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Global satellite observations (NOAA/AVHRR) available for an extended time series (10 years) provide measurements of spectral reflectances in the red and near-infrared, which are useful to monitor the phenology of green vegetation. The methodology applied to study the phytophenological rhythms of vegetation involved the application of the Fourier analysis, used to summarize nine years of monthly NDVI-global/local area coverage (GAC/LAC) composites in a few parameters, ie, amplitude and phase values of the dominant periodic components. Continental-scale maps of vegetation units, each one characterized by homogeneous foliar phenology (maps of isogrowth zones), were calculated by using Fourier attributes such as the mean NDVI, the amplitude and the phase values at 12 and six months. Maps of isogrowth zones were obtained at continental scale for Southern Africa, South America and Europe by means of different temporal NOAA-AVHRR NDVI datasets. The main Fourier spectra of the isogrowth zones were compared with various ancillary data, ie, land cover maps, meteorological data, etc. The results highlight the importance of the Fourier analysis of the time series of NDVI observations as a reliable method of characterizing vegetation dynamics and correlating them with land cover characteristics at regional and continental scales.

The impact of the insecticide lindane (gamma-hexachlorocyclohexane) on the clearance rate (CR) of Daphnia magna was investigated using artificial beads. CR (24-h EC50: 65mgr;g L-1) was found to be a more sensitive endpoint than acute lethality for D. magna (48-h LC50: 516mgr;g L-1). The onset of the effect was rapid; after 2 h of exposure to approximately 241mgr;g L-1 of lindane a significant decrease in CR was observed. Daphnids recovered rapidly after transfer to clean water; after 24 h of exposure to approximately 250mgr;g L-1 lindane, transfer into clean water resulted in recovery to 80% of control levels within 2 h and complete recovery within 24 h.

Microbial activity in soils depends on the status or the soil water, which is expressed by pressure head (h) or water content (θ). There is no unique relationship between θ and h because moisture relations exhibit hysteresis. For convenience microbial activity has usually been related to the main drying curve but in general this will lead to bias in computer models that aim to simulate microbial activity. This article aims to evaluate the magnitude of the bias resulting from the use of computer simulation models of mineralization coupled to a model that follows the hysteresis of water relations in soil. Simulations of mineralization were found to accumulate a bias of more than 15% of the mean annual mineralization by harvest. This bias was found to be most serious where soil is continually subject to severe wetting and drying cycles such as can be found in irrigated and tropical agriculture. | Microbial activity in soils depends on the status of the soil water, which is expressed by pressure head (h) or water content (θ). There is no unique relationship between θ and h because moisture relations exhibit hysteresis. For convenience microbial activity has usually been related to the main drying curve but in general this will lead to bias in computer models that aim to simulate microbial activity. This article aims to evaluate the magnitude of the bias resulting from the use of computer simulation models of mineralization coupled to a model that follows the hysteresis of water relations in soil. Simulations of mineralization were found to accumulate a bias of more than 15% of the mean annual mineralization by harvest. This bias was found to be most serious where soil is continually subject to severe wetting and drying cycles such as can be found in irrigated and tropical agriculture.

The spatial variability of turbulence above a forest has been examined. Two measurement towers were erected 800 m apart within a heterogeneous mixed forest located in the north east of the Netherlands. The measurements of u*/u were analysed and subsequently used to test a surface layer model. The model simulated the magnitude of the measurements reasonably well, but measured trends were not always reproduced by the model. The variable (du/dz)/u did not adapt as quickly to the new surface as u*/u. This is in agreement with Schmid (1994), and can be explained by a local decrease in mixing length. It is recommended to adapt the mixing length near a surface transition to improve the accuracy of surface layer models of heterogeneous landscapes.

A method is presented to calibrate transfer function-noise (TFN) models, operating at the same frequency as the input (auxiliary) variables, to sparsely or irregularly observed time series of the output (target) variable. Once calibrated, the TFN models can be used to predict or simulate the output variable at the same frequency as the input variable. Consequently, the method provides a useful tool for filling in gaps of irregularly or sparsely observed hydrological time series. Although generic and suitable for any type of time series, the method is described through the modeling of a time series of groundwater head data with precipitation surplus (precipitation minus potential evapotranspiration) as input variable. First, the TFN model is written in vector notation, yielding the state equation of a linear discrete stochastic system. Subsequently, the state equation is embedded in a Kalman filter algorithm. The Kalman filter is then combined with a maximum likelihood criterion to obtain estimates of the parameters of the TFN model for small time steps (e.g., 1 day) while using sparsely (e.g., two times a month) or even irregularly observed time series of groundwater head data. The method is illustrated using (subsets of) time series of groundwater head data with varying regular and irregular observation intervals.

The effect of the rate and method of nitrogen application on nitrogen uptake and utilization by broccoli (Brassica oleracea var. italica) was studied in four field experiments. The methods of application were broadcast application vs band placement and split application. Maximum uptake of nitrogen by the crop was around 300 kg ha-1. In one experiment, band placement positively influenced nitrogen uptake. Split application did not influence nitrogen uptake. Nitrogen application resulted in a higher head dry matter production, but the efficiency of nitrogen utilization for the production of head dry matter decreased with higher amounts of nitrogen applied. Nitrogen application decreased the dry matter content of the heads. In half of the experiments, band placement of nitrogen fertilizer resulted in extra head dry matter production and lower head dry matter contents. At the optimum rates of band placed nitrogen application, the nitrogen harvest index in the experiments ranged from 27 to 30%. The amount of mineral nitrogen in the soil at harvest generally increased with increasing amounts of nitrogen applied. Band placement resulted only in one experiment in lower amounts of mineral nitrogen in the soil at harvest. The mineral nitrogen in the soil at harvest can be unevenly horizontally distributed, both with broadcast application and band placement of nitrogen fertilizer. The amount of nitrogen unaccounted for at harvest increased with increasing amounts of nitrogen applied, but was always less than the amount of nitrogen in crop residues. At the optimum rates of band placed nitrogen application, the amount of nitrogen in crop residues ranged from 120 to 155 kg ha-1. With broccoli cultivation, the nitrogen in the crop residues forms the single largest source of potential loss of nitrogen to the environment.

Studies have been carried out in two fully stocked, fast growing Douglas-fir plantations of the Dutch ACIFORN project in three consecutive years, to obtain information on fine root densities (Olsthoorn 1991). For the present paper, data collected in early summer 1987 were used to study the relation of fine root density and proximity to the nearest tree or the dominant tree. A large number of samples (37 in one site and 55 in another) was collected in a small plot (10 x 1 m). Two distances were measured at each sampling point: the distance to the nearest tree and the distance to the tree with a dominant crown above that point. There was large variability in fine root density in the samples. Tests with different regression models showed a distinct rooting pattern for one of the two locations. It is concluded that systematic errors in the assessment of fine root density can arise when sampling points are chosen at a constant distance from trees. For Douglas-fir, this systematic error could have been an overestimation of the fine root density by up to 10%. These systematic errors can be avoided easily, using a stratified random design or a random sampling design. When trees are spaced irregularly, a grid sampling design is also appropriate.

Conversion of arable land (maize) to pasture will affect the soil organic matter (SOM) content. Changes in the SOM content were studied using a size- and density-fractionation method and 13C analysis. Twenty-six years of maize cropping had resulted in a depletion of carbon stored in the macro-organic fractions (>150 μm) and an increase in the 250 μm), light (b.d. 150 μm) and light (b.d. < 1.13 g cm-3) fractions were almost completely regenerated by input of root-derived SOM. The amount of medium-weight and heavy macro-organic fractions (>150 μm; b.d. >1.13 g cm-3) in the 0- to 20-cm layer was still 40-50% lower than in the continuous pasture plots. Average half-life times calculated from 13C analyses ranged from 7 years in the light fractions to 56 years in heavy fractions. Fractionation results and 13C data indicated that mechanical disturbance (plowing) during maize cropping had resulted in vertical displacement of dispersed soil carbon from the 0- to 20-cm layer down to 60-80 cm. Conversion of arable land to pasture, therefore, not only causes a regeneration of the soil carbon content, it also reduces the risk of contaminant transport by dispersed soil carbon.

In the last decades, Dutch landscape conservation policy has developed from a sectoral into an integrated one, in which the values concerning the conservation of landscapes are strongly linked to the development of new local economic activities. This change is reflected in a new policy for the development of so-called valuable man-made landscapes (VMLs) that combines an integrated approach with a bottom-up strategy. Since 1994, 11 areas in the Netherlands have been designated as VMLs. The aim of this study is to identify and explain the differences in development between these areas. VML areas can be distinguished into three groups on the basis of their stages of development. Five main factors were found that explain the differences between these three groups: their history of regional co-operation (social), financial problems in agriculture (economic), awareness of regional identity (cultural), distance to the cities (physical, geographical) and size of the area (administrative). The problems in agriculture, regional identity and administrative size of the area explain why certain VMLs had a better start than other ones. For the future of VML-like-areas it is important that policy goals remain unchanged for decades and that VML policy is connected to urban developments.

In sandy regions of the Netherlands, current agricultural practices result in high nitrate concentrations in the upper groundwater, endangering water resources. A methodology to estimate costs of region specific solutions is applied to a groundwater protection area. It requires that for existing farms in the region variants of management are described that differ in nitrogen surplus. Costs of variants relate to investments and losses in income due to changes in farm management additional to those required by existing legislation. The expected nitrate concentration for each combination of variant and groundwater class is calculated from nitrogen and precipitation surplus and a leaching fraction. A linear programming model indicates the optimal spatial allocation of variants such that desired regional and sub-regional nitrate concentrations are reached at minimum regional costs. In the case study, current land use is grouped into non-dairy farms and various types of dairy farms and non-agricultural land use. Management variants are described for most of these groups. The LP model was used to analyze the effect of the following conditions: (a) the scale at which the tolerated maximum NO 3 concentration is effectuated; (b) the level of this maximum; (c) the hydrological situation; (d) the way farms are grouped; and (e) the biophysical situation of farms. Total regional costs show a negative, but not necessarily linear, relation with the resulting regional nitrate concentration. This relation is strongly influenced by the actual situation regarding management at the farms and the groundwater table depths in the region. For each region a specific cost–benefit relation might apply. Higher groundwater tables result in strongly reduced costs for required changes in farm management. Depending on costs and physical feasibility, measures that result in a higher groundwater table could therefore be used in addition to or replacement of measures at farm level. Calculated costs are influenced by the method of grouping of farms into farm types, the largest effect due to differentiating intensities of production. Differences between farms need therefore be considered when analyzing possibilities for and costs of measures to reduce nitrate leaching. Political decisions, specifically about the scale at which the maximum nitrate level should be effectuated (farm-type or region) also have a strong effect on the cost–benefit relation. Requiring all farm types to comply with a given maximum nitrate concentration, resulting in a specific regional average, is more costly than setting a similar regional maximum and allowing regional averaging. Methodology and preliminary results were discussed with policy-makers and representatives of farmers, environmentalists and groundwater extracting companies. They felt that these discussions helped focussing the interaction between the various groups of stakeholders. This is due to the fact that assumptions about effects of changes in land use and costs involved are made explicit and to the possibility of a more objective weighing of interests since wishes and conditions are translated into goals and constraints of the linear programming model.

A soil sampling scheme for estimating the mean extractable P concentration of fields is designed to be used as a tool for environmental regulation of the application rates of manure. The field to be sampled, is split up into geographically compact blocks of equal area that are used as strata. From each stratum one sampling point is selected by Simple Random Sampling. These samples are bulked into one composite for the field. The geographical stratification is performed by restricted least-squares clustering of raster cells using the coordinates of the midpoints as classification variables and the within-group sum of squares as the minimisation criterion. Using a variance model and a cost model, the numbers of sample points and laboratory analyses are optimised simultaneously, given a maximum allowed variance of the total error (sampling error plus measurement error). To predict the sampling variance, variograms have been estimated for 16 fields differing in land-use, soil parent material and phosphate level. A pooled relative variogram was used to predict the sampling variance for various sample sizes (5 to 50), field-areas (1 to 10 ha) and phosphate levels (for grassland 20 to 80 mg P 2 O 5 extracted in ammonimum lactate per 100 g soil, for arable land 20 to 80 mg P 2 O 5 extracted in water per 1 dm 3 soil). The cost model consists of three components: (i) fieldwork cost; (ii) field equipment cost and, (iii) laboratory cost. For the 16 fields, the predicted sampling variance of the Stratified Sampling design is 0.8 to 0.4 times the predicted variance of Simple Random Sampling if 40 points were sampled. To estimate the mean extractable P concentration with a total variance ≤9, replicate measurement of the composite only pays if the mean extractable P concentration of the field exceeds 40 to 50. This critical phosphate level increases with the maximum allowed variance of the total error.

When pesticides leach through the soil to the upper groundwater zone, it is important to know whether further transformation occurs before the pumping wells for drinking water are reached. Atrazine and bentazone were incubated (at 10°C) in five water-saturated sandy subsoils (collected at depths between 1·5 and 3·5 m), simulating the conditions in the field. In three subsoils with comparatively low pH and intermediate to high redox potential, atrazine was transformed gradually, to leave 1·9%, 6·2% and 17·5% of the dose after about five years. In one of these subsoils, hydroxy-atrazine was detected; the amount corresponded to half of the dose of atrazine. In one anaerobic subsoil with high pH, the transformation of atrazine was comparatively fast (half-life about 0·15 year). Another anaerobic subsoil, with similar pH and a somewhat higher redox potential, however, showed hardly any transformation. Sterilization of the first anaerobic subsoil had no effect on the rate of transformation. In the course of about five years, bentazone in the first three subsoils was transformed gradually to leave

Dry water repellent soils are known to inhibit water infiltration, ultimately forcing water to flow via preferential paths through the vadose zone. To study water flow and transport in a water repellent sandy soil, a bromide tracer experiment had been carried out, which started in the fall after winter wheat had been sown. Despite the uniform tracer application, soil core sampling indicated that bromide concentrations varied largely from place to place. Wetter sites in the experimental field received more bromide, due to lateral transport through a thin top layer. Wetting fronts infiltrated deeper here, leading to perturbed wetting fronts in the experimental field. In contrast to what was expected, the wetting front perturbations did not grow to fingers. Numerical results indicate that this was attributed to the relatively high soil water contents during the experiment, which caused the soil to be wettable instead of water repellent. The water-entry capillary pressure of the secondary wetting branch exceeds the air-entry capillary pressure of the primary drainage branch in this case. In the opposite situation, with the water-entry capillary pressure of the secondary wetting branch beneath the air-entry capillary pressure of the primary drainage branch, perturbations would have grown to fingers. Such a situation occurs during infiltration in initially dry, water repellent soil. The results presented illustrate the effect of antecedent moisture conditions on the formation of stable and unstable wetting fronts, and its relation to the moment of tracer application.

Experiments in microcosms and mesocosms, which can be carried out in an advanced tier of risk assessment, usually result in large data sets on the dynamics of biological communities of treated and control cosms. Multivariate techniques are an accepted tool to evaluate the community treatment effects resulting from these complex experiments. In this paper two methods of multivariate analysis are discussed on their merits: 1) the canonical ordination technique Principal Response Curves (PRC) and 2) the similarity indices of Bray-Curtis and Stander. For this, the data sets of a microcosm experiment were used to simultaneously study the impact of nutrient loading and insecticide application. Both similarity indices display, in a single graph, the total effect size against time and do not allow a direct interpretation down to the taxon level. In the PRC method, the principal components of the treatment effects are plotted against time. Since the species of the example data sets, react in qualitatively different ways to the treatments, more than one PRC is needed for a proper description of the treatment effects. The first PRC of one of the data sets describes the effects due to the chlorpyrifos addition, the second one the effects as a result of the nutrient loading. The resulting principal response curves jointly summarize the essential features of the response curves of the individual taxa. This paper goes beyond the first PRC to visualize the effects of chemicals at the community level. In both multivariate analysis methods the statistical significance of the effects can be assessed by Monte Carlo permutation testing.

The fate of the urea-herbicide linuron was studied in experimental ditches. Eight ditches were sprayed three times at monthly intervals, while two served as controls. As a simulation of spray drift, four doses (nominal: 0.5; 5; 15 and 50 pg.dni3) were applied in duplicate. After each herbicide application, the ditches were kept stagnant for a week, followed by a flow period of three weeks with fresh water. No clear stratifican n of linuron could be found in the water compartment. During the stagnant period, half-lives ‘a for the disappearance of linuron in the water compartment ranged from 7.2 to 11.8 days. The rate of disappearance was slower in the ditches treated with the highest dose and in colder treatment periods. A maximum of 6% of the linuron dose was found in the sediment and 1% in the macrophyte compartment. Approximately 20% of the fraction in the sediment compartment was present in the pore water. 01998 Else&r Science Ltd. All rights reserved

There is growing awareness that grassland livestock production systems are major sources of nitrous oxide (N2O). Controlling these emissions requires a thorough understanding of all sources and controlling factors at the farm level. This paper examines the various controlling factors and proposes farm management measures to decrease N2O emissions from intensively managed grassland livestock farming systems. Two types of regulating mechanisms of N2O emissions can be distinguished, i.e. environmental regulators and farm management regulators. Both types of regulators may influence the number and size of N2O sources, and the timing of the emissions. At the field and farm scales, two clusters of environmental regulating factors have been identified, i.e. soil and climate, and three levels of management regulators, i.e. strategic, tactical and operational. Though the understanding of these controls is still incomplete, the available information suggests that there is large scope for diminishing N2O emissions at the farm scale, using strategies that have been identified already. For example, model calculations indicate that it may be possible to decrease total N2O emissions from intensively managed dairy farms in The Netherlands in the short term from a mean of about 19 to about 13 kg N per ha per year by more effective nutrient management, whilst maintaining productivity. There is scope for a further reduction to a level of about 6 kg N per ha per year. Advisory tools for controlling N2O emissions have to be developed for all three management levels, i.e. strategic, tactical and operational, to be able to effectively implement emission reduction options and strategies in practice. Some strategies and best management practices to decrease N2O emissions from grassland livestock farming systems are suggested.

Biological halogenation of aromatic compounds implies the generation of reducing equivalents in the form of e.g. NADH. Thermodynamic calculations show that coupling the halogenation step to a step in which the reducing equivalents are oxidized with a potent oxidant such as O2 or N2O makes the halogenation reaction thermodynamically feasible without the input of additional energy in the form of e.g. NADH. In a current model on the halogenation of tryptophan to 7-chloro-L-tryptophan NADH and O2 are proposed as co-substrates in a reaction in which the aromatic compound is oxidized via an epoxide as intermediate. The thermodynamic calculations thus indicate that such a route hinges on mechanistic insights but has no thermodynamic necessity. Furthermore the calculations suggest that halogenation of tryptophan and other aromatic compounds should be possible with N2O, and possibly even with nitrate replacing O2 as the oxidant.

To test how a dysfunctioning ecosystem of a severely metal-polluted soil responds to renewed plant growth, a pot experiment was conducted with soil from an experimental arable field with pH and copper gradients imposed 13 years ago. In this experimentfour pH/copper combinations from this field were either planted with a pH- and copper-resistant grass cultivar or remained fallow. During a 10-week period, the dynamics of the microbial activity and of the abundances of bacteria, protozoa, and nematodeswere measured, as were the dynamics of several chemical soil parameters. After 13 years of copper (750 kg copper per hectare) and/or pH stress (from 6.1 to 4.7), which had resulted in severely reduced crop growth, no effects were observed on bacterial numbers, respiration, or protozoan numbers, but bacterial growth (measured as [3H]thymidine incorporation into bacterial DNA) was strongly reduced in the low pH plots, and even more so in low pH plots enriched with copper. Of the organisms, only nematodeswere negatively affected under conditions of high copper load at low pH. In these plots, numbers belonging to all feeding categories were strongly reduced. Planting of a copper-tolerant grass variety, Agrostis capillaris L. var. Parys Mountain, resulted within 10 weeks in faster bacterial growth and more protozoa and bacterivorous nematodes in comparison with fallow controls; these effects were markedly strongest in the acidic, copper-enriched soils. During incubation, fungivorous nematodes increased inall treatments, in fallow and in planted pots and in the pots with high-copper, low-pH soil. The results of this experiment suggest that introduction of plant growth is one of the major causes of increased biological activity in acidic contaminated soils(approximately 1.2 times the current EC limits). Planting such soils with metal-tolerant plant species can reestablish the necessary food base to support soil organism growth, and this can lead to numerous positive effects, reversing the loss of soil functions due to the high copper levels under acidic conditions.

Different soil acidification models have been developed for use on different scales, i.e., NUCSAM for the local scale, RESAM for the regional (national) scale and SMART for the continental scale. This paper focuses on the uncertainties associated with scale transfer by a simpler model description by (i) temporal aggregation of process descriptions, (ii) neglection of processes associated with vertical aggregation of soil layers and (iii) the use of less detailed formulations of processes (process aggregation) and by spatial aggregation of input data. Results obtained for simulations in acid (sandy) soils indicate that (i) temporal aggregation and process aggregation have a limited impact on the long-term (decades) annual response of soil solution chemistry to atmospheric deposition, (ii) vertical aggregation mainly affects predictions of solutes which show a strong concentration gradient with depth and (iii) spatial aggregation hardly affects the average output for a given forest/soil combination. However, ignoring the variability in input parameters, largely affects the frequency distribution of model outputs in a region. Results imply that model simplification is an adequate step in the upscaling of modelling results from a local to a regional scale.

This paper that serves as a prelude to a series of papers on the theme 'Pesticide stress in freshwater ecosystems', has three main objectives. First it outlines the importance of ecological expertise assessing the risks of discharge of pesticides into aquatic environments; second, it discusses possible contributions of ecotoxicological experiments to ecological theory; and third, it advocates the need for an integrative approach in stress-ecology. This approach is based on descriptive ecology, experimentation in the laboratory and in model ecosystems, and on the development of computer simulation models.

The distribution of the Eurasian badger (Meles meles, L.) in the Netherlands is fragmented and adult mortality is high in many places because of traffic casualties. Both these facts affect the survival and dispersal of badgers in a negative way and are suggested to be the main causes of the decline of this species. For this reason the species receives special attention from the government in the national Nature Policy Plan and also from the lower administration in the provinces in their policy on physical planning and nature conservation. To evaluate changes in land use by means of spatial scenarios and conservation strategies in favour of the species, an individual-based simulation model was built that describes population dynamics in space and time. The model was used to evaluate three scenarios. The results indicate that the survival of groups benefits strongly from measures directed at lowering adult mortality. Also the (re)colonization of suitable but not inhabited areas increases the survival and is favoured by measures that encourage dispersal. The results indicate that simulation models as described are useful tools for establishing the comparative effectiveness of plans or measures aimed at increasing the viability of the species.

Soil nutrient depletion is increasingly regarded as a major constraint to sustainable food production in tropical environments. Research in the recent past focused on different scales, but few attempts were made to link them. In this paper, two cases are elaborated in Central America (CA) and Sub-Saharan Africa (SSA), in which the integration of different scales has been studied. Soil nutrient depletion has been calculated for fields, and has then been aggregated to farms, regions, and subcontinents. Key problems on aggregation of field nutrient balances to farms include nutrient flows between fields. Aggregation of farms to regions requires a generalization of individual farms into a farm typology. Aggregation of regions into subcontinents implies that the farm typology concept can mostly not be maintained, resulting in a generalized calculation based on national soil, climate and land use data bases. The field-farm step proved complicated for SSA due to the occurrence of a wide variety of nutrient flows between fields, whereas in CA these flows were much less pronounced; the farm-region step turned out to be manageable for both CA and SSA as farm typology adequately covered observed variation; the region-subcontinent step proved difficult for CA due to the considerable variation in management and input levels in farming systems, whereas this was less the case in SSA. The study shows that integration of spatial scales is constrained by both data availability (the tropical parameter crisis) and by scale-specific variability.

Farm-NUTMON is a research tool that integrates the assessment of stocks and flows of the macro-nutrients nitrogen, phosphorus and potassium on the one hand and economic farm analysis on the other. The tool is applicable at both the farm and the activity level. It includes a structured questionnaire, a database, and two simple static models (NUTCAL for calculation of nutrient flows and the ECCAL for calculation of economic parameters). Finally, a user interface facilitates data entry, data manipulation and extracts data from the database to produce input for both models. Farm-NUTMON allows (i) estimation of the extent to which farmers generate income from soil nutrient mining, (ii) assessment of the impact of changes in farm management techniques on nutrient balance and economic performance at activity level and farm level, and (iii) calculation of the economic impact of exogenous changes on the farm and activity level.

Soil heat flux is difficult to measure accurately and soil heat flux plates are difficult to calibrate. In this research the reference heat flux was calculated from the temperature gradient and independent thermal conductivity measurements. Reference conductivities, as measured by the non-steady state probe method, have an error of about 2%, while the temperature gradient was measured to an accuracy better than 1%. This results in very reliable reference measurements. Compared with this reference, commercially available heat flux plates have significant inaccuracies. The 1 mm thin TNO PU 43 T sensor was the most accurate with an average relative error of only 4%. A promising new technique is the in situ self calibration, as performed by the Huksefluks HFP-01-SC disc sensor. With self calibration this sensor achieves an accuracy of 5% and confers several advantages for field use. The MIDDLETON CN3 and TNO WS 31S sensors had relative errors of about 20%. The ring shaped sensor Huksefluks SH1 gave relatively poor results, because it measured, in fact, the temperature gradient instead of the heat flux. The results of this sensor remained poor after correcting for the thermal conductivity of the sand. For all sensors the same conclusions hold for non-steady state conditions with evaporation. The often used Philip (1961) correction factor is shown to be not very accurate: in only half of our experiments its use decreased the relative measuring error; and in some cases it made it worse. However, the correction remains useful as a tool for designing soil heat flux sensors; also a positive relation exists between the magnitude of the correction and the inaccuracy of the measurement.

In order to enable application of the critical load approach in the international heavy metal and POP abatement policies, critical loads of lead, cadmium, copper, zinc, lindane and benzo(a)pyrene have been calculated for Dutch forest soils. The environmental risk of atmospheric deposition of these substances on Dutch forest soils has been described in terms of the difference between the calculated present atmospheric loads and critical loads. Results indicate that the calculated critical loads strongly depend on the type of environmental quality criterium which serves as a basis for the calculation. More information on the quality criteria to be used is thus essential to gain insight in the risk of the loads of heavy metals and POPs on the terrestrial ecosystem.