Calculations of weathering rate and soil solution chemistry for forest soils in the Norwegian-Russian border area with the PROFILE model

Abstract

Potential response of forest soils to S deposition in the Norwegian-Russian border area in the surroundings of the Pechenganikel smelters, the major S emitters in northern Europe, has been assessed with the PROFILE model. The release rate of base cations due to weathering range from 0.05 to 0.28 kmol_c/ha/yr in the 0–50 cm soil layer, thus demonstrating the high sensitivity of the coarse and thin podzols studied. Calculated steady-state BC/Al values are significantly lower than the presumed critical value of 1, which indicate possible negative effect on vegetation through soil acidification. According to the model calculations future S deposition has to be very low in order to stop the ongoing acidification and prevent vegetation damage. However, model assumptions, uncertainty in input data and critical chemical values applied implies that modelling results must be interpreted carefully.

Introduction

The Norwegian-Russian border area and most of the Kola Peninsula receive high loads of S, the primary component of acid deposition in this area (Fig. 1). The surroundings of the Ni smelters in Nikel and Zapolyarnyy are especially polluted. Here the total emission of SO₂ was 380 000 tons in 1980 and decreased to 266 000 tons in 1990 (Pechenganikel smelter data). The smelter in Nikel is the 4th largest S emitter in Europe (Barrett and Protheroe, 1995).

The boreal forests in the border region are among the northernmost coniferous forests of the world. Under the prevailing extreme growth conditions where trees and vegetation are under strong natural stress, it can be presumed that even minor loads of air pollutants may have severe effects upon forest vitality. The capacity of ecosystems to withstand or buffer the effects of acid deposition varies widely according to their physical, chemical and biological properties. The critical load concept is now developed and widely used to assess the sustainability of ecosystems, to compare critical loads with the present pollutant deposition and to connect them with emission reduction strategy in the frame of the Convention on Long-Range Transmission of Air Pollution (CLRTAP). A critical acid load is defined as the highest deposition of acidifying compounds that will not cause chemical changes leading to long-term harmful effects on an ecosystem structure and function according to present knowledge (Nilsson and Grennfelt, 1988).

For forests the molar ratio between base cations (currently as Ca+Mg+K, previously as Ca+Mg or only Ca) and Al in the soil solution (the BC/Al ratio) in the upper part of the profile is commonly used as the critical parameter (Sverdrup et al., 1990; Hettelingh et al., 1991; Downing et al., 1993). It is generally assumed that the value should be larger than one to avoid harmful effects on growth and vitality (Ulrich, 1983; Schulze, 1989; Sverdrup et al., 1992a; Sverdrup and Warfvinge, 1993), however, this value is debated (Högberg and Jensén, 1994; Ilvesniemi and Starr, 1994; Løkke et al., 1996; Nygaard and Eldhuset, 1996). Among the processes which determine the BC/Al ratio in soil solutions in the long run weathering plays a fundamental role. To find a realistic value of the weathering is therefore of great concern in all work attempting to quantify the vulnerability of an ecosystem to acid deposition. In the work on `critical load' the model PROFILE has often been used to determine the weathering rate on a regional scale. However, published data from the use of the PROFILE model are limited to those from the UK and Sweden (Langan et al., 1995). Unfortunately, usually it is very difficult to estimate the uncertainty of a calculated value. The accuracy of the model calculated on the base of statistical deviation in actual tests against field data is stated as ±20% (Sverdrup and Warfvinge, 1993) and ±10% (Sverdrup and Warfvinge, 1995). Critical load of acid deposition for one site calculated by using sensitivity analysis and maximum estimated deviation in inputs varies within a range of ±40% (Jönsson et al., 1995). However, some authors have discussed and criticized the model (Langan et al., 1996; Teveldal and Jørgensen, 1998).

Critical loads of acid deposition for the study area have been assessed as part of the European work and included in several European critical load maps. The maps show somewhat different values:

• According to European maps of critical loads produced by the Coordination Center for Effects (CCE map) on the basis of national data in the EMEP grid cells of 150 by 150 km, critical loads of S (5 percentile) range from 0 to 0.2 kmol_c/ha/yr (Hettelingh et al., 1991; Kämäri et al., 1992; Downing et al., 1993). Present (1990) S deposition exceeds critical loads with 1–2 kmol_c/ha/yr, and the exceedance of critical S deposition is 0.5–1.0 kmol_c/ha/yr.

• The Stockholm Environment Institute (SEI) maps of sensitivity to acidic deposition and critical load values assigned to the sensitivity classes give a still larger range of critical loads, with the low ones, <0.2 kmol_c/ha/yr, mostly predominant (Hettelingh et al., 1991; Kämäri et al., 1992).

• According to maps based on the steady-state mass balance (SSMB) method, critical loads of actual acidity and of S (5 percentile) range mainly from 0.2 to 0.5 kmol_c/ha/yr (Kämäri et al., 1992; Koptsik and Koptsik, 1995). A grid net of 50 by 50 km² was used. Present S depositions are higher than critical values in the large part of the north-western Kola. The greatest excess (0.8–1.2 kmol_c/ha/yr) occurs in surroundings of Ni smelters in Nikel and Zapolyarnyy.

The differences between the maps originate both from differences in the spatial resolution of input data, from the methods used and from the different receptors considered. The CCE map reflects the sensitivity of a mixture of forest soils and surface water ecosystems, whereas the SSMB maps are computed for forest soils only.

The main objectives of the present study were to calculate weathering rates and solution chemistry for soils in the Norwegian-Russian border area with the PROFILE model and to estimate soil vulnerability to acid deposition. These results are important for the assessment of the anthropogenic influence on these marginal forest ecosystems (Løbersli and Venn, 1995).