Mineralisation of low concentrations of organic compounds and microbial biomass in surface and vadose zone soils from the Swan Coastal Plain, Western Australia

Abstract

Mineralisation rates for ring-labelled ¹⁴C-atrazine, benzene, and toluene were determined for a number of Swan Coastal Plain soils which had not been previously in contact with these contaminants. Microbial biomass was estimated by phospholipid techniques in soil samples from the same sites.

Mineralisation rates for the volatile aromatic hydrocarbons in the thin (up to 30 cm) surface soils (23·4-42·6 µmol/kg · day when fitted to zeroth-order rate kinetics) were appreciably faster than the mineralisation rates measured in soils collected from a depth of 1 m (0·11-3·0 µmol/kg · day). The pesticide atrazine was degraded slowly, with degradation rates in surface soils ranging from 1·22×10^-3 to 2·78×10^-4 µmol/kg · day, and those in soils at 1 m ranging from 5· 13×10^-4 to 3·16×10^-4 mol/kg · day. When mineralisation data were fitted to first-order kinetics then half-lives for atrazine mineralisation ranged from about 1 year in surface soils to 3·1-5·1 years in soils at 1 m. These rates were comparable to atrazine mineralisation rates measured in soils that had not been previously in contact with atrazine, as reported by others. The extent of mineralisation of the organic compounds v. time generally fitted better to zeroth-order kinetics than to first-order kinetics. Confidence in the determination of the mineralisation rate at slow rates of mineralisation was low (r² as low as 0·2 in plots of the extent of mineralisation v. time in zeroth-order and first-order plots for samples that showed slow mineralisation).

Biomass, expressed as stationary phase Escherichia coli equivalents (SPEE), ranged from 1·4 ×10⁷ to 1·2×10⁸ SPEE/g dry weight for surface soils, and from 8·6×10⁵ to 7·3×10⁶ SPEE/g dry weight for soils at 1 m. The phospholipids extracted from surface soils tended to contain higher proportions of unsaturated and hydroxy fatty acids than soils at 1 m, which contained higher relative concentrations of branched fatty acids, which is consistent with the microbiota moving, with greater depth, from a Gram-negative to a more Gram-positive dominated population. Biomass correlated well with mineralisation rates of the volatile aromatic compounds (r² = 0·96) but less so with atrazine mineralisation rates (r² = 0·86), although further experiments would be required to determine if biomass was consistently a good predictor of mineralisation rate for these organic contaminants.

The results suggest that it is important to maintain a fertile, ‘microbially rich’ soil in order to maximise the potential for mineralisation of organic contaminants above groundwater resources. The mineralisation rate measurements for atrazine obtained in this study should be useful in models for the prediction of the contamination threat to groundwater resources.