Inhibition of soil nitrifying bacteria communities and their activities by glucosinolate hydrolysis products

During microbial degradation of crucifer tissues in soil, a range of low molecular weight volatile S-containing compounds is produced. While a number of these compounds are known to have potent nitrification inhibiting properties, the effects of isothiocyanates (ITCs), which are derived from glucosinolates, are not known. We investigated the effects of glucosinolate hydrolysis products on communities and activities of nitrifying bacteria in bioassays using contrasting sandy- and clay-loam soils. In both soils, ITCs reduced populations of NH4+-oxidizing bacteria and inhibited their growth. ITCs had no apparent inhibitory effect on populations of NO2−-oxidizing bacteria in sandy-loam, but did reduce growth of these bacteria in clay-loam. Individual application of an aliphatic and an aromatic ITC inhibited nitrification of applied NH4+ in the two soils, with the effect being longer lived in sandy-loam relative to clay-loam. After 42 days, mineralization of N in sandy-loam amended with 2-phenethyl-ITC was greater than in unamended soil, suggesting that this compound had a general fumigant effect on the soil microbiota. ITCs were more effective inhibitors of nitrification than intact glucosinolates or nitriles. Phenyl-ITC was found to be the most toxic of the ITCs tested, but generally there were no differences between the nitrifying inhibitory properties of aliphatic and aromatic ITCs. The capacity of 2-propenyl-ITC to inhibit nitrification was shown to be less than that of dimethyl-disulphide. However, when concentrations of 2-propenyl-ITC and dimethyl-sulphide, which had no effect on nitrification when applied to soil individually, were mixed, nitrification was strongly inhibited. No such synergistic interaction was found for either of these compounds with dimethyl-disulphide. The significance of these findings is discussed.