Long-term amendment of Spanish soils with sewage sludge: Effects on soil functioning

Wastewater treatment processes generate highly biodegradable solid wastes. As their fate is an environmental issue of great concern, public administrations promote a sustainable management of urban wastes. The objective of the present study was to analyze the systematic and periodical use, for 16 years, of anaerobically digested sewage sludge as an agricultural fertilizer by assessing the effects on some soil physical–chemical, functional, and ecotoxicological properties. The results showed that the input of sludge enhances soil properties proportionally to the application doses and/or frequency. The organic amendments increased the organic matter content (and its aromaticity), the soil nitrogen, and the microbial activity, improving carbon and nitrogen mineralization processes and some enzymatic functions. However, a maximum dose was identified (40 Mg ha−1 year−1), beyond which soil properties do not improve, and may even worsen. Regarding environmental risks, although the bioluminescent bacteria test showed no toxicity on soil extracts, potential adverse effects such as some potentially toxic elements accumulation, phytotoxicity and the likelihood of groundwater pollution by nitrates or dissolved organic matter should be taken into account. The complementarity of studying soil functioning parameters and ecotoxicological effects, together with the analysis of pollutant content, must be enhanced. This assures a more realistic assessment of long-term effects of sewage sludge-amended soils.

► Levels of some toxic elements increased significantly in amended soils.
► Doses <80 Mg ha−1 year−1 do not affect soil C & N mineralization.
► Plant growth test shows inhibition at annual doses >40 Mg ha−1.
► Sustainable N supply is achieved by distributing sludge over time.