Nitrogen Saturation Research Articles

Synthesis of ratiometric fluorescent nanoparticles for sensing oxygen

A facile reprecipitation-encapsulation method is used for the preparation of ratiometric fluorescent nanoparticles (NPs) for sensing intracellular oxygen. The surface of the NPs is modified in-situ with poly-L-lysine, which renders good biocompatibility and enables easy internalization into living cells. The sensor NPs contain a red fluorescent probe whose fluorescence is sensitive to oxygen with a quenching response of 77 % on going from nitrogen saturation to oxygen saturation, and a reference dye giving a green signal that acts as an oxygen-independent reference. The ratio of the two emissions serves as the analytical information and is sensitive to dissolved oxygen in the 0–43 ppm concentration range. When incorporated into cells, the ratio of the signals increases by 400 % on going from oxygen-saturated to oxygen-free environment.

Structure and mechanical properties of iron after surface severe plastic deformation under friction with simultaneous nitrogen saturation: II. The mechanical properties of nano- and submicrocrystalline iron saturated with nitrogen during deformation

The effect of structure refinement of iron to submicro- and nanograins during intense plastic deformation under friction (SPDF) with simultaneous nitrogen saturation on the mechanical characteristics (hardness, plasticity, Young’s modulus) has been studied by nanoindentation. The nitrogen saturation of iron during SPDF is shown to increase the hardness of micro- and submicrocrystalline regions of the surface layer as compared to SPDF in argon as a result of solid-solution hardening. A high nitrogen concentration in an α-Fe[N] solid solution weakly affects the mechanical properties of nanocrystalline iron with grain sizes d < 50 nm, in which deformation is controlled by grain-boundary sliding.

Structure and mechanical properties of iron after surface severe plastic deformation under friction with simultaneous nitrogen saturation: I. structure formation

The structure of armco iron after severe plastic deformation under friction with simultaneous nitrogen saturation has been studied by microstructure analysis, X-ray diffraction, and transmission and scanning electron microscopy. A gradient surface layer, in which the grain size varies from micro- to submicro- and nanometer-scale values, is found to form. A mutual intensifying effect of the deformation-induced refinement of a grain structure and the diffusion of nitrogen atoms in iron is shown. The formation of a nanocrystalline state is discussed as a result of dynamic recrystallization.

Persistent surface water acidification in an organic soil-dominated upland region subject to high atmospheric deposition: The North York Moors, UK

The North York Moors National Park, in Northeast England, is one of the few upland areas of the United Kingdom located immediately downwind of major sulphur and nitrogen emission sources. Despite this, few studies of air pollution impacts have been undertaken, and there is no formal long-term upland water quality monitoring site. We examined the condition of surface waters in the National Park based on (1) a unique 20 year stream pH record from three locations; and (2) a snapshot survey of 51 surface waters draining moorland and conifer plantations. Interpretation was supported by sulphur isotope analysis of a subset of water samples, and a diatom survey of one of the monitoring streams. Long-term pH data for a stream draining the peat plateau demonstrate extreme, year-round acidification, with recovery only evident in the last few years. Lower-elevation sites are less acidic, but show similar temporal trends, and are characterised by frequent and severe acid episodes. The snapshot survey confirmed that acidification of the moorland area is widespread, to a degree observed in few other areas globally; out of 37 moorland streams sampled, 32 had an acid neutralising capacity (ANC) below −50μequiv.l−1. Sulphate was found to be (by far) the dominant cause of acidification, and sulphur isotope analysis confirmed that this derives primarily from atmospheric deposition. Nitrate concentrations remain low, indicating that the organic moorland soils continue to retain most incoming nitrogen. It appears that conifer planting has exacerbated acidification, leading to fivefold higher nitrate and threefold higher aluminium concentrations compared to the moorland sites. Available biological data suggest that waters in the region have been impoverished by acidification. We speculate that the slow recovery of surface waters in the North York Moors is due to the release of a legacy of stored sulphur from the peats, released during droughts. We conclude that: (1) acidification is far from being a solved problem in this sensitive near-source upland region, despite reductions in sulphur deposition; (2) plantation forestry has exacerbated the effects of atmospheric pollution, and triggered nitrogen saturation; (3) the lack of any formal long-term monitoring in the North York Moors represents a major gap in the current evidence base for the effects of long-range air pollutants on UK upland ecosystems.

Geologic nitrogen and climate change in the geochemical budget of Kärkevagge, Swedish Lapland

While nitrogen is an essential nutrient for plant growth in the terrestrial environment and generally fixed by biological processes in soils, in Arctic and alpine environments N is a severely limited nutrient because of both climatic and soil constraints. It is widely believed that geologic nitrogen is such a small component of the nitrogen cycle that this source is generally not viewed as being a significant component. However, it has been estimated that in fact as much as 20% of the total nitrogen budget may be stored in geological materials and where N is severely limited such as in Arctic and alpine environments geologic N may in fact be a dominant source of N for biological processes. Nitrogen saturation may therefore have substantial impacts on geochemical cycles involving both soil and water chemistry.In order to determine the potential for the rocks and soils of Kärkevagge to be significant sources of N samples of the principal rock units of the valley along with soil developed on a variety of rock types were sampled. Ammonia/ammonium contents were determined using ion chromatography. Ammonia/ammonium contents of the sampled rock materials display a wide range of abundances with marble containing a mean NH4 abundance of 4300mgNkg, quartz schist 1600mgNkg, biotite schist 4300mgNkg, and garnet mica schist 4500mgNkg. Soil ammonia/ammonium abundances were found to be 1600mgNkg. N-pool abundances range from 2.5kgm3 for marble to 8.7 for garnet mica schist. Soil N-pools were found to be 3.0kgm3.The inorganic N present as ammonia/ammonium measured in bedrock represents substantial N-input into the geochemical budget. The N in bedrock originates from organically bound nitrogen associated with sediment or geothermal waters representing a mixture of sedimentary, mantle and meteoric sources. Nitrogen in the soil originates from the weathering of N-rich rocks, the presence of unweathered nitrate minerals or fluid inclusions released to the soil and ultimately to plants by assimilation by biota or nitrification. Geologic nitrogen in Kärkevagge rocks and soils may represent a large and reactive pool with the potential for considerable impact on the geochemical system.To investigate the potential responses of nitrogen release, as well as other nutrients to increasing CO2 concentration scenarios, flow-through leaching experiments were conducted on crushed rock samples and soils from the study area. Leaching experiments revealed two to three fold increases in both N and K release with a tripling of carbon dioxide concentrations. Ammonium shows more rapid release than potassium which suggests substantial changes in nitrogen cycling with increasing CO2 enrichment of the atmosphere.

Atmospheric Deposition and Inorganic Nitrogen Flux

Flux of dissolved inorganic nitrogen (DIN—primarily nitrate) from terrestrial ecosystems has been considered an important contributor to acidification of linked aquatic systems. The basis of this concern is the nitrogen (N) saturation hypothesis, positing that additions of N to terrestrial ecosystems in excess of biological requirements will result in DIN leaching. There is a consensus (implicit hypothesis) in the literature that atmospheric deposition of DIN in excess of a threshold of approximately 10 kg ha−1 year−1 leads to significant flux. Diverse data from USA indicate that DIN flux is highly variable both in space and time; the spatial uncertainty as measured by the pooled coefficient of variation is about 0.95, and the temporal (inter-year) uncertainty is about 0.75. The relationship between atmospheric deposition of DIN and annual flux is near-linear within the range of current deposition for US sites (≤8 kg ha−1 year−1 wet deposition). If wet and dry depositions are approximately equal, over 85 % of total DIN deposition is retained. This is nearly equal to the retention reported by the US Geological Survey National Water-Quality Assessment Program, which considered all nonpoint sources of N as inputs and both DIN and organic N as fluxes. Although input–output data have high uncertainty, the 85 % retention of atmospheric DIN by terrestrial watersheds casts doubt on its importance as a contributor to aquatic acidification. There is no obvious threshold of deposition leading to DIN leaching. The nitrogen saturation hypothesis may not fully explain N behavior in terrestrial ecosystems.

Responses of benthic invertebrate assemblages to shifts in the acidity of subtropical mountain streams

Recent economic development in China has been remarkable and so has the scale of associated atmospheric pollutant loading. Forest soil acidification resulting from atmospheric pollutant deposition has become a major concern, particularly in tropical and subtropical forests. Nutrient saturation in southeastern Chinese forest ecosystems has been reported, and soil pH is now in the range of 3.8–4.0. This acidification in forest soil undoubtedly affects water chemistry, and hence forest stream ecosystems. There have been few studies of these impacts on streams. In order to better understand the risks of nitrogen (N) saturation on stream ecosystem in warm and humid climate, where seasonal tropical and subtropical forests are located, we undertook a comparative study of stream faunas in two regions of Dinghushan (DHS) where stream acidification has already occurred and in Heishiding (HSD) where stream acidification has not yet begun, but where forest soils are already acidified. Abundance and species richness o...

The effects of a changing pollution climate on throughfall deposition and cycling in a forested area in southern England

This study compares two sets of measurements of the composition of bulk precipitation and throughfall at a site in southern England with a 20-year gap between them. During this time, SO2 emissions from the UK fell by 82%, NOx emissions by 35% and NH3 emissions by 7%. These reductions were partly reflected in bulk precipitation, with deposition reductions of 56% in SO42−, 38% in NO3−, 32% in NH4+, and 73% in H+. In throughfall under Scots pine, the effects were more dramatic, with an 89% reduction in SO42− deposition and a 98% reduction in H+ deposition. The mean pH under these trees increased from 2.85 to 4.30. Nitrate and ammonium deposition in throughfall increased slightly, however. In the earlier period, the Scots pines were unable to neutralise the high flux of acidity associated with sulphur deposition, even though this was not a highly polluted part of the UK, and deciduous trees (oak and birch) were only able to neutralise it in summer when the leaves were present. In the later period, the sulphur flux had reduced to the point where the acidity could be neutralised by all species — the neutralisation mechanism is thus likely to be largely leaching of base cations and buffering substances from the foliage. The high fluxes are partly due to the fact that these are 60–80year old trees growing in an open forest structure. The increase in NO3− and NH4+ in throughfall in spite of decreased deposition seems likely due to a decrease in foliar uptake, perhaps due to the increasing nitrogen saturation of the catchment soils. These changes may increase the rate of soil microbial activity as nitrogen increases and acidity declines, with consequent effects on water quality of the catchment drainage stream.

大気降下窒素が渓流水に流出する過程

Nitrogen compound deposition as a result of human activities on the terrestrial surface has increased sharply during the last one hundred years, causing ecological problems. One problem is nitrogen saturation in forest ecosystems, which results in the forest ecosystem receiving more nitrogen than required. In recent years, nitrogen oxide emissions have been decreasing in Japan; however, the total deposition of nitrogen is not decreasing because of cross-boundary emissions from other countries. As depositions in forest ecosystems increase, the likelihood of nitrogen saturation increases. At the same time, certain species of trees, geological conditions, and climate will affect nitrogen saturation. In Japan, there is heavy precipitation in summer; therefore, decreases in nitrate concentrations of stream water cannot be observed. Stable isotope analysis is the key to elucidating nitrogen dynamics between the atmosphere and rivers. From the atmosphere to stream water, nitrogen compounds have many chemical and biological reactions. Because a stable isotope is an indicator of the progress of these reactions, frequent analyses of stable isotope compositions reveals how nitrogen moves from the atmosphere to stream

Vascularization of Air Sinuses and Fat Bodies in the Head of the Bottlenose Dolphin (Tursiops truncatus): Morphological Implications on Physiology

Cetaceans have long been considered capable of limiting diving-induced nitrogen absorption and subsequent decompression sickness through a series of behavioral, anatomical, and physiological adaptations. Recent studies however suggest that in some situations these adaptive mechanisms might be overcome, resulting in lethal and sublethal injuries. Perhaps most relevant to this discussion is the finding of intravascular gas and fat emboli in mass-stranded beaked whales. Although the source of the gas emboli has as yet to been ascertained, preliminary findings suggest nitrogen is the primary component. Since nitrogen gas embolus formation in divers is linked to nitrogen saturation, it seems premature to dismiss similar pathogenic mechanisms in breath-hold diving cetaceans. Due to the various anatomical adaptations in cetacean lungs, the pulmonary system is thought of as an unlikely site of significant nitrogen absorption. The accessory sinus system on the ventral head of odontocete cetaceans contains a sizeable volume of air that is exposed to the changing hydrostatic pressures during a dive, and is intimately associated with vasculature potentially capable of absorbing nitrogen through its walls. The source of the fat emboli has also remained elusive. Most mammalian fat deposits are considered poorly vascularized and therefore unlikely sites of intravascular introduction of lipid, although cetacean blubber may not be as poorly vascularized as previously thought. We present new data on the vasculature of air sinuses and acoustic fat bodies in the head of bottlenose dolphins and compare it to published accounts. We show that the mandibular fat bodies and accessory sinus system are associated with extensive venous plexuses and suggest potential physiological and pathological implications.

Changes in atmospheric deposition and streamwater chemistry over 25 years in undisturbed catchments in a Mediterranean mountain environment

Surface water chemistry has changed in response to reduced atmospheric deposition of sulphur and acidity in many regions of Europe and North America. Most of these studies come from acidic or low-alkalinity surface waters under high acidic deposition. Mediterranean climates offer a different biogeochemical context, characterised by streamwaters of higher alkalinity and low acid inputs. In this paper, we use surveys of streamwater chemistry conducted in 1981–1984 and again in 2007 in the Montseny natural park (NE Spain) to test whether streamwaters of these well-buffered catchments respond to changes in atmospheric deposition, which has declined for S during the last decades in NE Spain while remaining about stable for nitrogen. The 23 sampled streams drained heathland, beech forests and evergreen oak forests in relatively undisturbed small catchments underlain by silicate bedrock. Bulk deposition of sulphate at Montseny decreased by 54% while nitrate bulk deposition increased (non-significantly) by 30% in this period. Total N deposition is estimated in the range 15–30kg Nha−1y−1 for NE Spain. This is well above threshold values (e.g. 10kg Nha−1y−1) reported as starting nitrogen saturation symptoms in forest ecosystems in Europe. Baseflow sulphate concentrations decreased on average by 47μeqL−1 or 29% of early 1980s concentrations. Baseflow mean nitrate concentrations increased significantly but only from 5.5 to 8.9μeqL−1. Thus, despite decades of high N deposition, these ecosystems appear to be still far from N saturation. Baseflow alkalinity and base cation concentrations increased substantially, probably a combined result of decreased S deposition, enhanced silicate weathering under current higher temperatures, reduced plant cation uptake as vegetation matures, and slightly drier conditions in the survey of 2007. Overall, these well-buffered catchments have shown sizable changes in baseflow chemistry in response to changed atmospheric deposition and other environmental changes.

Predicting forest site productivity in temperate lowland from forest floor, soil and litterfall characteristics using boosted regression trees

The aim of this study is on the one hand to identify the most determining variables predicting the site productivity of pedunculate oak, common beech and Scots pine in temperate lowland forests of Flanders; and on the other hand to test whether the accuracy of site productivity models based exclusively on soil or forest floor predictor variables is similar to the accuracy achieved by full ecosystem models, combining all soil, vegetation, humus and litterfall composition related variables. Boosted Regression Trees (BRT) were used to model in a climatically homogeneous region the relationship between environmental variables and site productivity. A distinction was made between soil (soil physical and chemical), forest floor (vegetation and humus) and ecosystem (soil, forest floor and litterfall composition jointly) predictors. Our results have illustrated the strength of BRT to model the non-linear behaviour of ecological processes. The ecosystem models, based on all collected variables, explained most of the variability and were more accurate than those limited to either soil or forest floor variables. Nevertheless, both the soil and forest floor models can serve as good predictive models for many forest management practices. Soil granulometric fractions and litterfall nitrogen concentrations were the most effective predictors of forest site productivity in Flanders. Although many studies revealed a fertilising effect of increased nitrogen deposition, nitrogen saturation seemed to reduce species’ productivity in this region.

Linking Carbon Saturation Concepts to Nitrogen Saturation and Retention

Recent advances in soil C saturation concepts have increased our understanding of soil C storage and mineralization without explicit links to N retention and saturation theories. Here, we exploit soil texture and organic matter (OM) gradients in a Maryland, USA hardwood forest to test hypotheses that link soil organic C saturation with soil 15N retention and nitrification. At our site, mineral-associated OM (MAOM) N concentrations in the silt + clay particle fraction (g MAOM-N g silt + clay−1) were negatively correlated with the fraction of NH4-N transferred to MAOM during a 3-day in situ incubation (R = −0.85), but positively correlated with potential net nitrification (R = 0.76). Moreover, the fraction of NH4-N transferred to MAOM was negatively correlated with potential net nitrification (R = −0.76). Due to physico-chemical stabilization mechanisms, MAOM is considered to be resistant to mineralization. Carbon saturation theory suggests that the proportion of new C inputs that can be stabilized in MAOM decreases in proportion to the amount of C already present in the fraction; C inputs not stabilized in MAOM are susceptible to rapid mineralization. We demonstrate that NH4-N stabilization in MAOM is similar to C stabilization in MAOM and associated with nitrification, thereby extending soil C saturation theory to mineral N and linking it with N retention and saturation theories. These data and concepts complement N saturation models that emphasize vegetation type, N input levels, and microbial turnover. Incorporating the OM retention capacity of fine mineral particles into N saturation theory can improve predictions of N saturation rates and resolve inconsistent relationships between soil organic matter, texture, N mineralization, and N retention.

Assessment of impacts of charcoal production on soil properties in the derived savanna, Oyo state, Nigeria

This study evaluated the impacts of charcoal production on soil properties in the derived savanna zone of South Western, Nigeria. Ten soil samples were collected randomly at the depth of 0 to10 cm in each of the charcoal production sites (CPS) and adjacent field sites (AFS) which is the control site. All soil samples collected were subjected to laboratory analysis for soil pH, particle size composition, available phosphorus, organic carbon, total nitrogen, exchangeable potassium, calcium, sodium, magnesium, cation exchangeable capacity and base saturation. The mean of each of these soil properties was used for comparison and t-test was also used to determine the significant difference that exists in each soil property. The results of the analysis showed that the soils are texturally similar at both the CPS and AFS sites. The soil pH under CPS is 6.75 while it is 5.96 under AFS. The mean of the available phosphorous under CPS is 11.21 ppm while it is 2.07 ppm under AFS. The analysis reveals a slight increase in exchangeable calcium, sodium, magnesium, total nitrogen, organic carbon and base saturation contents of the soil in CPS while the amount of cation exchange capacity decreases in CPS with the mean value of 2.32 meq/100 g than the AFS with the mean value of 2.37 me/100 g. It is therefore recommended among others that there should be awareness to consider the age of trees, species and biomass before trees are cut down for charcoal production. Key words: Charcoal production, soil properties, assessment, derived savanna, Ibarapa East, Nigeria.

Climate-mediated nitrogen and carbon dynamics in a tropical watershed

[1] Climate variability affects the capacity of the biosphere to assimilate and store important elements, such as nitrogen and carbon. Here we present biogeochemical evidence from the sediments of tropical Lake Titicaca indicating that large hydrologic changes in response to global glacial cycles during the Quaternary were accompanied by major shifts in ecosystem state. During prolonged glacial intervals, lake level was high and the lake was in a stable nitrogen-limited state. In contrast, during warm dry interglacials lake level fell and rates of nitrogen concentrations increased by a factor of 4–12, resulting in a fivefold to 24-fold increase in organic carbon concentrations in the sediments due to increased primary productivity. Observed periods of increased primary productivity were also associated with an apparent increase in denitrification. However, the net accumulation of nitrogen during interglacial intervals indicates that increased nitrogen supply exceeded nitrogen losses due to denitrification, thereby causing increases in primary productivity. Although primary productivity in tropical ecosystems, especially freshwater ecosystems, tends to be nitrogen limited, our results indicate that climate variability may lead to changes in nitrogen availability and thus changes in primary productivity. Therefore some tropical ecosystems may shift between a stable state of nitrogen limitation and a stable state of nitrogen saturation in response to varying climatic conditions.

A New Conceptual Model of Nitrogen Saturation Based on Experimental Nitrogen Addition to an Oak Forest

The dominant conceptual model of nitrogen (N) saturation in forests predicts the temporal patterns of key N cycling indicators as an initially N-limited forest is progressively enriched in N. We present the results from a long-term N addition experiment in an oak forest in southeastern New York State, USA, which do not conform to the predictions of the conceptual model in several ways. In contrast to the predictions of the conceptual model, the foliar N concentrations in the N-treated stands of our study increased to about 20% above the levels in the control stands and then remained essentially constant, and nitrogen leaching from the treated stands increased almost immediately after the start of the experiment, prior to the onset of elevated nitrification. Concentrations of N in soil solution of the N-treated stands peaked at over 150-fold greater than the concentrations in the control stands. There were no significant changes in potential net N mineralization. Tree mortality increased in the treated stands, but the tree mortality did not appear to be the primary cause of the excess nitrate leaching. Based on these results and those of other recent studies, we present a new conceptual model of the N saturation process focused on the mass balance of N rather than the temporal dynamics of N cycling indicators. The mass balance is characterized by inputs of N from atmospheric deposition and fertilization, internal sinks in the vegetation and soils, and outputs to leaching and gaseous losses. The key points of the conceptual model are (1) added N can flow simultaneously to all sinks and losses in the system, (2) the fate of the added N and the temporal patterns of flow of N depend on the strength of the sinks and the factors that control them, and (3) the movement of N to the various sinks determines how N saturation is manifested in the ecosystem. We distinguish capacity N saturation, in which the sinks in the vegetation and soil are zero or negative, from kinetic N saturation, in which the sinks are positive but lower than the N input rate. The sink strengths in the vegetation and soil have two components, one due to carbon (C) accumulation in the system and the other due to change in the stoichiometry (C:N ratio) of the pool. Further work quantifying the magnitudes and controlling factors for the N sinks will allow better prediction of the dynamics of N saturation in different types of forested ecosystems.

Aboveground biomass response to increasing nitrogen deposition on grassland on the northern Loess Plateau of China

The fragile ecosystem of China's Loess Plateau is being exposed to increasing atmospheric nitrogen deposition but little information about the response of the region's natural vegetation is currently available. We studied the responses of aboveground biomass (AGB) to simulated nitrogen deposition in a field experiment conducted on natural grassland on sunny and shady slopes. Three levels of simulated nitrogen deposition were applied, and two treatments with phosphorus were included to test for secondary phosphorus limitation. For the same level of nitrogen deposition, grass generally grew better on the shady slope than on the sunny slope. Compared to a control treatment with no additional nitrogen, we found: (1) the 2.5 g N m−2 yr−1 treatment significantly increased biomass only on the sunny slope; (2) the total AGB increased significantly in the 5 g N m−2 yr−1 treatment on both the shady slope (by 31%) and the sunny slope (by 25%); and (3) for 10 g N m−2 yr−1, AGB was also significantly increased, however, phosphorus limitation became more apparent, and soil nitrate N levels increased significantly, suggesting nitrogen saturation and the potential for nitrate pollution. The AGB of Stipa bungeana (the dominant grass) was significantly increased by nitrogen, but not by phosphorus. The biomass of the second dominant species Lespedeza davurica Schindl., was not affected by increased nitrogen but addition of phosphorus had some positive impact. Therefore, nitrogen deposition was proven to have effects on plant growth in our study area on the Loess Plateau of China, but high level of nitrogen deposition would result in P limitation. Furthermore, increasing nitrogen deposition is likely to induce diversity change.

Early post-fire changes in Pinus brutia forests (Amanos Mountains, Turkey)

Early post-fire changes inPinus brutiaforests (Amanos Mountains, Turkey)We studied the species composition and soil nutrients in aPinus brutiaforest after a fire that occurred in 1989. Four permanent plots were created in the burnt and not burnt areas in the Amanos Mountains of Turkey. The floristic richness, biological spectra, above ground phytomass and soil features in the study areas were assessed during the first three years after the fire. After the fire, we found a reduced amount of organic matter (14.3%), total nitrogen (22%) and soil water saturation (13.1%), but an increased amount of available phosphorus (71%), acidity (3.6%), cation exchange capacity (9.9%), exchangeable sodium (20.8%) and exchangeable potassium (37.1%). The aboveground phytomass in the burned area reached 5284 kg ha-1, the third year after the fire. Forty-six pre-fire species were renewed in the first three years after the fire.Juniperus oxycedruscould not renew within three years after the fire. Pine phytomass has increased five times within three years after the fire.

Regional and local patterns of soil nutrients at Rocky Mountain treelines

The soils across treeline should vary because of direct effects of biological differences of coniferous subalpine forest and the herbaceous alpine tundra in Colorado. In addition, the change in life form may indirectly affect soils because of interactions of the vegetation and wind-driven deposition processes. This is particularly important as nitrogen (N) saturation is a growing concern in high elevation ecosystems, and treeline is predicted to be a deposition hotspot. The vegetation transition at treeline provides an opportunity to test the effects of vegetation, topography, and external inputs on soils at three spatial scales. First, a regional evaluation of soils at eleven abrupt treeline sites was made comparing sites on east and west aspects both east and west of the Continental Divide (CD). Second, soils were compared in the adjacent forest and tundra. Finally, edge effects were assessed along transects spanning treeline. At the regional scale, total soil N was higher east of the CD and on east aspects while exchangeable calcium was higher on east aspects and at sites west of the CD. Higher lead (Pb) concentration in the forest organic horizon was associated with lower 206Pb/ 207Pb ratios, an indication of greater anthropogenic Pb inputs. However, the spatial pattern in soil Pb suggests a different source area or transport mechanism than N. Within individual sites, the soils differed between the forest and tundra in almost every measured variable, but edge effects were minimal on both sides of these abrupt treelines. While a direct link between the observed soil patterns to deposition of external inputs cannot be made based on the study design, the observed soil patterns suggest that the impacts of acid deposition are amplified or attenuated by processes such as dust deposition.

Recovery of ectomycorrhiza after ‘nitrogen saturation’ of a conifer forest

Trees reduce their carbon (C) allocation to roots and mycorrhizal fungi in response to high nitrogen (N) additions, which should reduce the N retention capacity of forests. The time needed for recovery of mycorrhizas after termination of N loading remains unknown. Here, we report the long-term impact of N loading and the recovery of ectomycorrhiza after high N loading on a Pinus sylvestris forest. We analysed the N% and abundance of the stable isotope (15) N in tree needles and soil, soil microbial fatty acid biomarkers and fungal DNA. Needles in N-loaded plots became enriched in (15) N, reflecting decreased N retention by mycorrhizal fungi and isotopic discrimination against (15) N during loss of N. Meanwhile, needles in N-limited (control) plots became depleted in (15) N, reflecting high retention of (15) N by mycorrhizal fungi. N loading was terminated after 20yr. The δ(15) N and N% of the needles decreased 6yr after N loading had been terminated, and approached values in control plots after 15yr. This decrease, and the larger contributions compared with N-loaded plots of a fungal fatty acid biomarker and ectomycorrhizal sequences, suggest recovery of ectomycorrhiza. High N loading rapidly decreased the functional role of ectomycorrhiza in the forest N cycle, but significant recovery occurred within 6-15yr after termination of N loading.