Introduction of 2,4-Dichlorophenoxyacetic acid into soil with solvents and resulting implications for bioavailability to microorganisms

Authors: JOHNSON, T - The Ohio State University; Sims, Gerald

Submitted to: World Journal of Microbiology and Biotechnology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/3/2010
Publication Date: 9/18/2010
Citation: Johnson, T.A., Sims, G.K. 2010. Introduction of 2,4-Dichlorophenoxyacetic Acid Into Soil With Solvents and Resulting Implications for Bioavailability to Microorganisms. World Journal of Microbiology and Biotechnology. 27(5):1137-1143.

Interpretive Summary: Test chemicals, such as pesticides, are difficult to distribute uniformly in soils for the purpose of laboratory investigations, and several days may be required before the substance is redistributed sufficiently to reach many of the organisms involved in chemical degradation. Often, water insoluble chemicals are introduced dissolved in organic solvents (typically in very small volumes to avoid toxicity from the solvents), which further complicates achieving a uniform distribution. Our objective was to investigate relative effects of carrier solvent choice and volume on xenobiotic distribution, apparent solvent toxicity, and soil degradation of 2,4-dichlorophenoxy acetic acid (2,4-D). Incubations using four carrier solvents ranging in properties showed that the fraction of 2,4-D mineralized increased with solvent volume used (owing to increased herbicide bioavailability) and began to decrease with further increases in volume as solvent toxicity became more important than bioavailability. Substrate concentration influenced mineralization of herbicide introduced with organic carriers, but not water. A greater fraction of the added material could be released by extraction with water when water was the initial carrier. Solvent toxicity should thus be balanced with uniformity of substrate distribution when using organic carriers in soils. Substrate bioavailability remains a ubiquitous issue in terrestrial microbiology research, regardless of introduction method. We advocate the development of tools to characterize variable conditions among soil compartments, and linkage of this information to microbial data. The impact of this research is to enable the research community to obtain more dependable data on the fate of chemicals in soil, in order to develop management practices that achieve production goals without sacrificing environmental quality.

Technical Abstract: Slow equilibration of introduced chemicals through tortuous pore space limits uniform substrate distribution in soil biodegradation studies. The necessity of introducing poorly soluble xenobiotics via organic solvents, the volume of which is minimized to limit toxicity, likely also affects xenobiotic distribution. Our objective was to investigate relative effects of carrier solvent choice and volume on xenobiotic distribution, apparent solvent toxicity, and soil degradation of 2,4-dichlorophenoxy acetic acid. Incubations using four carrier solvents ranging in properties showed that the fraction of 2,4-D mineralized was a hyperbolic function of solvent volume used (0.02-10 microliter/g), attributed to compensating effects of herbicide bioavailability and solvent toxicity. Substrate concentration influenced mineralization of herbicide introduced with organic carriers, but not water. Fraction of material readily desorbed increased when water was the carrier. Solvent toxicity should be balanced with uniformity of substrate distribution when using organic carriers in soils. Substrate bioavailability remains a ubiquitous issue in terrestrial microbiology research, regardless of introduction method. Limitations observed herein broadly apply to microbiology questions about introduced substances in soil. We advocate the development of tools to characterize variable conditions among soil compartments, and linkage of this information to microbial data.