Lennart Rasmussen

Critical loads for nitrogen deposition on forest ecosystems

Critical loads for N deposition are derived from an ecosystems anion and cation balance assuming that the processes determining ecosystem stability are soil acidification and nitrate leaching. Depending on the deposition of S, the parent soil material, and the site quality critical N deposition rates will range between 20 to 200 mmol m−2 yr−1 (3 to 14 kg ha−1 yr−1) on silicate soils and reach 20 to 390 mmol m−2 yr−1 (3 to 48 kg ha−1) on calcareous soils.

Introduction to the NITREX and EXMAN projects

Abstract European concern over the cause and consequences of forest decline, acidification of soils and surface waters, and the nutrient enrichment of terrestrial and aquatic ecosystems, led to the establishment of the NITREX (Nitrogen saturation experiments) and EXMAN (Experimental Manipulation of Forest Ecosystems in Europe) projects, two research networks of large-scale manipulation experiments under the auspices of the EU Commission of European Communities. NITREX comprises 10 experiments at 8 sites in 7 countries at which nitrogen is either added to or removed from ambient atmospheric deposition to simulate major changes in nitrogen deposition. EXMAN comprises experiments at 6 sites in 4 countries at which ambient atmospheric deposition is experimentally altered in chemical composition and/or quantity. The ultimate goal of this research is to contribute to the scientific basis required for the refinement of EU policy on atmospheric quality, and the legislation which will emanate from that policy.

Nitrification in Forest Soils: Effects from Nitrogen Deposition on Soil Acidification and Aluminum Release

From the use of NH4 + fertilizers on arable land it is well known that nitrification is an acidifying process, when N is in excess of plant demand. A considerable amount of NO3 − is leached, and in agricultural practice lime is added regularly to buffer the protons produced from nitrification.

Nitrogen mobility in a nitrogen limited forest at Klosterhede, Denmark, examined by NH4NO3 addition

Abstract The fate and effects of increased N deposition were investigated by experimental manipulation in a mature Norway spruce plantation at Klosterhede, western Denmark. Ambient N deposition was 23 kg N ha −1 year −1 . Addition of 35 kg N ha −1 year −1 as ammonium nitrate to a 500 m 2 plot was carried out monthly by handspraying. The soil solution chemistry responded promptly to the nitrogen application. Nitrate concentrations increased at all depths, and nitrate leaching increased from 0.3 to 2.3 kg N ha −1 year −1 . Nitrate leached during winter and spring when water transport was highest. Despite the increased nitrate leaching, 92% of the nitrate input was retained. The ammonium input was completely retained within the system, and soil water concentrations of ammonium were only slightly changed by the nitrogen addition. Mineralisation was the major source of ammonium in the soil. An increase of exchangeable ammonium could account for 20% of the added ammonium. No changes in concentrations of other major ions due to the nitrogen addition were detectable. There was no detectable growth response or change in the nitrogen concentration of the foliage after the first growing season with nitrogen addition although the foliage was nitrogen deficient. A longer period of treatment is probably necessary to demonstrate the response of biological processes. The implications of the direct increase in nitrate leaching at nitrogen-limited conditions are discussed.

Atmospheric deposition and soil acidification in five coniferous forest ecosystems: a comparison of the control plots of the EXMAN sites☆

Abstract The five sites of the EXMAN project conducted in Ireland, Denmark, Netherlands, Northern and Southern Germany were compared regarding (1) the marine and anthropogenic components of deposition, (2) the acidification of soil and consequences for Al status, and (3) the nitrate load of seepage. The marine deposition decreases with decreasing rates from the coast inland. It vanishes at a distance of more than 600 km of the sea. The most part of sea salt input in conifer forests is due to dry deposition. Sea salt Mg2+ in throughfall near the coast by far exceeds the demand of trees. The anthropogenic deposition of N and S at the Irish site is about 20%; at the Danish site, about 60% of that at the inland sites. At the Irish site, the anthropogenic deposition is the two- to threefold of the preindustrial deposition. NH4+ prevails at all sites as acid component in throughfall, controlling the pH values that vary between 3.9 and 5.3. In drainage water leaving the root zone, the mean pH values vary only between 4.1 and 4.4. Proton budgets for the forest floor have shown that N turnover dominates as a proton source at the inland sites, whereas at the coastal sites the dominant source results from the production of organic acids. The main proton sink is due to H+ output. Proton budgets for the total root zone indicate that an important proton gain is caused at the most sites by proton excretion of the roots in connection with base cation uptake. In addition, at some sites, the release and output of SO42− appears to be a considerable proton source. At all sites, buffering and output of Al represent the main proton sink. The Al solubility of each layer of the EXMAN sites was compared with the solubility of a synthetic gibbsite. Al saturation exists only at the lower boundary of the main root zone. In the soil layers above, there is an undersaturation that is largest at the humus layer efflux. The relationship between Al dissolved and Al adsorbed, both expressed in cation percentages, is rather weak for the coastal sites in contrast to the inland sites. The importance of the ionic strength effect of sea salt input is discussed with respect to the deep reaching Al saturation and acidification of the soils. Moderate to strong Al stress is indicated at all sites in the mineral soil. The nitrate load of the seepage water depends on the N status of the ecosystems rather than N deposition when the throughfall exceeds 20 kg N ha−1 yr−1. An attempt was made to classify the EXMAN sites with respect to the N status with the aid of the N flux gradient by depth.

Effect of drought experiments using roof installations on acidification/nitrification of soils

Within the framework of the EU-funded EXMAN (EXperimental MANipulation of Forest Ecosystems) project drought experiments were conducted at four different European Norway Spruce (Picea abies (L.) Karst.) plantations in the year 1992–1995. The aim of the project was to investigate if there is an additional risk of soil acidification due to nitrification/acidification pulses after extended periods of summer droughts. The sites included (Ballyhooly/BH, southwest of Ireland. Klosterhede/KH, west coast of Denmark, Hoglwald/HW, northwest of Munich, and Solling/SL, central Germany) cover considerable gradients of climatic and air pollution regimes. Artificial droughts were produced by using different types of roof constructions installed below the forest canopy (non-permanent constructions in BH and HW, permanent installations in KH and SL). Each drought was started in spring time and dry conditions were maintained until the soil water tension was below a potential of −700 hPa at 70 cm depth in the mineral soil. Results given are focused on changes in the soil solution concentrations during the rewetting of severely dried out soils. No marked nitrification pulses were observed after any of the droughts carried out at any site. Only single lysimeters/sample locations showed the hypothesized reaction with increasing aluminum concentrations and decreasing pH values, but spatial heterogeneity was high during rewetting. In the first soil solution samples after the drought, occasionally distinct peaks of NH4+, DOC and K+, and partly also Norg and HPO42− appeared. For the BH site the respective potassium concentrations increased 10 to 20 times compared to the level of the control plot. Presented data indicate a certain potential for nitrification pulses at the BH and KH site, but probably due to a fast root uptake, below the rooting zone no nitrate was determined. At the HW site, the drought induced reactions in the soil solution composition were only visible in humus water samples. At SL, nitrate concentrations were generally reduced at the drought plot. During rewetting in 1992 samples from a single lysimeter indicated a clear nitrification pulse. Applied flux calculations for SL showed a significant increase of the nitrate budget but a distinct decrease of the potassium budget. It is concluded that drought phases can influence the element cycling, but there seems to be no risk that forest soils will be subjected to pronounced acidification after summer droughts.

A new method for estimation of dry deposition of particles based on throughfall measurements in a forest edge

Abstract Throughfall measurements are very often used to calculate the atmospheric input to ecosystems. Such attempts are normally complicated by canopy interaction processes which are difficult to assess. This study presents an approach to calculate dry deposition and canopy leaching of Ca 2+ , Mg 2+ and K + to forests. The calculations are based on throughfall measurements in a forest edge of a spruce stand and by use of Na + as a model substance for dry deposition of particles. This ‘forest edge approach’ is compared with an approach based on equal Na + to base cation ratios in wet and dry deposition (‘wet/dry ratio approach’), which has been widely used in the literature. Our calculations show that the wet/dry ratio approach may overestimate the dry deposition of Ca 2+ , Mg 2+ and K + by up to 100% and leaching will correspondingly be underestimated. The assumptions underlying the different approaches and the use of throughfall measurements to estimate dry deposition are discussed and it is suggested that throughfall measurements in forest edges may be a valuable improvement to studies of atmospheric deposition in forests.

Field-scale ‘clean rain’ treatments to two Norway spruce stands within the EXMAN project—effects on soil solution chemistry, foliar nutrition and tree growth

Abstract ‘Clean rain’ treatments were performed in spruce forests at Klosterhede, Denmark, and Solling, Germany, by replacement of ambient throughfall water with artificial throughfall reducing the load of S (55–95%), N (70–90%) and acidity (ca. 90%). The induced strong reductions in SO42− input caused corresponding strong reductions in soil solution concentrations at Klosterhede and to a minor extent at Solling. The difference in response to the reduced input is suggested to be largely a consequence of differences in storage capacity between the two sites. Reduced input of N at Solling caused strong and immediate reduction in the soil solution concentration. At Klosterhede, N concentrations in the soil solution were already low prior to the treatment and no effects were recorded. The effects on soil acidification reflected by Al and H concentrations were slow and small although a slight improvement in acid neutralising capacity (ANC) was observed at Klosterhede, mainly caused by a strong reduction in strong acid anions. The input reductions decreased the general ion activity in the soil solution and the accumulation of base cations at both sites were consequently increased. Within the 6 yr of treatment at Klosterhede and the 2 yr at Solling, the effects on the soil solution chemistry were generally moderate. Accordingly, no significant changes in vegetation were observed apart from a water induced increase in growth rate at Klosterhede.

Airborne heavy metal pollution in the environment of a danish steel plant

A survey of heavy metal deposition was carried out in the vicinity of a Danish steel plant. Bulk precipitation and transplanted lichen (Hypogymnia physodes (L.) Nyl.) were sampled at 12 stations in the environment before and after the production had been converted from open-hearth furnaces to electric-arc furnaces. The samples were analyzed for Cd, Cr, Cu, Fe, Mn, Pb and Zn.The results show that heavy metal pollution from the steelworks still is severe and that it follows a decreasing power curve when the distance to the steelworks is increased. However, a reduction in the deposition of heavy metals close to the steelworks has been observed, pointing to the conclusion that the change from a situation of emission through a 46 m stack without any filter to emission from the electric arc furnaces equipped with bag-filters has lead to changes in the emission. At the sampling stations with the highest deposition levels measured in bulk precipitation the corresponding concentrations in the lichens were relatively lower indicating a change in particle size distribution. Within each station there was a direct proportionality of metal concentrations in lichens and atmospheric fallout measured in bulk precipitation.

Biological response of five forest ecosystems in the EXMAN project to input changes of water, nutrients and atmospheric loads

Abstract In five coniferous forest ecosystems in Europe, water and nutrient supply, as well as atmospheric loads, were manipulated for 3 or 4 years. Water supply was optimised and nutrients were added according to tree demand in optimal proportions relative to the ambient N supply. Tree growth was strongly enhanced by optimal water supply but not further enhanced by nutrient additions. The nutritional balance in trees was improved for P and K. The increased water and nutrient supply retarded needle shedding in autumn and diminished root production. To date, the manipulated decrease in N input to the soil has decreased the N content in needles in one stand. Water additions tended to lower N contents at two sites. Large applications of N increased N content in needles even though the N nutrition was already optimal. Liming with dolomite stimulated tree growth only in the nutrient-poor stand, but has generally increased Ca content in needles. The number of species and cover of understorey vegetation has increased considerably by liming and, in some cases, by water addition. Ecosystem manipulation experiments have been shown to be a useful tool for the quantification of the growth effects of traditionally limiting factors. Additionally they give indications of the effects on forest ecosystem processes of future changes in atmospheric loads.