K. Blanck

Vegetation and soil biota response to experimentally-changed nitrogen inputs in coniferous forest ecosystems of the NITREX project

Enhancement of the atmospheric N deposition is a serious threat for the structure and function of ecosystems. Here we evaluate the ecological effects of excess N with respect to changes in vegetation and soil biota in a series of experiments along a N gradient across Europe. The aim of this project (NITREX: N saturation EXperiments) is to assess the risk of N saturation and the reversibility of N saturation. At the experimental sites with a low-to-moderate input, N was added (n = 3), while at sites with a high input, N was removed by means of a transparent roof (n = 4). The experiments started between 1989 and 1991. Across the N gradient a positive correlation was found between the N concentration in deposition or soil solution with the N concentration in the needles and in general a negative correlation with the base cations K and Mg. In the N-addition plots there was a tendency towards a decreasing nutrient status of the needles, whereas at one site N-removal led to an improvement. Addition of N hardly affected fine-root biomass production, whereas signs of growth increase were recorded when the input was reduced. Tree growth was accelerated upon input reduction at two of three sites. Manipulation of N input did not alter the decomposition rate, although significant differences between sites were noted. Manipulation of the N input hardly affected the biomass of fungi and bacteria, but a negative relation between the N-addition and part of the soil fauna may be present among sites.

Input-output budgets at the NITREX sites

The NITREX project entails large-scale manipulation of nitrogen deposition to whole, forested ecosystems at eight sites in Europe. Nitrogen is added at sites with low-to-intermediate ambient N deposition and removed at sites with high deposition. Changes in outputs of dissolved constituents reflect the integrated effects on ecosystem processes and changes in storage. At sites exhibiting clear symptoms of nitrogen saturation prior to treatment, the nitrate flux in leachate and runoff responded rapidly to changes in deposition. Reduced deposition gave immediate improvement in water quality. At sites with low nitrogen losses prior to treatment, the response to increased deposition was small and delayed. Together the results point to significant hysteresis in output response related to the nitrogen status of the ecosystem. The input-output budgets indicate that forest ecosystems require many years to adjust to changes in nitrogen deposition.

NITREX: responses of coniferous forest ecosystems to experimentally changed deposition of nitrogen

Abstract In large regions of Europe and eastern North America atmospheric deposition of inorganic nitrogen compounds has greatly increased the natural external supply to forest ecosystems. This leads to nitrogen saturation, in which availability of inorganic nitrogen is in excess of biological demand and the ecosystem is unable to retain all incoming nitrogen. The large-scale experiments of the NITREX project (nitrogen saturation experiments) are designed to provide information regarding the patterns and rates of responses of coniferous forest ecosystems to increases in N deposition and the reversibility and recovery of impacted ecosystems following reductions in N deposition. The nitrogen input-output data from the NITREX sites are consistent with the general pattern of nitrogen fluxes from forest ecosystems in Europe. At annual inputs of less than about 10 kg ha−1 year−1, nearly all the nitrogen is retained and outputs are very small. At inputs above about 25 kg ha−1 year−1 outputs are substantial. In the range 10–25 kg ha−1 year−1 these forest ecosystems undergo a transition to nitrogen saturation. The 10 kg ha−1 year−1 apparently represents the minimum threshold for nitrogen saturation. The NITREX experiments indicate that nitrogen outputs respond markedly across the 10–25 kg ha−1 year−1 range of inputs. In contrast, the nutrient concentrations in foliage, a measure of tree response, is delayed by several years. Nitrogen saturation can apparently be induced or reversed within only a few years, at least with respect to the commonly used diagnostic of nitrogen saturation-nitrogen output in leachate or runoff.

Response of soil water chemistry to experimental ‘clean rain’ in the NITREX roof experiment at Solling, Germany

Abstract In the Solling experimental forest in central Germany a manipulation study with three roof constructions of 300 m 2 area each is underway in a 60-year-old Norway spruce ( Picea abies (L.) Karst.) forest. The experimental treatments are: 1. (1) ‘clean rain’ roof with simulated pre-industrial throughfall; 2. (2) control roof for roof effects alone, without manipulation; 3. (3) drought/rewetting roof, for simulation of extreme drought; 4. (4) control without roof. The clean rain roof is part of the NITREX project; we investigate the time scales and biological reactions of the system which lead to reversal of nitrogen saturation. The data include soil water chemistry for 2 years prior to treatment and 1.5 years of treatment. The response to reduced inputs of N and S was lower concentrations in soil solution of N ions, sulfate and Al down to 40 cm depth. Nitrogen ions virtually disappeared from the soil water in the rooting zone (less than 70 cm soil depth) during the 1.5 years of clean rain application. If this trend continues, nitrogen leaching will cease within a relatively short period of time, and N-saturation will be reversed. Future studies will focus on the reaction of the forest to the significantly reduced N levels in soil solution.

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.

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.

Nitrex: The timing of response of coniferous forest ecosystems to experimentally-changed nitrogen deposition

In large regions of Europe and eastern North America atmospheric deposition of inorganic nitrogen (N) compounds has greatly increased the natural external supply to forest ecosystems. This leads to N saturation, in which availability of inorganic N is in excess of biological demand and the ecosystem is unable to retain all incoming N. The large-scale experiments of the NITREX project (NITRogen saturation EXperiments) are designed to provide information regarding the patterns and rates of responses of coniferous forest ecosystems to increases in N deposition and the reversibility and recovery of impacted ecosystems following reductions in N deposition.The timing of ecosystem response generally followed a hypothesized “cascade of response”. In all sites N outputs have responded markedly but to very different degrees within the first three years of treatment. Within this time significant effects on soil processes and on vegetation have only been detected at two sites. This delayed response is explained by the large capacity of the soil system to buffer the increased N supply by microbial immobilization and adsorption. We believe that this concept provides a framework for the evaluation and prediction of the ecosystem response to environmental change.

Hydrochemical input-output budgets for a clean rain and drought experiment at Solling

In order to evaluate the response of an acidified forest ecosystem to reduced atmospheric deposition and seasonally limited water supply, hydrochemical fluxes were estimated for the roof experiments at Solling, Germany. The experiments were carried out in a 58 year-old spruce (Picea abies (L.) Karst.) plantation from 1990 to 1994. Clean rain was applied to the soil surface by reducing the nitrogen, sulphur and proton inputs to 67%, 56% and 74% of the loads in ambient throughfall. The drought/rewetting experiment was carried out by artificially modifying weather extremes of drought and rewetting events. Hydrochemical input-output budgets, based on element fluxes in throughfall and in soil seepage water, have been established for major elements. Results show that on this highly acidified soil, atmospheric deposition widely influenced element fluxes in the soil seepage water. Consequently, hydrochemical budgets altered when inputs of acidity and nitrogen were reduced. After one year of the clean rain treatment nitrate output was considerably reduced. The nitrate and proton budgets changed from surplus to deficit. The sulphur deficit was further increased by reducing sulphur input. In the course of the drought/rewetting experiment, a distinct increase of nitrate budgets was found. A possible nitrification or an acidification pulse was not observed.

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.