Keywords

1 Introduction

The safe disposal of sewage sludge is one of the major concerns worldwide (Singh and Agrawel 2010). In Tunisia,. sewage sludge generation has significantly increased in recent years (245 thousands m3 of dried sewage sludge in 2010 compared to 99 thousand m3 in 2002) and its management is still a significant problem. Among the proposed strategies for sewage sludge management and according to the report of the National Urban Wastewater Treatment Program, agricultural application of sewage sludge remains one of the methods. It is used not only to solve the problem of disposal but also to increase productivity in agriculture thanks to its organic waste content as a source of nutrients together with other organic constituents (Martinez et al. 2003). However, this practice should be monitored for its environmental risk due to the presence of noxious substances such as heavy metals and pathogens. Moreover crops responses to sludge application vary by source, application rate, plant species, soil type, weathering conditions and application management (Al Zoubi et al. 2008). In this context, the present field study was conducted to assess the potential effect of three sewage sludge rates, provided from the sewage treatment plant of Beja (North-West of Tunisia), on a soil and forage crop, namely triticale “X Triticosecale Wittmack”.

2 Materials and Methods

The experiment was conducted on field at the experimental station of the Crops Regional Research Center Beja, North-West of Tunisia. Sewage sludge (SS) was collected from an urban sewage treatment plant (STP) located in Beja. SS parameters (i.e. metal contents and pathogens) were in the standard Tunisian range (NT106.2 2002). The treatments included three rates of SS (6, 12 and 18 t/ha), mineral fertilizer A (100 kg N/ha) and unamended plots (0 t/ha) which served as control, with four replicates for a final total of 20 plots. After 15 days of sewage sludge stabilization, soil samples were collected in triplicate (0–20, 20–40 cm) for some physico-chemical analysis in the beginning of triticale cycle. For plant growth parameters (shoot length) and biomass determination, one linear meter was selected randomly, from each treatment, and hand harvested at 60 and 90 days after sowing. The collected samples at final harvest were analyzed for nutrients (N and P) and trace metals (Cd, Cu, Zn). All physico-chemical analyses of soil and plants were carried out in accordance with standard methods.

3 Results and Discussion

The soil pH decreased but conductivity (EC) increased under the different sewage sludge treatments. This variation was significant at 12 and 18 t/ha of sewage sludge application compared to control (Table 1). Singh and Agrawal (2010) also found a reduction in pH and an increase of EC due to SS addition at different rates in the soil. The organic matter, P (P2O5) and K (K2O) contents of soil enhanced due to SS amendments with a significant increase in the presence of the highest rate of SS3 (18 t/ha). However no significant variation was observed after chemical fertilization (A).

Table 1 Effect of different sewage sludge treatments on some physico-chemical characteristics of soil
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Shoot length and plant biomass increased significantly at all SS treatments compared to control (Fig. 1). Maximum increase was observed at SS3 (18 t/ha) at 60 and 90 days after sowing. Increments in these parameters at different SS rates may be attributed to the availability of nutrients through sewage sludge. Indeed, the results showed that phosphor content in plants increased significantly after 60 days of sowing with the highest rate of sewage sludge: 18 t/ha. This result is in accordance with those of Mahdy et al. (2007) who reported a significant increase in total plant biomass of corn plant grown at different amendment rates of sewage sludge.

Fig. 1
figure 1

Effect of different SS treatments on shoot length and biomass of triticale before 60 (first harvest) and 90 days (second harvest) of sowing (Mean of three replicates), Bars with different letters in each group show significant difference at p < 0.05

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The N and P contents in root, shoot and grain of triticale plant at final harvest are compiled in Table 2. The results show that N and P concentrations increased with the increase of sewage sludge rates in the different organs of the triticale compared to control and significantly in the grains with 79% for N at SS3. Thus, it can be deduced that the majority of nitrogen provided from the sludge was accumulated in the above ground part of the triticale and essentially in grains. The improvement of nitrogen content after sewage sludge amendment has been widely reported in the literature (Moreno et al. 1996; Kchaou et al. 2011). These results corroborate also with those of Igoud (2001) who showed that the phosphorus content of Pinus maritima and Acacia cyanophyla increased with the increase of sewage sludge doses.

Table 2 Effect of treatments on N and P contents (%), on the root, shoot and grain of triticale at final harvest
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Heavy metal contents (Cd, Cu and Zn), at different parts of plants and at final harvest showed that their concentration were highest in roots than other plant parts (Table 3). The Cd values obtained in this work (0.11–1.46 mg/kg) are lower than the non toxic value in plants (1.9 mg/kg) implying that Cd was not present in the sewage sludge. Cu is an essential element in plants growth. However, it can cause toxic effects in stem and leaf tissues if its concentration exceeds 20 mg/kg (Borkert et al. 1998). Our results show that the highest value of Cu (11.08 mg/kg) was obtained in the roots in the presence of the highest sludge dose (18 t/ha) and which was found to be non-toxic. Although Zn presented the highest concentration in the different parts of the plant, its content remains below the toxic concentration (230 mg/kg) in the above ground part according to Borkert et al. (1998).

Table 3 Selected heavy metal concentrations in root, shoot and grain for different treatments and at final harvest (Mean of four replicates ±SE)
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4 Conclusion

Long-run experiments should be continued to evaluate the effect of using organic waste, sewage sludge, on triticale productivity and soil fertility without any environmental risk and for the development of a sustainable agriculture practice.