Differential degradation of pesticides in soils of different metabolic status

Pesticide degradation, or the breakdown of pesticides, is usually beneficial. The reactions that destroy pesticides change most pesticide residues in the environment to inactive, less toxic, and harmless compounds. However, degradation is detrimental when a pesticide is destroyed before the target pest has been controlled.

Three types of pesticide degradation are microbial, chemical, and photodegradation.

Microbial degradation is the breakdown of pesticides by fungi, bacteria, and other microorganisms that use pesticides as a food source. Most microbial degradation of pesticides occurs in the soil. Soil conditions such as moisture, temperature, aeration, pH, and the amount of organic matter affect the rate of microbial degradation because of their direct influence on microbial growth and activity.

The frequency of pesticide applications can also influence microbial degradation. Rapid microbial degradation is more likely when the same pesticide is used repeatedly in a field. Repeated applications can actually stimulate the buildup of organisms effective in degrading the chemical. As the population of these organisms increases, degradation accelerates and the amount of pesticide available to control the pest is reduced.

Chemical degradation is the breakdown of pesticides by processes that do not involve living organisms. Temperature, moisture, pH, and adsorption, in addition to the chemical and physical properties of the pesticide, determine which chemical reactions take place and how quickly they occur. Because of lack of light, heat, and oxygen in the water-saturated layers of the soil profile below the surface, chemical breakdown is generally much slower than at the surface. In northern states, the season influences groundwater temperatures from 5 to 10 feet below the ground surface, varying from 39 degrees F to 41 degrees F during the coldest part of the winter to 59 degrees F to 61 degrees F during the hottest part of the summer. Groundwater below 10 to 15 feet maintains a constant temperature of 50 degrees F to 53 degrees F. These low temperatures greatly reduce tile rate of chemical breakdown.

One of the most common pesticide degradation reactions is hydrolysis a breakdown process where the pesticide reacts with water. Depending on the pesticide, this may occur in both acid and alkaline conditions. Many organophosphate and carbamate insecticides are particularly susceptible to hydrolysis under alkaline conditions. Some are actually broken down within a matter of hours when mixed with alkaline water.
Photodegradation is the breakdown of pesticides by light, particularly sunlight. Photodegradation can destroy pesticides on foliage, on the soil surface, and even in the air.
Factors that influence pesticide photodegradation include the intensity of the sunlight, properties of the application site, the application method, and the properties of the pesticide.

Metolachlor [2-chloro-N-(2'-ethyl-6'-methyl phenyl)-N-(2-methoxy-1-methylethyl)-acetamide] is an acetanilide herbicide. Commonly used as pre-emergence application for the control of annual grasses and some broadleaved weeds in a variety of crops, including maize, sorghum, cotton, sugarcane, sugar beet, potato, peanuts, soybean, sunflower, safflower and some vegetables. Metolachlor is relatively more persistent compared to other acetanilide herbicides, alachlor and propachlor in the soil environment. Its degradation in soil is a biological rather than a chemical process and it is considered to be a co metabolic process. Metolachlor can be easily analysed by gas chromatography (GC), the polar nature of its transformation products prevents their detection by this technique. Metolachlor and its various ring hydroxylated and demethylated degradation products have previously been analysed by HPLC method using relatively elaborate solvent programming.

The research involves a wide range of subjects such as plant protection, environmental analysis and soil microbiology. It includes the degradation and metabolism pathways of selected pesticides in different kind of soils, which have been collected from Germany, USA, Portugal, Yemen and China.
Methodology includes the utilization of ELISA-Test, HPLC/UV and GC/NPD.