## 3 Results and Discussion
### Effect of Treated Sludge on Soil Fertility, and Chemical and Microbiological Properties
In most cases, N (NH\({}_{4}\)-N, NO\({}_{3}\)-N), P and K availability was higher in the inorganically fertilized treatments of both soils because N, P and K were applied in the forms of NH\({}_{4}\)NO\({}_{3}\) and KH\({}_{2}\)PO\({}_{4}\) solutions. Among the treatments with sludge of both soils, those with biochar-treated sludge (AcBc, AlBc) had the lowest available N content. For the acid soil, the highest NH\({}_{4}\)-N concentrations were observed in the treatments with air-dried sludge (AcD) and bentonite-treated sludge (AcB). Probably because of the presence of bentonite in the latter case, a considerable amount of NH\({}_{4}\)-N was retained as exchangeable, after its release from sludge during organic N mineralization. The treatment with the lined sludge had the highest concentration of NO\({}_{3}\)-N, probably because of the strongly alkaline pH of the particular treatment, which favors organic N mineralization-nitrification [41]. As far as the alkaline soil is concerned, the highest NH\({}_{4}\)-N concentration was obtained upon the addition of lined sludge, whereas the highest NO\({}_{3}\)-N concentration was observed in the AID treatment (Table 3). The significant increase in available P in all sludge treatments of both soils was expected (Table 3) since the sewage sludge contained considerable amounts of P [9; 22], and this is in agreement with other researchers' findings [7]. Moreover, inorganic P accounts for approximately 70-90% of total P in sewage sludge [42]. Available concentrations of K of both soils were mainly indigenous, while a small portion was attributed to sludge's K since its K content is usually very low [1]. As far as the available Ca and Mg are concerned, it is worth noting that for both soils, Ca concentration was the highest in the treatments with lined sludge (AcL, AIL) due to its Ca(OH)\({}_{2}\) content, whereas Mg concentration was the highest in the treatments with vermiculite-treated sludge (AcV, AIV) due to the interlayer Mg of the mineral (Table 3). After the pot experiment, the available concentrations of NH\({}_{4}\)-N, NO\({}_{3}\)-N and P of all treatments on both soils remained increased compared to the control, whereas those of K ranged at levels similar to the control (data not shown). The available concentrations of Cu and Zn increased in all treatments with the sludge of both soils compared to the control (Table 4) due to the considerable content of sludge [22]. As far as Zn is concerned, its availability increased 6.5-9.5 and 5.0-7.5 times upon the addition of treated or untreated sludge to the acid and alkaline soil, respectively, in comparison to the control. The highest increases were observed in the air-dried sludge treatments of both soils (Table 4). Nickel and Pb are scarcely added to soils through sewage sludge [1]; in the current study the results concerning these heavy metals were rather inconclusive (Table 4). It is worth mentioning that the available concentrations of Fe and Mn decreased or remained unchanged upon the addition of sludge to both soils (Table 4). This was attributed to the pH increase in the acid soil and to the alkaline pH in all cases of the alkaline soil since pH is the main soil factor that regulates the availability of the two micronutrients [21]. The available B of both soils significantly increased upon the addition of