Isothiocyanate release from soil-incorporated Brassica tissues

Isothiocyanates (ITCs) released from glucosinolates in Brassicaceous residues are thought to suppress soil-borne plant pests, however, little is known about ITC formation in soil. We conducted field and laboratory studies to determine the concentration and pattern of ITC production in soil following incorporation of rapeseed (Brassica napus L.) and Indian mustard (Brassica juncea Czern. & Coss). After tissue incorporation, ITC was extracted from soil with methanol, derivatised and quantified using HPLC. Maximum ITC concentrations near 1.0 nmol g−1 soil were measured immediately after tissue incorporation, with little production detected after 4 d. Only 1% or less of the ITC predicted from tissue glucosinolate concentrations was measured in soil amended with tissues of high glucosinolate rapeseed or mustard varieties. Tissue disruption at the cellular level afforded by freezing and thawing the tissues, resulted in maximum ITC concentrations ranging from 40 to 75 nmol ITC g−1 soil, thus increasing ITC release efficiencies to 14 and 26%. Our work indicates that soil-borne pest suppression is likely to be improved by choosing a high glucosinolate-containing variety of rapeseed or mustard and providing adequate moisture to increase ITC release and soil retention. However, the greatest improvements in the use of Brassica biofumigants to control soil-borne plant pests will be achieved by focussing on methods to increase cell disruption thereby maximising glucosinolate hydrolysis and ITC release.