Hydrogen isotope labeling unravels origin of soil-bound organic contaminant residues in biodegradability testing.

Biodegradability testing in soil helps to identify safe synthetic organic chemicals but is still obscured by the formation of soil-bound 'non-extractable' residues (NERs). Present-day methodologies using radiocarbon or stable (¹³C, ¹⁵N) isotope labeling cannot easily differentiate soil-bound parent chemicals or transformation products (xenoNERs) from harmless soil-bound biomolecules of microbial degraders (bioNERs). Hypothesizing a minimal retention of hydrogen in biomolecules, we here apply stable hydrogen isotope – deuterium (D) – labeling to unravel the origin of NERs. Soil biodegradation tests with D- and ¹³C-labeled 2,4-D, glyphosate and sulfamethoxazole reveal consistently lower proportions of applied D than ¹³C in total NERs and in amino acids, a quantitative biomarker for bioNERs. Soil-bound D thus mostly represents xenoNERs and not bioNERs, enabling an efficient quantification of xenoNERs by just measuring the total bound D. D or tritium (T) labeling could thus improve the value of biodegradability testing results for diverse organic chemicals forming soil-bound residues. Non-extractable residues formation limits biodegradability testing in soil. Here, the authors show that hydrogen isotope tracers are hardly retained in microbial biomass, enabling an efficient distinction between biogenic and xenobiotic residues. [ABSTRACT FROM AUTHOR]