title: Rate of hydrolysis and degradation of the cyanogenic glycoside - dhurrin - in soil.
abstract: Cyanogenic glycosides are common plant toxins. Toxic hydrogen cyanide originating from cyanogenic glycosides may affect soil processes and water quality. In this study, hydrolysis, degradation and sorption of dhurrin (4-hydroxymandelonitrile-beta-d-glucoside) produced by sorghum has been studied in order to assess its fate in soil. The log K(ow) of dhurrin was -1.18+/-0.08 (22 degrees C). Hydrolysis was a first-order reaction with respect to dhurrin and hydroxyl ion concentrations. Half lives ranged from 1.2h (pH 8.6; 25 degrees C) to 530d (pH 4; 25 degrees C). The activation energy of hydrolysis was 112+9kJ. At pH 5.8 and room temperature, addition of humic acids (50gl(-1)) increased the rate of hydrolysis tenfold, while addition of kaolinite or goethite (100-250gl(-1)) both decreased the rate considerably. No significant sorption to soil components could be observed. The degradation rates of dhurrin in top and subsoils of Oxisols, Ultisols, Alfisols and Mollisols were studied at 22 degrees C (25mgl(-1), soil:liquid 1:1 (w:V), pH 3.8-8.1). Half-lives were 0.25-2h for topsoils, and 5-288h in subsoils. Hydrolysis in solution explained up to 45% of the degradation in subsoils whereas the contribution in topsoils was less than 14%, indicating the importance of enzymatic degradation processes. The highest risk of dhurrin leaching will take place when the soil is a low activity acid shallow soil with low content of clay minerals, iron oxides and humic acids.
authors.affiliation: Department of Natural Sciences, The Royal Veterinary and Agricultural University (KVL), Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark.
authors.forename: Henrik
authors.initials: H
authors.lastname: Johansen
authors.forename: Lars Holm
authors.initials: LH
authors.lastname: Rasmussen
authors.forename: Carl Erik
authors.initials: CE
authors.lastname: Olsen
authors.forename: Hans Christian
authors.initials: HC
authors.lastname: Bruun Hansen
mesh_terms: D000327:Adsorption; D004784:Environmental Monitoring; D006868:Hydrolysis; D007700:Kinetics; D009570:Nitriles / Q000737:chemistry*; D012989:Soil Pollutants / Q000737:chemistry*; D045868:Sorghum / Q000378:metabolism
chemical_list: D009570:Nitriles; D012989:Soil Pollutants; C011220:dhurrin