Effect of synthetic dairy factory effluent containing different acids (H3PO4, HNO3 and CH3COOH) on soil microbial and chemical properties and nutrient leaching

The effects of addition of synthetic dairy factory effluent (DFE) containing phosphoric, nitric or acetic acids as cleaning agents on soil chemical and microbial properties and potential nutrient leaching from a dairy farm soil were studied in an 84-day open incubation/leaching experiment and in a short-term (24 days) closed incubation. Dairy factory effluent resulted in increased leaching of Na+ and Ca2+. The main anion leached in the DFE treatments was Cl−, whilst in the control treatments it was NO3−. Leachates from DFE-treated soil had a greater electrical conductivity (EC) and lower pH than those from controls, but type of acid had little effect on the ionic content or composition of leachates. At the conclusion of the experiment, soils from DFE treatments showed a large accumulation of exchangeable Na+, a reduction in exchangeable Ca2+ and significant increases in microbial biomass C, basal respiration and β-glucosidase and arylsulphatase activities. Microbial activity was greatest from the acetic acid-DFE treatment. There was a significant increase in extractable P in the phosphoric acid-DFE treatment, but addition of nitric acid-DFE did not increase leaching of NO3−. In the short-term incubation experiment, EC increased over time in all treatments, while pH, microbial biomass and basal respiration decreased. During the first few days after a one-off DFE addition, there was an increase in both microbial biomass C and basal respiration, but these differences dissipated after 3 days. It was concluded that while additions of DFE have significant effects on chemical and microbial soil properties, a change in the form of acid present in the DFE has relatively minor effects on these properties.