Safe food production on soils contaminated with organochlorine pesticides

Summary

State of the Art:
Pesticides: The uptake of organochlorine pesticides (OCP) into plants is influenced by several factors/parameters. In a comprehensive literature study of Werkbüro für Boden und Bodenschutz (1998) the results of many field, pot and lysimeter experiments were reviewed. The group of OCP is generally highly persistant in soils and can still be found nowadays after it has already been banned for 20-30 years. Within the group of OCP the stability ascends in the sequence Gamma HCH – Aldrin, Dieldrin, Endrin - Chlordan – DDT, HCB. In the group of the HCH-Isomeres Lindane (Gamma HCH) has the highest mobility. Aldrin is degraded into the epoxid Dieldrin. However, further contamination of groundwater by Dieldrin has not been observed. Both Aldrin and Dieldrin have been applied as insecticides in horticulture, fruit and arable crops.
Soil properties: In general no correlation was found between soil content and crop content of OCP. There was found a very broad range of transfer factors (content crop/content soil), ranging from 0 to 86, but mostly between 0 and 1 (Werkbüro für Boden und Bodenschutz, 1998). A high content of soil organic carbon enhances binding of OCP (Beestman et al., 1969; Gonzalez et al., 2003; Harris and Sans, 1971). No relation was found among the soil parameters cation exchange capacity, iron and clay content and plant uptake (Werkbüro für Boden und Bodenschutz, 1998).
Plant species: The uptake of OCP varies among different plant families. The results of several studies are not consistent. Vegetables from the family of the Cucurbitaceae (e.g. cucumbers, zucchini or pumpkins) accumulate to a much higher degree than root crops such as carrots, radish, or celariac, and sugar beet (Werkbüro für Boden und Bodenschutz, 1998) tomatoe (Gonzalez et al., 2003) and potatoes (Miglioranza et al., 1999). Plants with higher content of lipids or oils (pumpkins, carrots, cress) accumulate such pesticides in a higher amount because of their adipose structures (Mandl and Lindner, 1999; Topp et al., 1986). A transfer depending on the variety was found with carrots (Endrin; Hermanson et al., 1970) and rape (Lindan; Schlosserova, 1994).
Definition of the problem:
Background: Organochlorine pesticides (OCP) such as Dieldrin and DDT were applied world wide against pests in horticulture, fruit and arable crops. Although OCP are forbidden since more than twenty years in the European Union (EU), the highly persistent compounds are still detectable in soil. Several crops are known to accumulate OCP in edible parts up to critical levels. For instance Dieldrin is detected in high economic value crops such as cucumbers (Cucurbitaceae) and tomatoes (Solanaceae). Besides plant type and climatic conditions, soil properties, e.g. pH and soil organic carbon content influence the plant’s OCP uptake. In particular in premium food and feed stuff production, such as organic farming according to EU Regulation (EEC) No. 2092/91, consumer expectations in safe, high quality food are extremely high. Since organic farming is prospering – more than 10% of vegetable production is certified organic in Switzerland – the OCP residues in organic food stuff have become a major issue for food control authorities. Enquiries showed that some of the imported food from EU countries such as pumpkin seed and oil was highly contaminated with OCP. Hence, the OCP problem in food stuff is expected to occur EU wide or even world wide.
Preliminary study: Facing this problem we performed a case study and analysed the OCP content of 15 fields and their grown fruits from the most important Swiss organic cucumber growers. Two out of these 15 field soils revealed Dieldrin leading to surpassing of the tolerance level. It also occurred that the whole crop yield from an OCP contaminated field was rejected by food retailers and the crop had to be ploughed down. The Swiss organic label organisation has the option in their standard now to exclude polluted soils for production of highly OCP accumulating vegetables, but this may severely affect the grower’s economy.
Objectives: In this project we aim at developing sustainable safe food production on OCP contaminated soils, which is economically and ecologically sound.
First we will perform an in-depth field survey on 50 sites to assess the extent of OCP contamination on Swiss farms. Soils and Cucurbitacea crops will be analysed for OCP. Cucurbitacea are chosen because crops of this family accumulate strongly OCP and thus are suitable as indicator plant.
Second, we will develop strategies in bioassays in the glasshouse to lower OCP uptake from the contaminated field soils through immobilisation of OCP by organic carbon addition to the soil and by selection of OCP excluder plant varieties.
These experiments are organised as follows:
i) Test system optimization: We will optimize a previously used test system for the immobilisation and exclusion experiments (see below). We will grow cucumbers in OCP contaminated field soils (little, medium, high) and measure OCP in the growing cucumbers in a time course. As an alternative we will use cress as a test plant to shorten the duration of the experiment. 14C-labelled Dieldrin will be used to calculate fluxes in the soil-plant system. Based on the results of these experiments we will decide which soil and crop we will use in the following tests and when the test plant will be harvested for OCP analyses.
ii) Immobilisation of OCP by organic carbon: The aim of these experiments is to immobilise OCP in the soil by the addition of organic carbon to the soil. The hypothesis is that the OCP will be bound by the organic carbon and consequently the bioavailability is expected to be decreased. Organic carbon will be added to the soil in form of compost or as brown coal product (charcoal) at three levels each. The bioavailability of OCP will be assessed by measuring these molecules in the plant shoot.
iii) Exclusion of OCP by crops: The aim of these experiments is to identify plant varieties with a low OCP uptake. Since cucumbers are normally cultivated by grafted plants nowadays, where a robust and vital root stock is inoculated by a desired variety, we will select for OCP excluder root stocks. For cucumbers, root stocks from the groups of Cucurbita moschata x C. maxima, Sicyos angulatus and Cucurbit ficifolia are currently in use, but we will test further ones from the same family. The root stocks and the grafted plants will also be cultivated in contaminated field soils and OCP will be measured in the plant’s tissue. The most promising treatments from the immobilisation and the exclusion experiments will be applied in combination.
Finally we synthesize the results from the field survey and the glasshouse experiments and, including literature findings, we suggest field evaluations of the elaborated strategies for safe food production on OCP contaminated soils. We already expect after three project years a high degree of applicability of the strategies, since this project bridges basic and applied research and combines different strategies to prevent plant uptake of the harmful OCP.
Project aims including target group:
In the literature and in practice, several solutions for minimizing the uptake of OCP into plants are proposed. Techniques like the removal of contaminated soils and replacement with clean or ameliorated soils are heavily invasive and costly and therefore applicable only on restricted areas. In this project, the following two strategies are envisaged to exclude OCP uptake by the plant:
• Binding OCP to soil compounds:
A field study carried out in Austria used activated charcoal to reduce the bioavailability of HCB in the soil. Due to the binding of the highly lipophilic HCB onto the carbon surface, it is no longer available for the plant. This methodology reduced the amount of HCB in pumpkin seeds by about 30% (Mandl and Lindner, 1999).
• Use of exluder plants
Since tomatoes and cucumbers are very often cultivated by grafted plants, where a robust and vital root stock is occulated by a desired variety, one can select for excluder root stocks. For cucumbers, root stocks from the groups of Cucurbita moschata x C. maxima, Sicyos angulatus and Cucurbit ficifolia are currently in use (Lee, 2003). Also with heavy metal contaminations gentle remediation techniques such as the use of excluder plants were performed (Gupta et al., 2000).
The target group includes farmers.
Methodology:
The basic principles of binding and immobilising OCP as well as exclusion of OCP techniques are known and developed to a certain extent with heavy metals. They are hardly applied under practical farm condition with other organic pollutants such as OCP. The innovation of the project is the near to practice testing condition, the use of the grafting technique to exclude OCP by selected root stocks and the combination of binding/immobilising and exclusion to solve the OCP problem situation.
Accordingly, the project is organised in six work packages (figure 1). In WP 1 we will perform an in-depth literature study (WP 1) as a base for a detailed planning of a field survey on OCP (WP 2). The project follows consequently a “fork to field” approach and starts with the field survey, in order to clarify the degree of OCP contamination in Swiss soils and the edible plant parts of grown Cucurbitacea. After the optimization of a previously used test system (WP 3) the two basic strategies are investigated under near farming conditions in the glasshouse (WP 4 and 5). Finally, we synthesis the results from the field survey and the glasshouse experiments and suggest a field evaluation of the developed strategies to solve the OCP problematic in WP 6.
For all pot experiments in WP 3-5, OCP contaminated field soils will be used as potting ground and cucumbers (Cucumis sativus L. var. Aramon RZ), or alternatively cress (Lepidium sativum L.) will be grown on these soils. Pot size will usually be 5 Liter. Each treatment will have four replicates.
The following OCP will be measured in soil and plant: Aldrin, alpha-Endosulfan, beta-Endosulfan, Brompropylat, Captafol, Captan, cis-Heptachlorepoxid, Endosulfansulfat, Endrin, Heptachlor, Hexachlorbenzol, Lindan, p,p’-DDD, p,p’-DDE, p,p’-DDT, Pentachloranilin, Quintozen, trans-Heptachlorepoxid with special focus on Dieldrin. The method used for analysis of organochlorine pesticides in soil and plants has been modified from DFG-Multimethode S19 (Anonymous, 1991) and Methode L 00.00-34 (Anonymous, 2001). Soil and plant samples are extracted using a Dionex ASE 200 with acetone : hexan (65:35 v/v) and ethylacetate : cyclohexan (1:1 v/v), respectively. Extracts are subfractionated by silica gel chromatography as described by Metcalfe and Metcalfe (1997). A capillary gas chromatography equipped with a massspectrometer is used for detection of the compounds.
Data sets of the field survey (WP 2) will be analysed by multiple regression analyses. Data of WP 3-5 will be analysed by an analysis of variance, followed by a Tukey test.
Results, conclusion, state of the art:
The project has started in June 2005. So far a monitoring of various agricultural sites with organic and non-organic cucumber production has been carried out. Soil samples of defined areas and cucumber samples for those areas were taken. The analysis for organochlorine pesticides will follow in the next few weeks.