The Swiss Soil Monitoring Network: regular measurements of heavy metals in soil and field balances
A. Keller and A. Desaules
Introduction
The Swiss Soil Monitoring Network (NABO) started in 1985. One of the main driving forces for its implementation was the increasing awareness of environmental pollution caused by the forest decline. The network got a legal mandate in 1996, shortly before the Ordinance relating to impacts on soil tha t was established in 1998 (SAEFL 2001). The objective of NABO is to assess soil quality in the long term and to validate appropriate soil protection measures.
For this task regular soil measurements at defined observations sites are inevitable. These measurements provide the current status of soil quality. Another essential tool in soil monitoring is the mass balance that predicts temporal changes of soil quality, i.e. increasing metal flows through the soil system that may cause serious problems for soil fertility, ground water quality and food chains. Thus, element balances complement soil measurements as they enable the anticipation of metal accumulation in soil. On the other hand, a balance model is only meaningful if model predictions can be validated by regular soil measurements. This manuscript (i) briefly outlines the concept of the NABO and (ii) presents some selected results with respect to the assessment and reduction of heavy metal inputs into agro- ecosystems.
Concept of the NABO
Figure 1 delineates the modular concept of the NABO. In general, the monitoring network consists of four modules that are linked to each other. NABO Trend measures and assesses temporal changes of contaminants in soil. The network comprises currently 105 observation sites across Switzerland with various land use characteristics (34 arable land, 30 permanent grassland and rural land, 28 forest, 4 vegetables, 4 vineyards, 3 orchards, 2 urban). Four bulked soil samples from soil depth 0-20 cm are taken from each observation site (10 x 10 m2) every 5 years. Currently the fourth sampling period is being completed. Soil samples are analysed for nine inorganic pollutants (2 M HNO3-extractable concentrations of Cd, Co, Cr, Cu, Hg, Ni, Pb, Zn and total contents of F) according to a analytical quality assessment scheme and all samples stored in an archive. A detailed description of the sampling and analytical procedure is given in BUWAL (2000). Further information can also be obtained at the NABO website (www.nabo.admin.ch).
NABO Flux records annually the management data for the 48 agricultural sites and assesses the input and output fluxes of the heavy metals. This was done so far for the periods 1985- 1990 (BUWAL, 1993) and 1995-2001 (Keller et.al, 2004). Inputs considered in the balance model are atmospheric deposition and application of mineral fertilizers, animal manure, sewage sludge, compost and pesticides. In a first step only outputs by harvest is considered in the model. Outputs via leaching and erosion will be included in an advanced model concept.
Figure 1: Observation sites and the modular concept of the Swiss Soil Monitoring Network (NABO).
Quality data of all fertilizers and crops were taken from investigations in Switzerland and were organized in a database. In order to consider uncertainties arising from limited data quality, measurement errors, and from functions to derive model input parameters and from spatial and temporal variation of model parameters a stochastic balance approach is used.
Parameters can be treated as random variables, i.e. defined by probability distributions, which are then propagated to model outputs using a simulation method (Keller et al., 2001, 2002).
NABO Status links the results of the monitoring network with other soil surveys and land use data to evaluate the spatial distribution of soil quality indicators in Switzerland. Recently, about 330.000 soil measurements from about 14.000 sites across Switzerland were compiled and the frequency distribution of heavy metal concentrations analysed with regard to type of land use, type of studies and soil properties (Keller and Desaules, 2001a). Such statistics were used to relate the reference values of the monitoring network to the one frequently measured in Swiss soils. It is planned to arrange these site-specific data in a soil information system that couples soil contents, soil properties and land use data with a geographical information system. Such a soil information system provides calculating and mapping critical loads of heavy metals. As a first step, geogenic metal contents were combined with soil pH, organic matter and clay content to derive the (background) sensitivity of Swiss soils with respect to metal loads (Keller and Desaules, 2001b).
The essential requirement for all of the three modules of the NABO mentioned above is to assure the quality assessment of all soil measurements. This is the task of NABO Quality that determines reliability and accuracy of the sampling procedures and analytical methods used.
Analysis of temporal changes
Following the hypothesis that temporal changes of heavy metal in soils is the result of various interacting processes, three groups of such processes may be distinguished:
1. anthropogenic: changes resulting from agricultural management (e.g. fertilizers) and atmospheric deposition
2. soil dynamic: changes resulting from soil cultivation (e.g. tillage), biological activity (e.g.
earthworms), solute and particle transport, preferential flow, erosion and other processes that are yet unknown.
3. procedure errors: changes resulting from all kind of possible errors associated with soil sampling, physical soil preparation and analytical procedure.
While the processes of the first group are quantified by a stochastic input-output balance model, soil dynamic processes are not assessed yet. Furthermore, errors caused by analytical procedures were determined, whereas the errors resulting from soil sampling and soil preparation were sometimes not reproducib le. The only way to analyse the contribution of these three groups to the measured temporal variation of heavy metal concentration in soil is to validate an appropriate balance model with measured soil data based on an exhaustive quality assessment. Such a balance model has to consider both anthropogenic and soil dynamic processes and all relevant procedure errors have to be taken into account in the calculations.
However, balance models usually consider anthropogenic inputs and outputs and sometimes leaching but little is known, for example, how vertical soil transport of earthworms influences soil concentration in the top 20 cm soil layer in the long term. In other words, the main task of long-term monitoring of soil pollutants is to analyse the contribution of the processes given above to the temporal variation of the signal (regular soil sampling). This is illustrated below for some selected results of the NABO.
Regular measurements
Figure 2 shows the derivation of the HNO3- extractable Cd concentration in soil for the second and third soil sampling period of the NABO sites. The derivation is expressed relative to the guide value, e.g. + 5% corresponds to an increase of 0.04 mg/kg. Negative values indicate a measured decrease of Cd in soil.
observation site Cd HNO3 (guide value: 0.8 mg/kg)
relative change in % of guide value
after 5 years (0-5 cm) after 5 years (0-20 cm) after 10 years (0-20 cm)
Figure 2: Relative change of measured total Cd concentrations in soil of the observation sites.
For all elements relative changes above the ±5 % level (see dashed line) are considered as relevant. Both increasing and decreasing relative changes were found above the ±5 % level for Cd. This dynamic of soil concentration at NABO sites was also determined for the other heavy metals. In total, after five years 87 of 100 sites showed a significant increase or decrease involving at least one of the measured pollutants (Cd, Co, Cr, Cu, Hg, Ni, Pb, Zn, F) at a soil depth of 0-20 cm. After ten years, this was the case for all of the 25 agricultural sites investigated so far. Hg showed the greatest dynamics by far, characterised mainly by increases. Significant increases were also measured for Zn, while F and Co mainly showed decreases. We interpreted these patterns of temporal changes as the interaction of the anthropogenic and soil dynamic processes. It is not possible yet to relate changes in concentration clearly and quantitatively to individual causes at each site. Therefore, currently results can not directly be linked to soil protection measures.
Field balances of heavy metals
The distributions of the net Zn fluxes for the period 1995-2001 of the 48 agricultural NABO sites revealed large variations between and within the field balances (Figure 3). In particular, large net Zn fluxes in the range of 500-2000 g ha-1 yr-1 were found for husbandry farms.
Furthermore, large Zn inputs were also modelled for sites that belong to dairy farms, mixed farms and mixed dairy farms. For some extensive sites, e.g. permanent grassland, also a depletion of Zn was found.
466879102926374977868795 1 17282930333538546074788010310365669 3 111325314448636465 4 5 152051559496 -500
0 500 1000 1500 2000 2500 3000 3500 4000
Flux [g ha-1 yr-1]
site number
arable other mixed husbandry farms dairy farmsmixed dairy darms special crops
Figure 3: Net Zn fluxes of the field balances for the 48 agricultural NABO sites.
The coefficient of variation of the net Zn fluxes ranged between 40 % and 300 %. The Zn balances were very sensitive to variations of Zn concentration in various types of animal manure. Field balances of farms with intermediate and high livestock density were dominated by this uncertainty source (in particular manure from pig and calf breeding). Other important uncertainty sources for the Zn balances were the variation of deposition data and of various crop types. Furthermore, the balance model applied is particularly suited to
1. assess the uncertainty in metal balances and to reduce the uncertainty in balances by further investigations determining highly sensitive parameters more accurately,
2. evaluate the effect of preventive strategies against metal enrichment in soil, i.e. calculation of scenarios, such as the influence of improving fertilization management or changes in crop and livestock production on the heavy- metal cycles, for example, and
3. estimate steady state conditions in soil and derive critical metal inputs to prevent harmful effects on soil fertility and crop quality.
Comparing regular soil measurements with balances
The element balances are only meaningful if they could be validated by regular measurements. So far at six observation sites the results of both monitoring tools for the period 1995-2000 were compared (Figure 4). The fluxes were converted into accumulation rates using the soil depth 0-20 cm and the soil bulk density of the site. The measured changes of Cd and Zn concentration in soil could only be explained in some cases by anthropogenic inputs and outputs.
Figure 4: Comparison of regular soil measurements (square) and field balances (circle) at 6 NABO sites for Cd and Zn. The error bars indicate the standard deviation.
In particular, the measured decreases of metal concentration in soil were not predicted by the metal balances. These preliminary results suggest that besides the anthropogenic processes the soil dynamic processes are also of magnificant importance for the analysis of temporal changes of soil concentration in the long term.
Outlook
The stochastic balance model applied have to be extended. Leaching, erosion as well as other soil dynamic processes have to be included in the model. Therefore, research on modelling soil dynamic processes have to be intensified. Only validated element balances are meaningful. The extended balance model will be validated for different time periods with regular soil measurements of almost 20 years. This validation will take into account the quality assessment of all procedure errors. The validated balance model will be used to evaluate the effect of preventive strategies against metal enrichment in soil (model scenarios) and to evaluate critical metal inputs with respect to soil fertility, crop and water quality.
References
BUWAL (1993): Nationales Bodenbeobachtungsnetz – Messresultate 1985-1991. Schriftenreihe Umwelt Nr. 200. Bundesamt für Umwelt, Wald und Landschaft (Hrsg.), CH-3003 Bern.
BUWAL (2000): Nationales Boden-Beobachtungsnetz - Veränderungen von Schadstoffgehalten nach 5 und 10 Jahren. Schriftenreihe Umwelt Nr. 320. Bundesamt für Umwelt, Wald und Landschaft (Hrsg.), CH-3003 Bern.
Keller, A., von Steiger, B., van der Zee, S.E.A.T.M., Schulin, R. (2001): A stochastic empirical model for regional heavy metal balances in agroecosystems. J. Environ. Qual. 30: 1976-1989.
Keller, A., Abbaspour, K.C., Schulin, R. (2002): Assessment of Uncertainty and Risk in Modeling Regional Heavy-Metal Accumulation in Agricultural Soils. J. Environ. Qual. 31: 175-187.
Keller et al, (2004). Schadstoffbilanzen 1995-2001 auf Parzellen des Nationalen
Bodenbeobachtungsnetzes. Schriftenreihe. Eidg. Forschungsanstalt für Agrarökologie und Landbau (FAL), CH-8046 Zürich.
Keller, Th. & Desaules, A. (2001a): Böden der Schweiz – Schadstoffgehalte und Orientierungswerte (1990-1996). Umwelt-Materialien Nr. 139. Bundesamt für Umwelt, Wald und Landschaft (Hrsg.), CH-3003 Bern. 115 S.
Keller, Th. & Desaules, A. (2001b): Kartie rgrundlagen zur Bestimmung der Bodenempfindlichkeit gegenüber anorganischen Schadstoffeinträgen in der Schweiz. Nationale Bodenbeobachtung (NABO). Eidg. Forschungsanstalt für Agrarökologie und Landbau (FAL), CH-8046 Zürich.
SAEFL, 2001: Commentary on the Ordinance of 1st July 1998 relating to impacts on soil. Environment in practice. Swiss Agency for the Environment, Forests and Landscape (SAEFL), CH-3003 Bern.
