Glucosinolate and isothiocyanate concentration in soil following incorporation of Brassica biofumigants

The concentration of glucosinolates (GSLs) and isothiocyanates (ITCs) was monitored in soil following the incorporation of pulverised high and low GSL varieties of rape (Brassica napus) and mustard (Brassica juncea) biofumigant crops. The concentration of both GSLs and ITCs in soil was highest immediately (30 min) after incorporation and they could be detected for up to 8 and 12 d, respectively. Irrigating with 18 mm of water over 3 h had no effect on either GSL or ITC concentrations. The amounts detected were generally related to the amount of GSL added in the incorporated plant tissue. Maximum total GSL concentration detected in the soil was 13.8 and 22.8 nmol g−1 for rape and mustard, respectively, representing 7% and 13% of the original GSL present in the incorporated tissues. The non-ITC liberating GSLs (predominately indolyl GSLs) were found at lower concentrations than ITC-liberating GSLs, but tended to persist longer in the soil. Maximum total ITC concentration was 21.6 nmol g−1 and 90.6 nmol g−1 for rape and mustard, respectively. Calculated ITC release efficiency was 26% and 56% for high GSL rape and mustard, respectively at the time of the highest ITC concentration measured. These are the first reported measurements of GSLs in soil following biofumigant incorporation. They indicate that a significant proportion of plant GSL can persist un-hydrolysed in the soil for several days following Brassica incorporation. Further investigations of plant treatment and incorporation methods to maximise ITC release are warranted.