Non-extractable residues of pendimethalin in soil

  • Nicht-extrahierbare Rückstände von Pendimethalin im Boden

Luks, Ann-Katrin; Schäffer, Andreas (Thesis advisor); Hollert, Henner (Thesis advisor)

Aachen : RWTH Aachen University (2021, 2022)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2021


Two main scenarios were investigated regarding the formation of non-extractable residues and their characteristics of the herbicide pendimethalin in soil. First, the incubation in soil alone was examined and second, the incubation in soil amended with 20 weight-% compost. Incubation of pendimethalin in a LUFA 2.2 soil (loamy fine sand, USDA) at an application rate of 5 mg kg-1 soil led to a mineralization of 26.2 % AR and a formation of non-extractable residues of 32.0 % after 400 days (end of study), hence moderate amounts of non-extractable residues were found. The amount of non-extractable residues increased over time and 14CO2-development was observed over the whole time. 14C-pendimethalin was degraded to several metabolites of which one could be identified as M455H001 (2-methyl-3,5-dinitro-4-(pentan-3-ylamino) benzoic acid) amounting to 5 % AR after 400 days of incubation. Amendment with compost (incubation time until 217 days) to the soil led to a slightly higher formation of non-extractable residues (38.2 % AR, 217 days) and reduced the mineralization almost completely (1.7 % AR after 217 days of incubation). Also in this study, metabolite M455H001 was identified but detected in minor amounts (2.4 % AR after 217 days, highest amount detected). A tenfold application of pendimethalin to soil led to a delayed formation of non-extractable residues and the degradation of pendimethalin occurred at a lower rate, too, but this might have been due to the incorrect water content which was not adjusted the whole time. Humic matter fractionation revealed that in soil alone, the radioactivity was almost evenly distributed among the fractions of fulvic acids, humic acids and humins at all sampling dates investigated. When soil was amended with compost, the major part of the radioactivity in the residues was found in the fraction of humins. The purified fulvic acid fraction was analyzed with size-exclusion chromatography. The major part of the injected radioactivity of the fulvic acid fraction eluted rather early, together with the fulvic acids themselves. This amount increased with time. It amounted to 92 % of the radioactivity in the fulvic acids fraction or 9.4 % of applied radioactivity after 400 days of incubation. A minor portion of the radioactivity eluted at a later time point from the SEC column. This was assigned to radioactivity not associated firmly with fulvic acids. This portion amounted to 0.4 % AR (400 days of incubation). Comparing with the retention times of the parent pendimethalin and the metabolite M455H001, this portion of radioactivity was tentatively assigned to the metabolite M455H001 whereas no pendimethalin could be identified. When soil was amended with compost, the amount of radioactivity eluting in the range of radioactivity not associated firmly with fulvic acids was higher. A silylation reaction was conducted on extracted soil to evaluate whether the non-extractable residues were covalently bound to or rather sequestered in pores of the soil matrix. From soil alone (study B), ~8 - 20 % (or ~1.5 - 3.6 %AR) of the non-extractable residues could be released. Considering the amount released by chloroform treatment of the soil, the amount released by silylation alone would have been even lower. The amount of non-extractable radioactivity released by silylation decreased with increasing incubation time, hence suggesting a firmer association with the soil matrix. It is concluded that the major portion of the bound residues is indeed bound covalently to the soil matrix and only a minor portion was present rather in a sequestered form. The analysis of the obtained extracts additionally revealed that only part of the released (thus sequestered) radioactivity was present as parent pendimethalin. HPLC of a sample suggested the presence of metabolite M455H001 and radioactive residues associated with soil organic matter. In contrast to incubation in soil alone, silylation could release more radioactivity of the non-extractable residues formed in compost amended soil. However, higher amounts of radioactivity released might not be linked to silylation itself but to the initial treatment with CHCl3 prior to the silylation process. These leads to the suggestion that the methanolic extraction method suitable for extraction of pendimethalin and its metabolites from soil alone was not suitable for exhaustive extraction of pendimethalin and metabolites from soil with compost addition. The higher amount of organic matter resulted probably in the higher amount of pendimethalin (equivalents) being sorbed to the soil matrix due to the higher amount of possible adsorption locations in compost amended soil. In an additional experiment, the remobilization experiment assessed the potential of non-extractable residues to be released in the future. Small quantities were biodegraded, indicated by minor portions degraded completely to 14CO2 and minor amounts becoming extractable after incubation in fresh soil. It was concluded that only minor amounts could become bioavailable with time, provided that the organic matter matrix is thoroughly altered by degradation or oxidation. All in all, with the results of this investigation, the non-extractable residues formed by pendimethalin in this soil are considered to be mainly covalently bound to the soil matrix. Compost amendment enhanced the formation of non-extractable residues and thus, the herbicide was less bioavailable and was degraded to a lesser degree than in soil alone. The methods used in the present dissertation are a tool to investigate the formation and characteristics of xenobiotic non-extractable residues and to distinguish those being sequestered (type I) and covalently bound (type II).