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Current knowledge about how biochars affect the fate of pesticides in soil is based on studies that used pure biochars. After finding that an additional biological post-pyrolysis treatment, such as co-composting or lactic fermentation, is required for biochars for superior performance in temperate arable soils, a knowledge gap formed of how such further processed biochar products would affect the fate of pesticides in soil. This study compared the effects of a novel fermented biochar alone or mixed with biogas residues on the fate of two pesticides, 4-chloro-2-methylphenoxyacetic acid (MCPA) and metalaxyl-M, in a temperate arable soil to the traditional organic amendments wheat straw and compost. The fate of 14 C-labeled MCPA was markedly affected in different ways. Fermented biochar effectively reduced the water-extractability and mineralization due to adsorption that was comparable to adsorption strengths reported for pure biochars. However, this effect was weak for the biochar mixed with biogas residues. Straw reduced water-extractable amounts due to increased biodegradation and formation of likely biogenic non-extractable residues of MCPA. In contrast, compost decelerated mineralization and increased the water solubility of the MCPA residues due to released dissolved organic matter. The amendments' effects were minor regarding 14 C-metalaxyl-M, except for the fermented biochar which again reduced water-extractability and delayed degradation due to adsorption. Thus, the effects of the organic amendments differed for the two pesticide compounds with only the fermented biochar's effect being similar for both. However, this effect was no longer present in the mixed product containing 20% biochar. Our findings clearly show that biologically treated biochar-containing products can affect the fate of pesticides in soil very differently, also when compared to traditional organic amendments. Such impacts and their desirable and undesirable ecotoxicological implications need to be considered before the large-scale application of biochars to temperate arable soils., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 The Authors. Published by Elsevier Ltd.. All rights reserved.)

The influence of an ionic functional group on the fate of chemicals in the environment, especially the formation of non-extractable residues (NER), has not been systematically investigated. Using 4-n-dodecylphenol[phenylring- 14 C(U)], 4-n-dodecylbenzenesulfonicacid[phenylring- 1 4 C(U)] sodiumsalt ( 14 C-DS - ) and 4-n-dodecylbenzyltrimethylammoniumchloride[phenylring- 14 C(U)] ( 14 C-DA + ) all with a high structural similarity, the formation, classification and identification of NER of negatively ( 14 C-DS - ), positively ( 14 C-DA + ) and uncharged ( 14 C-DP) chemicals were investigated in a sterilized and non-sterilized soil. After 84 days of incubation in non-sterile soil, 40.6%, 21.7% and 33.5% of the applied radioactivity (AR) of 14 C-DP, 14 C-DS - and 14 C-DA + , respectively, were converted to NER. In contrast, in sterile soil NER formation was markedly lower. The NER were further investigated with respect to sequestered, covalently bound and biogenic residues (i.e. NER types I, II, and III). Silylation of 14 C-DP, 14 C-DS - and 14 C-DA + derived NER released 3.0-23.2% AR, indicating that these were sequestered, whereas the residual NER (12.9-33.1% AR) was covalently bound to the soil. Analysis of extracts derived by silylation showed that 14 C-DP, but neither 14 C-DS - nor 14 C-DA + , were released by silylation, suggesting that DP might be part of the sequestered NER. Acid hydrolysis of the NER containing soil and subsequent analysis of soil extracts for 14 C-aminoacids indicated that 2.5-23.8% AR were biogenic residues. Most DP and DS - derived NER were biogenically or covalently bound, whereas DA + predominantly forms sequestered NER in soil. From these results we propose that chemicals forming high amounts of NER should be investigated regarding types I-III NER because sequestered parent compounds should be considered in persistence assessments., (Copyright © 2019. Published by Elsevier Ltd.)