Quantitative Identification of Biogenic Nonextractable Pesticide Residues in Soil by ¹⁴C-Analysis

Abstract

Quantification of nonextractable residues (NER) of pesticides in soil is feasible by use of radioactively labeled compounds, but structural information on these long-term stabilized residues is usually lacking. Microorganisms incorporate parts of the radiolabeled ((14)C-) carbon from contaminants into microbial biomass, which after cell death enters soil organic matter, thus forming biogenic nonextractable residues (bioNER). The formation of bioNER is not yet determinable in environmental fate studies due to a lack of methodology. This paper focuses on the development of a feasible analytical method to quantify proteinaceous carbon, since proteins make up the largest mass portion of bacterial cells. The test substance (14)C-bromoxynil after 56 days forms more than 70% of NER in soil. For further characterization of NER the amino acids were extracted, purified, and separated by two-dimensional thin-layer chromatography (TLC). Visualization of the (14)C-amino acids was performed by bioimaging, unambiguous identification by GC-MS and LC-MS/MS. Our analysis revealed that after 56 days of incubation about 14.5% of the (14)C-label of bromoxynil was incorporated in amino acids. Extrapolating this content based on the amount of proteins in the biomass (55%), in total about 26% of the NER is accounted for by bioNER and thus is not environmentally relevant.

Critical evaluation of the Microbial Turnover to Biomass approach for the estimation of biogenic non-extractable residue

“…For the determination of tAA, proteins in NER were extracted by 6 M HCl, and the extract was cleaned up by a cation-exchange solid-phase extraction (SPE). In all studies, except those of Hennecke et al (2021) [29], selected amino acids present in the extract were analysed, and from the expected ratio of amino acids in microbiota, the tAA was calculated [35]. Hennecke et al…”

Influence of Flooding, Salinization, and Soil Properties on Degradation of Chlorantraniliprole in California Rice Field Soils

“…Because CAP is nonvolatile, possible explanations for lack of mass balance include mineralization and the formation of bound or biogenic nonextractable residues, although this cannot be stated with certainty without the application of unavailable radiolabels. Nonextractable residue formation can occur through chemical or biological mechanisms and can vary greatly based on chemical properties, soil organic matter content, pH, moisture content, redox state, and microbial activity. − Furthermore, increased contact time, or aging, of pesticides with soils has been shown to increase sequestration or decrease the extractability of organic compounds from soils. − Previous observation of an increase in the sorption of CAP to soils after brief aging suggests that the interaction of CAP with soils may be time-dependent . The formation of nonextractable residues may also decrease bioavailability and could explain the cessation of degradation observed after 63 days in these microcosm experiments.…”

A unified approach for including non-extractable residues (NER) of chemicals and pesticides in the assessment of persistence

“…However, for the final determination of the type I or II NER extent, the amounts of included bioNER need to be evaluated. In the type I NER containing fraction, this can either be done by quantification of the parent substances and transformation products, e.g., by MS, or the amount of bioNER [53]. The type II NER containing fraction needs to be calculated by the total NER after exhaustive extraction minus total bioNER and identified type I NER.…”

“…Both labeling with radioactive ( 14 C) [53] and stable isotopes ( 13 C, 15 N) [12] has been successfully applied to quantify the amounts of type III NER using the extraction method. Basically, the environmental matrix, e.g., soil, is hydrolyzed by concentrated HCl at elevated temperature.…”