Changes in light-textured soil parameters following two successive annual amendments with urban sewage sludge

Abstract

The generation of considerable amounts of sewage sludge during wastewater treatment is inevitable and should be carefully addressed. Among the various options available for urban sewage sludge management, the agricultural re-use as a fertilizer/soil conditioner has seen a wide acceptance because of the high amount of nutrients and organic matter contained in this biowaste. Accordingly, the aim of this field-scale study was to evaluate the changes in agricultural soil parameters following two successive annual amendments with increasing amounts of urban sludge (0, 40, 80 and 120tha−1year−1). Sludge addition improved the fertility of the topsoil, showing a significant dose-dependent increase of TOC, N, P, and K contents up to the excessive amount of 120tha−1year−1 (1.6%, 0.09, 233 and 21mgkg−1, respectively). Consequently, soil biological parameters such as microbial communities and enzymatic activities (dehydrogenase, phosphatase, protease and cellulase) were all enhanced as well. On the other hand, we noticed also a similar increasing trend of soil salinity (EC and Na+) and phytotoxic heavy metals namely, Cd and Cr (2.4 and 3.8mgkg−1 at 120tha−1year−1). This variation was nonetheless below thresholds; and metal bioavailability to microorganisms and lettuce seeds was likely to be hindered by the non change in soil pH, which varied within a neutral range for all treatments (7.43–7.87). According to the C/N ratio, which reflects organic matter evolution in soil, plots treated at 80tha−1year−1 resulted in C/N values below 15. Therefore, this excessive sludge dose for agricultural reuse is likely to be the best practice that ensures an adequate organic matter mineralization with less contamination after two successive annual amendments.