H.F.G. van Dijk

Ecosystem responses to reduced nitrogen and sulphur inputs into two coniferous forest stands in the Netherlands.

Abstract Atmospheric inputs of nitrogen and sulphur were reduced to pre-industrial levels in a nitrogen and sulphur saturated Scots pine (Pinus sylvestris) and Douglas fir (Pseudotsuga menziesii) stand in the Netherlands. Starting in 1989, throughfall water was intercepted by means of a roof and replaced by simulated, clean throughfall water. Underneath the roof two plots were designed to receive either clean water (roof clean) or ambient throughfall (roof control). Outside the roof a second control plot was established (ambient control). Until 1992 a significant roof effect was found owing to differences in water application, but automation of the watering regime significantly reduced this problem. Throughfall chemistry showed a dominance of ammonium to nitrate, whereas the reverse was observed in the soil solution. In the roof clean plots a quick response of soil solution chemistry was observed. The sulphur and nitrogen concentrations in the upper soil layers strongly decreased, as did the fluxes of these elements through the soil profile. As a result, leaching of base cations and ratios of ammonium to various cations decreased. Decomposition studies in the Scots pine stand showed a positive effect of nitrogen deposition on the decomposition rate in the roof control plot compared with the roof clean plot, whereas in the Douglas fir stand no differences between these plots were found. A reduction of atmospheric nitrogen and sulphur deposition in the Scots pine stand increased the species diversity of microarthropods, owing to a decreased dominance of some species at a constant species number. In the Scots pine stand fine root biomass and the number of root tips increased, indicating an increased nutrient uptake capacity. As a result potassium and magnesium concentrations and their ratios to nitrogen in the needles increased. After 4 years of treatment, nitrogen concentrations in the needles of the roof clean plot remained high, but were significantly lower than in the needles of the control plots. In the fourth year of the experiment nitrogen concentrations in the older needles of the clean plot became lower than in the current needles, which is typical for a nitrogen limited forest ecosystem. This is in agreement with the nitrogen flux via litterfall, which was lower onto the roof clean plot than onto the control plots. Until now, no significant changes in nutrient concentrations in the needles of the Douglas fir stand have been observed.

Distribution and decline of endangered herbaceous heathland species in relation to the chemical composition of the soil

AbstractHigh atmospheric deposition of ammonium affects the physical and chemical status of the soil, increasing nitrogen availability, soil acidity and the mobilization of toxic metal ions. To investigate whether and how the decline of several herbaceous plant species in Dutch heathlands is associated with these processes, the chemical composition of the soil on which these species grow has been compared with the soil on which heathland species such asCalluna vulgaris (L.) Hull,Erica tetralix L. andMolinea caerulea (L.) Moench dominate.The discrimination between both soil types was primarily based on differences in pH (H2O), pH (NaCl) and the aluminium/calcium ratio in the waterextracts. Within the group of endangered herbaceous heathland species these soil parameters also varied. This led to a division into 4 groups of species:uDominating species growing on acid soilsHerbaceous species growing together with dominating species on acid soilsHerbaceous species growing together with dominating species on moderately acid soilsHerbaceous species growing together with dominating species on weakly acid soils. This study indicated that, unlike the decline of heather species, the decline of herbaceous species is not likely to be due to increased competition from grass species as a result of eutrophication. Soil acidification and the changed mineral balance in the soil are most likely to be responsible for the decline of all three groups of herbaceous plant species. ei]R F Huettl

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.

Effects of a decrease in atmospheric deposition of nitrogen and sulphur on the mineral balance and vitality of a Scots pine stand in the Netherlands

Abstract Atmospheric deposition of sulphur and nitrogen was reduced to pre-industrial levels in a 45-year-old Scots pine ( Pinus sylvestris ) stand in the Netherlands. The reduction was achieved by throughfall interception by means of a transparent roof. In an experimental plot underneath the roof, trees were sprinkled with demineralized water to which all nutrients except for nitrogen and sulphur were added in the same amounts as present in the throughfall water. Below and outside the roof control plots were established. After 1 year of treatment the first preliminary results were available. The nitrogen and sulphur depositions were reduced from 40 to 2 and from 30 to 4 kg ha −1 year −1 , respectively. The chemical composition of the soil solution showed a quick response. The sulphur, nitrate and ammonium concentrations strongly decreased in the upper soil layers. Consequently, the ratios of ammonium to potassium and magnesium also became lower. The levels of nutrients in the needles have not yet changed. However, nutrient fluxes via the litterfall have decreased slightly, which might indicate some rellocation. No changes in the undergrowth have been noticed so far.

Assessment of the Exposure and Loads of Acidifying and Eutrophying Pollutants and Ozone, as well as their Harmful Influence on the Vitality of the Trees and the Speulder Forest Ecosystem as a Whole

Within the framework of the Dutch Priority Program on Acidification, 10 yr of research was conducted in a Douglas fir stand at Speulder forest. Research was conducted to establish the loads and levels of acidifying and eutrophying pollutants and ozone, to determine forest vitality characteristics and follow growth parameters and nutrient status in time and to determine the effects of reduction of loads and levels by manipulation experiments. Results indicate that during the last 20 yr critical levels for air pollutants have hardly been exceeded except for ozone, which slightly affected assimilation. Elevated nitrogen deposition has caused several adverse effects including (i) inhibited mycorrhizal development, leading to a decreased base cation and phosphorus uptake; (ii) elevated foliage/root (fine and coarse) ratios, making the forest more sensitive to drought and windthrow; (iii) elevated nitrogen and arginine concentrations in the foliage, associated with relative base cation and phosphorus deficiency, and (iv) elevated nitrate leaching polluting the groundwater. High inputs of acidity have caused elevated ratios of Al to base cations, affecting fine root (uptake) and depletion of the readily available Al pool, thus affecting the long-term sustainability. Despite these effects, forest vitality, in terms of defoliation/discoloration, is reasonable and forest growth even increased in response to nitrogen. The exceedances of critical loads for nitrogen and acidity, however, implies a (large) risk for the long-term sustainability of the Speulder forest.

Iron-deficiency chlorosis in Scots pine growing on acid soils

Scots pine (Pinus sylvestris L.) trees growing on acidic soils may show a special type of chlorosis. The current needles are yellowish and the discolouration is most pronounced at the needle base. The disease bears resemblance to iron chlorosis in trees on calcareous soils. In order to investigate the cause of the chlorosis, needle and soil samples were collected in 41 stands in the southern, central and eastern parts of the Netherlands. Needles of the chlorotic trees had low levels of chlorophylls and carotenoids and high ratios of carotenoids to chlorophylls and of chlorophyll a to chlorophyll b. These symptoms occur in plants suffering from iron deficiency. The current needles of chlorotic trees contained on average 14 mg kg−1 Fe, whereas unaffected trees from the same stands contained 24 mg kg−1. Healthy trees from stands without chlorotic trees had needle iron levels of 29 mg kg−1. Spraying the needles of chlorotic trees with ferric-EDTA containing solutions resulted in partly regreening within a few weeks. Chlorotic needles showed also high nitrogen and cadmium levels. Soils of chlorotic stands contained large amounts of extractable inorganic nitrogen and phosphorus. The possible causal relationships between the observed iron deficiency and the differences in elemental composition of needles and soils are discussed.

Research at the Speulder forest: assessment of the effects, acidification eutrophication and ozone

Publisher Summary This chapter discusses the major findings of the research at the Speulder forest that are used to assess the causal relations between loads and levels on the one hand and effects on the other. First, the atmospheric loads and levels at the Speulder forest are presented. Besides their evolution during the past years and the uncertainty in the estimates of atmospheric loads and levels are discussed. Critical loads and levels for the Speulder forest are derived and exceedances are determined in the chapter. Critical levels and critical loads refer to thresholds, which can be used to guide emission reduction policies until precise effect relations are available. They can serve to assess the possible occurrence of effects in natural ecosystems. A critical level is defined as the concentration of a pollutant in the atmosphere above which direct adverse effects on receptors may occur. A critical load is a quantitative estimate of an exposure to one or more pollutants below which significant harmful effects do not occur.