Non extractable residues (NER) so called “bound residues” of plant protection products are formed e.g. in soil as a result of degradation processes. Due to their inherent nature, analysis and further assessments of bound residues are challenging. Therefore an estimation of the generation and behaviour of NERs is mainly done indirectly. In order to evaluate the microbial impact on the formation of non extractable residues (NER) in addition to the microbial degradation experiments sterile control experiments are conducted to proof whether the formation of NERs is only due to adsorption processes or due to microbial degradation processes. If the formation is due to microbial degradation processes the nature of the NERs is of course different to nature of original parent compound and release of NER is very slow in time and strongly correlated to degradation processes of the matrix (e.g., soil). New investigations in the elucidation of the nature of NER have been published (Possberg et al. 2016, Nowak et. al. 2011) and evolved a new classification approach for NER (Kaestner et al. 2014): NER type I: Entrapped, strongly adsorbed, no covalent binding NER type II: Covalently bound not extractable residues NER type III: Biogenic bound residues Both evaluation mainly relayed on a digestion step of the soil using 6 N hydrochloric acid for 22h at 110° C following clean up procedures and derivatization for amino acids before analysing with radio thin-layer chromatography followed by bio-imaging and definitive identification by GC-MS and LC-MS/MS or with isotope ratio determination with GC-MS-MS. We investigated the method of Possberg et al. As a main result 16% of the generated NER could be assigned to amino acids and therefore clearly indicate an extensive microbial degradation of bromoxynil before incorporated into biomass of soil as amino acids. Amino acids identified were phenylalanine, valine, leucine and isoleucine, proline, alanine and glutamine. However, nearly 75% of the soil bound non extractable residues stuck in the soil and could not be made extractable or available even with soil destructing methods like extraction with 6 M HCl at 110° C. This strongly indicate that under natural environmental conditions even suggesting a real harsh climate change those bound residues will not be liberated or are extractable from the matrix. Therefore a future risk of those bound residues could not be foreseen and therefore NER is regarded as a “safe sink”.

Not extractable residues (NER): How extractable are they?

256th National Meeting (2018)

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