## 3 Results and Discussion
### Effect of Treated Sludge on Soil Fertility, and Chemical and Microbiological Properties
variable negative charge, due to deprotonation of structural and functional hydroxyls [39]. This agrees with the fact that the highest CEC increase was observed in the lined sludge treatment (AcL), which had a strongly alkaline reaction (Table 2). The EC\({}_{\text{se}}\) of both soils significantly increased in all treatments with the sludge compared to the control (Table 2), due to sludge's water-soluble salt content. Although salinity is not inhibitory towards the growth of white clover [36], it is notable that the EC\({}_{\text{se}}\) of treatments with air-dried sludge (AcD, AlD) of both soils exceeded the critical value of 2 dS m\({}^{-1}\) for a salinity hazard to sensitive crops [21], whereas it was lower in all other cases. The same treatments also had the significantly highest SAR values (Table 2), probably because of sludge's considerable water-soluble Na. However, in all cases, no sodicity hazard was evidenced. The results concerning Kjeldahl-N were rather inconclusive, especially for the alkaline soil, whereas the treatment of the acid soil with the untreated sludge (AcD) had the highest N content. However, OC content significantly increased in most of the treatments with sludge on both soils compared to the control because of sludge's organic matter content. Both soils with biochar-treated sludge (AcBc, AlBc) had the significantly highest OC content (Table 2) because of biochar's contribution. As is reported in the literature, both the total N and OC contents of sewage sludge are not strongly affected by sludge's treatment, whereas a major loss of water-soluble nutrients is denoted [1]. The beneficial effect of treated or untreated sludge on the pH and CEC of the acid soil also persisted after the pot experiment. Unfortunately, the treatment effect on the EC\({}_{\text{se}}\) of both soils also persisted. Specifically, not only did the treatments with air-dried sludge of both soils maintain EC\({}_{\text{se}}\) above the critical value of 2 dS m\({}^{-1}\)[21], but the treatments with lined sludge of both soils and those with sludge treated with clay minerals or biochar of the alkaline soil did too. In all these cases, EC\({}_{\text{se}}\) was almost below 3 dS m\({}^{-1}\). Since the biological experiment lasted for a short period of time and sewage sludge's degradation is very slow, the Kjeldahl-N and OC contents of all soil treatments after the experiment remained at levels similar to the initial values (data not shown). The initial heavy metal load of treated and untreated sludge [9,22] was either not detectable or below prohibitive levels for the soil application of sewage sludge according to the European Council Directive 86/278/EEC [3] and the third draft of the working document on sludge of the European Commission (2000) for Cr [23]. Consequently, upon soil application of treated or untreated sludge, although total concentrations of heavy metals slightly increased, they remained, in most cases, below the critical limits of legislation [3, 23]. In acid soil treatments, the total concentrations of Cu, Zn, Ni, Cr and Pb ranged between 24 and 32, 51 and 85, 27 and 32, 10 and 13 and 24 and 28 mg kg\({}^{-1}\), respectively, while in alkaline soil treatments, the corresponding concentrations ranged between 30 and 32, 44 and 55, 161 and 173, 107 and 112 and 9 and 11 mg kg\({}^{-1}\). In all cases, Cd and Hg were not detectable. However, total Ni and Cr concentrations of the alkaline soil treatments were above the critical limits of the aforementioned legislation [3,23] due to the soil's parent material. Both treated and untreated sludge had considerable readily available nutrient contents [9,22]. Therefore, available N (NH\({}_{4}\)-N, NO\({}_{3}\)-N), P and K significantly increased in all sludge treatments of both soils, which agrees with the findings of other researchers [7,40], as well as in the inorganic fertilization treatments, compared to the control (Table 3).