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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 120 t ha −1 year −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 120 t ha −1 year −1 (1.6%, 0.09, 233 and 21 mg kg −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 3.8 mg kg −1 at 120 t ha −1 year −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 80 t ha −1 year −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. [ABSTRACT FROM AUTHOR]

Abstract The agricultural reuse of urban sewage sludge in degraded croplands has seen a wide acceptance for biowaste recycling and for the improvement of soil fertility. However, soil degradation and health issues may arise from the fact that sludge addition causes chemical and biological contamination if mismanaged. To closely monitor the long term soil-sludge interactions, a field study was conducted over a three-year period under a semi-arid climate. More precisely, changes in topsoil properties were assessed in 4-m2 uncropped plots amended with sludge rates equivalent to 0, 40, 80 and 120 t ha−1 year−1 for two soil textures: soil A (sandy loam) and soil S (sandy). Results showed that all studied parameters changed significantly in a dose-dependent manner for both soils as compared to untreated controls. As such, sludge addition improved consistently TOC, N, P and K content up to soils treated with 120 t ha−1 year−1. The enhancement of soil nutrient status impacted positively on biological properties, including microbial biomass and soil enzyme activities (dehydrogenase, protease and phosphatase). However, the variation of soil properties was more important in soil A characterized by a higher fine fraction thus stronger retention capacity. Accordingly, XRD analysis of soil A revealed several identifiable peaks representing kaolinite and illite clays in contrast to soil S. Unexpectedly, high to excessive sludge doses of 80 and 120 t ha−1 year−1 did not provoke soil degradation after three repeated annual amendments. In fact, pH values in both soils remained within neutral to alkaline range (7.76–8.63); total heavy metals (Cu, Zn, Ni and Pb), soil salinity and fecal coliforms were all below threshold values for contaminated soils. Highlights • Tunisian croplands have low organic matter content due to overexploitation. • Three successive annual sewage sludge amendments improved soil fertility. • Soil biological activities were positively correlated with the sludge dose. • Soil degradation was not observed even at excessive sludge doses. • A finer texture had more impact on soil parameters after sludge addition. [ABSTRACT FROM AUTHOR]