Peter Blaser

Preferential flow paths: biological ‘hot spots’ in soils

The objective of this study was to investigate whether preferential flow paths have higher microbial biomass and different microbial community structures than the rest of the soil. The organic C concentrations in the preferential flow paths were 10 to 70% higher than in the matrix. The organic N concentrations were also enriched in the preferential flow paths, as well as the effective cation exchange capacity and the base saturation. Microbial biomass was 9 to 92% higher in the preferential flow paths than in the matrix, probably due to the better nutrient and substrate supply. The DNA concentrations and direct cell counts showed a similar pattern, while domain-specific genetic fingerprints did not reflect the differences between flow regions. However, Pseudomonas displayed different population structures between preferential flow paths and soil matrix. This indicated that possibly only few populations with a broad acceptance for substrates and aerobic as well as anaerobic growth specifically profit from the favourable conditions in the preferential flow paths.

Critical examination of trace element enrichments and depletions in soils: As, Cr, Cu, Ni, Pb, and Zn in Swiss forest soils

The aim of this study was to obtain an overview of trace element concentrations in Swiss forest soils and to critically assess the measured values with respect to anthropogenic input vs. lithogenic background. Twenty-three sites were selected which represent a broad range of natural forest sites, bedrock material and soil types of Switzerland. At each site, samples were collected from all genetic soil horizons down to a C or B/C horizon. Total concentrations of As, Cr, Cu, Ni, Pb, and Zn in all samples were determined by X-ray fluorescence spectrometry. There were distinct differences in the geological background values estimated from the concentrations measured in the samples from the lowest soil horizon. Background concentrations for Cr and Ni were lowest in granite and gneiss, whereas Pb and Zn were highest in limestone and marl. Enrichment or depletion of the trace elements was assessed using Zr as reference element. Within the same profile, the six trace elements showed completely different enrichment/depletion patterns with depth. The various natural processes and anthropogenic inputs that can lead to these patterns are critically discussed. Based on this critical assessment, pollution of the investigated forest soils was found to be most severe for Pb and Zn and to a somewhat lesser extent for As and Cu, whereas anthropogenic input of Cr and Ni seems to be less important. The data suggest that a critical evaluation of enrichment factors is a better tool to assess soil pollution with trace elements than the use of maximum allowable concentrations (MAC) for topsoil samples. The enrichment factors calculated as described here consider the effects of geological variation on metal abundances whereas the MAC does not. In order to obtain an estimate of soil solution concentrations, water extracts of the samples collected from a subset of 10 soil profiles were analyzed for the same trace elements. Solubility of all elements generally decreased with soil depth. An exception was Cr, Cu, and Ni solubility in the humus layer, which was lower than in the underlying mineral horizon. For all elements, solubility was higher for the collective of soil samples depleted in this element when compared to the samples, in which the element was enriched.

Spatial Distribution of 137CS in Forest SOils of Switzerland

In the framework of the second Swiss forest soil inventory, 137Cs-activity from 172 sites was measured systematically for the first time in the topmost soil layer (0–5 cm) and represented on a map. The spatial distribution of 137Cs contamination was similar to the pattern observed in 1986 from dose equivalent measurements following the Chernobyl nuclear power plant accident. Forest soils from regions with high precipitation in 1986 showed a higher 137Cs activity than regions with low precipitation. At sites with high caesium activities it was possible to discriminate between 137Cs originating from global fallout of the fifties and sixties and 137Cs from the Chernobyl accident. The results indicate that radiocaesium persists in the top soil layers and is recycled in forest ecosystems.

Equilibrium ion exchange method: methodology at low ionic strength and copper(II) complexation by dissolved organic matter in a leaf litter extract.

The equilibrium ion exchange method (EIM) is a powerful tool for the investigation of metal cation complexation by dissolved organic matter (DOM) in natural systems. Tests with different ion exchange resins demonstrated that under low ionic strength conditions (0.01 mol/kg) and in the presence of DOM, equilibration times of at least 24 h are required for experiments with Cu(II). The classical approach to the EIM was modified by using nonlinear reference adsorption isotherms in order to expand the method to a broader range of experimental conditions. For Cu(II) at low ionic strength (0.01 mol/kg), the reference isotherms between pH 4 and 6 were identical and were mathematically modeled in terms of Langmuir adsorption parameters. The EIM using nonlinear reference isotherms was validated between pH 4 and 6 by the correct determination of the stability constants for the complexes CuOxalate and Cu(Oxalate)(2). Then the method was used to quantitatively characterize the Cu(II) complexation behavior of DOM in an aqueous chestnut leaf litter extract between pH 4 and 6. In contrast to the classical approach to the EIM, data were analyzed by using plots [Cu](bound)/[Cu](free)vs. [Cu](bound). This allowed the determination of both, conditional stability constants and metal binding capacities for two different binding site classes. The logarithmic values of the stability constants were about 8 for the strong binding sites and 5.5-6 for the weak binding sites. The total Cu(II) binding capacity increased from 0.22 mol/(kg C) at pH 4 to 2.85 mol/(kg C) at pH 6.

Heavy metals in Swiss forest soils: modification of lithogenic and anthropogenic contents by pedogenetic processes, and implications for ecological risk assessment

Abstract We investigated the occurrence and effective mobility of heavy metals (HM) in a representative collective of Swiss forest soils. The total HM contents of pedogenetic horizons were analysed, and the enrichment or depletion of a given HM relative to the original lithogenic content was assessed. The latter was calculated using the contents of Zr as an immobile reference. Chromium, Ni, Cu and Zn were mainly lithogenic, with a wide range of contents reflecting the diverse geology of Switzerland, while anthropogenic input of Pb was detected in most topsoils. Pedogenetic processes exerted a strong influence on the translocation or leaching of the HM. In acidic soils Ni, Cu and Pb were more mobile than Cr and Zn. We relate this behaviour to the strong affinity of the former HM to dissolved or colloidal organic matter. On the other hand, nutrient cycling by the vegetation probably led to an apparent reduction of Zn downward mobility. In many soils, guide levels as specified by Swiss legislation, or threshold values for effects on micro-organisms, were exceeded, indicating a potential risk for long-term soil fertility. As a consequence of translocation, guide levels for Cr, Ni and Cu were exceeded more often in subsoils than in topsoils.

Monitoring of Water Chemistry in Forest Soils: An Indicator for Acidification

Acid atmospheric deposition can affect the chemistry of soils and drainage waters in forest ecosystems and accelerate the acidification of soils. In acidic soils, the input of acidifying compounds increases the mobility of aluminum that can reach toxic levels for sensitive plant species. In addition, leaching losses of nutrients such as Ca, Mg and K may increase as a result of acidic deposition. These cations are important for tree nutrition and a depletion can affect both biomass production and, by an imbalanced nutrition, tree health and sensitivity to pests. The soil water chemistry in a chestnut forest at Copera near Monte Ceneri in Ticino has been monitored since 1987 to measure the soil response to atmospheric acid deposition. This area, with its mainly acidic bedrock, is very sensitive because the soil is poorly buffered. It has received high loads of acidifying compounds due to local and long-range emissions from the industrial Po Valley in Italy. Acid deposition declined through the 1980s and 1990s, mainly because of the reduction of SO2 emissions following air pollution abatements. This raised the question whether the forest soil has recovered since then. The ratio of base nutrient cations (BC = Ca2+ + Mg2+ + K+) to dissolved aluminum (BC/Al) in the soil solution was used to assess soil acidification and the associated ecological risks. Water samples were collected fortnightly with tension lysimeters. The main chemical characteristics of the soil solutions were measured routinely: pH, electric conductivity, dissolved organic carbon, concentrations of major cations and anions. A significant decrease in BC/Al ratios has been observed since 1987, indicating a rapid soil acidification. However, initial signs of recovery were detected recently below the litter layer. In the mineral horizons, the ratios have stabilized since the late 1990s, suggesting that acidification has slowed down.

Analytical Problems in the Determination of Inorganic Soil Contaminants

Most of the inorganic materials designated as pollutants in soil are trace elements that occur naturally in all soils in a wide range of concentrations. They are considered harmful only where their concentrations exceed certain threshold values. In most cases there is no method that specifically characterizes these elements according to their origin. An assessment of inorganic pollution, based on total element concentrations of top-soil samples only, is therefore almost impossible. More promising results can be obtained using radio isotope techniques or from chemical analysis of entire soil profiles. The latter approach is demonstrated in a case study with lead, extracted with nitric acid and NH4-acetate-EDTA from soil samples collected by horizons in 100 profiles from Switzerland. The total (extracted by nitric acid) and the extractable (extracted by NH4-acetate-EDTA) amounts of lead varied significantly among the different parent materials.