Environmental Pollution 102, S1 (1998) 327–335 reformated layout similar to printed version Spatial variation in annual nitrogen deposition in a rural region in Switzerland a b a a a Werner Eugster ∗, Silvan Perego , Heinz Wanner , Alex Leuenberger , Matthias Liechti , Markus Reinhardt c , Peter Geissbühler d, Marion Gempeler a , and Jürg Schenk a a University of Bern, Institute of Geography, Hallerstrasse 12, CH-3012 Bern, Switzerland bSwiss Federal Institute of Technology, Air and Soil Pollution Laboratory, CH-1015 Lausanne, Switzerland c Swiss Meteorological Office, Krähenbühlstrasse 58, CH-8004 Zürich, Switzerland dPaul Scherrer Institute, CH-5232 Villigen PSI, Switzerland Received 27 March 1998; accepted 9 September 1998 Abstract We studied the spatial variation in annual nitrogen deposition to the Seeland region of Switzerland by combining (1) mesoscale atmospheric modeling of gaseous dry deposition, (2) wet deposition data measured within the study area as well as published data, and (3) estimates of dry deposition of aerosol particles based on concentration measurements and sedimentation samples from a near-by region. Gaseous 1 1 dry deposition shows greatest regional variation. Total nitrogen input to this region is in the range of 22–51 kg N ha− yr− (13–42 kg N gaseous dry deposition; 7–8 kg N wet deposition; 2 kg N dry deposition of aerosol particles). Gaseous dry deposition is dominated ≈ 1 1 by reduced nitrogen components from agricultural sources. The highest nitrogen inputs were found over lakes (51 kg N ha− yr− ). Our 1 1 results indicate that current nitrogen loads exceed the nutrient critical loads by several kg N ha− yr− in all nitrogen-sensitive ecosystem types occurring in our 70 50 km2 study area. × Keywords: Nitrogen input; dry deposition; pollutant dispersion modeling; critical loads; Switzerland Introduction loads for various ecosystem types in our study area. Nitrogen is essential for biomass production in undisturbed natural ecosystems as well as in intensely managed agro- Study area ecosystems. The nitrogen cycle is therefore regarded as one of the major ecological cycles, together with the carbon and Selection of a study area water cycles (Larcher, 1995), and has received great atten- In a previous study (Hesterberg et al., 1996; Eugster and tion in scientific research since the very beginning of eco- Hesterberg, 1996; Neftel et al., 1994; Eugster, 1994) con- logical studies. ducted in a non-urban, densely populated rural area of In this paper we focus on the atmospheric inputs of ni- Switzerland in the lower Reuss valley north of Lucerne trogen in three forms, (1) gaseous dry deposition, (2) wet and west of Zürich, it was concluded that reduced nitro- deposition (input by precipitation), and (3) dry deposition gen compounds, primarily gaseous ammonia (NH , 45.9%) of aerosol particles (sedimentation and impaction/diffusion 3 and wet deposited ammonium (NH+, 19.8%), were more of dry and wettened aerosol particles, but excluding those 4 abundant than oxidized nitrogen compounds in the total an- incorporated in precipitation), to obtain the current nitrogen nual average deposition of 28.3 kg N ha 1 year 1. Be- loads in a densely populated rural part of Switzerland. The − − cause of the local importance of pig production, high am- current loads are then compared with the nutrient critical monia losses from stables and slurry applications raised the question whether the results obtained in that area were also ∗Corresponding author. Tel.: +41 31 631-8551; fax: +41-31-631-8511; e-mail: [email protected] representative of other rural parts of Switzerland. There- 327 328 W. Eugster et al. / Environmental Pollution 102, S1 (1998) 327–335 fore we selected a study area that had (1) lower cattle den- Methods and data sity, (2) more diverse agricultural land use, (3) less influ- ence from large urban centers (like Zürich), and (4) in- To obtain annual nitrogen deposition amounts on the re- corporated both source regions and potential receptor areas gional scale, we used various scale- and process-specific with semi-natural and natural undisturbed ecosystems that concepts and methods for the three forms of nitrogen de- are expected to be most vulnerable, i.e. have lowest critical position listed in the introduction. Because of the high de- loads of nitrogen. mand for input data and the heterogeneous quality, avail- ability and temporal coverage of such data, we tried to de- termine an annual average of the time period 1990–1996. The Seeland region For such a 7-year period, we assumed that land use and land cover changes can be neglected whenever information was These criteria were all met in the Seeland region, where only available for a single year. there is an abrupt change from intensive farming in the rural plains of the Seeland in the Southeast to the steep south slope of the Jura mountains in the Northwest. The Dry deposition of trace gases Jura south slope contains extensive mesic to xeric grass- The dynamic mesoscale Metphomod (Meteorology and lands at lower elevations intermixed with deciduous and Photochemistry Model) numerical model (version 1.1; mixed forests on calcareous soil with thin organic soil hori- Perego, 1996) was used to model gaseous dry deposition. zons, small soil nitrogen pools, and negligible influence of Metphomod is a three-dimensional Eulerian model based on groundwater nitrogen supplies due to the porous and well- the primitive equations of fluid dynamics. The atmospheric drained calcareous geology of the Jura mountains. In ad- chemistry module used 39 substances with 82 chemical re- dition, the three lakes of Neuchâtel, Biel and Murten in actions, plus NH3 that does not participate in the photo- this region add a non-terrestrial ecosystem component to the chemical cycle but is essential to obtain credible amounts of study area that was not previousely recognized as receiving gaseous nitrogen deposition in our study area. The chem- considerable inputs of nitrogen from the atmosphere (Rihm, istry follows the RADM mechansim described in Stockwell 1997). (1986) which does not consider aerosol formation. The model domain size was 70 50 km2, with 22 ver- tical layers. Horizontal grid resolution× was 1 1 km2 and Climate vertical spacing was 100 m between 450 and 2650× m a.s.l. The Seeland region has a temperate climate with a bi-modal Topography was described by the RIMINI digital elevation precipitation distribution. Annual precipitation measured at model (Bundesamt für Landestopographie, 1992), which has a horizontal resolution of 250 250 m2 with 1 m ver- the Swiss Meteorological Office (SMO) weather station in × Biel (434 m a.s.l.; Fig. 1a) totals 1090 mm (30-year av- tical resolution. erage 1966–1995) with a primary maximum in December We defined 17 turbulent transport classes and modeled a (120.9 mm) and a secondary maximum in August (115.3 representative day for each class selected from the period 1990–93 (see below). The frequency of occurrence was mm). Annual average air temperature is 9.23◦C with an ab- used as a weighting factor to obtain annual deposition val- solute maximum and minimum of 30.70◦C and –7.90◦C, respectively. ues. For each representative day we worked out emission in- ventories of all trace gas species of interest, and all available meteorological input data. Each model run started at noon Land use of the previous day and was stopped at midnight of the se- lected day (36-hour runs). The first 12 hours were used for The land use classification is based on a multispectral- initialization of the model with the current meteorological multitemporal analysis of four Landsat 5 TM scenes conditions and obtaining the appropriate spatial distribution (Liechti, 1996) on 30 April, 17 June, 3 July and 4 August of locally emitted and advected trace gases within the model 1994. It revealed the following land use pattern in the cen- domain. tral section of our study area: 45.2% arable lands; 32.7% forests and woodlands; 12.4% lakes and rivers; 6.9% ur- Deposition module ban areas and infrastructure; 2.8% unclassified land. The arable land consists of 62.2% meadows and pastures; 15.9% The deposition module of Metphomod is a three-resistor cereals; 11.4% maize; 4.5% sugar beets; 2.5% orchards; chain with an aerodynamic resistance Ra, a laminar 1.3% vineyards; 1.1% potatoes; and 1.1% other agricultural boundary-layer resistance Rb, and a canopy resistance Rc crops (especially vegetables that were not distinguishable (Erisman et al., 1994; Eugster and Hesterberg, 1996). For on Landsat TM images). Rb the approximation by Wesely and Hicks (1977) was W. Eugster et al. / Environmental Pollution 102, S1 (1998) 327–335 329 a d Basel Zürich 4 Biel 3 Lucerne 1 Bern 2 5 Geneva b e c f Figure 1: (a) Location of the model domain in Switzerland (red box), with the weather stations (triangles) used in the classification of tur- bulent transport classes (see text for details). The size of the red box corresponds to the model domain shown in panels b–f. (b) Total annual nitrogen deposition of gaseous dry deposition, wet deposition, and aerosol particle deposition (sedimentation and impaction/diffusion) in 1 1 1 1 kg N ha− year− . (c) Total annual dry deposition of oxidized gaseous nitrogen (NOy) in kg N ha− year− . (d) Ratio of dry deposition of oxidized nitrogen (NOy-N) vs. dry deposition of reduced nitrogen (NH3-N), annual average. (e) Ratio of emission vs. deposition of total nitrogen in the study area, annual average. Thin lines in panels (a)–(e) indicate the 1:1 ratio; broken lines are topographic contour lines; and bold lines are rivers and lake shore lines. (f) Regions defined for regional averaging. Yellow indicates Seeland rural plains; red indicates the Jura south slope; deep blue indicates lakes; pink indicates urban area including Bern; green indicates forested hills (Jolimont, Schaltenrain, Bargenholz); gray shading shows the topography.