Critical Loads Of Nutrient Nitrogen Research Articles

Acid wet-deposition modeling sensitivity to WRF-CMAQ planetary boundary layer schemes and exceedance of critical loads over an industrializing coastal valley in northwestern British Columbia, Canada

Computational tools used to implement the critical-load approach of atmospheric deposition impact often do not elucidate modeling uncertainty making it difficult for environmental policy-makers to know how much confidence to put in its results, also hampering aspects that may need improving. This study evaluated acid deposition modeling for various parameterizations of the planetary boundary-layer (PBL) over the Terrace-Kitimat valley, a physiographically complex region of northwestern British Columbia. Of five schemes, simulations with the Mellor-Yamada-Nakanishi-Niino Level 3 and Mellor-Yamada-Janjic PBL schemes best captured weekly wet deposition fluxes of acidifying ions (SO42−, NO3−, NH4+) within a factor of 2 of observations at an industrial fence line station. Alongside the Yonsei University PBL scheme, these two schemes slightly overestimated the chemical species at a station that was distant from major anthropogenic precursor sources in the valley, hence useful for worst-scenario projections of atmospheric deposition on the natural environment. Forest soils in the vicinity of a large aluminum smelter in Kitimat was estimated to be in exceedance of critical load of acidity by 30.1–53.5 kg S ha−1 yr−1. Exceedance of critical load of nutrient nitrogen restricted to the Terrace area (≤ 7 km2), ranged between none and 0.71 kg ha−1 yr−1. This work provides guidance for using PBL schemes in the Weather Research and Forecasting model that is coupled to a deposition model when calculating critical-load exceedance over temperate, rugged, coastal geographies.

Vegetation community change points suggest that critical loads of nutrient nitrogen may be too high

It is widely accepted that elevated nitrogen deposition can have detrimental effects on semi-natural ecosystems, including changes to plant diversity. Empirical critical loads of nutrient nitrogen have been recommended to protect many sensitive European habitats from significant harmful effects. In this study, we used Threshold Indicator Taxa Analysis (TITAN) to investigate shifts in vegetation communities along an atmospheric nitrogen deposition gradient for twenty-two semi-natural habitat types (as described under Annex I of the European Union Habitats Directive) in Ireland. Significant changes in vegetation community, i.e., change points, were determined for twelve habitats, with seven habitats showing a decrease in the number of positive indicator species. Community-level change points indicated a decrease in species abundance along a nitrogen deposition gradient ranging from 3.9 to 15.3 kg N ha−1 yr−1, which were significantly lower than recommended critical loads (Wilcoxon signed-rank test; V = 6, p < 0.05). These results suggest that lower critical loads of empirical nutrient nitrogen deposition may be required to protect many European habitats. Changes to vegetation communities may mean a loss of sensitive indicator species and potentially rare species in these habitats, highlighting how emission reductions policies set under the National Emissions Ceilings Directive may be directly linked to meeting the goal set out under the European Union’s Biodiversity Strategy of “halting the loss of biodiversity” across Europe by 2020.

Critical loads and nitrogen availability under deposition and harvest scenarios for conifer forests in Ireland

In this study we calculated the critical load of nutrient nitrogen (N) for Irish forest plots (n=380) under two harvesting scenarios: conventional stem-only harvest (SOH) and stem plus branch harvest (SBH) and two deposition scenarios: current and with a 10% increase in reduced-N. In addition, current N status was assessed using the following data from forest monitoring plots: forest floor C:N, foliar N and plant root simulation (PRS™) probe N supply rate. Average critical loads were 15.3kgNha−1year−1 under SOH and 19.5kgNha−1year−1 under SBH. Average total (wet+dry) N deposition was 18kgNha−1year−1, ranging from 8.6 to 26kgNha−1year−1. As a result, critical loads were exceeded at 67% of sites under SOH and 40% of sites under SBH. However, there was little evidence of exceedance at monitored plots. Foliar and forest floor C:N data indicated that most of these sites had low to intermediate N status. There were considerable differences in N cycling between soil types. Plant root simulation (PRS™) probe data indicated that this was likely due to differences in net N-mineralization and nitrification. Our results indicate that many sites are currently N limited but critical load exceedance suggests that these systems will accumulate N over time. The findings have implications for forest management, allowing for the assessment of nutrient management under different harvest scenarios.

Relationship between critical load exceedances and empirical impact indicators at Integrated Monitoring sites across Europe

Critical loads for acidification and eutrophication and their exceedances were determined for a selection of ecosystem effects monitoring sites in the Integrated Monitoring programme (UNECE ICP IM). The level of protection of these sites with respect to acidifying and eutrophying deposition was estimated for 2000 and 2020. In 2020 more sites were protected from acidification (67%) than in 2000 (61%). However, due to the sensitivity of the sites, even the maximum technically feasible emission reductions scenario would not protect all sites from acidification. In 2000, around 20% of the IM sites were protected from eutrophication. In 2020, under reductions in accordance with current legislation, about one third of the sites would be protected, and at best, with the maximum technically feasible reductions, half of the sites would be protected from eutrophication. Data from intensively monitored sites, such as those in ICP IM, provide a connection between modelled critical thresholds and empirical observations, and thus an indication of the applicability of critical load estimates for natural ecosystems. Across the sites, there was good correlation between the exceedance of critical loads for acidification and key acidification parameters in runoff water, both with annual mean fluxes and concentrations. There was also evidence of a link between exceedances of critical loads of nutrient nitrogen and nitrogen leaching. The collected empirical data of the ICP IM thus allow testing and validation of key concepts used in the critical load calculations. This increases confidence in the European-scale critical loads mapping used in integrated assessment modelling to support emission reduction agreements.

Modelling of atmospheric nitrogen deposition effects to Polish terrestrial ecosystems for various emission scenarios until the target year 2020

Biogeochemical effects to Polish terrestrial ecosystems resulting from atmospheric nitrogen deposition were forecasted until the target year 2020. To this end recently updated critical loads of nutrient nitrogen were applied and the nitrogen deposition projections for the sequence of decades from 1980 until the target year 2020, based on the Current Legislation (CLE) and Maximum Feasible Reductions (MFR) emission scenarios. The predictions were done by use of the Very Simple Dynamic (VSD) Model developed within the Working Group on Effects of the UN ECE Convention on the Long-Range Transboundary Air Pollution CLRTAP. The calculations were done for three main forest ecosystems and three selected semi-natural ecosystems encompassing the whole territory of Poland with the spatial resolution defined by a grid cell of 1×1 km size. The study concluded with maps of CLnut(N) exceedances and expected nitrogen concentrations in soil as chemical criterion, assigned to different eutrophication risk categories for each deposition scenario. The obtained results show that in spite of the realistic (CLE scenario) and extreme (MFR) nitrogen emission reductions until 2020, more than 99% and 80% of total area of terrestrial ecosystems of Poland, respectively, will be exposed to excessive nitrogen deposition. Results of this study as well as studies done on the European scale reveal that the nitrogen emission reductions determined by the Gothenburg Protocol are still insufficient and may lead to negative ecological effects including loss of ecosystems biodiversity. This substantiates a demanding need for the revision of the CLRTAP Gothenburg Protocol.

Choice of control of sulfur and/or nitrogen deposition based on critical loads

In order to design more reasonable and effective acid deposition control policies, the critical load function (CLF) was established and a two-dimensional critical load function figure was drawn based on the steady-state mass balance (SMB) method. The figure could be divided into five different regions: critical load region (non-control region), S deposition control region, N deposition control region, S and N deposition selective control region and S and N deposition simultaneous control region. According to the region into which the deposition point fell, we could choose the corresponding control approach. With further consideration of high base cation deposition (BCdep) in China, a three-dimensional critical load function surface could be drawn by taking BCdep as a variable. As a case study, critical loads of sulfur and nitrogen for the lateritic red earth in Guangzhou were calculated with current deposition data, vegetation data and soil data obtained by field sampling and laboratory analysis. Results showed that the current deposition point fell into the critical load region and a 75% reduction in BCdep caused the deposition point falling into S deposition control region, which indicated the importance of controlling sulfur deposition. While taking the critical load of nutrient nitrogen into account, the current deposition point fell into N deposition control region and with the reduction of BCdep, the deposition point entered S and N deposition simultaneous control region, which meant sulfur and nitrogen deposition should be controlled at the same time.

Critical Loads of Sulphur and Nitrogen for Terrestrial Ecosystems in Europe and Northern Asia Using Different Soil Chemical Criteria

A critical load data base was developed for Europe and Northern Asia using the latest data bases on soils, vegetation, climate and forest growth. Critical loads for acidity and nutrient nitrogen for terrestrial ecosystems were computed with the Simple Mass Balance model. The resulting critical loads are in accordance with critical loads from previous global empirical studies, but have a much higher spatial resolution. Critical loads of acidity are sensitive to both the chemical criterion and the critical limit chosen. Therefore a sensitivity analysis of critical loads was performed by employing different chemical criteria. A critical limit based on an acid neutralizing capacity (ANC) of zero resulted in critical loads that protect ecosystems against toxic concentrations of aluminium and unfavourable Al/Bc ratios, suggesting that ANC could be an alternative to the commonly used Al/Bc ratio. Critical loads of nutrient nitrogen are sensitive to the specified critical nitrate concentration, especially in areas with a high precipitation surplus. If limits of 3–6 mg N l−1 are used for Western Europe instead of the widely used 0.2 mg N l−1, critical loads double on average. In low precipitation areas, the increase is less than 50%. The strong dependence on precipitation surplus is a consequence of the simple modelling approach. Future models should explore other nitrogen parameters (such as nitrogen availability) instead of leaching as the factor influencing vegetation changes in terrestrial ecosystems.

Acidic loadings in South Korean ecosystems by long-range transport and local emissions

Exceedances of sulfur and nitrogen critical loads in South Korean ecosystems caused by long-range transport and local emissions of sulfur and nitrogen have been estimated using the maximum critical load of sulfur and the critical load of nutrient nitrogen. The long-term-averaged deposition of sulfur and nitrogen is estimated with a simplified chemical model and the K -mean clustering technique. The three consecutive days of gridded daily mean National Center for Environmental Protection (NCEP) reanalyzed 850 hPa geopotential height fields with and without precipitation on the last day over South Korea are used for clustering of synoptic patterns for the period of 1994–1998. Two emission conditions are simulated for each cluster to estimate long-term averaged depositions of sulfur and nitrogen by long-range transport and local emissions over South Korea. One condition takes all emissions within the simulated domain into account as a base case and the other condition excludes all South Korean emissions but includes all of the other emissions, as a control case. The results of the present study indicate that the contribution of long-range transport to the annual total deposition over South Korea is found to be about 40% ( 530 eq ha - 1 yr - 1 ) for sulfur and 49% ( 650 eq ha - 1 yr - 1 ) for nitrogen, of which 55% for sulfur and 58% for nitrogen are contributed by wet deposition. This suggests the importance of wet deposition through the transformed acidic precursors for long-range transport to South Korea's total deposition of sulfur and nitrogen. The estimated exceedance for South Korean ecosystems indicates that the current estimate of total sulfur deposition affects about 42% of the South Korean ecosystems adversely, of which 14% is attributed to South Korean source only and the rest 28% is attributed to long-range transport together with South Korean source. Long-range transport of sulfur itself does not exceed the maximum critical load of sulfur. On the other hand, the current estimate of total nitrogen deposition is found to affect all South Korean ecosystems adversely. About 65% and 15% of the Korean ecosystems are, respectively, found to be affected adversely by South Korean nitrogen emissions and by long-range transport, suggesting that the reduction of nitrogen deposition from both South Korean sources and long-range transport is a prerequisite to ensure South Korean ecosystem sustainability.

Impacts of climate change on critical loads and their exceedances in Europe

As part of a project on the integrated assessment of regional air pollution and climate change in Europe (AIR-CLIM) this paper reports the impacts of climate change on the sensitivity of forest ecosystems to the deposition of sulfur and nitrogen. This sensitivity is characterized by critical loads of acidity and nutrient nitrogen, which have been widely used for setting emission reduction targets in Europe. Using a consistent database on European soils and forests, it is shown that higher temperatures, changed precipitation patterns and modified net primary production (NPP) generally increase critical loads, although decreases are observed in mountainous and arid regions. Comparing critical loads with the deposition of sulfur and nitrogen, it is shown that the exceedances of the critical loads are declining under all eight scenarios developed within AIR-CLIM, although exceedances of critical loads of nutrient nitrogen remain substantial, even under the most stringent scenario. A sensitivity study of some key parameters corroborates the conclusion that research should focus on the effects of nitrogen in the environment, especially under conditions of climate change, whereas policies should concentrate on further reductions of nitrogen emissions.

Estimation of Critical Loads of. Sulfur and Nitrogen for the Korean Ecosystem

The critical loads of sulfur and nitrogen and their exceedances by the sulfur and nitrogen deposition of 1994-1998 were mapped for South Korea for the first time with a spatial resolution of 11km×14km (0.12 5° Long.×0.125° Lat.) using the steady-state mass balance model. The Korean soil and geological maps with the map scale of 1: 250, 000 and the Korean forestry statistical yearbooks were used as basis for the estimations of weathering rate of base cations, critical alkalinity leaching, denitrification fraction and uptakes of base cation and nitrogen. Wet depositions of sulfur, nitrogen and non-sea-salt base cations were derived from measured concentrations in precipitation and precipitation rates using optimum regression equations while dry depositions of sulfur and base cations were estimated using the inferential method. However, dry deposition of nitrogen due to NO, NO2, NH3 and NH4+ species was estimated using the MM5 numerical model, a modified Regional Acid deposition Model (RADM) and the K-mean clustering technique using three consecutive days of gridded daily mean 850 hPa geopotential height fields without precipitation on the last day over South Korea for 5 years. The predominant ranges of base cation deposition, weathering and uptake were estimated to be 300-500 eq ha-1 yr-1, 200-500 eq ha-1 yr-1 and 200-400 eq ha-1 yr-1, respectively. Critical alkalinity leaching was mainly in the range of 1, 0002, 000 eq ha-1 yr-1. Consequently, a large percentage of the maximum critical load of sulfur was found to be in the range of 1, 0002, 000 eq ha-1 yr-1. The estimated predominant ranges of nitrogen immobilization and uptake were 120-180 eq ha-1 yr-1 and 300-400 eq ha-1 yr-1, respectively, indicating low values of the minimum critical load of nitrogen. The maximum critical load of nitrogen was found to be relatively high (predominantly 2, 0004, 000 eq ha-1 yr-1) while the critical load of nutrient nitrogen was low (predominantly 400-800 eq ha-1 yr-1). Exceedance of the maximum critical load of sulfur was found at 85% of the Korean ecosystems considered mainly in the southeastern part of Korea, whereas that of the critical load of nutrient nitrogen was found in the whole Korean ecosystems. This implies that Korean ecosystems are very susceptible to the combined loadings of sulfur and nitrogen.

Past and future exceedances of nitrogen critical loads in Europe.

Critical loads of acidity and nutrient nitrogen — simple measures of the sensitivity of ecosystems to deposition — have been widely used for setting emission reduction targets in Europe. In contrast to sulfur, the emissions of nitrogen compounds remain high in the future. This is also true for the exceedances of critical loads until 2010. Looking further into the future, climate change is likely to influence ecosystem sensitivity, and thus critical loads. It is shown that higher temperatures, changed precipitation patterns, and modified net primary production mainly increase critical loads, except in mountainous and arid regions. Using consistent scenarios of climate change and air pollution from a recently completed European study (AIR-CLIM), it is shown that the exceedances in 2100 of the critical loads are declining in comparison to 2010. However, exceedances of critical loads of nutrient nitrogen remain substantial, even under the most stringent scenario. This confirms the increasing role nitrogen plays in environmental problems in comparison to sulfur. Thus research should focus on the effects of nitrogen in the environment, especially under conditions of climate change, to support nitrogen-emission mitigating policies. This not only reduces acidification and eutrophication, but also helps curb the formation of tropospheric ozone.

Uncertainties in Large Scale Assessments of Critical Load Exceedances

In the 1999 Gothenburg protocol to the UN/ECE LRTAP Convention andin the negotiations for an EU acidification strategy the area withexceedances of critical loads has been the preferred measure forenvironmental impacts. The aim of this study has been to assessthe influence of the uncertainty and spatial variation of both thecritical loads and deposition values on the calculated area withexceedances of critical loads. This has been done on a nationalscale for Denmark and on the European scale based on the dataincluded in the RAINS model. It is demonstrated that includinguncertainty and spatial variation in exceedance calculations, ingeneral gives larger exceeded area for the critical load ofacidity, CL(A). The picture for the critical load of nutrientnitrogen, CLnut(N), is more mixed because of the higherproportion of exceeded areas. A further point of interest is thepossibility of validating relationships between critical loadexceedances and observable damage based on large scale monitoringand model data. It is demonstrated that it will probably not bepossible to use exceedance calculations on European scale as basisfor validation exercises, linking exceedances to observable damage.

Evaluating critical loads of nutrient nitrogen and acidity for terrestrial systems using ecosystem-scale experiments (NITREX)

Critical loads (CL) for nitrogen as a nutrient (CL nut (N)) and total acidity (CL(S + N)) for the terrestrial ecosystem have been calculated and evaluated for all NITREX sites. NITREX constitutes a series of N addition or removal experiments in coniferous forest stands and one alpine site across a European deposition gradient (2–60 kgN ha −1 yr −1 ). Experimental manipulation of S and N inputs at these sites provides a valuable opportunity to test the validity of the mass balance approach to computing CLs at the ecosystem scale. CL nut (N) values calculated using the sustainability (nutrient limitation) approach were 3–8 kgN ha −1 yr −1 for sites on acidic geology, and 19 kgN ha −1 yr −1 for sites on basic geology. Where N uptake was estimated using N removed in bolewood, CL values increased to 11–15 kgN ha −1 yr −1 at acidic sites. These values will increase further if whole-tree rather than stem-only harvesting is employed and will be greater in areas where stand rotation lengths are relatively short (e.g., UK). Mean annual uptake rates of N into bolewood could not be sustained by available P at any of the sites indicating the probable need for fertiliser applications in the long-term. Problems associated with estimating values for other parameters in the CL equation (N i(crit) , N de(crit) , N le(crit) ) are discussed and preferred approaches identified. CL(S + N) values were computed as 1.7–6.9 keq ha −1 yr −1 reflecting differences in weathering rates, deposition and uptake of base cations, and the (Ca + Mg + K) Al crit ratio in soil solution for the dominant vegetation. Comparison of ambient deposition loadings and calculated CL values indicated that the sites were in three categories: (1) sites with deposition below both CL nut (N) and CL(S + N) (Sogndal and Alptal), (2) sites where deposition was equal to or exceeded CL nut (N) but not CL(S + N) (Gårdsjön, Aber and Klosterhede), and (3) sites in which both CL nut (N) and CL(S + N) were exceeded (Speuld, Solling and Ysselsteyn). Response of soil solution (Ca + Mg + K) Al ratio following changes in deposition at the sites is discussed with respect to observed ecological changes. Nitrate leaching was positively related to exceedance of CL nut (N) if the larger CL nut (N) values were used, highlighting the importance of this removal pathway for excess N in coniferous forest ecosystems. The study has demonstrated the value of long-term, ecosystem scale experiments if current approaches to calculating CLs are to be rigorously tested.