Michel Schiavon

Phytotoxicity of ancient gaswork soils. Effect of polycyclic aromatic hydrocarbons (PAHs) on plant germination

Abstract The phytotoxicity of various contaminated soils was assessed by plant inventories on ancient industrial fields and by phytotoxicity tests. Industrial fields are well colonised by numerous weedy plants. Phytotoxicity was tested with pure PAHs, ancient industrial soils, soil leaches, liquid tar and tar volatile compounds. Both field studies and toxicity tests show that contaminated samples can be classified into two categories: first, a recently excavated soil/liquid tar that was foul-smelling and phytotoxic and second, an ‘aged’, surface soil that was weathered and non-phytotoxic. Plant germination and growth are strongly inhibited by the presence of volatile, water-soluble low molecular-weight hydrocarbons (

Sorption and leaching of 14C-glyphosate in agricultural soils

Glyphosate (N-(phosphonomethyl)glycine) is one of the most widely used herbicides in the world to control weeds in agricultural and urban areas. Its increasing use requires special attention to its transfer from terrestrial to aquatic environments. However, knowledge on the leaching of glyphosate and its metabolite aminomethylphosphonic acid (AMPA) is scarce. Here we aimed to assess the dynamic interactions between glyphosate sorption and leaching; and to identify the main factors that influence the two processes in three undisturbed agricultural soils using microlysimeters under outdoor conditions. We studied the sorption, desorption and leaching of 14C-labelled glyphosate on three soils using batch experiments in the laboratory and lysimeters under natural conditions for 11 months. The laboratory results showed that glyphosate was strongly adsorbed, yielding empirical constants of Freundlich sorption isotherms (Kf) of 16.6 for the clay loam soil, 33.6 for the silt clay loam soil and 34.5 for the sandy loam soil, with nf close to 1 in all three cases. Glyphosate was also weakly desorbed, i.e. 5 to 24% (w) of initially sorbed glyphosate. Sorption and desorption were only pH-dependent. The outdoor results showed that nearly 70% of the initial glyphosate was present in the soil in a non-extractable form at the beginning of the experiment. Conversely, only less than 20% of the initial glyphosate is present in the soil in a non-extractable form after 11 months. These findings suggest that the non-extractable residues become available and take part in biodégradation and leaching. The amounts of 14C-glyphosate derivatives leached were less than 0.28% of the initially applied glyphosate. HPLC analyses showed that the AMPA metabolite generally represented up to 100% of the residues present in the leachates. The results of leaching were highly influenced by the hydrodynamic properties and the biodegradation capacities of the soils. Although glyphosate residues were found in low concentrations in the leachates for almost a year, the contamination of groundwater does not seem to be a concern, regardless of the soil type, if the herbicide is used in accordance with good agricultural practice.

Leaching of atrazine and some of its metabolites in undisturbed field lysimeters of three soil types

The movement of 14C-atrazine was studied in lysimeters (10×70 cm) under field conditions in loamy clay, calcareous clay and a high clay soils. Ten months after atrazine treatment and with a cumulative rainfall of 502 mm, the leachates from the calcareous clay soil contained 3.3% of applied s-triazine radioactivity, while those from the loamy clay and high clay soils contained only 0.9% and 1.1%, respectively. The mobility of the s-triazine residues was not related to the distribution of organic carbon content withh depth. The proportion of extractable residues in the upper levels of the lysimeters was lower for the calcareous clay soil, 19.2% compared to 30.0 and 28.6% in the loamy clay and high clay soils. The extraclable residues increased with depth in the calcareous clay soil, 62.8% in the 54-60 cm layer, whereas it decreased in the loamy clay and high clay soils down to 16.3 and 17.6%, respectively. Atrazine was observed to a depth of 36 cm in the loamy clay and high clay soils, and to a depth of 54 cm in the calcareous clay soil. Diamino-atrazine was detected in some places while deethyl-atrazine and deisopropyl-atrazine were present over a large part of the soil profile, sometimes to depths greater than that of the parent molecule. The results suggest a greater mobility of the s-triazine residues in the calcareous clay soil.

Degradation of 14C-glyphosate and aminomethylphosphonic acid (AMPA) in three agricultural soils

Glyphosate (N-phosphonomethyl glycine) is the most used herbicide worldwide. The degradation of 14C-labeled glyphosate was studied under controlled laboratory conditions in three different agricultural soils: a silt clay loam, a clay loam and a sandy loam soil. The kinetic and intensity of glyphosate degradation varied considerably over time within the same soil and among different types of soil. Our results demonstrated that the mineralization rate of glyphosate was high at the beginning of incubation and then decreased with time until the end of the experiment. The same kinetic was observed for the water extractable residues. The degradation of glyphosate was rapid in the soil with low adsorption capacity (clay loam soil) with a short half-life of 4 days. However, the persistence of glyphosate in high adsorption capacity, soils increased, with half-live of 19 days for silt clay loam soil and 14.5 days for sandy loam soil. HPLC analyses showed that the main metabolite of glyphosate, aminomethylphosphonic acid (AMPA) was detected after three days of incubation in the extracts of all three soils. Our results suggested that the possibility of contamination of groundwater by glyphosate was high on a long-term period in soils with high adsorption capacity and low degrading activities and/or acid similar to sandy loam soil. This risk might be faster but less sustainable in soil with low adsorption capacity and high degrading activity like the clay loam soil. However, the release of non-extractable residues may increase the risk of contamination of groundwater regardless of the type of soil.

Seasonal dynamics of atrazine in three soils under outdoor conditions

The fate of 14C labelled atrazine was followed in the first centimeters of soil throughout one year in a microlysimeter experiment under outdoor conditions. Leaching of atrazine reached 40 to 60 % and depends on the soil type and soil structure. As a whole, the pool of reversibly adsorbed residues decreased with time from 100 to less than 5 %, according to the rainfall distribution. Hydroxyatrazine was the main degradation product of atrazine and was preferentially adsorbed rather than leached in the three soils studied. Bound atrazine residues were formed from the first 45 days and their amount increased in the order : pelosol < brown soil < rendzina according to the soil organic matter content. These residues were involved in partial degradation processes after 180 days.

The Role of Plants in the Remediation of Contaminated Soils

Accumulation of organic or mineral micropollutants in soils and waters may alter the functioning of ecosystems and contaminate the food chain. The remediation of these contaminated environments by currently available physico-chemical methods is either costly or impossible. Phyto-remediation, i.e. remediation based on the use of plants, would be more economic and more environmentally friendly, leaving soil material without major alterations in biological properties. Living plant roots transform the soil environment through many processes including uptake of water and elements and release of organic compounds, i.e. exudates, in the surrounding soil. Presence of exudates stimulates the soil microflora and induces changes in the soil structure as well as in the mobility of mineral ions. Hence, plants significantly alter the fate of pollutants in soils, and are suitable candidates for management of contaminated soils, i.e. phytoremediation. Phytoremediation utilizes the numerous capabilities that plants have to change their close environment. Covering of contaminated soils by adapted plants, i.e. phytostabilisation, helps the stabilization of the soil surface, and reduces the risk of transport to water streams of pollutants adsorbedon the fine solid phase. Also, growing tolerant plants reduces the water movement into the soil profile, thus limiting the leaching of soluble pollutants. Plants have also the ability to extract and accumulate non-essential trace elements in their tissues making it possible to removed metals from polluted environments. Hyperaccumulators of metals are a specialized class of plants able to accumulate metals to very high concentrations (up to 1 % by dry weight) in their above-ground tissues. They proved to be efficient for removing significant amounts of metals from soils polluted by sewage sludge or industrial activities, with little changes in other soil properties, i.e. phytoextraction. Plants are not only a sink for pollutants, they exert changes in the compounds present in their rhizosphere. The release of exudates modifies the chemistry and physics of the soil and may subsequently alter the mobility of metals Enhanced microbial activity is also observed in the rhizosphere, which makes plants useful in the management of environments contaminated with organic pollutants. In soils and waters, pesticides and hydrocarbons are degraded at a rate that depends on molecule type, soil properties, and the state of the microflora. In presence of plants, the process of degradation of organic pollutants, e.g. pesticides and hydrocarbons, is accelerated. Extraction by plants of organic compounds at high rates has not been demonstrated uniquevoquely yet. Phyto-remediation can be suitable for many polluted sites, and research is underway to make this approach a routine technique for soil and water remediation.