Environ. Sci. Technol. 2006, 40, 3305-3312
Quantifying the Effect of Soil Moisture on the Aerobic Microbial Mineralization of Selected Pesticides in Different Soils R E I N E R S C H R O L L , * ,† HANS HEINRICH BECHER,‡ ULRIKE DO ¨ RFLER,† SEBASTIAN GAYLER,† SABINE GRUNDMANN,† HANS PETER HARTMANN,† AND JU ¨ RGEN RUOSS† GSF-National Research Center for Environment and Health Institute of Soil Ecology, Ingolsta¨dter Landstrasse 1, D-85764 Neuherberg, Germany, and Institute for Soil Science, Technical University of Munich/Weihenstephan, Am Hochanger 2, D-85354 Freising, Germany
A standardized quantitative approach was developed to reliably elucidate the effect of increasing soil moisture on pesticide mineralization. The mineralization of three aerobically degradable and chemically different 14Clabeled pesticides (isoproturon, benazolin-ethyl, and glyphosate) was studied under controlled conditions in the laboratory at an identical soil density of 1.3 g cm-3. The agricultural soils used are characterized by (i) large variations in soil texture (sand content 4-88%) and organic matter content (0.97-2.70% org. C), (ii) fairly diverse soilwater retention curves, and (iii) differing pH values. We quantified the effect of soil moisture on mineralization of pesticides and found that (i) at soil water potential e -20 MPa minimal pesticide mineralization occurred; (ii) a linear correlation (P < 0.0001) exists between increasing soil moisture (within a soil water potential range of -20 and -0.015 MPa), and increased relative pesticide mineralization; (iii) optimum pesticide mineralization was obtained at a soil water potential of -0.015 MPa, and (iv) when soil moisture approximated water holding capacity, pesticide mineralization was considerably reduced. As both selected pesticides and soils varied to a large degree, we propose that the correlation observed in this study may be also valid in the case of aerobic degradation of other native and artificial organic compounds in soils.
Introduction On a global scale, the availability of water designed for human consumption is limited. As such, ensuring the protection of drinking water reservoirs against contamination is of paramount importance. This is threatened by the leaching of pesticide residues into groundwater after pesticide application on agricultural soils, a practice extensively used in modern-day food production. Studies (e.g., 1) have dem* Corresponding author phone: 0049 89 3187 3319; fax: 0049 89 3187 3376, e-mail:
[email protected]. † GSF-National Research Center for Environment and Health Institute of Soil Ecology. ‡ Technical University of Munich/Weihenstephan. 10.1021/es052205j CCC: $33.50 Published on Web 04/12/2006
2006 American Chemical Society
onstrated that surface and groundwater could be contaminated by leached pesticide residues. Since adequate pesticide transformation in soil may reduce the extent of leached chemicals, improved understanding of the factors underlying pesticide mineralization in soils is necessary to develop suitable risk assessment of chemical leaching to groundwater, as well as establishing effective water protection strategies. Pesticides degradation in soils occurs via either microbiological or chemical processes, of which microbiological processes predominate (2). Furthermore, mass transfer of chemicals toward the degrading microbes is in most cases the critical factor in biodegradation (3). Factors such as soil moisture, pH, and percent of organic matter affect microbial activity and chemical diffusion in soils and the subsequent degradation of chemicals. Several methods are used to characterize soil water content (4). The German pesticide registration authority suggests in the guideline for pesticide degradation studies (5) that, in disturbed soils, equilibration should occur at 40% water holding capacity (WHC). To our knowledge, only a few studies addressing pesticide degradation in different soils equilibrate soil material at defined water potentials: e.g., -33 kPa in a study on removal of isoproturon (6) or -12 kPa in a study on removal of nematizides (7). One of the first quantitative analyses studying the effect of increasing water potentials on chemical removal was conducted determining depletion rates of toluene caused by Pseudomonas putida biofilms in two-compartment glass film reactors (8). Water availability is one of the most important parameters regulating biological activities in soils. “To absorb water, an organism must be able to overcome the forces tending to retain the water in the soil” (9) and, therefore, water uptake by microorganisms becomes energy-linked (10). Thus, soil water potential measurements should be incorporated into pesticide degradation studies to allow comparison of data obtained in soils with different texture and properties (10). Additionally, diffusion of solutes (e.g., nutrients, pesticides) is directly related to the cross section for flow; “therefore, water content, not water potential, is the proper variable to use in comparing microbially related reaction rates in systems which are diffusion limited” (11), emphasizing the importance of reporting water content and water potential of various soils in degradation studies. The use of 14C-labeling in pesticide studies allows identification and quantification of the various processes of pesticide removal (mineralization, volatility, metabolites, and nonextractable residues) (12). A study addressing the behavior of the worldwide-used pesticide isoproturon was determined in both long-term experiments with outdoor lysimeters as well as in the laboratory (12) at the GSF-research center. Isoproturon is widely used in cereal cultures for pre- and post-emergence control of weeds and has been already detected in surface water and groundwater (1). In parallel work, the degradation behavior of benazolin-ethyl in soils was investigated in a German-wide study of the Helmholtz Association of National Research Centers, aimed to determine soil parameters accounting for herbicide behavior. This pesticide was selected because of its relatively high persistence and leaching tendency. According to the German guideline for pesticide degradation studies (5) all participants decided to assess soil water content on % WHC. Evaluation and comparison of results from both independent studies by standardizing the specification of soil moisture in terms of water potential demonstrated an apparent effect of soil water potential on pesticide mineralization. These results VOL. 40, NO. 10, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Soil Characteristics (0-10 cm) and Location natural site name: classification
Bad Bad Bad Merzenhausen Scheyern 1 Lauchstaedt 1 Lauchstaedt 2 Lauchstaedt 3 Feldkirchen Kelheim Scheyern 2 Hohenwart Neumarkt Haplic Haplic Haplic Eutric Eutric Calcaric Humic Mollic Aric Haplic Chernozem Chernozem Chernozem Luvisol Cambisol Regosol Cambisol Gleysol Anthrosol Arenosol
abbreviation
