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Tobor-Kapl on, M. A., Bloem, J., Rö mkens, P. F. A. M. and de Ruiter, P. C. 2005. Functional stability of microbial communities in contaminated soils. Á/ Oikos 111: 119 Á/129.Functional stability, measured in terms of resistance and resilience of respiration and growth rate of bacteria and fungi, was studied in soils that have been exposed to copper and low pH for more than twenty years. We used treatments, consisting of soil with no or high copper load (0 or 750 kg ha (1 ) and low or neutral pH (4.0 or 6.1). Stability was examined by applying an additional stress in the form of lead or salt. After addition of lead, respiration (decomposition of freshly added lucerne meal) showed lower resistance at low than at neutral pH and at high copper than at low copper. The most acid and contaminated soil was the least resistant. Respiration showed no resilience after addition of lead. Bacterial growth rate (thymidine incorporation) also showed resistance at low pH but only in soils that were not contaminated with copper.After addition of salt, respiration showed no differences in resistance but the soils without copper contamination showed higher resilience. Bacterial growth rate showed lower resistance at low pH than at neutral pH, the latter in which the growth rate increased by on average 123%. This increase at high pH was faster in soil without copper than in soil with copper contamination in which the growth rate initially decreased and then increased. The effects of secondary stress depended on the nature of the stress (lead or salt) and on the parameter measured (respiration or bacterial growth rate). In general the highest resistance and/or resilience were found in the least contaminated soils with neutral pH and/or no copper contamination. Thus, the microbial communities in the cleaner soils showed the highest functional stability. The results seem to confirm the notion that environmental stress alters ecosystems such that supplementary stress will have stronger impacts than in an unstressed system. The results may also confirm the insurance-hypothesis that reduced biodiversity due to the first stress negatively affected community stability. As an alternative, we discuss the observed effects in terms of altered energy budget.

Flocculation of soil-derived dissolved organic carbon (DOC) was used as a tool to study the molecular size distribution of DOC and the native amount of copper bound to different size fractions. DOC was extracted from arable soils that had received varying amounts of animal manure, inorganic fertilizer, or CuSO 4 . Addition of calcium to the DOC extracts resulted in flocculation of up to 50% of the DOC originally present in the samples. High performance size exclusion chromatography (HPSEC) analysis revealed that the DOC removed mainly consisted of high molecular weight (HMW) organic acids. Low molecular weight (LMW) substances remained soluble even at high calcium concentrations. Copper solution concentrations decreased almost linearly with decreasing DOC levels, suggesting that copper remained bound to the flocculated material despite increasing calcium concentrations. The amounts of copper bound to both LMW and HMW components depended on the copper content of the soil and were described by two Langmuir sorption equations. Maximum binding capacities varied between 250 µmol of copper g -1 of C for HMW components and 450 µmol of copper g -1 of C for LMW components, but binding affinities were higher for HMW components. The Langmuir equation described the measured copper solution concentrations from the batch experiments very well (R 2 ) 0.994). The observation that up to 50% of the DOC in solution was not removed by calcium indicates that a significant part of copper is potentially highly mobile in soils and can be prone to DOC-facilitated transport.

Background, aim, and scope Ongoing industrialization has resulted in an accumulation of metals like Cd, Cu, Cr, Ni, Zn, and Pb in paddy fields across Southeast Asia. Risks of metals in soils depend on soil properties and the availability of metals in soil. At present, however, limited information is available on how to measure or predict the directly available fraction of metals in paddy soils. Here, the distribution of Cd, Cu, Cr, Ni, Zn, and Pb in 19 paddy fields among the total, reactive, and directly available pools was measured using recently developed concepts for aerated soils. Solid-solution partitioning models have been derived to predict the directly available metal pool. Such models are proven to be useful for risk assessment and to derive soil quality standards for aerated soils. , and directly available metal pools (0.01 M CaCl 2 ) were determined. Solid-solution partitioning models were derived by multiple linear regressions using an extended Freundlich equation using the reactive metal pool, pH, and the cation exchange capacity (CEC). The influence of Zn on metal partitioning and differences between both sampling events (May/November) were evaluated. Results Total metals contents range from background levels to levels in excess of current soil quality standards for arable land. Between 3% (Cr) and 30% (Cd) of all samples exceed present soil quality standards based on extraction with AR. Total metal levels decreased with an increasing distance from the irrigation water inlet. The reactive metal pool relative to the total metal content is increased in the order Cr << Ni = Zn < Pb < Cu < Cd and ranged from less than 10% for Cr to more than 70% for Cd. Despite frequent redox cycles, Cd, Pb, and Cu appear to remain rather reactive. The methods to determine the reactive metal pool in soils yield comparable results, although the 0.43 M HNO 3 extraction is slightly stronger than HCl and EDTA. The close correlation between these methods suggests that they release similar fractions from soils, probably those reversibly sorbed to soil organic matter (SOM) and clay. The average directly available pool ranged from less than 1% for Cu, Pb, and Cr to 10% for Ni, Zn, and Cd when compared to the reactive metal pool. For Cd, Ni, Zn, and to a lesser extent for Cu and Pb, solid-solution partitioning models were able to explain up to 93% (Cd) of the observed variation in the directly available metal pool. CaCl 2 extractable Zn increased the directly available pool for Ni, Cd, and Cu but not that of Pb and Cr. In the polluted soils, the directly available pool was higher in November compared to that in May. Differences in temperature, rainfall, and changes in soil properties such as pH are likely to contribute to the differences observed within the year. The solid-solution partitioning model failed to explain the variation in the directly available Cr pool, probably because Cr is present in precipitates rather than being adsorbed onto SOM and clay. Despite obvious differences in parent material, source of pollu...

Models to predict the solid-solution partitioning of trace metals are important tools in risk assessment, providing information on the biological availability of metals and their leaching. Empirically based models, or transfer functions, published to date differ with respect to the mathematical model used, the optimization method, the methods used to determine metal concentrations in the solid and solution phases and the soil properties accounted for. Here we review these methodological aspects before deriving our own transfer functions that relate free metal ion activities to reactive metal contents in the solid phase. One single function was able to predict free-metal ion activities estimated by a variety of soil solution extraction methods. Evaluation of the mathematical formulation showed that transfer functions derived to optimize the Freundlich adsorption constant (K f ), in contrast to functions derived to optimize either the solid or solution concentration, were most suitable for predicting concentrations in solution from solid phase concentrations and vice versa. The model was shown to be generally applicable on the basis of a large number of independent data, for which predicted free metal activities were within one order of magnitude of the observations. The model only over-estimated free-metal ion activities at alkaline pH (>7). The use of the reactive metal content measured by 0.43 m HNO 3 rather than the total metal content resulted in a close correlation with measured data, particularly for nickel and zinc.

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