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

Comparison of vertical and horizontal atmospheric deposition of nitrate at Central European mountain-top sites during three consecutive winters

Nitrogen (N) deposition, a key process of atmospheric self-cleaning, represents an important pathway for nutrients and pollutants to ecosystems. Enhanced N deposition flux contributes to acidification, eutrophication and loss of biodiversity. N-NO₃^- concentrations in rime and snow were measured at 10 Czech plots situated in borderline mountains in 2009-2011 winters. The results were put in context with data-driven geostatistical modelling results of annual wet vertical and horizontal deposition. Our hypotheses were that: (i) rime and snow would be more polluted in the highly industrialized north than in the south, (ii) the N-NO₃^- concentrations would differ in the three winters studied, and (iii), that N-NO₃^- rime deposition is not negligible in Central European mountain ranges. Our results indicated that winter N-NO₃^- concentrations were significantly higher in rime than in snow and that there were much larger between-site differences in N-NO₃^- concentrations for rime than for snow. Relatively large differences were found between individual years. Atmospheric input of N-NO₃^- in winter was dominated by vertical deposition, i.e., snow. Modelled results showed that mean winter rime deposition corresponded to about 6-25 %, and mean winter snow deposition made up 25-72.5 % of mean annual N-NO₃^- wet-only deposition. Model N-NO₃^-occult deposition estimated from throughfall and total (wet and dry) deposition is highly uncertain, however: N throughfall is not a relevant proxy for estimation of realistic total N deposition due to N exchange between the tree canopy and atmosphere.

Long-term stabilization of 15N-labeled experimental NH4+ deposition in a temperate forest under high N deposition

High nitrogen (N) deposition levels, currently present in many industrial and agricultural regions of the world, can strongly affect the functioning of forest ecosystems. In a pine forest with strong N leaching, located in the Netherlands, we studied the long-term fate of a year-long NH₄⁺ deposition cohort labeled with ¹⁵N. A high ambient and a low N deposition treatment had been established at the site by means of a roof and sprinklers. Resampling the N pools 19 years after labeling and 11 years after the last sampling, we found similar ¹⁵N deltas in needles, twigs and the LF1 organic soil layer of each treatment, indicating intensive N cycling among these pools. In the last 11 years, label recovery decreased in these labile pools, while recovery remained constant in wood and increased in bark. Together these aboveground vegetation pools retained less than 3% of the labeled N. In the organic layers, label recovery after 19 years decreased to 23% in both treatments, while in the mineral soil it increased from 4% to 13% (high N) and from 3% to 29% (low N treatment). Within the mineral soil of the high N treatment the labeled N was mainly found in fine roots, while in the low N treatment most N was incorporated in the two soil density fractions, shifting to the high density fraction with depth. This suggests a low capacity of the mineral soil at high N deposition to incorporate N. After the labeled N had been lost substantially in previous years, especially in the first, its presence remained constant in the last 11 years at 38% (high N) and 54% (low N treatment). Apparently, even in this strongly N leaching ecosystem, N once incorporated, was retained well and did not affect the input-output fluxes of the system.

Throughfall reduction diminished the enhancing effect of N addition on soil N leaching loss in an old, temperate forest

Soil nitrogen (N) leaching is recognized to have negative effects on the environment. There is a lack of studies on different simultaneously occurring drivers of environmental change, including changing rainfall and N deposition, on soil N leaching. In this study, a two factorial field experiment was conducted in a Korean pine forest with the following four treatments: 30% of throughfall reduction (TR), 50 kg N ha^-1 yr^-1 of N addition (N+), throughfall reduction plus N addition (TRN+) and natural forest (CK). The zero-tension pan lysimeter method was used to assess the response of soil N leaching loss to manipulated N addition and throughfall reduction. The results showed that the soil N leaching loss in natural forest was 5.0 ± 0.4 kg N ha^-1yr^-1, of which dissolved organic nitrogen (DON) accounted for 48%. Compared to natural forest, six years of N addition (NH₄NO₃, 50 kg N ha^-1 year^-1) significantly (P < 0.05) increased soil N leaching losses by 122%, especially in the form of NO₃^-; a 30% reduction in throughfall slightly decreased N leaching losses by 23%; in combination, N addition and throughfall reduction increased N leaching losses by 48%. There was a strong interaction between N addition and throughfall reduction, which decreased N leaching loss by approximately 2.5 kg N ha^-1 yr^-1. Our results indicated that drought would diminish the enhancing effect of N deposition on soil N leaching. These findings highlight the importance of incorporating both N deposition and precipitation and their impacts on soil N leaching into future N budget assessments of forest ecosystems under global environmental change.

Evaluation of Forest Conversion Effects on Soil Erosion, Soil Organic Carbon and Total Nitrogen Based on 137Cs Tracer Technique

Soil erosion can affect the horizontal and the vertical distribution of soil carbon at the landscape scale. The 137Cs tracer technique can overcome the shortcomings of traditional erosion research and has proven to be the best method to study soil erosion. To understand the responses of soil organic carbon and nitrogen to soil erosion and forest conversion in the development of slope economic forests in rocky mountain areas, three representative types of economic forests that were all formed after clear-cutting and afforestation on the basis of CBF (coniferous and broad-leaved mixed forests) were selected: CF (chestnut forests) with small human disturbance intensity, AF (apple forests), and HF (hawthorn forests) with high interference intensity. The results showed that all land use types have significantly eroded since 1950; the average annual loss of soil was 0.79 mm in the CBF, 2.31 mm in the AF, 1.84 mm in the HF, and 0.87 mm in the CF. The results indicated aggravation of soil erosion after the transformation of the CBF into an economic forest. The economic forest management reduced the average carbon storage and accelerated nutrient loss. The better vegetation coverage and litter coverage of CF made them stand out among the three economic forest varieties. Therefore, when developing economic forests, we should select species that can produce litter to ensure as much soil conservation as possible to reduce the risk of soil erosion.

Experimental addition of nitrogen to a whole forest ecosystem at Gårdsjön, Sweden (NITREX): Nitrate leaching during 26 years of treatment

Chronic high deposition of nitrogen (N) to forest ecosystems can lead to increased leaching of inorganic N to surface waters, enhancing acidification and eutrophication. For 26 years nitrogen has been added as ammonium nitrate (NH₄NO₃) at 40 kg N ha^-1 yr^-1 to a whole forested catchment ecosystem at Gårdsjön, Sweden, to experimentally simulate the transition from a N-limited to N-rich state. Over the first 10 years of treatment there was an increasing amount of nitrate (NO₃^-) and to a lesser extent ammonium (NH₄⁺) lost in runoff, but then N leaching stabilised, and for the subsequent 16 years the fraction of N added lost in runoff remained at 9%. NO₃^- concentrations in runoff were low in the summer during the first years of treatment, but now are high throughout the year. High frequency sampling showed that peaks in NO₃^- concentrations generally occurred with high discharge, and were enhanced if high discharge coincided with occasions of N addition. Approximately 50% of the added N has gone to the soil. The added N is equivalent to 140 years of ambient N deposition. At current ambient levels of N deposition there thus appears to be no immediate risk of N saturation at this coniferous forest ecosystem, and by inference to other such N-limited forests in Scandinavia.

Phytoplankton response to whole lake inorganic N fertilization along a gradient in dissolved organic carbon

Global change has increased inorganic nitrogen (N) and dissolved organic carbon (DOC; i.e., "browning") inputs to northern hemisphere boreal lakes. However, we do not know how phytoplankton in nutrient poor lake ecosystems of different DOC concentration respond to increased N availability. Here, we monitored changes in phytoplankton production, biomass and community composition in response to whole lake inorganic N fertilization in six boreal unproductive Swedish lakes divided into three lake pairs (control, N enriched) at three DOC levels (low, medium, high), with one reference year (2011) and 2 impact yr (2012, 2013). We found that phytoplankton biomass and production decreased with DOC concentration before N fertilization. Further, phytoplankton community composition also differed with respect to DOC, with a dominance of non-flagellated autotrophs at low DOC towards an increasing dominance of flagellated autotrophs with increased lake DOC concentration. The N fertilization increased phytoplankton biomass and production in all lakes, but did not affect phytoplankton community composition. However, the net response in biomass and production to N fertilization declined with increasing DOC, implying that the lake DOC concentration is critical in order to infer phytoplankton responses to N fertilization, and that the system switches from being primarily nutrient limited to becoming increasingly light limited with increased DOC concentration. In conclusion, our results show that browning will reduce phytoplankton production and biomass and influence phytoplankton community composition, whereas increased inorganic N loadings from deposition, forestry or other land use will primarily enhance phytoplankton biomass and production. Together, any change in the landscape that enhances inorganic N availability will increase phytoplankton production and biomass, but the positive effects of N will be much weaker or even neutralized in browner lakes as caused by light limitation.

Nitrogen effects on the pelagic food web are modified by dissolved organic carbon

Global environmental change has altered the nitrogen (N) cycle and enhanced terrestrial dissolved organic carbon (DOC) loadings to northern boreal lakes. However, it is still unclear how enhanced N availability affects pelagic food web efficiency (FWE) and crustacean zooplankton growth in N limited boreal lakes. Here, we performed in situ mesocosm experiments in six unproductive boreal Swedish lakes, paired across a DOC gradient, with one lake in each pair fertilized with N (2011: reference year; 2012, 2013: impact years). We assessed how zooplankton growth and FWE were affected by changes in pelagic energy mobilization (PEM), food chain length (phytoplankton versus bacterial production based food chain, i.e. PP:BP), and food quality (seston stoichiometry) in response to N fertilization. Although PP, PEM and PP:BP increased in low and medium DOC lakes after N fertilization, consumer growth and FWE were reduced, especially at low DOC—potentially due to reduced phytoplankton food quality [increased C: phosphorus (P); N:P]. At high DOC, N fertilization caused modest increases in PP and PEM, with marginal changes in PP:BP and phytoplankton food quality, which, combined, led to a slight increase in zooplankton growth and FWE. Consequently, at low DOC (<12 mg L⁻¹), increased N availability lowers FWE due to mismatches in food quality demand and supply, whereas at high DOC this mismatch does not occur, and zooplankton production and FWE may increase. We conclude that the lake DOC level is critical for predicting the effects of enhanced inorganic N availability on pelagic productivity in boreal lakes.

Spatially valid data of atmospheric deposition of heavy metals and nitrogen derived by moss surveys for pollution risk assessments of ecosystems

For analysing element input into ecosystems and associated risks due to atmospheric deposition, element concentrations in moss provide complementary and time-integrated data at high spatial resolution every 5 years since 1990. The paper reviews (1) minimum sample sizes needed for reliable, statistical estimation of mean values at four different spatial scales (European and national level as well as landscape-specific level covering Europe and single countries); (2) trends of heavy metal (HM) and nitrogen (N) concentrations in moss in Europe (1990-2010); (3) correlations between concentrations of HM in moss and soil specimens collected across Norway (1990-2010); and (4) canopy drip-induced site-specific variation of N concentration in moss sampled in seven European countries (1990-2013). While the minimum sample sizes on the European and national level were achieved without exception, for some ecological land classes and elements, the coverage with sampling sites should be improved. The decline in emission and subsequent atmospheric deposition of HM across Europe has resulted in decreasing HM concentrations in moss between 1990 and 2010. In contrast, hardly any changes were observed for N in moss between 2005, when N was included into the survey for the first time, and 2010. In Norway, both, the moss and the soil survey data sets, were correlated, indicating a decrease of HM concentrations in moss and soil. At the site level, the average N deposition inside of forests was almost three times higher than the average N deposition outside of forests.

Surface runoff and nitrogen (N) loss in a bamboo (Phyllostachys pubescens) forest under different fertilization regimes

Nitrogen (N) losses from agricultural fields have been extensively studied. In contrast, surface runoff and N losses have rarely been considered for bamboo forests that are widespread in regions such as southern China. The thriving of bamboo industries has led to increasing fertilizer use in bamboo forests. In this study, we evaluated surface runoff and N losses in runoff following different fertilization treatments under field conditions in a bamboo (Phyllostachys pubescens) forest in the catchment of Lake Taihu in Jiangsu, China. Under three different fertilization regimes, i.e., control, site-specific nutrient management (SSNM), and farmer's fertilization practice (FFP), the water runoff rate amounted to 356, 361, and 342 m(3) ha(-1) and accounted for 1.91, 1.98, and 1.85% of the water input, respectively, from June 2009 to May 2010. The total N losses via surface runoff ranged from 1.2 to 1.8 kg ha(-1). Compared with FFP, the SSNM treatment reduced total nitrogen (TN) and dissolved nitrogen (DN) losses by 31 and 34%, respectively. The results also showed that variations in N losses depended mainly on runoff fluxes, not N concentrations. Runoff samples collected from all treatments throughout the year showed TN concentrations greater than 0.35 mg L(-1), with the mean TN concentration in the runoff from the FFP treatment reaching 8.97 mg L(-1). The loss of NO3(-)-N was greater than the loss of NH4(+)-N. The total loss of dissolved organic nitrogen (DON) reached 23-41% of the corresponding DN. Therefore, DON is likely the main N species in runoff from bamboo forests and should be emphasized in the assessment and management of N losses in bamboo forest.

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.

Nitrogen leaching and acidification during 19 years of NH₄NO₃ additions to a coniferous-forested catchment at Gårdsjön, Sweden (NITREX)

The role of nitrogen (N) in acidification of soil and water has become relatively more important as the deposition of sulphur has decreased. Starting in 1991, we have conducted a whole-catchment experiment with N addition at Gårdsjön, Sweden, to investigate the risk of N saturation. We have added 41 kg N ha(-1) yr(-1) as NH(4)NO(3) to the ambient 9 kg N ha(-1) yr(-1) in fortnightly doses by means of sprinkling system. The fraction of input N lost to runoff has increased from 0% to 10%. Increased concentrations of NO(3) in runoff partially offset the decreasing concentrations of SO(4) and slowed ecosystem recovery from acid deposition. From 1990-2002, about 5% of the total N input went to runoff, 44% to biomass, and the remaining 51% to soil. The soil N pool increased by 5%. N deposition enhanced carbon (C) sequestration at a mean C/N ratio of 42-59 g g(-1).

The evolution of the science of Bear Brook Watershed in Maine, USA

The Bear Brook Watershed in Maine (BBWM), USA is a paired watershed study with chemical manipulation of one watershed (West Bear = WB) while the other watershed (East Bear = EB) serves as a reference. Characterization of hydrology and chemical fluxes occurred in 1987-1989 and demonstrated the similarity of the ca. 10 ha adjacent forested watersheds. From 1989-2010, we have added 1,800 eq (NH(4))(2)SO(4) ha(-1) y(-1) to WB. EB runoff has slowly acidified even as atmospheric deposition of SO4(-2) has declined. EB acidification included decreasing pH, base cation concentrations, and alkalinity, and increasing inorganic Al concentration, as SO4(-2) declined. Organic Al increased. WB has acidified more rapidly, including a 6-year period of increasing leaching of base cations, followed by a long-term decline of base cations, although still elevated over pretreatment values, as base saturation declined in the soils. Sulfate in WB has not increased to a new steady state because of increased anion adsorption accompanying soil acidification. Dissolved Al has increased dramatically in WB; increased export of particulate Al and P has accompanied the acidification in both watersheds, WB more than EB. Nitrogen retention in EB increased after 3 years of study, as did many watersheds in the northeastern USA. Nitrogen retention in WB still remains at over 80%, in spite of 20+ years of N addition. The 20-year chemical treatment with continuous measurements of critical variables in both watersheds has enabled the identification of decadal-scale processes, including ecosystem response to declining SO4(-2) in ambient precipitation in EB and evolving mechanisms of treatment response in WB. The study has demonstrated soil mechanisms buffering pH, declines in soil base saturation, altered P biogeochemistry, unexpected mechanisms of storage of S, and continuous high retention of treatment N.

Modeling future acidification and fish populations in Norwegian surface waters

Despite great progress made in the past 25 years, acid deposition continues to cause widespread damage to the environment in Europe and eastern North America. Legislation to limit emissions of sulfur and nitrogen compounds in Europe is now under revision. The most recent protocol was based in part on the critical loads concept. The new protocol may also take into consideration the time delays between dose and response inherent in natural ecosystems. Policy decisions to reduce adverse effects on ecosystems entail a trade-off: quick response will require deeper cuts in emissions and thus higher costs, whereas lower costs with lesser cuts in emissions will give slower response. Acidification of lakes and damage to fish populations in Norway is used as an example. Under current legislation for emission reductions, surface waters will continue to slowly recover, but for many decades lakes in about 18% of Norway will continue to have water quality insufficient to support healthy populations of brown trout and other indicator organisms. Additional emission reductions can speed up the rate and degree of recovery.

Rapid immobilisation and leaching of wet-deposited nitrate in upland organic soils

Nitrate (NO3-) is often observed in surface waters draining terrestrial ecosystems that remain strongly nitrogen (N) limited. It has been suggested that this occurs due to hydrological bypassing of soil or vegetation N retention, particularly during high flows. To test this hypothesis, artificial rain events were applied to 12 replicate soil blocks on a Welsh podzolic acid grassland hillslope, labelled with 15N-enriched NO3- and a conservative bromide (Br-) tracer. On average, 31% of tracer-labelled water was recovered within 4 h, mostly as mineral horizon lateral flow, indicating rapid vertical water transfer through the organic horizon via preferential flowpaths. However, on average only 6% of 15N-labelled NO3- was recovered. Around 80% of added NO3- was thus rapidly immobilised, probably by microbial communities present on the surfaces of preferential flowpaths. Transitory exceedance of microbial N-uptake capacity during periods of high water and N flux may therefore provide a mechanism for NO3- leaching.

Modelling nitrogen saturation and carbon accumulation in heathland soils under elevated nitrogen deposition

A simple model of nitrogen (N) saturation, based on an extension of the biogeochemical model MAGIC, has been tested at two long-running heathland N manipulation experiments. The model simulates N immobilisation as a function of organic soil C/N ratio, but permits a proportion of immobilised N to be accompanied by accumulation of soil carbon (C), slowing the rate of C/N ratio change and subsequent N saturation. The model successfully reproduced observed treatment effects on soil C and N, and inorganic N leaching, for both sites. At the C-rich upland site, N addition led to relatively small reductions in soil C/N, low inorganic N leaching, and a substantial increase in organic soil C. At the C-poor lowland site, soil C/N ratio decreases and N leaching increases were much more dramatic, and soil C accumulation predicted to be smaller. The study suggests that (i) a simple model can effectively simulate observed changes in soil and leachate N; (ii) previous model predictions based on a constant soil C pool may overpredict future N leaching; (iii) N saturation may develop most rapidly in dry, organic-poor, high-decomposition systems; and (iv) N deposition may lead to significantly enhanced soil C sequestration, particularly in wet, nutrient-poor, organic-rich systems.