Dissipation of pure and broccoli-released glucosinolates in soil under high and low moisture content

Abstract

Glucosinolates (GSLs) are secondary metabolites found in Brassica species. Upon tissue disruption GSLs are hydrolyzed by myrosinase enzymes to isothiocyanates (ITCs) which are highly toxic to microbes. Therefore the incorporation of fresh Brassicaceae tissues into soil for the control of soil-born plant pathogens is viewed as a biofumigation process. Its efficacy relies on the rate of GSL conversion to ITCs, but also on environmental factors controlling GSL availability in the soil matrix. We studied the dissipation of GSLs, applied either by broccoli leaves or as pure compounds, in a clay loam soil at two soil moisture content levels. Regardless of their mode of application, GSLs were rapidly dissipated in soil with half-life values ranging from 3.2 to 15.5 h. Increasing moisture from 20 to 90% of the soil water holding capacity significantly accelerated their dissipation. Indolyl broccoli-derived GSLs dissipated faster than aliphatic GSLs at high moisture levels, while at low moisture levels a three-fold reduction in their dissipation rates was observed. The dissipation of aliphatic GSLs was less affected by soil moisture levels. Application of pure GSLs resulted in increased soil metabolic quotients (qCO2), suggesting a decline is microbial metabolic efficiency. Their dissipation was related to myrosinase activity apparently derived from soil microbes.