Search

In order to investigate the long-term fate of the herbicide chloridazon and its polar metabolites desphenyl-chloridazon and methyl-desphenyl-chloridazon, a lysimeter experiment was conducted. The plant protection product Pyramin WG, which contains chloridazon, was applied to a weighable, monolithic lysimeter and a backfilled, gravitation lysimeter. Leachate, soil and maize samples were analyzed, and the data from the monolithic lysimeter was used to simulate the transport through the soil profile with the pesticide leaching model Pesticide Emission Assessment at Regional and Local scales (PEARL). Chloridazon rapidly degraded to desphenyl-chloridazon and methyl-desphenyl-chloridazon. In leachate, chloridazon was therefore detected at concentrations up to 3.5 μg L−1, whereas the metabolites desphenyl-chloridazon and methyl-desphenyl-chloridazon were present for more than 2 years at higher concentrations up to 24 and 6.1 μg L−1. The concentrations of chloridazon in soil decreased significantly with depth and time, whereas both metabolites increased up to 370 and 16 μg kg−1. The high concentrations, even 916 days after the application, clearly indicate a continuous downward migration and degradation in soil. In maize, chloridazon and purely the metabolite desphenyl-chloridazon were detected in leaves and stems. Desphenyl-chloridazon was additionally found in grains. It was noted that the design and setup of the two lysimeters lead to significant variations in terms of transformation rate, soil retention time and accumulation by plants. A comparison of the simulated results and the leaching rates measured from the monolithic lysimeter yielded adequate results for the metabolites, but rather poor correlation for chloridazon. The results obtained suggest persistence and high dispersion of chloridazon, and especially its metabolites, in leachate and soil. In maize, the migration in leaves and stems and the accumulation by grains might be relevant in terms of food security.

Humic substances are ubiquitous in the environment and have manifold functions. While their composition is well known, information on the chemical structure and three-dimensional conformation is scarce. Here we describe the Vienna Soil-Organic-Matter Modeler, which is an online tool to generate condensed phase computer models of humic substances (http://somm.boku.ac.at). Many different models can be created that reflect the diversity in composition and conformations of the constituting molecules. To exemplify the modeler, 18 different models are generated based on two experimentally determined compositions, to explicitly study the effect of varying e.g. the amount of water molecules in the models or the pH. Molecular dynamics simulations were performed on the models, which were subsequently analyzed in terms of structure, interactions and dynamics, linking macroscopic observables to the microscopic composition of the systems. We are convinced that this new tool opens the way for a wide range of in silico studies on soil organic matter.