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Urban sewage sludge (USS) is increasingly applied to agricultural soils, but mixed results have been reported because of variations in reuse conditions. Most field trials have been conducted in cropping systems, which conceal intrinsic soil responses to sludge amendments due to the rhizosphere effect and farming practices. Therefore, the current field study highlights long‐term changes in bare soil properties in strict relationship with soil texture and USS dose. Two agricultural soils (loamy sand [LS] and sandy [S]) were amended annually with increasing sludge rates up to 120 t ha−1 yr−1 for 5 yr under unvegetated conditions. Outcomes showed a USS dose‐dependent variation of all studied parameters in topsoil samples. Soil salinization was the most significant risk related to excessive USS doses. Total dissolved salts (TDS) in saturated paste extracts reached the highest concentrations of 37.2 and 43.1 g L−1 in S soil and LS soil, respectively, treated with 120 t USS ha−1 yr−1. This was also reflected by electrical conductivity of the saturated paste extract (ECe) exceeding 4,000 µS cm−1 in both treatments. As observed for TDS, fertility indicators and bioavailable metals varied with soil texture due to the greater retention capacity of LS soil owing to higher fine fraction content. Soil phytotoxicity was estimated by the seed germination index (GI) calculated for lettuce, alfalfa, oat, and durum wheat. The GI was species dependent, indicating different degrees of sensitivity or tolerance to increasing USS rates. Lettuce germination was significantly affected by changes in soil conditions showing negative correlations with ECe and soluble metals. In contrast, treatment with USS enhanced the GI of wheat, reflecting higher salinity tolerance and a positive effect of sludge on abiotic conditions that control germination in soil. Therefore, the choice of adapted plant species is the key factor for successful cropping trials in sludge‐amended soils.

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Sewage sludge is increasingly used as an organic amendment to agricultural soils, especially to soils containing little organic matter. However, little is known on the impact of this biowaste on seasonal changes of nickel and cadmium toxicity in a sandy loam soil. Accordingly, the aim of this field-scale study was to evaluate the seasonal phytotoxicity according to Cd, Ni, and dehydrogenase variation in an agricultural soil during two successive annual amendments with increasing amounts of urban sludge (0, 40, 80, and 120 t ha year). Sampling was carried out at the end of dry season (EDS) and at the end of wet season (EWS) during 2 years 2012/2013. Sludge application significantly increased the amount of organic matter and dehydrogenase activity in the soil. In order to explain the seasonal variation of Cd and Ni, pH and electrical conductivity were also monitored in this study. The increased rate of sewage sludge addition slightly reduced the pH but soil remained above neutrality. The electrical conductivity which reflects soil salinity was strongly correlated with Cd and Ni content that increased with sludge dose. Salinity and heavy metals were highest at EDS 2013. In addition, soil phytotoxicity testing was performed by the evaluation of lettuce seed germination for 120 h. Although heavy metal content did not generally exceed Tunisian thresholds (3 and 75 mg kg for Cd and Ni, respectively), the seed germination index decreased with sewage sludge dose at all seasons. In general, we observed a significant effect of seasonal variation for all studied parameters. Sewage sludge reuse could be a feasible way to improve soil organic matter but toxicity risks consistently increased with time.

Wastewaters are increasingly used for irrigation of cropping systems in Tunisia. However, to develop environmentally sound practices the contribution of wastewater to crop N nutrition needs to be clarified, especially in cropping systems already receiving mineral fertilizers. For a better understanding of the interaction between fertilizer N and N originating from wastewater, experiments using 15 N were conducted. 15 N-labeled fertilizer was applied at different rates (0, 60, 100 and 140 kg N•ha-1) and with different water irrigation qualities (tap water or treated wastewater) to sorghum grown in lysimeters during 1998 and 1999. Recovery of 15 N-labeled fertilizer in the above-ground crop at final harvest in treated wastewater irrigation was higher at the lowest rate of fertilizer application (54%), with the amount recovered in the crop increasing as the rate of 15 N-labeled fertilizer application increased up to the rate of 100 kg N•ha-1. Nevertheless, in spite of this increase in 15 N-labeled fertilizer in the crop, total plant N uptake did not differ between rates. Treated wastewater irrigation had no negative effect on the recovery of 15 N-labeled fertilizer. About 62 and 55% of 15 N-labeled fertilizer was removed by Sudangrass in either tap water or treated wastewater. Neither fertilizer N rate nor water quality had an effect on the 15 N-labeled fertilizer remaining in the soil at final harvest. On average 20% in the wastewater treatment (19-24%) and 30% in the tap water treatment (26-31%) of the 15 N fertilizer applied were in the 0-60 cm layer of soil at final harvest in 1998 and 1999, respectively, and mostly present in the 0-20 cm layer. The proportion of applied 15 Nlabeled fertilizer remaining in the soil at final harvest increased with increasing N rates. About 60, 69 and 72% of 15 N left in the soil at final harvest was in the surface 0-20 cm layer. Residual 15 N was greatly higher in soil following the first harvest than after the final harvest, with the greatest value (38%) measured at the lowest rate of 15 N-labeled fertilizer (30 kg N•ha-1). Losses of 15 N-labeled fertilizer increased with application rate, but were unaffected by water quality irrigation. Approximately 13% of the applied 15 N fertilizer was lost following the application of 100 kg N•ha-1 with either treated wastewater or tap water irrigation.

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