Dissipation of cyanogenic glucosides and cyanide in soil amended with white clover (Trifolium repens L.)

The use of white clover (Trifolium repens L.) as green manure is common practice due to its high nitrogen content. White clover produces the two cyanogenic glucosides linamarin and lotaustralin, which release toxic hydrogen cyanide upon hydrolysis. The hydrolysis of cyanogenic glucosides and release of cyanide were studied in batch experiments with ground white clover added to soil at loadings of 75 g leaves per kg soil. Linamarin and lotaustralin were quickly hydrolysed with first-order rate constants of 0.026–0.0062 h−1 (corresponding to half lives of 11–27 h) in sandy and loamy soils at natural moisture contents and 11 °C. Experiments with addition of pure cyanogenic glucosides and with sterilized soil material as well as addition of white clover to inert quartz showed that hydrolysis by plant glucosidases is partly inhibited in the soil matrix, but also that soil glucosidases present in soil contribute to degradation of the cyanogenic glucosides, and more so for linamarin than for lotaustralin. Cyanide was produced during the hydrolysis of the cyanogenic glucosides as seen by formation of HCN(g) and WAD-CN (Weak Acid Dissociable Cyanide) amounting to max. 5 and 50% of total-CN in the systems with white clover added to natural soils. However, the increase in WAD-CN was transient, due to subsequent dissipation of the compound caused by abiotic and microbial CN degradation. Due to WAD-CN dissipation in the top soils studied, long term effects of cyanide on sensitive microorganisms and plants are not expected. However, knowledge on the stability of WAD-CN in subsoil is lacking and warrants further investigations.