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

Delayed recovery of surface water chemistry from acidification in subtropical forest region of China

The three largest acid rain regions of current earth are located in northern and western Europe, eastern North America, and East Asia. Sulfur and nitrate concentrations in headwater streams in Europe and North America decreased as atmospheric sulfur and nitrogen deposition decreased, albeit with a considerable delay. However, how water chemistry responds to the declining sulfur and nitrogen deposition in China is unclear. The regional survey of surface water chemistry during 2010 and 2018 within the Sichuan Basin in southwestern China showed that the recovery of the surface water chemistry was delayed for at least 5 years owing to the release of previously deposited sulfur and nitrogen stored in the soil. After sulfur deposition declined from its peak value, the subregions of purplish soil with low sulfate adsorption capacity still exhibited a net sulfur release in 2010, but this release was no longer evident by 2018. The subregions with the red and yellow soils, which have a high sulfate adsorption capacity, operated as sulfur sinks during 2010 and 2018, indicating a continuous immobilization process through sulfate reduction despite a decrease in sulfur deposition. Additionally, this sulfate reduction countered the release of sulfate caused by sulfur desorption. There was a substantial nitrogen sink within the Sichuan Basin. Nitrogen leaching decreased slowly with the declined nitrogen deposition, except in regions where nitrogen deposition exceeded the critical threshold. Compared to temperate forest regions in Europe, the Sichuan Basin and its surrounding areas have experienced higher decline rates in the leaching of sulfur and nitrogen, highlighting that the subtropical forest region undergoes a faster restoration of surface water chemistry.

Towards a mechanistic understanding of the impacts of nitrogen deposition on producer-consumer interactions

Nitrogen (N) deposition has increased substantially since the second half of the 20th century due to human activities. This increase of reactive N into the biosphere has major implications for ecosystem functioning, including primary production, soil and water chemistry and producer community structure and diversity. Increased N deposition is also linked to the decline of insects observed over recent decades. However, we currently lack a mechanistic understanding of the effects of high N deposition on individual fitness, species richness and community structure of both invertebrate and vertebrate consumers. Here, we review the effects of N deposition on producer-consumer interactions, focusing on five existing ecological frameworks: C:N:P ecological stoichiometry, trace element ecological stoichiometry, nutritional geometry, essential micronutrients and allelochemicals. We link reported N deposition-mediated changes in producer quality to life-history strategies and traits of consumers, to gain a mechanistic understanding of the direction of response in consumers. We conclude that high N deposition influences producer quality via eutrophication and acidification pathways. This makes oligotrophic poorly buffered ecosystems most vulnerable to significant changes in producer quality. Changes in producer quality between the reviewed frameworks are often interlinked, complicating predictions of the effects of high N deposition on producer quality. The degree and direction of fitness responses of consumers to changes in producer quality varies among species but can be explained by differences in life-history traits and strategies, particularly those affecting species nutrient intake regulation, mobility, relative growth rate, host-plant specialisation, ontogeny and physiology. To increase our understanding of the effects of N deposition on these complex mechanisms, the inclusion of life-history traits of consumer species in future study designs is pivotal. Based on the reviewed literature, we formulate five hypotheses on the mechanisms underlying the effects of high N deposition on consumers, by linking effects of nutritional ecological frameworks to life-history strategies. Importantly, we expect that N-deposition-mediated changes in producer quality will result in a net decrease in consumer community as well as functional diversity. Moreover, we anticipate an increased risk of outbreak events of a small subset of generalist species, with concomitant declines in a multitude of specialist species. Overall, linking ecological frameworks with consumer life-history strategies provides a mechanistic understanding of the impacts of high N deposition on producer-consumer interactions, which can inform management towards more effective mitigation strategies.

Three Decades of Changing Nutrient Stoichiometry from Source to Sea on the Swedish West Coast

European ecosystems have been subject to extensive shifts in anthropogenic disturbance, primarily through atmospheric deposition, climate change, and land management. These changes have altered the macronutrient composition of aquatic systems, with widespread increases in organic carbon (C), and declines in nitrogen (N) and phosphorus (P). Less well known is how these disturbances have affected nutrient stoichiometry, which may be a more useful metric to evaluate the health of aquatic ecosystems than individual nutrient concentrations. The Swedish west coast has historically experienced moderate to high levels of atmospheric deposition of sulfate and N, and eutrophication. In addition, coastal waters have been darkening with damaging effects on marine flora and fauna. Here, we present three decades of macronutrient data from twenty lakes and watercourses along the Swedish west coast, extending from headwaters to river mouths, across a range of land covers, and with catchments ranging 0.037–40,000 km2. We find a high degree of consistency between these diverse sites, with widespread increasing trends in organic C, and declines in inorganic N and total P. These trends in individual macronutrients translate into large stoichiometric changes, with a doubling in C:P, and increases in C:N and N:P by 50% and 30%, showing that freshwaters are moving further away from the Redfield Ratio, and becoming even more C rich, and depleted in N and P. Although recovery from atmospheric deposition is linked to some of these changes, land cover also appears to have an effect; lakes buffer against C increases, and decreases in inorganic N have been greatest under arable land cover. Our analysis also detects coherently declining P concentrations in small forest lakes; so called (and unexplained) “oligotrophication.” Taken together, our findings show that freshwater macronutrient concentrations and stoichiometry have undergone substantial shifts during the last three decades, and these shifts can potentially explain some of the detrimental changes that adjacent coastal ecosystems are undergoing. Our findings are relevant for all European and North American waters that have experienced historically high levels of atmospheric deposition, and provide a starting point for understanding and mitigating against the trajectories of long-term change in aquatic systems.

Spatial lag effect of aridity and nitrogen deposition on Scots pine (Pinus sylvestris L.) damage

Scots pine (Pinus sylvestris L.) is a widespread tolerant forest tree-species; however, its adaptability to environmental change differs among sites with various buffering capacity. In this study, we compared the spatial effects of aridity index (AI) and nitrogen deposition (ND) on biomass density in natural and man-made pine stands of differing soil fertility using geographically weighted multiple lag regression. Soil fertility was defined using soil series as zonal trophic (27.9%), acidic (48.2%), gleyed (15.2%) and as azonal exposed (2.5%), maple (2.4%), ash (0.8%), wet (2.1%) and peat (0.9%) under pine stands in the Czech Republic (Central Europe; 4290.5 km²; 130-1298 m a.s.l.). Annual AI and ND in every pine stand were estimated by intersection between raster and vector from 1 × 1 km grid for years 2000, 2003, 2007 and 2010 of severe non-specific forest damage spread. Biomass density was obtained from a MODIS 250 × 250 m raster using the enhanced vegetation index (EVI) for years 2000-2015, with a decrease in EVI indicating non-specific damage. Environmental change was assessed by comparing predictor values at EVI time t and t+λ. Non-specific damage was registered over 51.9% of total forest area. Less than 8.8% of damaged stands were natural and the rest (91.2%) of damaged stands were man-made. Pure pine stands were more damaged than mixed. The ND effect prevailed up to 2007, while AI dominated later. Temporal increasing ND effect under AI effectiveness led to the most significant pine stand damage in 2008 and 2014. Predictors from 2000 to 2007 afflicted 58.5% of non-specifically damaged stands at R² 0.09-0.76 (median 0.38), but from 2000 to 2010 afflicted 57.1% of the stands at R² 0.16-0.75 (median 0.40). The most damaged stands occurred on acidic sites. Mixed forest and sustainable management on natural sites seem as effective remediation reducing damage by ND.

Responses of forest ecosystems in Europe to decreasing nitrogen deposition

Average nitrogen (N) deposition across Europe has declined since the 1990s. This resulted in decreased N inputs to forest ecosystems especially in Central and Western Europe where deposition levels are highest. While the impact of atmospheric N deposition on forests has been receiving much attention for decades, ecosystem responses to the decline in N inputs received less attention. Here, we review observational studies reporting on trends in a number of indicators: soil acidification and eutrophication, understory vegetation, tree nutrition (foliar element concentrations) as well as tree vitality and growth in response to decreasing N deposition across Europe. Ecosystem responses varied with limited decrease in soil solution nitrate concentrations and potentially also foliar N concentrations. There was no large-scale response in understory vegetation, tree growth, or vitality. Experimental studies support the observation of a more distinct reaction of soil solution and foliar element concentrations to changes in N supply compared to the three other parameters. According to the most likely scenarios, further decrease of N deposition will be limited. We hypothesize that this expected decline will not cause major responses of the parameters analysed in this study. Instead, future changes might be more strongly controlled by the development of N pools accumulated within forest soils, affected by climate change and forest management.

Multiyear dual nitrate isotope signatures suggest that N-saturated subtropical forested catchments can act as robust N sinks

In forests of the humid subtropics of China, chronically elevated nitrogen (N) deposition, predominantly as ammonium (NH₄⁺ ), causes significant nitrate (NO₃^- ) leaching from well-drained acid forest soils on hill slopes (HS), whereas significant retention of NO₃^- occurs in near-stream environments (groundwater discharge zones, GDZ). To aid our understanding of N transformations on the catchment level, we studied spatial and temporal variabilities of concentration and natural abundance (δ¹⁵ N and δ¹⁸ O) of nitrate (NO₃^- ) in soil pore water along a hydrological continuum in the N-saturated Tieshanping (TSP) catchment, southwest China. Our data show that effective removal of atmogenic NH₄⁺ and production of NO₃^- in soils on HS were associated with a significant decrease in δ¹⁵ N-NO₃^- , suggesting efficient nitrification despite low soil pH. The concentration of NO₃^- declined sharply along the hydrological flow path in the GDZ. This decline was associated with a significant increase in both δ¹⁵ N and δ¹⁸ O of residual NO₃^- , providing evidence that the GDZ acts as an N sink due to denitrification. The observed apparent ¹⁵ N enrichment factor (ε) of NO₃^- of about -5‰ in the GDZ is similar to values previously reported for efficient denitrification in riparian and groundwater systems. Episode studies in the summers of 2009, 2010 and 2013 revealed that the spatial pattern of δ¹⁵ N and δ¹⁸ O-NO₃^- in soil water was remarkably similar from year to year. The importance of denitrification as a major N sink was also seen at the catchment scale, as largest δ¹⁵ N-NO₃^- values in stream water were observed at lowest discharge, confirming the importance of the relatively small GDZ for N removal under base flow conditions. This study, explicitly recognizing hydrologically connected landscape elements, reveals an overlooked but robust N sink in N-saturated, subtropical forests with important implications for regional N budgets.

Effects of Simulated Nitrogen Deposition on Soil Net Nitrogen Mineralization in the Meadow Steppe of Inner Mongolia, China

Effects of simulated nitrogen (N) deposition on soil net nitrogen mineralization (NNM) were examined in situ during two growing seasons, using the resin-core technique in the semiarid meadow steppe in Inner Mongolia, China. The aim of this study is to clarify the effect of N levels (0, 10, and 20 kg N ha⁻¹yr⁻¹) and forms (NH₄⁺ and NO₃^-) on soil mineral N and NNM. Our results showed that N levels had no significant differences on soil mineral N and NNM. In the first year, three N treatments ((NH₄)₂SO₄, NH₄Cl and KNO₃) increased soil NH₄⁺ concentrations but had no significant effects on soil NO₃^- concentrations. In the second year, (NH₄)₂SO₄ treatment increased soil NO₃^- concentrations, NH₄Cl and KNO₃ treatments decreased them. Three N treatments significantly decreased soil NH₄⁺ concentrations in the later stages of the second year. As for the soil NNM, three N treatments had no significant effects on the rates of soil NNM (R_m) and net nitrification (R_n) in the first year, but significantly decreased them in the second year. The contribution of N addition to R_m was higher from (NH₄)₂SO₄ than from NH₄Cl and KNO₃. However, Soil R_m was mainly affected by soil water content (SWC), accumulated temperature (T_a), and soil total N (TN). These results suggest that the short-term atmospheric N deposition may inhibit soil NNM in the meadow steppe of Inner Mongolia.

Assessing the influence of topography and canopy structure on Douglas fir throughfall with LiDAR and empirical data in the Santa Cruz mountains, USA

Atmospheric inputs to forest ecosystems vary considerably over small spatial scales due to subtle changes in relief and vegetation structure. Relationships between throughfall fluxes (ions that pass through the canopy in water), topographic and canopy characteristics derived from sub-meter resolution light detection and ranging (LiDAR), and field measurements were compared to test the potential utility of LiDAR in empirical models of atmospheric deposition. From October 2012 to May 2013, we measured bulk (primarily wet) deposition and sulfate-S, chloride (Cl(-)), and nitrate-N fluxes beneath eight clusters of Douglas fir trees differing in size and canopy exposure in the Santa Cruz Mountains, California. For all trees sampled, LiDAR data were used to derive canopy surface height, tree height, slope, and canopy curvature, while tree height, diameter (DBH), and leaf area index were measured in the field. Wet season throughfall fluxes to Douglas fir clusters ranged from 1.4 to 3.8 kg S ha(-1), 17-54 kg Cl(-) ha(-1), and 0.2-4 kg N ha(-1). Throughfall S and Cl(-) fluxes were highest under clusters with large trees at topographically exposed sites; net fluxes were 2-18-fold greater underneath exposed/large clusters than all other clusters. LiDAR indices of canopy curvature and height were positively correlated with net sulfate-S fluxes, indicating that small-scale canopy surface features captured by LiDAR influence fog and dry deposition. Although tree diameter was more strongly correlated with net sulfate-S throughfall flux, our data suggest that LiDAR data can be related to empirical measurements of throughfall fluxes to generate robust high-resolution models of atmospheric deposition.

Surface water quality is improving due to declining atmospheric N deposition

We evaluated long-term surface water nitrate and atmospheric nitrogen (N) deposition trends for a group of nine predominantly forested Appalachian Mountain watersheds during a recent multidecadal period (1986-2009) in which regional NOx emissions have been progressively reduced. Statistical analysis showed unexpected linear declines in both annual surface water nitrate-N concentrations (mean =46.4%) and yields (mean =47.7%) among the watersheds corresponding to comparable declines in annual wet N deposition (mean =34.4%) resulting from U.S. NOx emission control programs during the same time period. Nitrate-N concentration trends were robust across a large geographical region and appeared insensitive to watershed size across several orders of magnitude-suggesting that the improvements in water quality are probably propagated to surface and estuarine waters downstream. Surface waters are thus responding to declining atmospheric N deposition in much the same way they responded to declining sulfur deposition-although only one watershed showed a 1:1 relationship. Application of a kinetic N saturation model indicated that all nine forested watersheds are exhibiting signs of N saturation as evidenced by a limited, but variable, efficiency of demand for N. Further reductions in N deposition would be expected to produce additional reductions in streamwater N loads.

Long-term nitrogen additions increase likelihood of climate stress and affect recovery from wildfire in a lowland heath

Increases in the emissions and associated atmospheric deposition of nitrogen (N) have the potential to cause significant changes to the structure and function of N-limited ecosystems. Here, we present the results of a long-term (13 year) experiment assessing the impacts of N addition (30 kg ha(-1)  yr(-1) ) on a UK lowland heathland under a wide range of environmental conditions, including the occurrence of prolonged natural drought episodes and a severe summer fire. Our findings indicate that elevated N deposition results in large, persistent effects on Calluna growth, phenology and chemistry, severe suppression of understorey lichen flora and changes in soil biogeochemistry. Growing season rainfall was found to be a strong driver of inter-annual variation in Calluna growth and, although interactions between N and rainfall for shoot growth were not significant until the later phase of the experiment, N addition exacerbated the extent of drought injury to Calluna shoots following naturally occurring droughts in 2003 and 2009. Following a severe wildfire at the experimental site in 2006, heathland regeneration dynamics were significantly affected by N, with a greater abundance of pioneering moss species and suppression of the lichen flora in plots receiving N additions. Significant interactions between climate and N were also apparent post fire, with the characteristic stimulation in Calluna growth in +N plots suppressed during dry years. Carbon (C) and N budgets demonstrate large increases in both above- and below-ground stocks of these elements in N-treated plots prior to the fire, despite higher levels of soil microbial activity and organic matter turnover. Although much of the organic material was removed during the fire, pre-existing treatment differences were still evident following the burn. Post fire accumulation of below-ground C and N stocks was increased rapidly in N-treated plots, highlighting the role of N deposition in ecosystem C sequestration.

Modelling soil nitrogen: the MAGIC model with nitrogen retention linked to carbon turnover using decomposer dynamics

We present a new formulation of the acidification model MAGIC that uses decomposer dynamics to link nitrogen (N) cycling to carbon (C) turnover in soils. The new model is evaluated by application to 15-30 years of water chemistry data at three coniferous-forested sites in the Czech Republic where deposition of sulphur (S) and N have decreased by >80% and 40%, respectively. Sulphate concentrations in waters have declined commensurately with S deposition, but nitrate concentrations have shown much larger decreases relative to N deposition. This behaviour is inconsistent with most conceptual models of N saturation, and with earlier versions of MAGIC which assume N retention to be a first-order function of N deposition and/or controlled by the soil C/N ratio. In comparison with earlier versions, the new formulation more correctly simulates observed short-term changes in nitrate leaching, as well as long-term retention of N in soils. The model suggests that, despite recent deposition reductions and recovery, progressive N saturation will lead to increased future nitrate leaching, ecosystem eutrophication and re-acidification.

Negative responses of Collembola in a forest soil (Alptal, Switzerland) under experimentally increased N deposition

The response of specific groups of organisms, like Collembola to atmospheric nitrogen (N) deposition is still scarcely known. We investigated the Collembola community in a subalpine forest (Alptal, Switzerland) as subjected for 12 years to an experimentally increased N deposition (+25 on top of ambient 12 kg N ha(-1) year(-1)). In the 0-5 cm soil layer, there was a tendency of total Collembola densities to be lower in N-treated than in control plots. The density of Isotomiella minor, the most abundant species, was significantly reduced by the N addition. A tendency of lower Collembola group richness was observed in N-treated plots. The Density-Group index (dDG) showed a significant reduction of community diversity, but the Shannon-Wiener index (H') was not significantly affected by the N addition. The Collembola community can be considered as a bioindicator of N inputs exceeding the biological needs, namely, soil N saturation.

Changes in nitrogen cycling during the past century in a northern hardwood forest

Nitrogen (N) availability, defined here as the supply of N to terrestrial plants and soil microorganisms relative to their N demands, limits the productivity of many temperate zone forests and in part determines ecosystem carbon (C) content. Despite multidecadal monitoring of N in streams, the long-term record of N availability in forests of the northeastern United States is largely unknown. Therefore, although these forests have been receiving anthropogenic N deposition for the past few decades, it is still uncertain whether terrestrial N availability has changed during this time and, subsequently, whether forest ecosystems have responded to increased N deposition. Here, we used stable N isotopes in tree rings and lake sediments to demonstrate that N availability in a northeastern forest has declined over the past 75 years, likely because of ecosystem recovery from Euro-American land use. Forest N availability has only recently returned to levels forecast from presettlement trajectories, rendering the trajectory of future forest N cycling uncertain. Our results suggest that chronic disturbances caused by humans, especially logging and agriculture, are major drivers of terrestrial N cycling in forest ecosystems today, even a century after cessation.

Increased nitrogen in runoff and soil following 13 years of experimentally increased nitrogen deposition to a coniferous-forested catchment at Gårdsjön, Sweden

Beginning in 1991, we have added nitrogen (N) to the 0.5-ha, N-poor, coniferous-forested catchment G2 NITREX at Gårdsjön, Sweden, to investigate the consequences of chronic elevated N deposition. We have added 40 kg N ha-1 yr-1 in fortnightly doses of NH4NO3 to the ambient 15 kg N ha-1 yr-1 by means of a sprinkling system. NO3 concentrations in runoff increased during 13 years from<1 to 70 microeq L-1, and in 2004 comprised about 10% of N input. Inhibition of NO3 immobilisation due to increased availability of NH4 might explain the increased leaching of NO3. C and N pools in the forest floor increased but C/N ratio has not changed. The increase in NO3 leaching thus occurred independently of change in C/N ratio. The results from Gårdsjön demonstrate that increased leaching of inorganic N and decrease in C/N ratio respond to increased N deposition at greatly different time scales.

Detrital control on the release of dissolved organic nitrogen (DON) and dissolved inorganic nitrogen (DIN) from the forest floor under chronic N deposition

The role of detrital quantity and quality in forest floor N leaching was investigated in a litter manipulation experiment at a deciduous forest under chronic N deposition. Dissolved inorganic nitrogen (DIN) comprised the bulk of nitrogen leaching from the control except a short period following autumn litterfall. The dominance of DIN was strengthened by litter exclusion, whereas the addition of glucose or fresh litter led to a small increase in dissolved organic nitrogen (DON) and either a temporary or gradual reduction in NO(3)(-) release, respectively. Changes in soluble organic C and microbial C in the forest floor implied that increased availability of C sources might have enhanced microbial immobilization of DIN, either temporarily following glucose application or over the longer term following litter addition. The results suggest that detrital quantity and quality can play a crucial role in determining the balance between DIN and DON in N-enriched forest soils.

Modelling the effect of climate change on recovery of acidified freshwaters: relative sensitivity of individual processes in the MAGIC model

The MAGIC model was used to evaluate the relative sensitivity of several possible climate-induced effects on the recovery of soil and surface water from acidification. A common protocol was used at 14 intensively studied sites in Europe and eastern North America. The results show that several of the factors are of only minor importance (increase in pCO(2) in soil air and runoff, for example), several are important at only a few sites (seasalts at near-coastal sites, for example) and several are important at nearly all sites (increased concentrations of organic acids in soil solution and runoff, for example). In addition changes in forest growth and decomposition of soil organic matter are important at forested sites and sites at risk of nitrogen saturation. The trials suggest that in future modelling of recovery from acidification should take into account possible concurrent climate changes and focus specially on the climate-induced changes in organic acids and nitrogen retention.

Are N and S deposition altering the mineral composition of Norway spruce and Scots pine needles in Finland?

Data from a large-scale foliar survey were used to calculate the extent to which N and S deposition determined the mineral composition of Scots pine and Norway spruce needles in Finland. Foliar data were available from 367 needle samples collected on 36 plots sampled almost annually between 1987 and 2000. A literature study of controlled experiments revealed that acidifying deposition mediates increasing N and S concentrations, and decreasing Mg:N and Ca:Al ratios in the needles. When this fingerprint for N and S elevated deposition on tree foliage was observed simultaneously with increased N and S inputs, it was considered sufficient evidence for assuming that acidifying deposition had altered the mineral composition of tree needles on that plot in the given year. Evidence for deposition-induced changes in the mineral composition of tree foliage was calculated on the basis of a simple frequency model. In the late eighties the evidence was found on 43% of the Norway spruce and 27% of Scots pine plots. The proportion of changed needle mineral composition decreased to below 8% for both species in the late nineties.

Effects of increased deposition of atmospheric nitrogen on an upland Calluna moor: N and P transformations

This study determined the effects of increased N deposition on rates of N and P transformations in an upland moor. The litter layer and the surface of the organic Oh horizon were taken from plots that had received long-term additions of ammonium nitrate at rates of 40, 80 and 120 kg N ha(-1) yr(-1). Net mineralisation processes were measured in both field and laboratory incubations. Soil phosphomonoesterase (PME) activity and rates of N(2)O release were measured in laboratory incubations and root-surface PME activity measured in laboratory microcosms using Calluna vulgaris bioassay seedlings. Net mineralisation rates were relatively slow, with net ammonification consistently stimulated by N addition. Net nitrification was marginally stimulated by N addition in the laboratory incubation. N additions also increased soil and root-surface (PME) activity and rates of N(2)O release. Linear correlations were found between litter C:N ratio and all the above processes except net nitrification in field incubations. When compared with data from a survey of European forest sites, values of litter C:N ratio were greater than a threshold below which substantial, N input-related increases in net nitrification rates occurred. The maintenance of high C:N ratios with negligible rates of net nitrification was associated with the common presence of ericaceous litter and a mor humus layer in both this moorland as well as the forest sites.

Effects of increased deposition of atmospheric nitrogen on an upland moor: leaching of N species and soil solution chemistry

This study was designed to investigate the leaching response of an upland moorland to long-term (10 yr) ammonium nitrate additions of 40, 80 and 120 kg N ha(-1) yr(-1) and to relate this response to other indications of potential system damage, such as acidification and cation displacement. Results showed increases in nitrate leaching only in response to high rates of N input, in excess of 96 and 136 kg total N input ha(-1) yr(-1) for the organic Oh horizon and mineral Eag horizon, respectively. Individual N additions did not alter ammonium leaching from either horizon and ammonium was completely retained by the mineral horizon. Leaching of dissolved organic nitrogen (DON) from the Oh horizon was increased by the addition of 40 kg N ha(-1) yr(-1), but in spite of increases, retention of total dissolved nitrogen reached a maximum of 92% and 95% of 80 kg added N ha(-1) yr(-1) in the Oh and Eag horizons, respectively. Calcium concentrations and calcium/aluminium ratios were decreased in the Eag horizon solution with significant acidification mainly in the Oh horizon leachate. Nitrate leaching is currently regarded as an early indication of N saturation in forest systems. Litter C:N ratios were significantly lowered but values remained above a threshold predicted to increase leaching of N in forests.

N : P ratios in terrestrial plants: variation and functional significance

Nitrogen (N) and phosphorus (P) availability limit plant growth in most terrestrial ecosystems. This review examines how variation in the relative availability of N and P, as reflected by N : P ratios of plant biomass, influences vegetation composition and functioning. Plastic responses of plants to N and P supply cause up to 50-fold variation in biomass N : P ratios, associated with differences in root allocation, nutrient uptake, biomass turnover and reproductive output. Optimal N : P ratios - those of plants whose growth is equally limited by N and P - depend on species, growth rate, plant age and plant parts. At vegetation level, N : P ratios <10 and >20 often (not always) correspond to N- and P-limited biomass production, as shown by short-term fertilization experiments; however long-term effects of fertilization or effects on individual species can be different. N : P ratios are on average higher in graminoids than in forbs, and in stress-tolerant species compared with ruderals; they correlate negatively with the maximal relative growth rates of species and with their N-indicator values. At vegetation level, N : P ratios often correlate negatively with biomass production; high N : P ratios promote graminoids and stress tolerators relative to other species, whereas relationships with species richness are not consistent. N : P ratios are influenced by global change, increased atmospheric N deposition, and conservation managment. Contents Summary 243 I Introduction 244 II Variability of N : P ratios in response to nutrient  supply 244 III Critical N : P ratios as indicators of nutrient  limitation 248 IV Interspecific variation in N : P ratios 252 V Vegetation properties in relation to N : P ratios 255 VI Implications of N : P ratios for human impacts  on ecosystems 258 VII Conclusions 259 Acknowledgements 259 References 260.

Gaseous nitrogen losses from a forest site in the North Tyrolean Limestone Alps

Microorganisms are responsible for the mineralisation of organic nitrogen in soils. NH4+ can be further oxidised to NO3- during nitrification and NO3- can be reduced to gaseous nitrogen compounds during denitrification. During both processes, nitrous oxide (N2O), which is known as greenhouse gas, can be lost from the ecosystem. The aim of this study was to quantify N2O emissions and the internal microbial N cycle including net N mineralisation and net nitrification in a montane forest ecosystem in the North Tyrolean Limestone Alps during an 18-month measurement period and to estimate the importance of these fluxes in comparison with other components of the N cycle. Gas samples were taken every 2 weeks using the closed chamber method. Additionally, CO2 emission rates were measured to estimate soil respiration activity. Net mineralisation and net nitrification rates were determined by the buried bag method every month. Ion exchange resin bags were used to determine the N availability in the root zone. Mean N2O emission rate was 0.9 kg N ha(-1) a(-1), which corresponds to 5% of the N deposited in the forest ecosystem. The main influencing factors were air and soil temperature and NO3- accumulated on the ion exchange resin bags. In the course of net ammonification, 14 kg NH4+-N ha(-1) were produced per year. About the same amount of NO3--N was formed during nitrification, indicating a rather complete nitrification going on at the site. NO3- concentrations found on the ion exchange resin bags were about 3 times as high as NO3- produced during net nitrification, indicating substantial NO3- immobilisation. The results of this study indicate significant nitrification activities taking place at the Mühleggerköpfl.

Simulation of soil hydrology and establishment of a nitrogen budget of a mountain forest

The water fluxes through the mountainous forest ecosystem 'Mühleggerköpfl' were simulated by means of the mechanistic soil physical model Hydrus 1D. The objective was to set up a nitrogen budget in order to decide if the ecosystem accumulates nitrogen or if nitrogen leaks from the site. The simulated annual loss of N by percolation ranges between 0.4 and 1 g N m(-2) yr(-1) and is smaller than the annual input by bulk and occult deposition, which combines to approx 1.2-1.5 g N m(-2) yr(-1). Obviously the forest soil presently accumulates N. With an N input-rate exceeding the N output, the operationally defined status of N saturation is not yet reached. Comparing the magnitude of the N pool in the soil (several kg N m(-2)) with the rate of the annual increase (a few g N m(-2) yr(-1)), the process of N saturation is apparently slow.

Nitrogen fluxes on an intensive investigation plot in the North Tyrolean Limestone Alps

In the framework of this study, nitrogen fluxes on a limestone site are investigated. The major goals are the assessment of the nitrogen status, the estimation of the nitrogen budget and the evaluation of the nitrogen saturation. The investigation area, the intensive investigation plot and the research equipment are described.

Limitations of controlled experimental systems as models for natural systems: a conceptual assessment of experimental practices in biogeochemistry and soil science

Experimental systems in which phenomena are studied under controlled conditions allow scientists to infer causal relationships from observable effects. When investigating ecosystems, however, scientists face complex systems. The conventional approach is to divide the system into conceptual units and to prepare experimental systems accordingly. Experimental systems are used as models for ecosystems: initially, scientists assume an analogy between the experimental system and ecosystem, then encode the experimental system into a formal system by measuring variables, and decode statements from the formal system to the ecosystem. We distinguish three types of experimental systems, i.e. laboratory, container and field set-ups, further divided into seven subtypes. Starting from the premises of experimental systems, we comment on the possibilities and limitations of experimentally derived causal relationships and on their significance for ecosystem understanding and prediction, illustrated by examples from soil science and the environmental sciences. Experimental set-ups have a characteristic duration, degree of structural integrity, internal variability and boundaries, which relate to conceptual closure and experimental control: control tends to be maximum on short time scales, in homogeneous set-ups with analytical boundaries, and in systems with few parameters to be observed. Complexity is increased at the expense of control. The higher the degree of manipulation, however, the better is reproducibility, but the larger is the deviation from unique ecosystems with their infinite number of factors. The material realization of closed systems is preceded by a conceptual closure of the system. Closure is relative to the domain of phenomena of interest, the theory and the list of variables selected by the scientist. Successful decoding from experimental systems to ecosystems largely depends on the validity of the chosen analogy. Laboratory systems are idealized systems which contain a limited number of a priori defined variables, and which are shielded from environmental influences. In contrast, ecosystems are materially and conceptually open, non-stationary, historical systems, in which system-level properties can emerge, and in which variables are produced internally. We conclude that when conducting experiments, causal factors can be identified, but that causal knowledge derived from insufficiently closed systems is invalid. In ecosystems, innumerous factors interact, which may enhance, reduce or neutralize the effect of an experimentally determined factor. Thus, experimental model systems need to be evaluated for concrete, well-defined ecosystems with a concrete history. Increasingly detailed studies of isolated phenomena in the laboratory will probably not contribute much to ecosystem-level understanding. When conducting experiments, scientists should aim at the maximum degree of complexity they can actually handle and they should justify the chosen analogy.