Calculating critical loads for acidification for five forested catchments in China using an extended steady state function

The critical load concept has become widely accepted as an important theoretical basis for establishing effective acid deposition control strategies. In the critical load calculations, the influence of variation in base cation (BC) deposition, which plays an important role in mitigating acidification, was seldom considered. In this manner, high uncertainty and over-estimation might be caused in those areas where current BC deposition is very high and of significant anthropogenic origin since anthropogenic deposition can change due to human activity. In this study, an extended sulfur(S)-nitrogen (N)-BC function based on the Steady State Mass Balance (SSMB) method is applied to calculate the critical loads for five sampled catchments in southern China under variable S, N, and BC deposition. The ceiling of S deposition (when N deposition is zero; CL(max)(S)) under current BC deposition varies from 4.5 to 10.8 keq ha(-1) yr(-1) among the five catchments, and the ceiling of N deposition (when S deposition is zero; CL(max)(N)) varies from 23.2 to 54.5 keq ha(-1) yr(-1). A 75% reduction in BC deposition is estimated to cause a 46%-86% decrease of CL(max)(S) and 45%-81% decrease of CL(max)(N). The critical loads for acidification are not exceeded in any of the five catchments under the current base cation deposition, despite extremely high S deposition in some places. However, if the BC deposition decreases to 25% of current while S remains unchanged, critical loads will be exceeded at all sites except one. A sensitivity analysis confirms that the long-term future BC deposition is among the most important parameters to the uncertainty of critical load, together with the dose-response relationship between ecosystem health and soil solution chemistry.     

Acidification sensitivity and critical loads of acid deposition for surface waters in China

Although decades of severe acid deposition have not resulted in serious surface water acidification in China, at present, the risk of some freshwaters becoming acidified cannot be neglected. To know more clearly about the possible impact acid deposition would have on Chinese surface waters, it is necessary to study the sensitivity of those surface waters to acidification and their critical loads of acid deposition on a national scale. Here we assess the acidification sensitivity of Chinese surface waters using an approach based on geology, soils, land use and hydrological conditions. Critical loads of S, N and acidity were also evaluated by a first-order acidity balance (FAB) model. Results show that most surface waters in China have very high critical loads of S and acidity and are not susceptible to acidification. Surface waters can be divided into three groups according to both sensitivity classes and critical loads. The few most sensitive surface waters are located in the northern part of Daxinganling region, with critical loads of S deposition and acidity lower than 2 keq ha(-1) year(-1). Surface waters in the northeastern region draining dark brown forest soils and in southern China belong to the second class of acidification sensitivity and their critical loads of S and acidity are generally between 2 and 15 keq ha(-1) year(-1), indicating they are not likely to be acidified under any flow conditions. Surface waters in other parts of China will not be acidified to any degree, with critical loads much higher than 15 keq ha(-1) year(-1). The magnitude and spatial distribution pattern of acidification sensitivity have significant similarity to the critical loads of S and acidity for Chinese surface waters. Although most surface waters are not likely to be acidified, attention should still be paid to the possible adverse impact acid deposition would have, especially in northeastern China, where the surface waters are the most sensitive, and the southern region, where acid pollution is among the most severe in China.     

Vegetation uptake of nitrogen and base cations in China and its role in soil acidification

Vegetation uptake of nitrogen and base cations may be an important source of soil acidity. In this study, the uptake rate of nitrogen and base cations was estimated for each major vegetation type in China. Although the uptake rates of most vegetation types are lower than 2.0 keq ha(-1) a(-1), great variations exist because of the significant differences in net primary productivity and elemental composition. Uptake rates decreased gradually from southeast to northwest, except for the wide-distributed agricultural areas in the east of China, where the nitrogen and base cation uptake rates are near zero. It is evident that this pattern coincides well with the distribution of the annual mean precipitation. However, the geographical distribution of the base cation uptake rate is not entirely the same as that of nitrogen, which indicates the importance of soil conditions in determining the chemical component and hence the uptake rate of different vegetation. The acidity input caused by vegetation uptake is quite high (>0.5 keq ha(-1) a(-1)), and even higher than the current acid deposition and the alkalinity produced by soil weathering in some areas, which indicates that the growth of the plants may contribute significantly to the soil acidification problem in China.     

Major nutrients and acidity: budgets and trends at four remote boreal stands in Finland during the 1990s

Major nutrients and acidity were studied at six plots in four Integrated Monitoring catchments in Finland, located along a south-north deposition gradient. The quality and quantity of bulk precipitation, throughfall, soil water, and litterfall were monitored during the period 1989-1997. The results showed a marked decline in SO4 and H+ concentrations overall. Trends in throughfall ANC values were positive, while those in soil water were mostly negative. The increase in throughfall ANC values indicates recovery from acid deposition. The adsorption of base cations and release of organic acids was implicated in the decrease in soil water ANC trends. For all variables, the changes tended to be the greatest in the most southerly catchment, which received the most deposition loads. Mass balance budgets showed that total deposition inputs were greater than soil leaching outputs (at 40 cm) for NO3, NH4 and SO4, and were attributed to nutrient uptake and cycling processes. In contrast, for Ca, Mg, K and Na, and S (Valkea-Kotinen3 and Vuoskoj?rvi2 plots only), inputs were less than outputs, indicating net leaching. Throughfall + litterfall inputs to the forest floor showed that the organic layer is particularly important in the retention of Ca, Mg and N. Sulfur was retained in the lower soil layers, and attributed to Al and Fe hydroxide adsorption in the B horizon.     

Uncertainties in critical loads and target loads of sulphur and nitrogen for European forests: Analysis and quantification

An analysis of the uncertainties in critical loads and target loads of sulphur (S) and nitrogen (N) for 182 European forest soils was carried out using the Very Simple Dynamic (VSD) model. The VSD model was calibrated with a Bayesian approach using prior probability functions for model parameters based on literature data, data from 200 Dutch forest sites and from simulated denitrification rates from a detailed ecosystem model. The calibration strongly improved the fit of the model to observed soil and soil solution concentrations, especially for pH and base saturation. Calibration also narrowed down the ranges in input parameters. The uncertainty analysis showed which parameters contribute most to the uncertainty in the critical loads and target loads. Base cation weathering and deposition and the parameters describing the H-Al equilibrium in the soil solution determine the uncertainty in the maximum critical loads for S, CL_max(S), when based on the aluminium to base cation (Al/Bc) criterion. Uncertainty in CL_max(S) based on an acid neutralizing capacity (ANC) criterion is completely determined by base cation inputs alone. The denitrification fraction is the most important source of uncertainty for the maximum critical loads of N, CL_max(N). N uptake and N immobilisation determine the uncertainties in the critical load for N as a nutrient, CL_nut(N). Calibration of VSD reduced the uncertainty: the coefficient of variation (CV) was reduced for all critical loads and criteria. After calibration, the CV for CL_max(S) was below 0.4 for almost all plots; however for CL_max(N) high values occurred for plots with high denitrification rates. Model calibration also improved the robustness of target load estimates: after calibration, no target loads were needed in any of the simulations for 40% of the plots, with the uncalibrated model there was a positive probability for the need of a target load for almost all plots.     

Simulation of surface water chemistry change resulting from soil acidification: a strontium isotope approach

Soil monoliths were treated with artificial acid rain (pH 3.71). The acidity load was 40 times higher than the present acidity load. Surface water chemistry was modelled on the basis of atmospheric deposition and organic and mineral soil horizon contributions to drainage water. Contributions were calculated using ⁸⁷Sr/⁸⁶Sr ratios measured in Sr sources and drainage water. In the early stages of acidification the contribution of organic horizons to surface water chemistry increased by up to 70%. This increase was accompanied by increases in Ca²⁺, Mg²⁺, K⁺, Na⁺, and dissolved organic carbon (DOC) concentrations. The amount of DOC washed out from mineral horizons was higher than from organic horizons, in contrast to base cations. In the range of acidity loads from 1.4 to 2.1 keq/ha, organic anions were predominant in solution. Further acidification resulted in a reduction of all solute concentrations and an increase in the contributions by deposition and mineral horizons.     

A review of the calculation procedure for critical acid loads for terrestrial ecosystems

Target loads for acid deposition in the Netherlands, as formulated in the Dutch environmental policy plan, are based on critical load calculations at the end of the 1980s. Since then knowledge on the effect of acid deposition on terrestrial ecosystems has substantially increased. In the early 1990s a simple mass balance model was developed to calculate critical loads. This model was evaluated and the methods were adapted to represent the current knowledge. The main changes in the model are the use of actual empirical relationships between Al and H concentrations in the soil solution, the addition of a constant base saturation as a second criterion for soil quality and the use of tree species-dependant critical Al/base cation (BC) ratios for Dutch circumstances. The changes in the model parameterisation and in the Al/BC criteria led to considerably (50%) higher critical loads for root damage. The addition of a second criterion in the critical load calculations for soil quality caused a decrease in the critical loads for soils with a median to high base saturation such as loess and clay soils. The adaptation hardly effected the median critical load for soil quality in the Netherlands, since only 15% of the Dutch forests occur on these soils. On a regional scale, however, critical loads were (much) lower in areas where those soils are located.     

Evaluation of different approaches for modeling effects of acid rain on soils in China

Acid deposition is an environmental problem of increasing concern in China. Acidic soils are common in the southern part of the country and soil acidification caused by acid deposition is expected to occur. Here we test and apply two different approaches for modeling effects of acid deposition and compare results with observed data from sites throughout southern China. The dynamic model MAGIC indicates that, during the last few decades, soil acidification rates have increased considerably due to acid deposition. This acidification will continue if sulfur deposition is not reduced, and if reduced more rapidly than base cation deposition. With the Steady State Mass Balance model (SSMB), and assuming that a molar ratio of Ca2+/Al3+ < 1 in soil water is harmful to vegetation, we estimate a slight probability for exceedance of the critical load for present deposition rates. Results from both modeling approaches show a strong dependence with deposition of base cations as well as sulfur. Hence, according to the models, changes in emission control of alkaline particulate matter prior to sulfur dioxide will be detrimental to the environment. Model calculations are, however, uncertain, particularly because available data on base cation deposition fluxes are scarce, and that model formulation of aluminum chemistry does not fully reproduce observations. An effort should be made to improve our present knowledge regarding deposition fluxes. Improvements to the model are suggested. Our work indicates that the critical loads presented in the regional acid deposition assessment model RAINS-Asia are too stringent. We find weaknesses in the SSMB approach, developed for northern European conditions, when applying it to Chinese conditions. We suggest an improved effort to revise the risk parameters for use in critical load estimates in China.     

Calculation and mapping of critical loads of sulphur and nitrogen for forest soils in Galicia (NW Spain)

Calculations of critical loads of acidity within Europe have shown Galicia to be one of the regions in Spain with the highest risk of soil acidification by deposition of atmospheric pollutants. We used the simple mass balance method (SMB) to derive critical loads of sulphur and nitrogen for forest soils in Galicia using available site-specific information. Exceedance of acidic deposition was calculated using the deposition levels of N and S registered in 2001. Most forest ecosystems analysed can support acid loads of more than 2000 eq N ha(-1) year(-1). The lowest critical loads of acidity (1483 eq ha(-1) year(-1)) occurred in coniferous forest stands growing on shallow soils developed on slates, in the interior zone. The highest critical load values (9386 eq ha(-1) year(-1)) were observed in coniferous stands in zones with elevated precipitation rates, in the littoral area. The critical load for acidity was exceeded in 5% of the forest soils. Such exceedances were mainly caused by deposition of nitrogen compounds. The contribution of sulphur deposition to acid exceedance was important in the surroundings of the city of A Coru?a. Analysis of rainfall bulk composition revealed that N deposition in Galicia was mainly due to ammonium emissions, probably derived from agricultural and cattle breeding activities.     

Steady state critical loads of acidity for sulphur and nitrogen: a multi-receptor, multi-criterion approach

The critical load approach to emission reductions has become an important element of the United Nations Economic Commission for Europe (UNECE) Convention on Long-Range Transboundary Air Pollution (CLRTAP). The steady state mass balance approach (SSMB) is the most widely used method in Europe for estimating critical loads, typically applied to forest ecosystems on mineral soils in conjunction with a base cation to aluminium (Bc:Al) ratio using a default critical limit of 1.0. The 'typical' approach has been expanded in this paper by: inclusion of a broader range of natural and semi-natural receptor ecosystems; inclusion of multiple chemical criterion for each receptor; and an attempt to include organic soils (peatlands). Critical loads of acidity were estimated for the Republic of Ireland using four receptor ecosystems (coniferous forest, deciduous forest, natural grasslands and moors and heathlands) and seven chemical criteria. The dominant chemical criteria, in the determination of critical loads, were based on a critical pH limit for mineral soils or a critical pH shift in relation to pristine conditions for organic soils. Approximately 68 and 26% of the final distribution of critical loads were estimated using these criteria, respectively. The 5th-percentile critical loads of acidity were more sensitive than those previously estimated for Ireland due to the inclusion of organic soils. Furthermore, coniferous ecosystems had the lowest critical loads due to the high percentage occurring on organic soils, and the removal of base cations through harvesting. The results demonstrate that it may be more appropriate to use multiple criteria and receptors to ensure adequate protection of biological indicators. However, it is important that appropriate critical limits are chosen to protect the biological indicators and receptor ecosystems from long-term damage.     

Time series of long-term annual fluxes in the streamwater of nine forest catchments from the Swedish environmental monitoring program (PMK 5)

International cooperation has contributed to major declines in SO(2) emission and S deposition during the last two decades in Europe. The chemical recovery from already anthropogenically acidified sites however, has been slow. In the present paper, long-term trends of chemical fluxes from nine selected forested reference catchments were studied, to detect recovery from acidification and leaching of S and base cations (BC). A decline in S deposition resulted in a decreased flux of non-marine sulfate (SO(4)*) in seven of nine streams, with statistically significant changes in four. The two cases with slight increases in SO(4)* flux resulted from increasing water flow. The SO(4)* decrease was followed by a recovery from acidification in terms of increased ANC flux in those sites in southern Sweden that were chronically acidified. The recovery was probably slowed down by leaching of SO(4) from the soil and a decrease in the flux of BC. A better understanding of the processes for leaching of SO(4) and BC is needed in order to quantify the need for further restrictions of sulfur emissions to allow a long term recovery of acidified catchments. The flux calculations available from small catchments such as those in this study, are of value for that understanding.     

Temporal variations of mineral character of acid-producing pyritic coastal sediments, Southeast Queensland, Australia

The lower Pimpama River catchment possesses many features of morphology, geology and landuse, typical for Southeast Queensland. Pimpama River and its main tributary Hotham Creek meander over a coastal plain which developed during the last several thousand years as a result of sea level fluctuations and changing fluvial and estuarine regimes which provided ideal conditions for the formation of sedimentary pyrite. A complex mixture of natural and human factors triggers and controls the oxidation and hydrolysis of this pyrite. The consequent production of sulfuric acid and leaching of metals from the pyrite-rich sediments represent main environmental issues of this coastal setting. This study aimed to determine the lithological character of the coastal unconsolidated sediments, and identify changes produced by acidity over a long period of time in a natural system and over a short period of time in a laboratory system. The mineral composition of the estuarine sediments of the coastal plain reflects the lithology of their source, the geological basement, and also the enhanced weathering rate due to acid production. The primary minerals present in the sediments consist of quartz and feldspars (primarily albite, K-feldspars to a lesser extent and minor anorthite) the product of physical weathering of bedrock material, mainly sandstone. Kaolinite is the dominant clay mineral and represents the product of intense leaching of feldspars promoted by acid conditions. Kaolinite is fairly ordered in structure in fresh and weathered bedrock and it becomes disordered in the estuarine sediments. Illite is mainly present in bedrock-related samples and the highest amounts are associated with muscovite. In the tidal sediments, illite is present to a lesser extent and is not found in the lower estuary. The distribution of mixed layers of smectite-illite is highly variable and their deposition is mainly controlled by the hydrodynamic conditions of the environment. Only low energy tidal settings favour the deposition of smectites. Pyrite is present at many sites and is always accompanied by oxidation products such as jarosite and gypsum. The laboratory experiment showed that in the short-term, changes induced by acid production mainly involved the: (1) decomposition of oxidation products such as gypsum and jarosite, (2) weathering of feldspars (mainly plagioclase), and (3) increased amount of kaolinite and possible increased structural disorder. These observations are of significance when considering wetlands as a method of remediating acid-producing areas by re-establishment of reducing conditions. Jarosite is quite widespread and can represent a source of acid over a short period of time in water-saturated, partially oxidized pyrite-rich sediments. The initial several months of a wetland establishment may, however, be dominated by secondary acid production and metal leaching from preexisting oxidation products like jarosite and natrojarosite.     

Estimating uncertainty in terrestrial critical loads and their exceedances at four sites in the UK

Critical loads are the basis for policies controlling emissions of acidic substances in Europe and elsewhere. They are assessed by several elaborate and ingenious models, each of which requires many parameters, and have to be applied on a spatially-distributed basis. Often the values of the input parameters are poorly known, calling into question the validity of the calculated critical loads. This paper attempts to quantify the uncertainty in the critical loads due to this "parameter uncertainty", using examples from the UK. Models used for calculating critical loads for deposition of acidity and nitrogen in forest and heathland ecosystems were tested at four contrasting sites. Uncertainty was assessed by Monte Carlo methods. Each input parameter or variable was assigned a value, range and distribution in an objective a fashion as possible. Each model was run 5000 times at each site using parameters sampled from these input distributions. Output distributions of various critical load parameters were calculated. The results were surprising. Confidence limits of the calculated critical loads were typically considerably narrower than those of most of the input parameters. This may be due to a "compensation of errors" mechanism. The range of possible critical load values at a given site is however rather wide, and the tails of the distributions are typically long. The deposition reductions required for a high level of confidence that the critical load is not exceeded are thus likely to be large. The implication for pollutant regulation is that requiring a high probability of non-exceedance is likely to carry high costs. The relative contribution of the input variables to critical load uncertainty varied from site to site: any input variable could be important, and thus it was not possible to identify variables as likely targets for research into narrowing uncertainties. Sites where a number of good measurements of input parameters were available had lower uncertainties, so use of in situ measurement could be a valuable way of reducing critical load uncertainty at particularly valuable or disputed sites. From a restricted number of samples, uncertainties in heathland critical loads appear comparable to those of coniferous forest, and nutrient nitrogen critical loads to those of acidity. It was important to include correlations between input variables in the Monte Carlo analysis, but choice of statistical distribution type was of lesser importance. Overall, the analysis provided objective support for the continued use of critical loads in policy development.     

Comparison of different critical load approaches for assessing streamwater acid-sensitivity to broadleaf woodland expansion

Due to its potential adverse effects on freshwater acidification, risk assessments of the impacts of forest expansion on surface waters are required. The critical load methodology is the standard way of assessing these risks and the two most widely used models are the Steady-State Water Chemistry (SSWC) and First-order Acidity Balance (FAB) models. In the UK the recommended risk assessment procedure for assessing the impact of forest expansion on freshwater acidification uses the SSWC model, whilst the FAB model is used for guiding emission policy. This study compared the two models for assessing the sensitivity of streamwater to acidification in 14 catchments with different proportions of broadleaf woodland cover in acid-sensitive areas in the UK. Both models predicted the exceedance of streamwater critical loads in the same catchments, but the magnitudes of exceedance varied due to the different treatment of nitrogen processes. The FAB model failed to account for high nitrogen leaching to streamwater, attributed to nitrogen deposition and/or fixation of nitrogen by alder trees in some study catchments, while both models underestimated the influence of high seasalt deposition. Critical load exceedance in most catchments was not sensitive to the use of different acid neutralising capacity thresholds or runoff estimates, probably due to the large difference between critical load values and acidic deposition loadings. However, the assessments were more sensitive to differences in calculation procedure in catchments where nitrogen deposition was similar to the availability of base cations from weathering and/or where critical load exceedance values were < 1 keq H⁺ ha^− 1 yr^− 1. Critical load exceedance values from both models agreed with assessments of acid-sensitivity based on indicator macroinvertebrates sampled from the study catchments. Thus the methodology currently used in the UK appears to be robust for assessing the risk of broadleaf woodland expansion on surface water acidification and ecological status.     

Calculation and mapping of critical loads of sulfur and nitrogen in Flanders, Belgium

Up to now, critical loads calculations for the Flemish Region were based on European background data of surrounding countries. A first attempt has been made to calculate and map critical loads for forest ecosystems in Flanders using available site-specific information. Values of current deposition were used to calculate and map exceedances. The lowest critical loads for acidification (697 eq ha(-1)year(-1)) occur in the Campine and the north of Limburg where ecosystems largely consist of coniferous forests on poor sandy soils. The dominance of coniferous forest types in the Campine is also responsible for low critical load values for eutrophication (between 536 and 971 eq ha(-1)year(-1)). In 75% of the receptor points that have been considered an exceedance of the critical load for acidification is noted, primarily in areas with high SO2 and NOx depositions, such as the north of the provinces East and West Flanders and Antwerp. The critical load for eutrophication is exceeded in all points considered. Exceedances are particularly high in coniferous forests in West Flanders, and in the north of the provinces of Antwerp and Limburg, where especially NHx depositions amount to high values. Data needed for the calculation of critical loads are still sparse in Flanders, e.g. for: (1) weathering rates of soil minerals; (2) interception and evaporation of forest ecosystems; and (3) uptake of N and basic cations by vegetation. This supplementary information will contribute to a further refining of the calculated critical loads, which constitute indispensable information in developing an emission abatement policy.     

Predicting dissolved inorganic nitrogen leaching in European forests using two independent databases

Regional-scale databases can be particularly useful for identifying relationships between dissolved inorganic nitrogen (N) leaching in forests and environmental drivers, which in turn allow an assessment of the risk of ecosystem damage, such as forest acidification and eutrophication of downstream water bodies. However, detecting the ‘signal’ of a significant correlate to N leaching against a background of wide variability in other factors requires a large number of sites, and the validation of models developed requires a similarly large number of independent sites. Here we use two large and fully independent databases of forest ecosystems across Europe to develop and validate indicators of N saturation and leaching. One database was used for model development and the other for validating these models.Among 35 variables considered, the most significant indicators of N leaching in the model development database were: the flux of dissolved inorganic N in deposition, mean annual temperature, mean altitude, the site drainage (plot vs catchment), needle- and litter-N concentration, organic horizon C:N ratio, and subsoil pH. Altitude was not a consistent predictor (it was significant in the development database but not in the validation database), and needle and litter N concentration, plot vs catchment, and subsoil pH all showed high intercorrelation with N deposition and so were not significant in models already including N deposition. The most consistent and useful indicators of N leaching were throughfall N deposition, organic horizon C:N ratio and mean annual temperature. Sites receiving low levels of N deposition (< 8 kg N ha^− 1 y^− 1) showed very low output fluxes of N and were simulated separately from more polluted forests. In general, the models successfully predicted N leaching (mean of ± 5 kg N ha^− 1 y^− 1 between observed and predicted) from forests at early to intermediate stages of nitrogen saturation but not from nitrogen-saturated sites. Thus, simple relationships developed from combining (1) external drivers (deposition, temperature) and (2) site conditions (nitrogen status of soils) can successfully estimate nitrogen leaching from forests that have not yet been highly damaged by N deposition.     

Testing the steady-state water chemistry model predictions of pre-industrial lake pH with paleolimnological data from northern Sweden

Criteria are needed for distinguishing naturally acid water from that acidified by air pollution, especially in the organic-rich waters of northern Sweden. The Steady-State Water Chemistry Model (SSWC) was augmented to include organic acidity so that it could predict pre-industrial pH in organic-rich waters. The resulting model predictions of pre-industrial ANC and pH were then tested against diatom predictions of pre-industrial pH and alkalinity in 58 lakes from N. Sweden (after alkalinity was converted to ANC using the CBALK method).The SSWC Model's predictions of pre-industrial lake pH in N. Sweden did not correspond well with the diatom predictions, even when accounting for the uncertainty in the diatom model. This was due to the SSWC's sensitivity to short-term fluctuations in contemporary water chemistry. Thus the SSWC Model is not suitable for judging the acidification of individual lakes in areas such as northern Sweden where the degree of chronic acidification is small, or without a good average value of contemporary water chemistry. These results should be considered when assessing the accuracy of critical loads calculated using SSWC.     

Input–output analysis of macroelements in ICP-IM catchment area, Estonia

Using bulk deposition, throughfall, stemflow, soil infiltration, runoff water, litterfall data, ion mass budgets were calculated for a catchment area and for mature spruce and pine stands on it. The ions considered in mass balances were Na⁺, K⁺, Ca²⁺, Mg²⁺, SO₄²⁻, NO₃⁻, NH₄⁺, HCO₃⁻, and H⁺. Corresponding fluxes for the budgets were calculated as an average for 6 years of studies (1995–2000). Annual input–output balances of all nutrients were positive at the plot-scale, so that leaching into soil water was less than the corresponding deposition load. Deposition of Ca, Mg, Na and S into soil by precipitation exceeded input through litterfall. A proton budget approach shows that the main soil buffering process is retention of sulphate, which clearly exceeds weathering. At the catchment-scale, input–output analysis shows essential output of cations due to weathering from the soil. A distinct change in input–output balance of sulphate during study period was evident. The retention of sulphur has been replaced by its release from the catchment area.     

Atmospheric deposition of nitrogen at five subtropical forested sites in South China

Elevated concentrations of reactive nitrogen (N) in precipitation have been reported for many cities in China. Due to increased use of fossil fuels and expansion in agriculture, further increases in deposition of ammonia (NHx) and reactive N oxides (NOy) are predicted. Increased deposition of reactive N is likely to affect N dynamics and N runoff in forest ecosystems. Yet, in China little work has been done to quantify the levels of atmospheric N deposition in such systems. Here, we assess the deposition of inorganic N (ammonium, NH4+ and nitrate, NO3-) for five subtropical forest ecosystems in remote and urban areas of South China. Annual volume-weighted concentrations in bulk precipitation range from 0.18 to 1.55 mg NH4+ -N L(-1) and from 0.12 to 0.74 mg NO3- -N L(-1). These values are large and several times greater than those reported for remote sites of the world. The fluxes of total inorganic N (TIN) in wet-only deposition range from 0.8 to 2.3 g N m(-2) yr(-1), with NH4+ -N contributing 54% to 77%. Both the tree canopy and the ground vegetation layer are important in determining the net N flux reaching the forest floor, but the net effect varies from site to site. At TieShanPing (TSP), close to Chongqing city, and at CaiJiaTang (CJT), near Shaoshan (Hunan province), the canopy represents a net source of N, probably due to dry deposition. At the other three sites (LiuChongGuan (LCG), LeiGongShan (LGS), both in Guizhou province, and LiuXiHe (LXH) in Guangdong), a net loss of reactive N from precipitation water occurs in the canopy, probably due to uptake processes. The total annual atmospheric TIN load is estimated to range from at least 0.8 g N m(-2) yr(-1) to 4.0 g N m(-2) yr(-1), with a considerable contribution from dry deposition. Concentrations and fluxes of inorganic N in tree canopy throughfall are greater than those in North America. Also the contribution of NH4+ -N to TIN fluxes in throughfall (40% to 70%) is greater than in North America. Our sites have N deposition levels and NH4+ -N contributions similar to those found in Northwest and central Europe. The TIN loads at TSP and CJT far exceed 2.5 g N m(-2) yr(-1), which is the N deposition load above which NO3- leaching is expected in temperate and boreal forests.     

Spatial heterogeneity of the spring flood acid pulse in a boreal stream network

Spatial and temporal patterns in streamwater acidity are ecologically important, but difficult to measure in parallel. Here we present the spatial distribution of streamwater chemistry relevant to acidity from 60 stream sites distributed throughout a 67 km² boreal catchment, sampled during a period of winter baseflow (high pH) and during a spring flood episode (low pH). Sites were grouped based on pH level and pH change from winter baseflow to spring flood. The site attributes of each pH group were then assessed in terms of both stream chemistry and subcatchment landscape characteristics. Winter baseflow pH was high throughout most of the stream network (median pH 6.4), but during the spring flood episode stream sites experienced declines in pH ranging from 0-1.6 pH units, resulting in pH ranging from 4.3-6.3. Spring flood pH was highest in larger, lower altitude catchments underlain by fine sorted sediments, and lowest in small, higher altitude catchments with a mixture of peat wetlands and forested till. Wetland-dominated headwater catchments had low but stable pH, while the spring flood pH drop was largest in a group of catchments of intermediate size which contained well-developed coniferous forest and a moderate proportion of peat wetlands. There was a trend with distance downstream of higher pH, acid neutralizing capacity (ANC) and base cation concentrations together with lower dissolved organic carbon (DOC, strongly negatively correlated with pH). This apparent scale-dependence of stream chemistry could be explained by a number of environmental factors which vary predictably with altitude, catchment area and distance downstream—most notably, a shift in surficial sediment type from unsorted till and peat wetlands to fine sorted sediments at lower altitudes in this catchment. As a result of the combination of spatial heterogeneity in landscape characteristics and scale-related processes, boreal catchments like this one can be expected to experience high spatial variability both in terms of chemistry at any given point in time, and in the change experienced during high discharge episodes. Although chemistry patterns showed associations with landscape characteristics, considerable additional variability remained, suggesting that the modeling of dynamic stream chemistry from map parameters will continue to present a challenge.