Filip Moldan

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.

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.

Recovery from acidification in central Europe--observed and predicted changes of soil and streamwater chemistry in the Lysina catchment, Czech Republic.

The geochemical model MAGIC was applied to estimate streamwater and soil chemistry between 1851 and 2030 at the Lysina catchment, an acid-sensitive granitic catchment covered by planted Norway spruce monoculture in the western Czech Republic. The total deposition of sulfur to the catchment was 164 meq m(-2) in 1991, but had declined to 52 meq m(-2) by 2000. Although SO2 emissions in the region declined by 90% compared to the 1980s, acidification recovery was small within the period 1990-2000. Stream pH increased only slightly (from 3.92 to 4.07), although SO4 concentration declined sharply from 568 microeq l(-1) (1990) to 232 microeq l(-1) (2000). Organic acids played an important role in streamwater buffering. According to the MAGIC prediction using deposition measured in 1999-2000, streamwater pH will increase to 4.3 and soil base saturation will increase to 6.2% by 2030 (from 5.7% in 2002). Pre-industrial pH was estimated to be 5.5 and soil base saturation 24.7%. The loss of base cations (Ca, Mg, Na, K) was caused predominantly by atmospheric acidity, but intensive forestry was responsible for approximately one third of the net base cation loss via accumulation in harvested biomass. Severely damaged sites, under continued pressure from forestry, will not return to a good environmental status in the near future (if ever) when the acid deposition input is only partially reduced.

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.

Nitrogen saturation at Gårdsjön, southwest Sweden, induced by experimental addition of ammonium nitrate

To investigate the risk and consequences of nitrogen saturation in coniferous forests typical of southern Scandinavia, a whole-catchment experiment is presently being conducted whereby nitrogen is added to throughfall at Gardsjon, southwest Sweden. The Gardsjon experiment is part of the European NITREX project (nitrogen saturation experiments). From April 1991, about 35 kg NH4NO3-N ha−1 year−1 in 5% extra water has been added in weekly doses to the ambient 12 kg NH4NO3-N ha−1 year−1 in throughfall. Elevated concentrations of nitrate in runoff occurred during the first 2 weeks of treatment, but then were very low during the growing season. Nitrate leakage appeared again in mid-November 1991 and continued during the winter and throughout the second year of treatment. The nitrate lost during the first and second years was about 0.6% and 1.1%, respectively, of the total inorganic nitrogen input. Of the other ions analysed in runoff, sulphate, inorganic aluminium and total phosphorus showed statistically significant changes, with lower concentrations during the treatment period relative to the untreated control catchment. The nitrogen input-output data indicate that the forested catchment ecosystem G2 NITREX is proceeding rapidly through several stages of ‘nitrogen saturation’. Both the frequency and the magnitude of nitrate peaks in runoff increased during the non-growing season in the first year (stage 1) and throughout the second year (stage 2). The results from NITREX Gardsjon demonstrate that nitrogen saturation can be induced over a relatively short time by increasing atmospheric deposition of nitrogen. The rate of response suggests that at ambient nitrogen deposition of 13 kg inorganic N ha−1 year−1, the ecosystem is near the threshold at which additional N inputs cause significant nitrate leaching.

Nitrogen, organic carbon and sulphur cycling in terrestrial ecosystems: linking nitrogen saturation to carbon limitation of soil microbial processes

Elevated and chronic nitrogen (N) deposition to N-limited terrestrial ecosystems can lead to ‘N saturation’, with resultant ecosystem damage and leaching of nitrate (NO3−) to surface waters. Present-day N deposition, however, is often a poor predictor of NO3− leaching, and the pathway of the ecosystem transition from N-limited to N-saturated remains incompletely understood. The dynamics of N cycling are intimately linked to the associated carbon (C) and sulphur (S) cycles. We hypothesize that N saturation is associated with shifts in the microbial community, manifest by a decrease in the fungi-to-bacteria ratio and a transition from N to C limitation. Three mechanisms could lead to lower amount of bioavailable dissolved organic C (DOC) for the microbial community and to C limitation of N-rich systems: (1) Increased abundance of N for plant uptake, causing lower C allocation to plant roots; (2) chemical suppression of DOC solubility by soil acidification; and (3) enhanced mineralisation of DOC due to increased abundance of electron acceptors in the form of

Changes in runoff chemistry after five years of N addition to a forested catchment at Gårdsjön, Sweden

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