Effects of digested, composted, and thermally dried sewage sludge on soil microbiota and mesofauna

Sewage sludge production increases in Europe proportionally to the number of municipalities that put wastewater treatment plants into service. Recycling to soil is among the main sludge outlets, since amending with sludge meets requirements of efficient recycling of resources while providing organic matter for poor or degraded soils. As raw sludge contains substances potentially harmful for environmental and human health, it must be stabilized before application to the soil. We tested three kinds of anaerobically digested sludge, differing by the post-treatments they had undergone (dewatering, composting or thermal drying), for their effects on soil microbial activity, microarthropod density and biological soil quality quantified as QBS indicator. Our experimental treatments were set on plots located in a burnt Mediterranean forest and consisted of spreading sludge on the soil surface to achieve a dose of 6 Mg o.m. ha−1. Although amendment with any of the three kinds of sludge increased soil organic matter and nitrogen, thermally dried sludge caused a transitory increase of the soil microbial metabolic coefficient that we considered indicative of soil disturbance. One year after soil amendment, all sludge effects on the soil microbial activity had disappeared. When working with microarthropods’ density and biodiversity, no sludge improved soil biological quality, fresh digested sludge being the most detrimental, since it caused the decline of the most sensitive taxa (oribatid mites) and increased the density of soil forms resistant to disturbance and pollution (astigmatic mites and symphypleon collembolans). Sludge effects on soil microarthropods were still evident three years after soil amendment, which suggests that soil invertebrate diversity is less resilient to soil disturbance than global microbial metabolism.