Sustainability Assessment after Twenty Years of Sewage Sludge Application on Calcareous Soil Following N or P Criterion

Abstract

Sewage sludge is a valuable source of nutrients when applied to the soil. Research on its agricultural use has been focused on chemical parameters to prevent heavy metal buildup. However, soil quality includes a wider spectrum of indicators. Our aim was to evaluate the impacts of sludge application on the biological and physical soil properties of calcareous soil when sludge is applied in maize monoculture using fertilization dosage criteria determined by N input or soil P thresholds. A control based on mineral fertilization was also included. After 20 years, no differences were found in the biological indicators: earthworm and soil oribatid mite abundances. Five oribatid species were identified, but three predominated: Acrotritia ardua americana, Oribatula (Zygoribatula) excavata and Tectocepheus sarekensis. The latter two showed a maximum number of individuals five months after maize stalks were buried in soil, independent of fertilization treatment. Sludge significantly increased water-stable aggregates (up to 30%), but there was no difference in macroporosity (apparent pore diameter > 30 μm). Under irrigated calcareous soil, sewage sludge applied as fertilizer is a sustainable option, independent of the threshold criterion (N or P) used. Nevertheless, as the P threshold criterion allows for reduced P inputs, it is more sustainable over the long term.