Sewage sludge processing determines its impact on soil microbial community structure and function

Composting and thermal drying are amongst the most commonly used post-digestion processes for allow-ing sanitation and biological stabilization of sewage sludge from municipal treatment plants, and makingit suitable as soil conditioner for use in agriculture. To assess the impact of sludge-derived materials onsoil microbial properties, fresh (LAF), composted (LAC) and thermally dried (LAT) sludge fractions, eachresulting from a different post-treatment process of a same aerobically digested sewage sludge, wereadded at 1% (w/w) application rate on two contrasting (a loam and a loamy sand) soils and incubatedunder laboratory conditions for 28 days. Soil respiration, microbial ATP content, hydrolytic activitiesand arginine ammonification rate were monitored throughout the incubation period. Results showedthat soil biochemical variables, including the metabolic quotient (qCO2), were markedly stimulated aftersludge application, and the magnitude of this stimulatory effect was dependent on sludge type (preciselyLAT > LAF > LAC), but not on soil type. This effect was related to the content of stable organic matter, whichwas lower in LAT. Genetic fingerprinting by PCR–DGGE revealed that compositional shifts of soil bacterialand, at greater extent, actinobacterial communities were responsive to the amendment with a differingsludge fraction. The observed time-dependent changes in the DGGE profiles of amended soils reflectedthe microbial turnover dependent on the sludge nutrient input, whereas no indications of adverse effectsof sludge-borne contaminants were noted. Our findings indicate that composting rather thermal dryingcan represent a more appropriate post-digestion process to make sewage sludge suitable for use as soilconditioner in agriculture.