation in leguminous plantsdominate the global production of reactive N. Global increases inreactive N may be detected in a range of natural reservoirsthroughout the world. For example, air trapped in ice cores inAntarctica and glaciers in Switzerland have demonstrated how theatmospheric N2O and NH4 concentrations, respectively, have bothincreased, concomitant with the rise in N consumption during thelast century (Zardini et al., 1989; Fagerli et al., 2007). The enhanceddeposition of atmospheric N leads to a range of environmentaleffects including: (1) eutrophication of aquatic and terrestrialsystems and in N sensitive ecosystems species loss (e.g. Pitcairnet al., 2002), (2) increased production of the greenhouse gas N2O(Skiba et al., 2004), (3) increased tropospheric O3 production and (4)increased production of secondary inorganic aerosol throughformation of ammonium sulphates and ammonium nitrate, withimplications for the Earth’s climate system and human health.

The global demand for N continues to rise. An analysis of globalchange effects and related mitigation measures on globalagriculture projected that global N fertilizer use will increasefrom roughly 83 Mt in 2000 to 205 Mt in 2080 (Tubiello and Fisher,2007). Recent scenario estimates by Erisman et al. (2008) give arange for 2080 of 90–180 Mt. While less extreme than theestimates of Tubiello and Fisher (2007), they further demonstratethe difficulty of reducing global N emissions.

Given the range of effects and the number of N-species involved,it is necessary to quantify all components, to provide an overallsynthesis of the consequences of human perturbation of theatmosphere N cycle. When compared with the carbon cycle, inwhich ecosystem exchange of CO2 plays a dominant role, the N cycleis driven by the biosphere–atmosphere exchange of many N species,which include N2O, NO, NO2, HNO3, HONO, NO3

�, NH4+ and NH3.

Atmospheric production and consumption processes and bio-sphere–atmosphere exchange processes are reasonably well knownfor some of the major species. For example, we know thatatmospheric NH3 concentrations and fluxes are related to livestockproduction and manure spreading, and regional differences in thisactivity very much influence the deposition of NH3 to itssurrounding environment (Sutton and Fowler, 2002). We knowthat soil is the single largest source of N2O (Kroeze et al., 1999) and inareas away from traffic and other combustion sources also thelargest source of NO (Yienger and Levy, 1995). However, what is lesswell understood is the contribution of these reactive N compoundsto the whole N budget for a given ecosystem, and how thecontribution varies in response to external natural and anthro-pogenic drivers, such as changing climate and land management.Studying the key reactive N species simultaneously and over a rangeof ecosystems is a major challenge. Whereas net CO2 exchange isroutinely measured by eddy covariance at over 50 sites in Europe bythe CarboEurope IP (e.g. Valentini et al., 2000), such a number ofhigh-resolution flux measurements is not feasible for the full suite ofreactive N compounds with current technology.

In this paper we describe the flux measurement strategydeveloped to quantify N fluxes and net greenhouse gas exchange in

the NitroEurope Integrated Project. We then report on results fromthe core network of flux ‘‘Super Sites’’ obtained during the first tworeporting periods (1.6.2006–31.12.2006 and 1.1.2007–30.6.2007).Together with modelling and upscaling tools, which are developedand tested in separate work packages of NitroEurope, thesemeasurements will provide information required to improveestimates of Europe’s current and future N budget.

2. Measurement strategy and experimental methods

The NitroEurope project aims to improve understanding of theN cycle at the continental scale and quantify the major fluxes ofreactive N by a combination of high and low frequency reactive Nmeasurements and modelling activities (Sutton et al., 2007). Giventhe non-feasibility of deploying a multi-site network for allreactive N measurements, a measurement strategy has beendeveloped that matches the different data objectives to three levelsof measurement. This allows for extremely detailed measurementsat a few intensive sites, decreasing to basic measurements at manysites across Europe.

At 13 sites (Level-3 ‘Super Sites’) (Table 1) the objective is tomeasure the major components of the N budget at a high spatialand temporal frequency for 4.5 years using detailed methods. Thisincludes a combination of classical micrometeorological andchamber methods, while some micrometeorological methodsare also improved for some reactive N species, for example theeddy covariance system for NH3 and HNO3 measurements. Thecore objective of the Level-3 sites is to provide a detailedassessment of the different components of the N budget and netgreenhouse gas exchange, which is also suitable as a basis fordeveloping and testing process-based models.

Low-cost N flux measurements, are deployed at a further 8locations (Level-2, ‘Regional Sites’). Most flux measurementsystems have been developed to sample at high frequency, whichtypically makes them technologically or labour expensive. Whilesuch intensive measurements are needed for model development(e.g. at the Super Sites), very often the need is simply to quantifynet N fluxes over longer time periods of months to years, extendingthe regional coverage of networks to other sites. To address thisneed, the Level-2 network of NitroEurope is developing and testinglow-cost methods for measuring reactive N fluxes, includingconditional time average gradient systems, and low-cost chamberapproaches. These integrate samples over periods of 2–4 weeks,thereby reducing analysis time and associated costs.

At a further 58 sites, monthly measurements of reactive N airconcentrations, potential soil N2O/CH4 emissions and soil C and Ncontents are measured for 4 years (Level-1, ‘Inferential Sites’). TheLevel-1 network links the NitroEurope and CarboEurope networks,providing a basis to infer N deposition fluxes at sites where CO2

fluxes are measured (Tang et al., 2009).To maximise synergy, the NitroEurope flux network combines

intensive measurements led by site-based partners with cross-cutting measurement conducted by a single partner across allLevel-3 sites (Sutton et al., 2008). These are complemented byspecific laboratory studies of key processes, to understand theinteractions and impacts of ecosystem-specific key drivers onreactive N and greenhouse gas fluxes. For example, soil and plant Npools and processes are studied in greater detail by specialists thatinclude (1) potential NO and greenhouse gas fluxes and microbial

Table 1The intensive measurement ‘Super Sites’ of the NitroEurope IP (Level-3 sites).

Ecosystem and location Lat/Lon Site code Soil texture Vegetation

Forest

Hyytiala, FIN 618510N 248170S FI-Hyy Sandy loam Pinus sylvestris (pine)

Sorø, DK 558 290N 118380S DK-Sor Sandy loam Fagus sylvaticus (beech)

Speulder Bos, NL 528220N 058320S NL-Spe Sand Pseudotsuga menziesii, some Quercus robur (Douglas fir, oak)

Hoglwald, DE 488300N 118110S DE-Hog Sandy loam Picea abies (Norway spruce)

Grassland

Easter Bush, UK 558520N 038120S UK-Ebu Clay loam Lolium perenne (grazed grass)

Oensingen, CH 478170N 078440S CH-Oen Clay loam Lolium perenne, Trifolium repens (grass and clover)

Bugac, HU 468410N 198360S HU-Bug Sandy loam Festuca spp. (lightly grazed, Festuce spp.)

Arable land

Gebesee, DE 518060N 108550S DE-Geb Silt loam Beta vulgaris./Solanum/cereal/Brassica napus

(sugar beet, potato, cereal and rapeseed)

Grignon, FR 488510N 018580S FR-Gri Silt loam Zea mays/Triticum/Hordeum vulgare/Sinapis alba (intercrop)

(maize, wheat, barley and mustard)

Castellaro, I 458120N 098040S IT-Cas Sandy loam Zea mays/Oryza sativa (maize and rice)

Borgo Cioffi, I 408310N 148570S IT-BCi Clay Zea mays/Medicago sativa/Foeniculum/Lolium spp.

(maize, alfalfa, fennel and grass)

Wetland

Lompolojankka, FIN 688000N 248130S FI-Lom Peat Species of Cyperaceae, Sphagnum, Eriophorum, Vaccinium, Equisetum,

Salix and Betula nana (rich fen species)

Auchencorth Moss, UK 558480N 038020S UK-AMo Peat Species of Juncacea, Cyperaceae, Sphagnum and Calluna vulgaris

(grass dominated moorland)

Fig. 1. Distribution of the sites in the NitroEurope flux network. Level-3: ‘‘Super

Sites’’(squares), Level-2: ‘‘Regional Sites’’ (triangles) and Level-1: ‘‘Inferential Sites’’

(circles).

U. Skiba et al. / Agriculture, Ecosystems and Environment 133 (2009) 139–149 141

community composition from soils collected from each Level-3site; (2) N concentration in the main soil aggregate fractions (i.e.macro-, micro-aggregates and silt and clay) were measured atselected Level-3 sites in Italy and (3) denitrification potential to N2

at all Level-3 sites using the method developed by Butterbach-Bahlet al. (2002).

For each Level, agreed lists of measurements were establishedtogether with the modelling community of NitroEurope to makesure that the data essential for model application are beingmeasured. The data generated are submitted via a uniform datatemplate to the NitroEurope data manager every 6 months, but the1st reporting period was only 5 months. The template for the Level-3 measurements, for instance, is organised into separate sheetsaccording to the frequency of measurements made: meta-data,one-off, annual, seasonal/monthly/weekly, four times per day,30 min measurements.

In addition to the above ‘routine’ measurements, ‘special topic’studies, which go beyond the standard protocols, are carried out atselected Level-3 sites according to ecosystem relevance and localexpertise. This includes continuous measurements of gradients ofNO/NO2, O3, NH3 or particulate N compounds and long-term eddycovariance flux measurements of N2O or CH4.

The NitroEurope flux network is complemented by two parallelnetworks. Firstly, a ‘Manipulation Network’ of ecosystem experi-ments is designed to investigate the N and net greenhouse gasexchange responses to a wide range of global change drivers.Secondly, a network of six European landscape study areas (circa20–40 km2) provides a focus for integrated measurements of Nfluxes and budgets in relation to rural management strategies),such as intensive poultry management, irrigation and ploughing(Sutton et al., 2007).

In the present paper, we focus on the activities and first resultsin the Level-3 ‘Super Site’ network. The distribution of sites wasdesigned to cover four main ecosystem types: forests, grasslands,arable lands and wetlands/shrublands across the main climatezones in Europe (Fig. 1). The Level-3 study sites cover four forests,three grasslands, four arable and two wetland sites (Table 1).

For each ecosystem type a range of wet to dry climates isrepresented (Table 2). Total rainfall for the period August 2006 toJune 2007 ranged from 327 mm for the semiarid Puszta grasslandin Hungary (HU-Bug) to 1377 mm for the managed grassland in

Switzerland (CH-Oen). Average annual air temperatures during thestudy period ranged from �1 8C at the Finnish wetland site (FI-Lom) to +16 8C at the arable field in Central Italy, IT-BCi.

Measurements started at the Level-3 sites on the 1st August2006, providing high resolution N2O, NO (also CH4, CO2) fluxes, wetand dry N deposition, leaching of N, soil mineral N and total C and Nconcentrations in plant, litter and soil are measured using the bestavailable, but well tested technology (Table 3). Description of themethods employed at the Level-3 sites are all published and arereferred to in the footnotes to Table 3, by Pilegaard et al. (2003) andin accompanying papers in this special issue (Ammann et al., 2009;Tang et al., 2009). Eddy covariance is used for CO2 fluxmeasurements following the CarboEurope protocol (Papaleet al., 2006), the flux gradient method for NOx (Kramm et al.,1991), static and dynamic chambers for N2O, CH4, NO fluxes andsoil respiration rates (e.g. Butterbach-Bahl et al., 1997; Pumpanen

Table 2Precipitation and average, minimum and maximum air temperature measured

every 30 min for the period August 2006 to June 2007.

Ecosystem Precipitation (mm) Tair (8C) Tmin (8C) Tmax (8C)

Forest

FI-Hyy 502.3 4.5 �26.2 26.8

DK-Sor 889 9.8 �6.5 27.7

NL-Spea 848 10.9 �6.7 29.8

DE-Hog 427.5 9.3 �7.2 25.6

Grassland

UK-EBu 1136.6 9.1 �5 24.9

CH-Oen 1377.4 8.3 �9.4 26.7

HU-Bug 327 9.7 �11.8 31.7

Arable land

DE-Geb 478.7 15.5 �9.2 28.2

FR-Gri 559.5 11.5 �5.9 30.9

IT-Cas 576.4 13.2 �4.1 33.0

IT-BCi 1366.3 16.3 0.4 38.7

Wetland

FI-Lom 737.7 �1.1 �35.7 24

UK-Amo 1244.8 8.1 �6.8 24.2

a For January 07–June 07.Fig. 2. The contribution of aerosol and gaseous reduced and oxidised N to

atmospheric N deposition rates at some NitroEurope Level-3 sites for the period

November 2006 to December 2007.

Fig. 3. Wet deposition of NO3� (black bars) and NH4

+ (grey bars). Measurement

periods varied for the sites and are bulk wet deposition, unless otherwise stated. FI-

Hyy: wet only, weekly in 06, at a representative site, Ahtari, 50 km from FI-Hyy; DK-

Sor: bulk throughfall, monthly in 07; NL-Spe: bulk throughfall, monthly 06 and 07;

DE-Geb: weekly in 07; FR-Gri: wet only; IT-BCi: September 06 to January 07, NH4+ not

shown, due to contamination by birds; UK-EBu: monthly September 06 to November

06, CH-Oen: weekly August 06 to June 07; HU-Bug: wet only, weekly 06 and 07; FI-

Lom: wet only, monthly in 06; UK-Amo: wet only, daily August 06 to June 07.

U. Skiba et al. / Agriculture, Ecosystems and Environment 133 (2009) 139–149142

et al., 2004). Monthly time-integrated measurements of atmo-spheric ammonia (NH3) and nitric acid (HNO3) and aerosolammonium (NH4

+) and nitrate (NO3�) are made using the DELTA

(DEnuder for long-term atmospheric monitoring) sampling system(Sutton et al., 2001) with subsequent aqueous analysis by sevenlaboratories in Europe and application of an inferential model tocalculate deposition fluxes (Tang et al., 2009).

3. Results

The simultaneous measurements of reactive N compounds inthe vegetation, soil and atmosphere at the NitroEurope Level-3sites provide a unique new database. It is too early for a completeanalysis of the data and comparison of the differences in N fluxes,pools and processes between sites and ecosystems. However, thedata shown here is from the first two reporting periods, 1.8.2006–31.12.2006 and 1.1.2007–30.6.2007, illustrating the nature of thedata collected and already highlighting key differences betweenthe sites. In addition, this first analysis illustrates gaps in themeasurement strategy that require more attention in theremaining study period.

3.1. N inputs

3.1.1. Atmospheric N deposition

The most commonly occurring species of N deposited ingaseous form (NH3, HNO3, HONO or HNO2) or as aerosols (NH4

+

and NO3�) were analysed for the period November 2006 to

December 2007 for all 13 Level-3 sites (Figs. 2 and 3). Gaseous Nspecies accounted for 57% of the total atmospheric N concentra-tions and aerosol N for 43%. Of the total N concentrations 77% wereas reduced N (NH4

+ and NH3) and 23% as oxidised N (HNO3, NO2�

and NO3�). The largest concentrations were measured in

intensively managed agricultural areas and the smallest at thetwo Finnish sites, the pine forest at Hyytiala (FI-Hyy) and the sedgefen at Lompolojankka (FI-Lom) (Tang et al., 2009). Preliminaryestimates of total reactive N (Nr) net turbulent fluxes from aselection of the Level-3 sites calculated using the site concentra-tion measurements and the EMEP model dry deposition moduleshow a range from 0.5 to 18 kg N ha�1 y�1 (Fig. 2). For the threeforest sites wet deposition increased in the same order: FI-Hyy,DK-Sor, NL-Loo (Loobos, a CarboEurope pine forest site, adjacent to

NL-Spe) (Fig. 3) as dry deposition (Fig. 2), primarily due to largeincreases in reduced N (Fig. 3).

Estimates of total N deposition, wet and dry, based on the dataprovided in Figs. 2 and 3 and/or on previous measurementsprovided by the partners ranged from 1.2 kg N ha�1 y�1 at theFinnish wetland, Lompolojankka (FI-Lom) to 52 kg N ha�1 y�1 atthe Dutch forest NL-Loo (close to NL-Spe) and 40 kg N ha�1 y�1 theGerman forest DE-Hog. Largest N deposition rates were observedfor the forests sites and the smallest for the wetlands, while thearable sites had slightly larger N deposition rates than thegrasslands (Figs. 2 and 3, Table 4).

3.1.2. N fertiliser application

Annual N fertiliser applications to the grasslands UK-EBu andCH-Oen and to the arable fields FR-Gri, IT-Cas and IT-BCi rangedbetween 170 and 350 kg N ha�1 y�1. Only two of the agriculturallymanaged sites, the arable crop rotation at Gebesee, Germany (DE-Geb) and the extensively grazed grassland at Bugac, Hungary (HU-Bug) had low agricultural N inputs. The loess at Gebesee is onlyfertilised with 31 kg N ha�1 y�1. Nitrification rates are very high

Table 3List of measurements at the NitroEurope Level-3 sites.

Location Frequency Measurement

Above canopy 30 min Fluxes of CO2, NO, NO2, O3, latent and sensible heat, momentum, CO2 storage in canopy air layera

Cumulative monthly NH3, HNO3, NH4, NO3 fluxes for 2 years, concentrations for 4.5 yearsb

Meteorology 30 min Global and net radiation, air and bole temp., pressure, precipitation, humidity, canopy wetness

and temp., photosynthetic photon flux density, direct and diffuse

In canopy Cumulative monthly/weekly Bulk wet deposition, throughfall, bulk NH4 and NO3 depositionc

Soil fluxes 30 min Soil heat fluxd

�4 times/day NO, N2O, CH4, soil respiratione

30 min Soil temperature and moisturef

Soil parameters Monthly Soil NH4 and NO3, NO3 leachingg

Once Soil description, texture, bulk density, moisture at field capacity, PDF curve, stone fraction,

pH, total C and Nh

Vegetation As required for ecosystem Mean canopy height, tissue C and N, wood biomass, residue after management, plant species

composition, standing leaf biomass, wood increment, leaf litter production, litter fraction, C and N in litter

a Gradient method for NO, NO2 and O3 and for the rest eddy covariance using high frequency gas analysers (infrared absorption for CO2, chemiluminescence for NO, NO2

and UV absorption for O3) and ultrasonic anemometers.b DELTA Denuders (Sutton et al., 2001) with conditional time average gradient flux measurements.c Tipping bucket rain gage and bulk collector (minimum collecting area = 20 cm) for NH4 and NO3 analysis by ion chromatography or colorimetric assays.d Hukseflux thermal sensors.e Automated dynamic chambers for NO and CO2 and in situ analysis by chemiluminescence and infrared absorption, respectively. Automated closed chambers for N2O/CH4.

In situ analysis by gas chromatography or photoacoustic instruments, or storage of samples in vials followed by analysis in the laboratory (Butterbach-Bahl et al., 1997).f Thermistor and time domain reflectometer, respectively, installed at 4 depths from surface (0.02 m) to below the rooting depth (0.5 m).g Soil collected at top two functional layers. KCl extraction and analysis of NH4 and NO3 (see footnote ‘c’). NO3 leaching samples collected below rooting zone using porous

suction cups (Weihermuller et al., 2007).h Soil description 0–1 m, rest at 3–5 depths, depending on soil type.

Fig. 4. Fertiliser induced NH3 emissions from the grassland UK-Ebu measured by

dual tunable laser adsorption spectroscopy. Six-month-old cattle slurry (49 kg total

available N ha�1) was applied on the 27th April.

U. Skiba et al. / Agriculture, Ecosystems and Environment 133 (2009) 139–149 143

here and the annual contribution of N deposited from theatmosphere matches that supplied by fertilisation (Figs. 2 and3). The Hungarian Puszta grassland is not fertilised and onlyreceives N through atmospheric deposition (9.5 kg N ha�1 y�1

August 2006–July 2007). The stocking density is low (0.56 cattle/ha) and, via excretion, the cattle are estimated to redistribute anequivalent of 16 kg of available N ha�1 y�1 to the grassland.

3.1.3. Biological N fixation rates

Estimates of biological N fixation are not yet available, and willbe measured by the 15N pool-dilution approach at selected sites.Field scale estimates will be made by direct estimates of plant Naccumulation with and without legumes, combined with empiricalmodelling. For most of the Level-3 sites, biological N fixation isanticipated to be small (<1–2 kg N ha�1 y�1), though largerfixation rates is expected for sites where legumes are present(CH-Oen, IT-BCi) (Ammann et al., 2009).

3.2. N losses

3.2.1. N loss after application of mineral and organic N fertiliser

Measurements of N loss during fertiliser application arecovered by the routine measurements listed in Table 3, however,the temporal scale of these measurements may not be sufficient toinvestigate the often fast and short lived response of N emissionsduring fertilisation. Therefore, at key sites where these emissions,particularly NH3, appear to be important, campaign measurementsat high temporal frequency and large spatial scale are carried out.For example, over the managed grassland UK-EBu, NH3 emissionswere measured before and after slurry application, using dualtunable diode laser absorption spectroscopy by eddy covariance(Whitehead et al., 2008). Application of 6-month-old cattle slurrybegan at 07:45 (GMT) on the 27th April 2005 and 2 h later thelargest flux was observed (35 mg m�2 s�1) based on 15 min fluxmeasurements (Fig. 4). Such intensive campaign measurementsfollowing fertilization are supported by the application of long-term conditional time average gradient (CoTAG) measurements(Fowler et al., 2001) on a lower time resolution at some sites.

3.2.2. N loss from soil microbial processes

The largest emissions of N2O were measured from theintensively grazed grassland at UK-EBu, which received 171 kgNH4NO3-N ha�1 y�1, spread over three applications in March, Mayand June, and a redistribution of approximately 155 kg organic Nestimated to be excreted by the grazing sheep (Table 4). Theorganic N input is characteristically very variable, both in spaceand time. The sheep population changes throughout the year,lambs are only present from spring to autumn and their N input tothe soil changes throughout this period. For this reason N2O fluxesfrom the eight manual flux chambers, which were removedbetween measurements to allow free grazing, are very variable, asindicated by the large error bars in Fig. 5. During peak emissionevents, usually for 2–3 weeks after mineral N fertilisation, N2Ofluxes were also measured at the field scale by eddy covarianceusing a tunable diode laser to analyse the N2O concentrations (Di

Table 4Variation in nitrogen and greenhouse gas fluxes illustrated for eight sites in the NitroEurope Level-3 network. Gaseous losses of NO, N2O, CH4 were measured by

autochambers (auto) or manual chambers (manual) as identified in the footnotes, with annual values based on rescaling from the measurement periods noted. The CO2 flux

was measured by eddy covariance. Data presented here are the mean and standard deviation (in brackets) for the NEU reporting periods August–December 2006 and January–

June 2007 scaled up to annual averages.

Ecosystem N input Gaseous losses Footnote

Atmospheric depositiona

(kg N ha�1 y�1)

Fertiliser and grazing

(kg N ha�1 y�1)

NO-N

(kg N ha�1 y�1)

N2O-N

(kg N ha�1 y�1)

CH4

(kg ha�1 y�1)

CO2

(kg ha�1 y�1)

Forest

FI-Hyy 5.0 0 <0.001d 0.25 (0.16) �9.84 (1.34) �6,078 g

DK-Sor 26.6 0 0.23d 0.45 (0.48) �3.90 (1.71) �777e h

Grassland

UK-EBu 10.0 326b na 12.47 (18.09) 1.67 (7.70) �3,788 i

CH-Oen 15.8 230 0.37 (0.24) 0.24 (0.91) Insignificant �18,333 j

HU-Bug 10.4 16c 1.20 (2.00) 0.16 (0.29) 0.25 (0.93) �3,691 k

Arable land

FR-Gri 14.2 175 0.32 (0.3) 0.27 (0.88) <�0.0 �19,114 l

Wetland

FI-Lom 1.2 0 na 0.09 (0.17) 171 (40) +2,300f m

UK-AMo 9.6 0.8 na 0.00 (0.39) 2.42 (2.93) �4,250 n

a Estimate of wet and dry deposition from local measurements and inferential modelling (cf. Figs. 2 and 3).b 171 kg NH4NO3 ha�1 y�1 plus 155 kg N redistributed as excreta of grazing sheep, average grazing density 1.55 livestock unit (LSU) ha�1.c Redistribution of N via recycling of excreta by grazing Hungarian grey cows, 0.65 LSU ha�1.d Pilegaard et al. (2006).e Annual average including July 07 = �3987 kg ha�1 y�1.f Annual average including July 07 = �800 kg ha�1 y�1.g Six manual and one autochamber, September 06–June 07.h Six manual and one autochamber.i Eight manual chambers.j Autochambers, NO and N2O August 06–October 06, NO April–June 07, N2O January–July 07.k Two autochambers on 12 positions for NO, eight manual chambers for N2O.l Autochambers on six positions for N2O and NO January 07–June 07; harvesting removed 18500 kg CO2 ha�1.m For N2O, 12 manual chambers in growing season, six in winter, eddy covariance for CH4 and CO2.n Nine manual chambers, na = estimates not currently available.oTo convert to N2O, multiply data by 1.57.

U. Skiba et al. / Agriculture, Ecosystems and Environment 133 (2009) 139–149144

Marco et al., 2004). After the March 2007 fertilisation, fluxesmeasured by eddy covariance and the four chambers that weresituated within the footprint of the eddy covariance measurementsshowed a very good agreement (r2 = 0.69) (Fig. 5).

For the remaining Level-3 sites, average annual fluxes weremuch smaller. N2O emissions were measured by auto-chambers atleast four times daily from the barley in FR-Gri and winter wheat inDE-Geb. Over the period (January 2007–June 2007) N2O emissionswere very similar (FR-Gri: 0.3 � 0.9 and DE-Geb 0.3 � 2.9 kg N2O-N ha�1 y�1, where standard deviation here is an indicator of thetemporal variability over the year) in spite of different N fertilisationrates of 108 and 32 kg N ha�1 y�1, respectively.

Fig. 5. Comparison of eddy covariance (solid line) and manual chambers (dotted

line) for N2O flux measurements from the grassland UK-EBu. Only the four

chambers within the same wind sector measured by the eddy covariance footprint

were considered. The bars are the standard error of the mean for the four chambers.

N fertiliser was applied at a rate of 69 kg NH4NO3-N ha�1 on 14th March.

The N2O fluxes measured from an irrigated crop rotation, fenneland maize in Central Italy, IT-BCi, were much larger than from thosemeasured at the French and German sites. The fennel cropped fieldwas fertilised on the 15th November, and in December 2006measured N2O emissions ranged from �0.8 to 184 mg N2O-N m�2 h�1 (average = 3.8 � 3.0 kg N ha�1 y�1, n = 8 manual cham-bers). In May and June 2007, when the field was cropped with maize, Nfertiliser was applied together with a nitrification inhibitor, delayingthe fertiliser induced N2O emission peak by 30 days. In spite offavourable warm temperature and moisture conditions, provided byirrigation, cumulative N2O emissions were less than 1 kg N ha�1 y�1 bythe end of the second reporting period (30th June 2007). For the ricecrop at IT-Cas, N2O emissions peaked after fertilisation at1187 � 1292 mg N2O-N m�2 h�1 (n = 8 chambers), but then declinedto zero, when the field was flooded (Fig. 6).

The largest NO fluxes were measured from the two Level-3forests, DE-Hog and NL-Spe. At both sites atmospheric N depositionrates exceed 40 kg N ha�1 y�1. For DE-Hog average fluxes during thestudy period (Aug 2006 to 20th June 07) were 8.1 � 5.9 kg N ha�1 y�1

from five chambers measured hourly. Previous measurements fromDE-Hog and NL-Spe in 2002–2003 were similar (7.2 kg N ha�1 y�1 atDE-Hog and 6.6 kg N ha�1 y�1 at NL-Spe, Pilegaard et al., 2006). Forboth forests NO emissions were almost an order of magnitude largerthan N2O emissions (Fig. 7 for DE-Hog). The semi-arid Puszta grasslandHU-Bug also had larger average NO fluxes compared to N2O (Table 4), asa consequence of aerobic soil conditions at this site.

3.2.3. Nitrate leaching and runoff

Nitrate concentrations below the rooting zone are measured atall Level-3 sites. Identical equipment was used to collect NO3

� indrainage water at UK-EBu, DK-Sor, FR-Gri, and NL-Loo (Loobos, aCarboEurope pine forest site, adjacent to NL-Spe) (Siemens and

Fig. 6. N2O and CH4 fluxes (average � SD) from the rice paddy field at IT-Cas, Italy. The

field was ploughed on 9.3, fertilised on 20.3 (application and incorporation of

2250 kg ha�1 of sugar-beet industrial slops (N 0.3%; K2O 0.6%) plus 75 kg ha�1 of urea

(46% of N)) and flooded from 12.4 to 15.5 and 5.6 to 31.6 (solid line). The N2O

(diamond) and the CH4 (square) fluxes are measured with automatic and manual

chambers. The horizontal dotted line indicates the zero point for the right y-axis.

Fig. 8. Soil water NO3� concentrations measured at example NitroEurope Level-3

sites. Measurements were made at 0.2–0.4 m, depending on site (black bars) and at

1 m depth (grey bars) by an identical set up of suction cups with samples collected

every 2–3 weeks. The values are averages and standard deviations for the period

December 06 to April 07.

U. Skiba et al. / Agriculture, Ecosystems and Environment 133 (2009) 139–149 145

Kaupenjohann, 2002). Samples were analysed in the samelaboratory and NO3

� leaching rates were calculated from NO3�

concentrations and modelled water fluxes (Weihermuller et al.,2007). Very low NO3

� concentrations were found at the beech forest(DK-Sor), which resulted in low leaching fluxes of0.3 kg NO3

� N ha�1 y�1 at 15 cm depth and 0.6 kg ha�1 y�1 at 1 mdepth. In line with larger atmospheric N inputs, much larger NO3

�

concentrations were found in soil leachates at the Dutch Loobosforest near NL-Spe (Fig. 8). In this coniferous forest, leaching losses ofNO3

� from topsoil amounted to 62 kg NO3� N ha�1 y�1, whereas

23 kg NO3� N were leached below 120 cm depth. Fluxes are not yet

calculated for UK-EBu and FR-Gri, but the measured concentrationswere comparable to those in the Dutch forest (NL-Loo). Also in theGerman forest, DE-Hog, leaching rates (22 kg NO3

� N ha�1 y�1)were comparable to those from NL-Loo. But NO3

� leaching losseswere negligible for the coniferous forest in Finland FI-Hyy, thegrasslands CH-Oen and HU-Bug and the arable site at DE-Geb. Forthe moorland, UK-AMo, runoff and leaching losses accounted for2 kg NH4

+ and NO3�-N ha�1 y�1, of which 70% was as NH4

+.

3.2.4. Fluxes of CH4 and CO2

Methane fluxes are usually measured together with N2O usingthe same chambers. Some sites were net CH4 sinks and others netemitters (Table 4). Largest sink activity occurred at the 46-year-old

Fig. 7. NO (black squares) and N2O (open triangles) fluxes in the spruce forest

Hoglwald (DE-Hog). Fluxes were measured by autochambers closing every hour for

NO flux measurements and every 2 h for N2O measurements (Butterbach-Bahl et al.,

1997). Each data point is the average for time periods 0:00–11:00 and 12:00–23:00.

Finnish pine forest (FI-Hyy, Table 4) followed by the 90-year-oldbeech forest (DK-Sor, Table 4) and the 100-year-old German spruceforest (DE-Hog, �2.26, range �6.8 to 0.7 kg CH4 ha�1 y�1, mea-sured by autochambers 12 times per day). Small sinks weremeasured at the arable sites FR-Gri (Table 4) and DE-Geb (–0.41,range �25.2 to 22.9 kg CH4 ha�1 y�1).

Small rates of net CH4 emissions (<2.5 kg CH4 ha�1 y�1) weremeasured from the grasslands HU-Bug, UK-EBu and the wetlandUK-AMo (Table 4) Large CH4 emissions (2500–6050 mg CH4 m�2 h�1) of short duration were measured fromthe rice field, IT-Cas, after flooding (Fig. 6) and from the Finnishwetland (Fi-Lom) in summer (Fig. 9). At FI-Lom methane wasmeasured by eddy covariance and analysis by spectroscopy using afast methane analyser (Los Gatos Research, Inc., Mountainview, CA,USA). Fluxes at this site tend to be small during the 9 cooler months(<1080 mg CH4 m�2 h�1), but as temperatures rose, CH4 emissionsincreased and remained high for approximately 3 months (average5544 mg CH4 m�2 h�1).

Fluxes of CO2 as measured at all sites by eddy covarianceshowed the largest sink strength for the French arable field (FR-Gri)and the Swiss grassland (CH-Oen) (Table 4). However, for thesetwo sites, the vegetation was removed at harvest/grass cutting,which made the site balance much lower; for example in FR-Gri,the harvested parts (grains and straw) were comparable with theCO2 flux (18.5 t ha�1 versus 19.1 t ha�1, respectively).

The first two reporting periods considered in this paper coveronly 11 months (August 2006–June 2007) and do not include themonth of July. This introduces a significant uncertainty to the

Fig. 9. Increase in CH4 fluxes at the start of summer in the Finnish wetland FI-Lom.

Fluxes are 30 min averages of eddy covariance flux measurements, using a fast

methane analyser.

U. Skiba et al. / Agriculture, Ecosystems and Environment 133 (2009) 139–149146

annual CO2 budget, especially for the Finnish wetland site, wheremost of the biological activity is confined to circa 3 summermonths. For the period August 2006–June 2007 the CO2 flux at FI-Lom was +2300 kg CO2 ha�1 y�1, but the average flux at this siteover a 2-year period (January 2006–December 2007) was�800 kg CO2 ha�1 y�1. Equally for the beech forest in Denmark(DK-Sor) the CO2 flux for the 11 months study period was�777 kg CO2 ha�1 y�1, but the annual flux for the period August2006 to July 2007 was �3987 kg CO2 ha�1 y�1. These differenceshighlight the role of inter-annual variability, which will beinvestigated through the ongoing measurements.

4. Discussion

4.1. N inputs

4.1.1. N fertiliser application and biological N2 fixation

Nitrogen inputs to the ecosystems may occur by N fertilisation,biological N fixation and atmospheric deposition. In mostagricultural systems N input is dominated by the application ofmineral and organic fertilisers and return of N to the system bygrazing livestock. The N content of fertiliser and grazing inputs canbe accounted for relatively easily. Unfortunately, this is not thecase for biological N fixation. Rates of biological N2 fixation aresuppressed by large mineral N concentrations, therefore it isimportant to measure actual N2 fixation rates, e.g. as estimatedusing 15N pool-dilution methods rather than the most commonlyused acetylene reduction to ethylene, which only determinespotential fixation rates (Vitousek et al., 2002). N fixation rates, bothin symbiotic associations with plants and by free-living hetero-trophs, can under ideal conditions add as much as100 kg N ha�1 y�1. Heterotrophic N fixation is particularly impor-tant in wetlands (Vitousek et al., 2002), though it should be notedthat in the high latitude oligotrophic wetlands examined in theLevel-3 network, much smaller N fixation rates are expected (<1–5 kg N ha�1 y�1). Nevertheless, these rates remain highly uncer-tain, and it is possible that N fixation is not just important at siteswith legumes (CH-Oen, IT-BCi), but also those devoid of legumes,and therefore need to be accounted for at the relevant Level-3 sites.

4.1.2. Atmospheric N deposition

In non-agricultural ecosystems, atmospheric deposition is themain source of N added, causing problems of eutrophication,acidification and species change when deposition rates rise beyondthe threshold value for the particular ecosystem (e.g. Pitcairn et al.,2002). Nitrogen is deposited as reduced or oxidised N, which isdissolved in precipitation or deposited as a gas, aerosols, either inmineral or in organic form. The wide range of deposition ratespresented in Table 4 reflects expected values over Europe. Due tothe much larger surface area of canopies compared with shortervegetation, atmospheric deposition to the forest floor tends to belarger (compare FI-Lom and FI-Hyy) (Fowler et al., 1989). Atmo-spheric N deposition rates also are larger in high N-emittingregions (compare FI-Lom with UK-AMo, or FI-Hyy with any of theother forest ecosystems in Table 4). In order to assess and simulateatmospheric N deposition rates for each ecosystem and the likelydamage caused to the ecosystems, it is essential to understand therelative partitioning of atmospheric N into the different forms.These appear to be linked to emission source and strength, landuse,seasonal and meteorological conditions (Sutton et al., 2004). Forthe Level-3 sites in general, the reduced form of N was moreimportant than oxidised N, as shown in Figs. 2 and 3.

Water-soluble organic N compounds (DON) are not routinelymeasured in many studies, in spite of reports that theircontribution to the total wet deposition rate can be significant(Zhang et al., 2008). A survey in the UK, for example has shown that

organic N contributes between 24 and 40% to the total annual wetdeposition (Cape et al., 2004). At the managed grassland at EasterBush (UK-EBu) organic N accounted for 33% of the total N inprecipitation in the period 2000–2002 and for the 38 weeks from6th July 06 to 28th March 07 to 13% (Cape, pers. comm). At theforest DK-Sor, dissolved organic N accounted for 9% of the total Ndeposition in the period December 2006 to November 2007(Ibrom, pers. comm.). It will be valuable to atmospheric models tounderstand how the contribution of DON to the total depositionchanges across Europe and across different ecosystems. Thereforewet deposition samples from a large range of Level-3 and Level-1sites will be analysed for organic N as part of the ongoingprogramme of NitroEurope.

4.2. N turnover in plants and soils

The N deposited to an ecosystem can have many fates, dependingon ecosystem type, climate and N status of the system. The majorplant, litter and soil N pools and interactions, the microbial C and Nbiomass, N mineralization rates and potential N2O, NO and CH4

fluxes are measured at the Level-3 sites by the site operators andspecialised laboratories within the NEU community, collecting soiland plant materials from all or a subsection of the Level-3 sites fordetermining the above parameters. These data are largely notavailable yet, but are essential for the full understanding of the Ntransformations occurring in each ecosystem and for creating Nbudgets. An example of the data collected are potential NO andgreenhouse gas fluxes from soil cores collected from all Level-3 sitesand incubated at different temperatures and water contents(Schaufler and Zechmeister, pers. comm.; Sutton et al., 2008). Thepotential trace gas emissions support the flux trends obtained fromthe field measurements.

4.3. Losses of N

Nitrogen is lost from the ecosystem as gaseous emissions to theatmosphere, removal of vegetation by harvesting and grazing,leaching, runoff and soil erosion. Losses by gaseous emissionsoccur as direct losses from decomposing vegetation and duringmineral or organic fertiliser application. The dominant gases ofconcern here are NH3 release from organic N fertilisers and urea(Sutton et al., 2000; Bless et al., 1991).

4.3.1. Losses of NO, N2O, N2

The microbial processes of nitrification and denitrification insoil are the main sources of the gases NO, N2O and N2 (Davidson,1991, Barnard et al., 2005). Production of these compounds isstimulated by increasing the mineral N concentration, which inturn is directly related to the mineral N applied and themineralization rate of organic N in the soil. The rate of emissionof NO, N2O and N2 and leaching of NO3

� depends on soil physicaland chemical properties, and climate. The pattern of NO and N2Oemissions observed at the Level-3 sites are as expected and followgeneral trends reported (i.e., Pilegaard et al., 2006; Flechard et al.,2007; Skiba and Ball, 2002; Davidson, 1991): (1) N emissions arerelated to N input, with smallest emissions measured from thelowest N input ecosystems, the non-agricultural wetlands andforests in unpolluted regions. (2) The NO/N2O emission ratiodepends on the redox potential of the soil, as demonstrated here bylarge NO emissions at HU-Bug, but emissions not significantlydifferent from zero for the less porous soil subjected to muchhigher rainfall CH-Oen (Table 4). The rather small N2O emissions atCH-Oen during the 11 months period summarized here are notrepresentative of the long-term average (Ammann et al., 2009, thisissue), indicating that long-term studies, are needed to provide anaccurate picture of the emissions.

U. Skiba et al. / Agriculture, Ecosystems and Environment 133 (2009) 139–149 147

Although there remain many uncertainties, measurements of Nlosses as NO and N2O are generally feasible using chambermethods, and in some instances micrometeorological approaches.By contrast, this is not the case for the final product ofdenitrification N2, because of the need to detect relatively smallamounts of N2 emissions against a large background of atmo-spheric N2. The methods available are either restricted to thelaboratory, as is the case for the direct measurements in a N2 freeenvironment (Butterbach-Bahl et al., 2002); are limited to high Ninput systems and expensive, as is the case for 15N tracer studies(Ryden, 1983); or are difficult to administer evenly throughout thesoil, as for the most widely used method, the acetylene blocktechnique (Arah et al., 1991). Developing a method that canmeasure denitrification in the field, and not be hampered by theabove listed restrictions, is a major challenge for the future.Denitrification to N2 at CH-Oen (Ammann et al., 2009), UK-EBu(Arah et al., 1991) and DE-Hog (Bruggemann, pers. comm.),appears to be important and must be included in studies of Nbudgets at all sites. For example, the acetylene block methodapplied to Macmerry soil (same soil series as at UK-EBu, collectedfrom a field within the same 1 km2) showed that in this soil theN2O/N2 ratio was at least 1, and highly variable in space and time(Arah et al., 1991).

4.3.2. Nitrate leaching

Nitrate leaching loss rates are determined by N input rate,nitrification and mineralisation rates, soil and vegetation type anddensity (Hansen et al., 2007; Lord et al., 1999). The pattern ofleaching rates at the Level-3 sites appears to follow this generalobservation. In agricultural soils N leaching rates are related to Nfertiliser input, as also seen here by comparing DE-Geb (fertiliserrate = 32 kg N ha�1 y�1) with FR-Gri (fertiliser rate = 175 -kg N ha�1 y�1). The two forests NL-Spe/NL-Loo and DE-Hog, whichhave persistently been exposed to high atmospheric N depositionrates (>40 kg N ha�1 y�1) and therefore are N saturated, havelarger leaching rates than forests, that are still net sinks foratmospheric N (DK-Sor and FI-Hyy). In low N input systems, Nleaching is linked to periods of high mineralization rates duringsnow melt and during high rainfall periods in summer, as wasreported for the pine forest FI-Hyy (Pihlatie, pers. comm.). Lossesfrom fertilised arable soils can be minimised by using nitrificationinhibitors (as done at IT-BCi), applying urea rather than NH4NO3

fertiliser (although this will increase NH3 emissions), maintaininggood soil structure and not applying fertiliser to cold, wet soilswhen competition by the crop for the N is expected to be minimal.

4.3.3. The influence of N on CH4 fluxes

Forests are generally recognized as the largest sinks for CH4,while CH4 uptake by disturbed soils tends to be small (Smith et al.,2000). This is demonstrated here by the large CH4 sink of the pine

Table 5A preliminary calculation of greenhouse gas (GHG) emissions based on the data in Table 4

for CH4 25, over the 100-year-time period, as defined by the IPCC (2007).

Ecosystem Site CO2 (kg ha�1 y�1) N2O (as equiv. CO2) (kg

Forest FI-Hyy �6,078 117

DK-Sor �3,987a 211

Grassland UK-EBu �3,788b 5840

HU-Bug �3,691 75

Arable land FR-Gri �19,114b 126

Wetland FI-Lom �800a 42

UK-AMo �4,250 0

a Average of measurements for 12 months August 06–July 07.b Export of C by harvest (vegetation and animals) is not included.c CH4 emissions from sheep are not included.

forest FI-Hyy, compared to the insignificant fluxes(<�1 kg ha�1 y�1) measured at most arable and grasslands. Thesmaller CH4 oxidation rates at DE-Hog (�2.26 � 0.54 kg ha�1 y�1)and at DK-Sor (�3.9 � 1.71 kg ha�1 y�1, Table 4) may partly beinfluenced by much larger atmospheric N deposition rates (40 and27 kg N ha�1 y�1, respectively). Grasslands in high rainfall regionstend to switch from being net CH4 sources or sinks, depending on theredox potential of the upper soil layers (van den Pol, 1999). In thisstudy, annual CH4 fluxes were very similar for the UK wetland (UK-AMo) and the UK grassland (UK-EBu), both within 10 km of each other(Table 4), but with very different soil and management conditions.Methane emissions from the Finnish wetland, FI-Lom, were muchlarger and of similar order of magnitude, as reported for otherwetlands in this area (Huttunen et al., 2003). Emissions from the FI-Lom wetland are driven by temperature, water table height andpresence of vegetation. The strong temperature dependence is thereason for very pronounced annual cycles.

4.4. N budgets and net greenhouse gas exchange

The ultimate aim for the Level-3 sites is to construct total N andnet greenhouse gas budgets. Although such budgets based on thefirst year of measurement remain uncertain, they are very valuablein providing insight in the importance of each the measurementsmade, revealing gaps in the measurements, and highlighting themain features between sites. Nevertheless, it is clear that the N andC fluxes described here have very large temporal variability, andseveral years of measurements are required to generate a picture ofthe N and C pools and interactions between atmosphere, soil andvegetation.

Preliminary greenhouse gas balances, expressed as CO2 equiva-lent, for selected sites in the Level-3 network (Table 5), show thatover the 100-year-time horizon soil emissions of N2O and CH4 cantip the balance from greenhouse gas sink to be a net greenhouse gassource, as demonstrated for the wetland FI-Lom and grazedgrassland UK-EBu. The CH4 sink strength of the two forestsameliorates some (DK-Sor) or all (FI-Hyy) of the warming effectof N2O. The crop rotation on the lightly tilled silt loam at FR-Gri,appears to be the largest sink for CO2 in this group of sites. The lack ofdense vegetation cover and tillage probably contributed to this. Forsuch a crop system the full rotational cycle and export of C by theharvested vegetation needs to be included in this analysis. Althoughthe budgets reported in Table 5 represent a relatively short period,they clearly illustrate the major differences between the systemsstudied, highlighting the role of climatologically, soil, landuse andmanagement differences. The large differences observed provide amajor challenge for simulation of net greenhouse gas exchange byprocess-based models in the ongoing work of NitroEurope.

It should be noted that routinely global warming potentialswere calculated over the 100-year-time horizon (Table 5). But for

from selected Level-3 sites. The global warming potential used for N2O was 298 and

ha�1 y�1) CH4 (as equiv. CO2) (kg ha�1 y�1) Total GHG (t ha�1 y�1)

�246 �6.2

�96 �3.9

42c 2.1

6 �3.6

0 �18.9

5000 4.2

61 �4.2

U. Skiba et al. / Agriculture, Ecosystems and Environment 133 (2009) 139–149148

CH4 emitting peat accumulating wetlands, the persistent seques-tration of CO2 over millennia needs to be considered. Lengtheningthe time span from 100 years to thousands of years has shown thatmost of the northern wetlands would be net greenhouse sinks,thereby contributing to global climatic cooling (Frolking et al.,2006).

In agricultural systems, N budgets are dominated by the N inputthrough fertilisation and loss through harvest. For example, at thearable crop rotation field, FR-Gri, differences in N budget wereobserved for the different crops within the rotation (Cellier, pers.comm.). This was due to both the succession of crops (e.g. slurryapplication is made once every 3 years, but contributes to the Nbalance of the whole rotation by organic N input) and to theclimatic conditions of each year. In 2004, there was a small surplusof N for barley. For maize there was a large surplus of N because thecattle slurry applied before maize sowing was very rich in mineraland organic N. For wheat, the balance was unexpectedly positivebecause fertilizer application rate was calculated for the expectedstandard yield of 8.5 tons, but the observed yield was only 7.5 tons.Moreover, when calculating the fertilisation rate, the farmer doesnot account for any inputs by atmospheric N deposition. Forintensively grazed grasslands, livestock numbers, growth rates ofthe young livestock and biomass yield need to be quantified well inorder to produce accurate N budgets, while for legume richgrasslands N fixation remains one of the major uncertainties inoverall N budgets (Ammann et al., 2009).

N budgets for each of the forests, low input grasslands andwetlands studied are estimated here to be dominated by atmo-spheric N deposition. For the DE-Hog forest, N input by deposition(40 kg N ha�1 y�1) is similar to the N loss by leaching, nitrificationand denitrification (37 kg N ha�1 y�1). This forest appeared to havereached saturation. In contrast the beech forest DK-Sor, the Pusztagrassland, HU-Bug, and the wetland UK-AMo are estimated to be Nsinks. Inputs by deposition and export by gaseous loses andleaching as kg N ha�1 y�1 were 26.6/2.4 for DK-Sor, 14.5/0.7 forHU-Bug and 10/2 for UK-AMo. While there remain uncertainties ineach of the component estimates listed in Table 4, apart from theneed to obtain longer datasets, the remaining challenges are toprovide robust estimates of N fixation and denitrification to N2 toeach of the Level-3 sites.

The data presented here show that for each Level-3 site chosenin this network the flow of N differs, depending on ecosystem,climate and pollution load. As a result the measurementprogramme of reactive N species needs to be tailored to thespecific requirements of each site.

5. Conclusion

The NitroEurope Level-3 sites have started to provide a detailedpicture of the reactive N and greenhouse gas exchange acrossvegetation types, agricultural practices and the European climate.The data will be used to assess and improve existing plot modelsthat simulate reactive N and greenhouse gas exchange (e.g. DNDC,Century, DayCent). These models will form the basis for upscalingto the European level and will supply a very detailed picture ofpresent and future N transformations and N budgets in Europe. Thecomponents of the N budgets are very different according to theecosystems and management practices, with the latter demon-strating the central role of human intervention in the N cycle.

Acknowledgements

The authors gratefully acknowledge funding of the NitroEuropeIP by the European Commission, as well as supporting funds fromnational funding bodies, including the UK Natural EnvironmentResearch Council and the UK Department for Environment Food

and Rural Affairs. This paper represents a contribution to theinternational land ecosystems atmosphere process study (iLEAPS)of the International Geosphere Biosphere Programme (IGBP). Weare grateful for supporting travel funds through the EuropeanScience Foundation NinE programme and COST 729.

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