Andries W. Boxman

Predicting the Effects of Atmospheric Nitrogen Deposition in Conifer Stands: Evidence from the NITREX Ecosystem-Scale Experiments

ABSTRACT The NITREX project, which encompasses seven ecosystem-scale experiments in coniferous forests at the plot or catchment level in northwestern Europe, investigates the effect of atmospheric nitrogen (N) deposition in coniferous forests. The common factor in all of the experiments is the experimentally controlled change in N input over a period of 4–5 years. Results indicate that the status and dynamics of the forest floor are key components in determining the response of forests to altered N inputs. An empirical relationship between the carbon–nitrogen (C/N) ratio of the forest floor and retention of incoming N provides a simply measured tool through which the likely timing and consequences of changes in atmospheric N deposition for fresh waters may be predicted. In the terrestrial ecosystem, a 50% increase in tree growth is observed following the experimental reduction of N and sulfur inputs in a highly N-saturated site, illustrating the damaging effects of acidifying pollutants to tree health in some locations. Few biotic responses to the experimental treatments were observed in other NITREX sites, but the rapid response of water quality to changes in N deposition, and the link to acidification in sensitive areas, highlight the need for N-emission controls, irrespective of the long-term effects on tree health. The observed changes in ecosystem function in response to the experimental treatments have been considered within the framework of the current critical-load approach and thus contribute to the formulation of environmental policy.

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.

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.

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.

Ecosystem responses to reduced and oxidised nitrogen inputs in European terrestrial habitats

While it is well established that ecosystems display strong responses to elevated nitrogen deposition, the importance of the ratio between the dominant forms of deposited nitrogen (NH(x) and NO(y)) in determining ecosystem response is poorly understood. As large changes in the ratio of oxidised and reduced nitrogen inputs are occurring, this oversight requires attention. One reason for this knowledge gap is that plants experience a different NH(x):NO(y) ratio in soil to that seen in atmospheric deposits because atmospheric inputs are modified by soil transformations, mediated by soil pH. Consequently species of neutral and alkaline habitats are less likely to encounter high NH(4)(+) concentrations than species from acid soils. We suggest that the response of vascular plant species to changing ratios of NH(x):NO(y) deposits will be driven primarily by a combination of soil pH and nitrification rates. Testing this hypothesis requires a combination of experimental and survey work in a range of systems.

Ecosystem recovery after a decrease in nitrogen input to a Scots pine stand at Ysselsteyn, the Netherlands

In a highly N-saturated Scots pine stand atmospheric N input to the forest floor was reduced to pre-industrial levels by means of a transparent roof and application of clean, artificial throughfall water. The N input was reduced from ≈ 60 kg N ha−1 yr−1 to <5 kg N ha−1 yr−1. The experiment began in 1989 and is part of the NITREX project. The aim is to assess the reversibility of N saturation. The results showed a close correlation between the input and output of N. The timing of this response is fast in contrast to the vegetation response which lags for some years. After this lag-phase signs of ecosystem recovery were noticed. The N concentration in the needles declined and the nutritional balance improved. Free arginine-N concentrations in the needles may be indicative of tree response to changes in N input. Although the total-N concentration in the needles is still high, the rapid drop in arginine-N concentrations may be the first sign of recovery. The trees have reacted with a growth improvement which is significantly correlated to decreasing arginine-N concentrations. The above-ground biomass of the nitrophilous ground vegetation decreased markedly during the experimental period, whilst there was a re-colonization and increase of fruitbodies of mycorrhizal fungi.

Nitrogen saturation experiments (NITREX) in coniferous forest ecosystems in Europe: a summary of results

The effect of changes in dissolved inorganic nitrogen (N) deposition on ecosystem functioning was investigated in the NITREX (NITRogen saturation EXperiments) project. Field-scale manipulation experiments were carried out over four to six years in seven coniferous forest ecosystems in northwestern Europe. At sites with low or moderate ambient N deposition, N was experimentally added to throughfall. At sites with high N deposition, N was removed from throughfall. We found that the capacity of the ecosystem to retain N was correlated to its internal N status. Some of the components of this N status like the N concentrations in foliage and forest floor are relatively easy to measure. The C/N ratio of the forest floor is especially closely related to the onset of nitrate leaching. Changes in N input may, in the long run, change the N status of an ecosystem due to for instance a decrease in C/N ratio in the forest floor. Decreased N input resulted in a rapid and large reduction in N concentration in drainage water. Significant improvement in tree nutritional status, tree growth, fine root biomass and diversity of ground vegetation and mycorrhizal fungi population were observed in one site only. The time period of four to six years of manipulated N deposition may have been too short for changes to be manifested in the other sites.

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 ammonium and aluminium on the development and nutrition of Pinus nigra in hydroculture.

Application of ammonium and aluminium to young Pinus nigra var. maritima (Ait.) Melville trees resulted in a variety of negative effects. Excess ammonium led to an increase in shoot/root ratio. The biomass of the fine roots declined, resulting in an increase of the coarse/fine root ratio. The degree of mycorrhizal infection of the roots decreased. The nitrogen content of the trees increased considerably, whereas particularly the levels of calcium magnesium, manganese and zinc decreased sharply. Excess aluminium resulted in a simultaneous reduction of root and shoot biomass, a decline of the fine root system, an increase in the coarse/fine root ratio and a decrease in the degree of mycorrhizal infection. Uptake of the divalent cations calcium, magnesium, iron, manganese and zinc was restricted substantially, The nitrogen and phosphorus contents of the trees were hardly affected, whilst the potassium content of the shoot increased and of the roots decreased. This implicates that a deteriorating fine root system has to supply water and nutrients to a more demanding shoot. In the long term, high ammonium inputs and aluminium dissolution in forest ecosystems will lead to substantial nutrient deficiencies, just as has been found in the field.

Biogeochemical constraints on the ecological rehabilitation of wetland vegetation in river floodplains

The European policy for river management during peak discharge periods is currently changing from exclusion strategies (reinforcement of dykes) to allowing a more natural situation by creating more floodplain space to reduce water levels during peak discharges. In addition, water retention and water storage areas have been created. The new measures are generally being combined with nature development strategies. Up till now, however, ecological targets of broadscale floodplain wetland restoration including sedge marshes, species-rich floodplain forests and carrs, riparian mesotrophic grasslands and other biodiverse riverine ecosystems, have hardly developed in these areas. Most studies on the conditions needed for sustainable ecological development of floodplains have focused on hydrological and geomorphological rather than biogeochemical issues (including nutrient availability and limitation). There are, however, large differences in the composition of river water and groundwater and in sediment quality between rivers in densely populated areas and those in more pristine areas, which serve as a reference. It is very likely that these factors, in combination with heavily altered hydrological regimes and the narrow areas confined between the dykes on both sides of the rivers, impose major constraints on sustainable ecological development of riverine areas. Another issue is that existing wetlands are generally considered to be very appropriate for water retention and conservation, although recent research has shown that this may pose a serious threat to their biodiversity. The present paper reviews the biogeochemical constraints on the combination of floodplain rehabilitation, water conservation and the conservation and development of wetlands. It is concluded that biogeochemical problems (mainly related to eutrophication) predominantly arise in less dynamic parts of the river system, to which the flood-pulse concept applies less. Sound knowledge of the biogeochemical processes involved will contribute to greater efficiency and a better prediction of the opportunities for restoration and development of riverine wetlands. This information can be directly applied in nature management, water management, policy-making and consultancy.