Harald Sverdrup

Calculating critical loads of acid deposition with profile : a steady-state soil chemistry model

A steady state soil chemistry model was used to calculate the critical load of acidity for forest soils and surface waters at Lake GÄrdsjön in S.W. Sweden. The critical load of all acid precursors (potential acidity) for the forest soil is 1.64 kmolc ha−1 yr−1, and 1.225 kmolc ha−1 yr−1 for surface waters. For the most sensitive receptor, the critical load is exceeded by 1.0 kmolc ha−1 yr−1, and a 80% reduction in S deposition is required, if N deposition remains unchanged. The critical load is largely affected by the present immobilization of N in the terrestrial ecosystem which is higher than the base cation uptake. The model, PROFILE, is based on mass balance calculations for the different soil layers. From measurable soil properties, PROFILE reproduces the present stream water composition as well as present soil solution chemistry. The model calculates the weathering rate from independent geophysical properties such as soil texture and mineral composition.

CALCULATING CRITICAL LOADS FOR ACIDITY WITH THE SIMPLE MASS BALANCE METHOD

The critical load for acid deposition to forest soils, groundwater and surface water can be calculated using a simple mass balance model, taking into account all sources of acidity and alkalinity in the system. The model is very simple to apply, and the major difficulty lies primarily in the estimation of reasonable values for the required input data such as the weathering rate. Methods for its estimation are indicated here. Examples are given for the application of the simple mass balance method (SMB) to Dutch and Swedish forest soils, lakes and shallow groundwater. This work outlines and explains the actual application of the consept as it is being carried out regionally for all European nations.

Modelling long-term cation supply in acidified forest stands

A dynamic soil chemistry model was used to explain the observed decrease in soil base saturation between 1949 and 1984 at three stands in southern Sweden. The results show that acid deposition has caused soil acidification. The model, SAFE (Soil Acidification in Forest Ecosystems), includes the fundamental physical processes such as leaching and accumulation, and chemical processes such as cation exchange, mineral weathering, nutrient uptake and solute equilibrium reactions. The sources and sinks of base cations in the soil system were quantified, showing that weathering, deposition of base cations and depletion of exchangeable base cations supply cations to the soil solution in similar amounts in the upper 1 m during the acidification phase. This demonstrates that budget studies alone cannot be used to distinguish between long-term capacity to resist acidification, represented by weathering, from short-term buffering caused by cation exchange.

Critical levels of atmospheric pollution: criteria and concepts for operational modelling of mercury in forest and lake ecosystems

Mercury (Hg) is regarded as a major environmental concern in many regions, traditionally because of high concentrations in freshwater fish, and now also because of potential toxic effects on soil microflora. The predominant source of Hg in most watersheds is atmospheric deposition, which has increased 2- to >20-fold over the past centuries. A promising approach for supporting current European efforts to limit transboundary air pollution is the development of emission-exposure-effect relationships, with the aim of determining the critical level of atmospheric pollution (CLAP, cf. critical load) causing harm or concern in sensitive elements of the environment. This requires a quantification of slow ecosystem dynamics from short-term collections of data. Aiming at an operational tool for assessing the past and future metal contamination of terrestrial and aquatic ecosystems, we present a simple and flexible modelling concept, including ways of minimizing requirements for computation and data collection, focusing on the exposure of biota in forest soils and lakes to Hg. Issues related to the complexity of Hg biogeochemistry are addressed by (1) a model design that allows independent validation of each model unit with readily available data, (2) a process- and scale-independent model formulation based on concentration ratios and transfer factors without requiring loads and mass balance, and (3) an equilibration concept that accounts for relevant dynamics in ecosystems without long-term data collection or advanced calculations. Based on data accumulated in Sweden over the past decades, we present a model to determine the CLAP-Hg from standardized values of region- or site-specific synoptic concentrations in four key matrices of boreal watersheds: precipitation (atmospheric source), large lacustrine fish (aquatic receptor and vector), organic soil layers (terrestrial receptor proxy and temporary reservoir), as well as new and old lake sediments (archives of response dynamics). Key dynamics in watersheds are accounted for by quantifying current states of equilibration in both soils and lakes based on comparison of contamination factors in sediment cores. Future steady-state concentrations in soils and fish in single watersheds or entire regions are then determined by corresponding projection of survey data. A regional-scale application to southern Sweden suggests that the response of environmental Hg levels to changes in atmospheric Hg pollution is delayed by centuries and initially not proportional among receptors (atmosphere >> soils not equal sediments>fish; clearwater lakes >> humic lakes). This has implications for the interpretation of common survey data as well as for the implementation of pollution control strategies. Near Hg emission sources, the pollution of organic soils and clearwater lakes deserves attention. Critical receptors, however, even in remote areas, are humic waters, in which biotic Hg levels are naturally high, most likely to increase further, and at high long-term risk of exceeding the current levels of concern: </=0.5 mg (kg fw)(-1) in freshwater fish, and 0.5 mg (kg dw)(-1) in soil organic matter. If environmental Hg concentrations are to be reduced and kept below these critical limits, virtually no man-made atmospheric Hg emissions can be permitted.

Developing principles and models for sustainable forestry in Sweden

1. Introduction to the SUFOR programme. 2. On Swedish forests. 3. Defining sustainability. 4. Principles of sustainable forest management. 5. Integrated modelling. 6. Models of the risk of windthrow and frost. 7. Biogeochemical processes and mechanisms. 8. Forest vitality and stress implications. 9. Biodiversity in sustainable forestry. 10. Forests, acidification and the socio-economic cost. 11. Assessing effects of wildlife on forestry. 12. Sustainability in spruce and mixed-species stands. 13. Productivity scenarios for the Asa Forest Park. 14. Assessment of sustainability in the Asa Forest Park. 15. Nutrient sustainability for Swedish forests. 16. General conclusions. 17. References.

Use of dynamic soil-vegetation models to assess impacts of nitrogen deposition on plant species composition: an overview

Field observations and experimental data of effects of nitrogen (N) deposition on plant species diversity have been used to derive empirical critical N loads for various ecosystems. The great advantage of such an approach is the inclusion of field evidence, but there are also restrictions, such as the absence of explicit criteria regarding significant effects on the vegetation, and the impossibility to predict future impacts when N deposition changes. Model approaches can account for this. In this paper, we review the possibilities of static and dynamic multispecies models in combination with dynamic soil-vegetation models to (1) predict plant species composition as a function of atmospheric N deposition and (2) calculate critical N loads in relation to a prescribed protection level of the species composition. The similarities between the models are presented, but also several important differences, including the use of different indicators for N and acidity and the prediction of individual plant species vs. plant communities. A summary of the strengths and weaknesses of the various models, including their validation status, is given. Furthermore, examples are given of critical load calculations with the model chains and their comparison with empirical critical N loads. We show that linked biogeochemistry-biodiversity models for N have potential for applications to support European policy to reduce N input, but the definition of damage thresholds for terrestrial biodiversity represents a major challenge. There is also a clear need for further testing and validation of the models against long-term monitoring or long-term experimental data sets and against large-scale survey data. This requires a focused data collection in Europe, combing vegetation descriptions with variables affecting the species diversity, such as soil acidity, nutrient status and water availability. Finally, there is a need for adaptation and upscaling of the models beyond the regions for which dose-response relationships have been parameterized, to make them generally applicable.

A model for the impact of soil solution Ca: Al ratio, soil moisture and temperature on tree base cation uptake

A model for tree base cation uptake has been developed, dependent on the soil solution concentration of Al3+, divalent base cations such as Ca2+, Mg2+ and H+ ions, modelled with a Mikaelis-Menten type of expression based on the molar BC∶Al ratio, where BC is the sum of the divalent non-toxic base cations Ca2+ and Mg2+. The expression has the form of counteracting adsorption isoterms for BC and Al to the tree root. The effect of toxic levels of Hg and Pb is incorporated into the model, using root adsorption as the mechanism, and parameterization against experimental data. The expression is modified with an expression accounting for the effect of heavy metal toxicity and soil water content. The dependence of the uptake rate on soil moisture content can be shown to have the form of a Freundlich adsorption isotherm for water. The available data indicate an activation energy of 47 kJ−1 mol for base cation uptake to trees. Data from the literature was used to estimate the rate coefficients and ion selectivity coefficients for typical coniferous and decideous trees in Sweden and Germany. The model indicates that Ca2+ and Mg2+ is effective in mitigating Al3+ toxicity to tree roots, and that increasing the Ca2++Mg2+ soil solution concentration has a promotive effect on base cation uptake. Above a certain limit base cation uptake becomes independent of the solution base cation concentration. This is consistent with field observations, and may be developed to become a tool for assessing the impact of soil chemistry changes on forest growth rate and health status. Field data from the Swedish Forest Survey indicate that uptake depend on the square root of the soil solution base cation availability originating from weathering and deposition input, which is consistent with the BC∶Al expression of the model.

Modelling recent and historic soil data from the Rothamsted Experimental Station, UK using SAFE

Abstract Historic soil data from 1883, 1906, 1964 and new data from 1991 from the Geesecroft Wilderness Experiment at Rothamsted Experimental Station, UK, was used as a test of the dynamic biogeochemical model SAFE in a situation where the available data can be used to confine the model completely. The test indicated that the model is capable of predicting the observed changes over time in soil chemistry, without calibration. This suggests that the model formulation and choice of significant processes are quantitatively correct. The model application at Rothamsted shows that deposition of acidity because of sulphur and nitrogen emissions during the last 110 years, is the major cause of soil acidification in Geesecroft Wilderness. Natural afforestation of the site has also contributed with a significant but smaller amount of acidity input to the soil. Acidification has caused a possibly irreversible decrease in the cation exchange capacity of the soil because of weathering of clay minerals.

Assessment of soil acidification effects on forest growth in Sweden

The results of mapping critical loads, areas where they have been exceeded and steady state (Ca+Mg+K)/Al ratios of soils in Sweden, has been used to assess the order of magnitude of the ecological and economic risks involved with acid deposition for Swedish forests. The results of the calculations indicate that 81% of the Swedish forested area receive acid deposition in excess of the critical load at present. Under continued deposition at 1990 level, forest die-back is predicted to occur on approximately 1% of the forested area, and significant growth rate reductions are predicted for 80% of the Swedish forested area. For Sweden, growth losses in the order of 17.5 million m−3 yr−1 is predicted, equivalent to approximately 19% of current growth. Comparable losses can be predicted for other Nordic countries. The soil acidification situation is predicted to deteriorate significantly during the next 5–15 years, unless rapid emission reductions can be achieved. A minimum deposition reduction over Sweden of 85% on sulphur deposition and 30% on the N deposition in relation to 1990 level is required in order to protect 95% of the Swedish forest ecosystems from adverse effects of acidification. A minimum reduction of 60% on sulphur deposition and 30% on the N deposition is required to keep forest harvest at planned levels.

Uncertainty in predicting weathering rate and environmental stress factors with the profile model

The PROFILE model is a steady state soil chemistry model which is used to calculate soil weathering rate. The model has also been used to calculate critical loads of acidity and N to forest soils, using the ratio of Ca+Mg+K to total inorganic aluminium in the soil solution as criterion, and to surface waters, using the ANC leached from the soil column as criterion.An uncertainty analysis of the PROFILE model was performed by Monte Carlo analysis, varying input parameter errors individually and simultaneously in ranges of ±10–100%, depending on parameter. The uncretainty in calculation of weathering rate, ANC leaching and ratio of Ca+Mg+K to inorganic Al in the soil solution was studied for three Nordic sites. Furthermore, the effect of uncertainty in estimates of critical load for forest soils was assessed.The analysis shows that the weathering rate can be calculated with high precision, provided that the errors of input parameter are within the range that has been reported in the literature. The model tend to be less sensitive to errors in input parameters for the range of conditions where forest damage is most likely to occur. Critical loads of acid deposition for one site calculated on the basis of the model varies within a largest range of ±40%. A study of one geographical grid included in the Swedish critical loads assessment shows that with the number of calculation points in the grid, the distribution of critical loads will stay stable independently of stochastic errors.