Characterisation of volatile sulphur-containing compounds produced during decomposition of Brassica juncea tissues in soil

Incorporation of sulphur rich crucifer tissues into soil is known to suppress a variety of soil-borne plant pathogens and pests. Although the `biofumigantʹ effect has been attributed to release of toxic isothiocyanates (ITC) during crucifer tissue decomposition, little is known of the dynamics of formation of these compounds, or other non-glucosinolate derived toxic volatile S compounds, during the decomposition process. We compared concentrations of these two groups of compounds during decomposition of leaf tissues of Brassica juncea in sandy-loam and clay-loam soils. The tissues were shown to be rich in 2-propenyl-glucosinolate, which is hydrolysed to 2-propenyl-ITC on tissue damage. Patterns of formation of the compounds differed in the two soils, with smaller amounts of all compounds detected in the clay-loam, in which microbial respiration was higher. In both soils, the non-glucosinolate derived volatile S-containing compounds carbon-disulphide, dimethyl-disulphide, dimethyl-sulphide and methanethiol were the dominant headspace components, with maximum concentrations reaching 88, 39, 406 and 992 nmol g−1 dw leaf incorporated, respectively, in sandy-loam and 152, 22, 119 and 473 nmol g−1 dw leaf added in clay-loam. Only small quantities of glucosinolate hydrolysis products were detected. Maximum headspace concentrations of 2-propenyl-ITC were 12 and 2 nmol g−1 dw leaf added in the sandy-loam and clay-loam, respectively. No 2-propenyl-ITC was detected in direct dichloromethane extracts of soil. Glucosinolates contained in the tissues were shown to be almost completely degraded over the course of the experiment. Peak formation of 2-propenyl-ITC and dimethyl-disulphide was associated with the initial stages of tissue colonization by the soil microbiota, whereas the other volatile S compounds were produced throughout decomposition. We suggest that the bio-fumigant properties of crucifer tissues represent the combined effect of the low quantities of highly toxic ITC, and large quantities of mildly toxic non-glucosinolate derived volatile S-containing compounds produced during decomposition.