Role of Wheat-Residue-Derived Char in the Biodegradation of Benzonitrile in Soil: Nutritional Stimulation versus Adsorptive Inhibition

Previous studies demonstrated that crop-residue-derived chars are effective adsorbents for pesticides and substantially reduce their biodegradation in soils. In contrast, the nutrients in the chars may stimulate cell growth and hence enhance the biodegradation. This work investigated the role of a wheat-residue-derived char in the nutritional stimulation and adsorptive inhibition of biodegradation of benzonitrile in a soil. The biodegradation was measured at 78 mg/L of benzonitrile, an initial concentration much higher than the half-saturation constant of the degrading organism (approximately 18 mg/L). The degradation was much faster in the extract of char-amended soil (CAS) than in those of soil and washed-char-amended soil (WCAS). The degradation was also faster in CAS slurry than in slurries of soil and WCAS until approximately 88% of benzonitrile degraded in char-containing slurries, where the aqueous-phase concentration was approximately 0.25 mg/L. Cell density was higher in extracts and slurries in the presence of the char nutrients than in the absence of them. Analysis of nutrient elements in extracts, along with measuring degradation in soil extract with nutrient supplements and subsequent stepwise multiple-regression, suggested that the char nutrients stimulated cell growth and degradation of benzonitrile, for which P was primarily responsible. Further degradation of benzonitrile beyond 88% in the presence of char was slow, suggestive of the adsorptive inhibition. The biodegradation of benzonitrile in CAS thus occurred through a fast-to-slow process, the former step being due primarily to the stimulation by soluble P of char and the latter phase to the adsorptive inhibition.