Deposition Loads of Sulphur and Nitrogen in Germany

High spatial resolution maps of deposition loads in Germanyare produced as an input for abatement strategy research andfor critical loads exceedance calculations on a nationalscale. In this paper methods ofmapping total deposition loads in Germany and preliminarymaps of nitrogen and sulphur deposition loads for the year1993 are presented. A comparison of these mapping resultswith EMEP deposition mapping results has been carried out.The differences in the results of the German national and theEuropean EMEP mapping, due to different databases anddifferent methods, are quantified and discussed. Highresolution maps of deposition loads are compared to Europeanlow resolution maps on the same temporal and spatial scale,assuming that on average both should lead to similar results.However, the average differencescalculated for 23 EMEP 150 × 150 km² grid cells over Germanywere found to be 33% higher for sulphur (S) total depositionby the German method 65% higher for S dry deposition and1% lower for S wet deposition. The German results fornitrogen (N) total deposition are 2% higher than the EMEPresult 22% higher for N dry deposition and 10% lower for Nwet deposition.     

Experimental Assessment of Atmospheric Ammonia Dispersion and Short Range Dry Deposition in a Maize Canopy

In order to study the effect of thevegetation structure on atmospheric ammonia(NH₃) dispersion and deposition, anexperiment was set up near Paris, in July 1997.Between 12 and 162 m downwind of a 200 m line-source releasing 600 to 1200 g NH₃hr^-1placed at the top of a maize canopy, NH₃concentration was measured, within and above thecanopy, with a set of 30 active, acid-coateddenuders over periods of 2 to 3 hr. Eight datasets were collected over a one-month period.NH₃ concentration decreased sharply withdistance to the source, from up to800 g NH₃ m^-3 at 12 m, to lessthan 10 g NH₃ m^-3 at 162 m andshowed strong vertical gradients. Within thecanopy, the concentration scaled using thefriction velocity, the canopy height, and thesource strength, exhibited a universal power lawrelationship as a function of the normaliseddownwind distance from the source. A mass balancemethod and a resistance model approach were usedas independent estimates of the cumulateddeposition at 162 m downwind from the source,which range between 1 and 29% of the emittedNH₃. Both methods agreed approximately inmagnitude. A sensitivity analysis showed that thecuticular uptake and the compensation point aremajor parameters that needs to be bettercharacterised under high NH₃ concentrationif one wants to improve NH₃ short-rangedeposition modelling.     

A Chronology of Nitrogen Deposition in the UK Between 1900 and 2000

Measurements of the concentrations of nitrogen compounds in air and precipitation in the UK have been made since the mid-19th century, but no networks operating to common protocols and having traceable analytical procedures were established until the 1950s. From 1986 onwards, a high-quality network of sampling stations for precipitation chemistry was established across the UK. In the following decade, monitoring networks provided measurement of NO₂, NH₃, HNO₃ and a satisfactory understanding of the dry deposition process for these gases allowed dry deposition to be quantified. Maps of N deposition for oxidized and reduced compounds at a spatial scale of 5 km × 5 km are available from 1986 to 2000. Between 1950 and 1985, the more limited measurements, beginning with those of the European Air Chemistry Network (EACN) provide a reasonable basis to estimate wet deposition of NO ₃ ^– –N and NH ₄ ⁺ –N. For the first half of the century, estimates of deposition were scaled with emissions assuming a constant relationship between emission and deposition for each of the components of the wet and dry deposition budget at the country scale. Emissions of oxidized N, which dominated total nitrogen emissions throughout the century, increased from 312 kt N annually in 1900 to a peak of 787 kt for the decade 1980–1990 and then declined to 460 kt in 2000. Emissions of reduced N, largely from coal combustion were about 168 kt N in 1900, increasing to a peak of 263 kt N in 2000 and by now dominated by agricultural sources. Reduced N dominated the deposition budget at the country scale, increasing from 163 kt N in 1900 to 211 kt N in 2000, while deposition of oxidized N was 66 kt N in 1900 and 191 kt N in 2000. Over the century, 68 Mt (Tg) of fixed N was emitted within the UK, 78% as NO _x , while 29 Mt of nitrogen was deposited (43% of emissions), equivalent to 1.2 t N ha^–1, on average, with 60% in the reduced form. Deposition to semi-natural ecosystems is approximately 15 Tg N, equivalent to between 1 and 5 t N ha^–1, over the century and appears to be accumulating in soil. The N deposition over the century is similar in magnitude to the total soil N inventory in surface horizons.     

Nitrous Oxide Concentrations in the Soil of a Mown Grassland: Comparison of Model Results with Soil Profile Measurements

Nitrous oxide (N₂O) concentrations in the soilprofile of a fertilized grassland on the Swiss plateauwere measured at irregular intervals during one year.Air samples were taken from air-permeable tubesinstalled at depths between 2 and 100 cm belowthe surface. Highest concentrations in thetopsoil were observed following precipitationafter fertilization. In the subsoil,concentrations were highest when the soil watercontent fell below about 92% after a wet periodand mineral nitrogen was available from priorfertilization. N₂O concentrations in thetopsoil were simulated with the process-basedgrassland ecosystem model PaSim 2.5 (PastureSimulation Model) and compared with measured soilN₂O concentrations. The model simulated wellthe concentration peaks after fertilizeradditions and the concentrations in winter. Butthe simulated baseline concentrations during thegrowing season were overestimated. Possiblecauses for this discrepancy are discussed andsuggestions are made to improve the model.     

An Automated Wet Deposition System to Compare the Effects of Reduced and Oxidised N on Ombrotrophic Bog Species: Practical Considerations

Critical N loads for ombrotrophic bogs, which often contain rare and N-sensitive plants (especially those in lower plant groups: lichens, mosses and liverworts), are based on very few experimental data from measured, low background N deposition areas. Additionally the relative effects of reduced versus oxidised N are largely unknown. This paper describes an automated field exposure system (30 km S. of Edinburgh, Scotland) for treating ombrotrophic bog vegetation with fine droplets of oxidised N (NaNO₃) and reduced N (NH₄Cl). Whim Moss exists in an area of low ambient N deposition (ca. 8 kg N ha^–1 y^–1), the sources and quantification of which are described. The wet N treatment system is run continuously, and is controlled/activated by wind speed and rainfall to provide a unique simulation of real world treatment patterns (no rain=no treatment). Simulated precipitation is supplied at ionic concentrations below4mMin rainwater collected on site. Treatments provide a replicated dose response to 16, 32 and 64 kg N ha^–1 y^–1 adjusted for ambient deposition (8 kg N ha^–1 y^–1). The 16 and 64 kg N ha^–1 y^–1 are duplicated with a P+K supplement. Baseline soil chemistry and foliar nutrient status was established for all 44 plots for Calluna vulgaris, Sphagnum capillifolium, Hypnum jutlandicum and Cladonia portentosa.     

Characterizing the Spatial Pattern of Soil Carbon and Nitrogen Pools in the Turkey Lakes Watershed: A Comparison of Regression Techniques

There is considerable spatial heterogeneity in organic carbon (C), total nitrogen (N), and potentially mineralizable nitrogen (PMN) pools in the soils of the Turkey Lakes Watershed. We hypothesized that topography regulates the spatial pattern of these pools through a combination of static factors (slope, aspect and elevation), which influence radiation, temperature andmoisture conditions, and dynamic factors (catenary position,profile and planar curvature), which influence the transport ofmaterials downslope. We used multiple linear regression (MLR)and tree regression (TR) models as exploratory techniques todetermine if there was a topographic basis for the spatialpattern of the C, N and PMN pools. The MLR and TR modelspredicted similar integrated totals (i.e., within 5% of eachother) but dissimilar spatial patterns of the pools. For thecombined litter, fibric and hemic layer, the MLR models explaineda significant portion of the variance (R² = 0.38, 0.23 and0.28 for C, N and PMN, respectively), however, the residuals werelarge and biased (the smallest contents were over-predicted andthe largest contents were under-predicted). The TR models (9-branch), in contrast, explained a greater portion of the variance (R² = 0.75, 0.67 and 0.62 for C, N and PMN, respectively) and the residuals were smaller and unbiased. Based on our sampling strategy, the models suggested that static factors were most important in predicting the spatial pattern of the nutrient pools. However, a nested sampling strategy that included scales where both static (among hillslopes) and dynamic (within hillslope) factors result in a systematic variation in soil nutrient pools may have improvedthe predictive ability of the models.     

The Effect of Nitrogen Enrichment on the Carbon Sink in Coniferous Forests: Uncertainty and Sensitivity Analyses of Three Ecosystem Models

Estimates of the global carbon sink induced by nitrogen enrichment range vary widely, from nearly zero to 2.3 Gt C year^-1. It is necessary to reduce this uncertainty if we are to make accurate predictions of the future magnitude of the terrestrial carbon sink. Here, we present a Monte Carlo approach to uncertainty and sensitivity analysis of three ecosystem models, Century,BGCand Hybrid. These models were applied to a coniferous forest ecosystem in Sweden. The best estimate of the change in total carbon content of the ecosystem with the cumulative change in nitrogen deposition over 100 years, C_total/N_deposition was 20.1 kg C (kg N)^-1 using the pooled mean, with a pooled standard deviation of 13.8 kg C (kg N)^-1. Variability in parameters accounted for 92% of the total uncertainty in C_total/N_deposition, and only 8% was attributable to differences between models. The most sensitive parameters were those which controlled the allocation of assimilate between leaves, roots and stem. In particular, an increase in allocation to fine roots led to a large reduction in C_total/N_deposition in all models, because the fine roots have a very high turnover rate, and extra carbon allocated there is soon lost through mortality and decomposition.