Critical loads for nitrogen deposition: Case studies at two northern hardwood forests

A critical load of a pollutant is the level of input below which no harmful ecological effects occur to a complex ecosystem. Critical loads are being used in policy decisions regarding air pollution emissions. In this paper, we applied four mass and charge balance methods of calculating critical loads to two northern hardwood forests in the northeastern United States. Critical loads for nitrogen deposition with respect to acidity ranged from 0–630 eq/ha-yr. Critical loads for nitrogen deposition with respect to effects of elevated nitrogen (eutrophication and nutrient imbalances) ranged from 0–1450 eq/ha-yr. At both the Hubbard Brook Experimental Forest (HBEF) and Huntington Wildlife Forest (HWF), the critical load for nitrogen with respect to acidity was exceeded. At the HBEF, due to reduced forest growth, the critical load for nitrogen with respect to nutrient imbalances and eutrophication was exceeded in recent years. At Huntington Wildlife Forest, the critical load with respect to nitrogen effects was also exceeded. This analysis demonstrated that the calculated critical load of nitrogen varies in response to changes in environmental conditions such as variations in atmospheric deposition of sulfate or changes in forest biomass accumulation.     

Critical loads of acid deposition on Scottish soils

The impact of acid deposition, attributable to sulphur and nitrogen pollutants, on the soils of Scotland has been analysed using a critical loads approach. The critical load of a soil (as an indicator of ecological damage) is calculated from the soil parent material controlling weathering and soil development. Using existing soil survey information national maps for critical loads of acidity and the sulphur fraction are presented for soils under natural and semi-natural ecosystems. The results show that highly sensitive soils, that is those derived from quartzite and granite are limited in occurrence. However, there are large areas of sensitive soils predominantly to the north and west of the Midland Valley and in the Southern Uplands, in receipt of acid deposition in excess of their critical load. Enhanced soil acidification should be widespread in these areas and consequently the ecosystems which they support will be adversely affected. The least sensitive soils, overlying limestone or marl, are restricted in occurrence and are confined to the major deposits of marine alluvium. The results of the analysis may be used to help policy makers derive emission abatement strategies in the context of the European Sulphur protocol renewal in 1993. In Scotland the maps may be used to aid the planning of large scale afforestation.     

Critical deposition levels for nitrogen and sulphur on dutch forest ecosystems

Critical loads for N and S on Dutch forest ecosystems have been derived in relation to effects induced by eutrophication and acidification, such as changes in forest vegetation, nutrient imbalances, increased susceptibility to diseases, nitrate leaching, and Al toxicity. The criteria that have been used are N contents in needles, nitrate concentrations in groundwater (drinking water), and NH₄/K ratios, Ca/Al ratios, and Al concentrations in the soil solution. Assuming an equal contribution of N and S, all effects seem to be prevented at a total deposition level below 600 mol_c ha^?1 yr^?1 due to N uptake by stemwood and acid neutralization by base cation weathering. The most serious effects will probably be prevented at total deposition levels between 1500 and 2000 mol_c ha^?1 yr^?1. The current average deposition in the Netherlands is 4900 mol_c ha^?1 yr^?1.     

The impact of nitrogen deposition on upland surface waters in Great Britain: A regional assessment of nitrate leaching

A national dataset of water chemistry collected for critical loads mapping is used to make a regional assessment of surface water nitrate concentrations in Great Britain. The primary data are dominated by high concentrations in lowland regions Where N inputs are dominated by non-atmospheric sources. Land cover data are used to screen out sites with potential catchment sources of N, allowing the evaluation of nitrate leaching due to atmospheric deposition alone. In the screened dataset several upland regions show elevated nitrate concentrations, notably Wales, the Pennines, Cumbria, Galloway and the Cairngorms, and there is a clear relationship between surface water nitrate and total N deposition.     

Critical loads of acidity and nitrogen,based on multiple criteria for different swedish ecosystems

The critical loads of acidity and nitrogen has been mapped for Swedish forest soils, using data from the Swedish Forest Inventory. The Swedish critical load map used in negotiations has been based on a number of ecological receptors. For terrestrial ecosystems criteria based on no adverse effect on growth, soil stability and groundwater quality was used. For surface waters, stream and lake biology was used as indicators for setting limits to acidification. A reduction of 75% of the acidity deposition in relation to 1988 is required in order to protect 95% of the forest resource in Sweden from effects of soil acidification. A reduction of 50% of the nitrogen deposition is required to avoid exceedance in more than 5% of the area. The mapping work was carried out by using the PROFILE model.     

Critical loads of sulfur and nitrogen for lakes I: Model description and estimation of uncertainty

Presently considerable effort is devoted to the development of methods for estimating critical loads of acidic deposition. In this paper a steady-state mass balance model for lakes is presented, allowing the simultaneous calculation of critical loads of acidifying S and N deposition and their exceedance. Special emphasis is given to the derivation of model inputs and parameters and the quantification of their uncertainties. The inclusion of rate-limited processes in the model leads to the dependence of the critical loads not only on catchment properties but also on the loading to the ecosystem. As a consequence, critical load values have to be re-calculated whenever deposition patterns change. The methods presented in this study are used in an accompanying paper to derive regional distributions of critical loads of S and N for lakes in Finland and to quantify their uncertainties.     

Critical loads of acidity for surface waters

The critical load of acidity for surface waters is based on the concept that the inputs of acids to a catchment do not exceed the weathering less a given amount of ANC. The Steady State Water Chemistry (SSWC) Method is used to calculate critical loads, using present water chemistry. To ensure no damage to biological indicators such as fish species a value for ANC_limit of 20 μeq/l has been used to date for calculating critical loads. The SSWC-method is sensitive to the choice of the ANC_limit. In areas with little acid deposition the probability of acid episodes leading to fish kills is small even if the ANC_limit is set to zero, while in areas with high acidic deposition fish kills may occur at this value. Thus, the ANC_limit can be a function of the acidifying deposition to the lake, nearing zero at low deposition and increasing to higher values at higher deposition. A formulation for such an ANC_limit has been worked out, and we have tested the effect of the ANC_limit as a linear function of the deposition, assuming ANC_limit = 0 at zero deposition with a linear increase to 50 ueq/l at a deposition of 200 meq.m^?2.yr^?1. For areas with high deposition the effect of a variable ANC_limit is small, while in areas with low deposition the effect is significant. For Norway the exceeded area decreases from 36 to 30% using a variable ANC_limit instead of a fixed value of 20 μeq/l.     

Long term effects of enhanced nitrogen deposition on a lowland dry heath in southern Britain

Experimental additions of ammonium sulphate to a nitrogen-poor dry heathland have been carried out since 1989. There are four nitrogen treatments: a control (receiving artificial rain only), a low treatment which receives an additional 7.7 kg N ha^?1 yr^?1, a high treatment receiving 15.4 kg N ha^?1 yr^?1 and an alternating treatment which receives either the control or the high nitrogen additions, in alternate years. The estimated background deposition at the study site is 13–18 kg N ha^?1 yr^?1, a value similar to the critical load that has been suggested for the conversion of lowland heath to grassland. Over the past 5 years there have been significant stimulations in shoot growth, flowering, canopy density and litter production. Flowering, in particular, strongly reflects nitrogen additions in the alternating treatment. Current models of the response of dry Calluna heathlaud to enhanced nitrogen deposition suggest that higher tissue nitrogen levels will occur and will be accompanied by heightened sensitivity to secondary stresses. This may in turn lead to canopy breakdown and replacement by grassland. The application of nitrogen at deposition rates only slightly in excess of the critical load over five years has produced small, non-significant increases in shoot nitrogen content. However, there is clear evidence of a large positive effect on shoot growth, flowering, litter production and canopy density of Calluna. The observation of these responses at the application rates used in this study supports the current proposals for critical loads of nitrogen for lowland heaths.     

Critical loads — Possibilities and constraints

Since critical thresholds for acidic deposition were first considered in the 1970s, the critical loads approach has rapidly evolved into a practical tool for addressing the problems of pollution control. It has gained acceptance from many scientists and policy makers, bridging the gap between science and policy and proving its use with the acidifying effects of the atmospheric pollutants of sulphur and nitrogen. For this there are examples at local, national and international scales and there appears to be scope for further applications as methods and databases improve. The UN/ECE used the approach to underpin the second Sulphur Protocol; for this, emission reductions were recommended on the basis of environmental effects as well as the costs of control measures. Although limited by a relatively coarse transport modelling scale, the advantages over non-effect-orientated approaches were evident. At a more local scale, the approach has often lacked information linking some of the chemical effects on soils and freshwaters to harmful effects to biota. In addition, the important consideration of rates of change, in particular recovery, of ecosystems with changing pollutant loads has still to be addressed nationally and internationally. The critical loads approach is a first step towards effects orientated pollution control. Already sophisticated methods, based upon similar dose-response data, are being developed; these lend themselves better to economic evaluation of damage. Despite limitations, the critical loads approach has proved a practical method for deriving pollution control strategies whose success will be judged in the years ahead.     

Critical nitrogen content and nitrogen nutrition index for sweetpotato crop

Abstract

Sweetpotato is an important tuber crop for the food security in Island countries of the South Pacific. The allometric relationship between tissue nitrogen (N) concentration and aerial dry matter is unknown. We determined critical N (N_c) content from vegetative stage to harvesting, and estimated the range of variation in N nutrition index (NNI) from two field experiments with varied rates of N (0, 25, 60, 125 and 180?kg N ha^?1 in 2015 and 0, 50, 125, 175 and 250?kg N ha^?1 in 2017). A unified critical N curve (N_c = 3.338?W^?0.307) where W?=?aerial dry matter with W?≥?1.38 t ha^?1, was constructed based on the N concentration in the aerial dry matter. The calculated NNI ranged from 0.69 to 1.23 in 2015 and 0.54 to 1.17 in 2017. The preliminary N_c dilution curve and NNI determined could potentially be used as a parameter for N management.     

Critical loads of acidity to surface waters

The critical load of acidity to surface water is based on the condition that the inputs of acids to a catchment do not exceed the weathering rate less a given amount of ANC (Acid Neutralizing Capacity). The Steady State Water Chemistry (SSWC) Method is used to calculate critical loads of acidity, using present water chemistry. To calculate the weathering, the so-called F-factor is used to estimate the part of the base cation flux that is due to soil acidification. The F-factor has been estimated empirically from historical data comparisons from Norway, Sweden, U.S.A. and Canada and is considered to be a function of the base cation concentration by the formula: F=sin(BC^*/S), where BC^* is the present base cation concentration and S the base cation concentration at which F=1. At higher values for BC^* F is set to 1. For Norway, Sweden and Finland S has been set to 400 μeq/l (ca. 8 mg Ca/l), giving F-values in the range 0.05–0.2. The importance of the F-factor in the calculations of the critical loads of acidity for Nordic surface waters was tested by calculating the magnitude of the area where the critical load of acidity is exceeded in Norway for different values of S. Similar calculations were carried out for the Finnish and Swedish lake data. Varying S from 100 μeq/l to 1200 μeq/l, the exceeded area in Norway decreases from 31,9 to 28,3%. For F=0 (S=∞, i.e. assuming no soil acidification), the exceeded area is reduced to 27,2%. For Finland and Sweden the the percent of lakes exceeded are reduced from 16,6 to 12,9% and 30 to 23,6%, respectively. For F = 0 the percent of lakes exceeded are reduced to 11,4 and 16,4, repectively. These results indicate that the F-factor is not of great importance for calculating critical load and critical load exceedances in Norway, Finland and Sweden.     

Critical loads for soils and waters in a selected Scottish catchment

In the UK, critical loads have been mapped for both soils and freshwaters and the maps indicate that discrepancies may occur between these two receptors over sensitive areas of the UK. Freshwater critical load maps were prepared by calculating the Henriksen critical load for the most sensitive water body in each 10 km grid square. Critical loads for soils were calculated according to the mineralogy and associated soil properties of the dominant soil at a 1 km resolution. To examine the differences between the soil and freshwater data sets it is necessary to calculate critical loads at a smaller scale using the catchment as the focus for study. This was done by selecting a catchment on granitic parent material in the North of Scotland. Data on water chemistry, collected on a weekly basis, was used to calculate temporal variations in critical loads for freshwaters using the Henriksen method. Soil sampling across the catchment was conducted on a grid based system to provide estimates of spatial variability in sensitivity. Profile characteristics and soil chemical data obtained from detailed soil sampling programmes were used in the PROFILE model to determine the spatial variation in critical loads for soils. In general, the results show that the critical loads for soils tend to be lower than those for freshwater. The spatial variation in the soil critical load tends to be small whilst the temporal variation in critical load for freshwaters is large. In order to account for these differences it is important to identify the key processes within the catchment which play a major role in controlling streamwater chemistry. This procedure improves the relationship between critical loads for soils and waters.     

Calculating nitrogen deposition in Europe

Nitrogen deposition calculations for Europe were performed by separate models describing the long-range transport of ammonia and oxidized N. A linearized version of a non-linear atmospheric chemistry model was used for calculating oxidized N. Model computations were found to be consistent with the observed spatial pattern of wet nitrate deposition in Europe. Interannual meteorological variability was estimated to cause a typical year-to-year variation in annual oxidized N deposition of about 6 to 10%. Nitrogen deposition was computed for several NO _x emissions reduction scenarios. These scenarios were derived from an OECD study and applied to the 27 largest countries in Europe. Most reduction scenarios affected the deposition pattern of oxidized N, but the most extreme NO _x emission reduction scenario did not change very much the overall pattern of total (oxidized N plus ammonia N) N deposition. Depending on the desired level of environmental protection, it may be necessary to reduce ammonia emissions in addition to NO _x emissions in order to reduce N deposition in Europe.     

Critical loads mapping in Poland: Lessons learned

Since 1990 the Institute for Ecology of Industrial Areas, acting as National Focal Center, is actively involved in an international research programme aimed at the calculation and mapping of critical loads of acidifying compounds. Following the methodological guidelines elaborated under the leadership of UN/ECE Task Force on Mapping and Coordination Center for Effects, national maps of critical loads and their exceedances for acidity, sulphur and nitrogen have been produced. These maps have already been utilized in derivation of European maps of critical loads of acidity and sulphur submitted to the UN/ECE LRTAP Convention as scientific input to the negotiations on the Second Sulphur Protocol. The lessons learned from the critical loads mapping exercise can be summarized as follow:

Contribution of dissolved organic nitrogen deposition to total dissolved nitrogen deposition under intensive agricultural activities

For elucidating the atmospheric deposition contribution of dissolved organic nitrogen (DON) to the total dissolved nitrogen (TDN) deposition rate, dissolved inorganic nitrogen (DIN: NH₄ ⁺ + NO₃ ^–) and DON deposition rates were annually and monthly estimated during 4 and half-yr monitoring period in an experimental multi-farm under intensive agricultural activities of N fertilizer use and animal husbandry in Central Japan. Annual NH₄ ⁺, DON and NO₃ ^– deposition rates in bulk and wet deposition data accounted for 48%, 32% and 20% of TDN deposition, respectively, which indicated that this area is strongly affected by the intensive agricultural activities. The DIN and DON deposition rates were respectively estimated at 21.6 and 10.1 kg N ha^?1 yr^?1, which ranked high in a worldwide regional data set. Consequently, this area has been exposed to a large amount of N deposition including DON with N fertilizer input. The difference between bulk and wet deposition rates (NH₄ ⁺ and DON) is one of important factors controlling the N deposition in this area. Monthly DON deposition showed positive correlations with DIN and NH₄ ⁺ deposition rates, respectively, with a significant linear regression curve. The linear regression curve of our monthly data (n = 127) indicates the same trend as the worldwide annual data set (n = 31).     

Linear programming for abatement of nitrogen oxides acid rain deposition

Linear programming is used to examine NO_x as an air quality management problem. Through the use of transfer coefficients, which translate source emissions of NO_x into receptor impacts, the model developed herein seeks to minimize the total cost of reducing emissions from 14 major Canadian point sources. It does this subject to the constraint that the wet deposition at 5 sensitive receptors must be reduced by at least some minimum amount. Several different reductions in wet deposition are used to observe the increase in costs associated with a systematic ‘tightening’ of the constraint relationships. A particular level of reduction is used to examine, on a more detailed level, the sources requiring some level of removal. These case study results and an analysis of the rationale for their solution are presented. Also presented is a comparison between this type of optimization strategy and the scenario of a single ‘across-the-board’ emissions cutback. From NO_x considerations alone, it would seem more expedient to concentrate current research efforts into developing technologies to further reduce vehicle emissions, since their magnitude considerably outweighs those from the point sources.     

Effects of nitrogen deposition on animal-mediated nitrogen mobilization in coniferous litter

Summary In microcosm studies the organic layers of coniferous forest soils show high nitrate and low ammonium mobilization, in accord with the presence of high numbers of autotrophic nitrifiers. The fungivorous collembolan Tomocerus minor (Lubbock) increases ammonium mobilization, probably through its excretion products, and has an indirect effect on nitrate mobilization. An input of N seems to have a negative effect on the number of nitrifiers and on nitrate mobilization; a decrease in N mobilization in the presence of T. minor is probably due to stimulation of microbial growth, which has an immobilizing effect.     

Critical loads of SO4 for surface waters in the Kola region of Russia

Precipitation falling on the Kola North of Russia is severely polluted by strong acids and heavy metals emitted from large local smelters operated by the Severonickel and Pechenganickel companies. Large areas in the affected region are considered sensitive to acid precipitation due to geological and climatological conditions. Extensive surface water acidification has taken place, particularly, in the mountain and remote tundra regions. Critical loads and their cxceedances have been calculated for 370 lakes sampled in the Kola peninsula. These data indicate that the surface waters in the Kola North are highly sensitive to acidic deposition. Using the minimum values determined for each NILU grid cell, critical loads of less than 20 meq/m²yr occur in about 50 percent of the area.     

Greenhouse gas mitigation potential of agricultural land in Great Britain

The aim of this paper is to assess the greenhouse gas (GHG) mitigation potential of croplands and grasslands in Great Britain under different management practices. We consider the feasible land management options for grass and cropland using county level land‐use data with estimates of per‐area mitigation potential for individual and total GHGs, to identify the land management options with the greatest cost‐effective mitigation potential. We show that for grasslands, uncertainties still remain on the mitigation potential because of their climatic sensitivity and also their less intensive management. For croplands in Great Britain, the technical mean GHG mitigation potentials for all cropland management practices range from 17 Mt CO₂‐eq. per 20 yr to 39 Mt CO₂‐eq. per 20 yr. There are significant regional variation in all cases, with the greatest potentials in England, negligible potential in Wales and intermediate potential in Scotland, with country differences largely driven by the areas of cropland and grassland in each country. Practices such as agronomic improvement and nutrient management are the most promising options because of their impact on N₂O emissions and also their larger potential at low cost. In terms of annual emissions from agriculture, calculated mitigation potentials are small, where the technical mitigation potential of agronomy and nutrient management strategies are ca. 4.5 and 3.8%, respectively (agricultural emissions account for ca. 9% or 47.7 Mt CO₂‐eq., of total Great Britain GHG emissions, Department of Energy and Climate Change, UK). However when compared with the land use, land‐use change and forestry sector (LULUCF) emissions, nutrient management would reduce further emission reductions by approximately half of the 2005 LULUCF sink (i.e. ?1.6 Mt CO₂‐eq. per year).