## 3 Results and Discussion
### Effect of Treated Sludge on Yield, Composition and Nutrient Uptake by White Clover
Concentrations of micronutrients in white clover plants grown in the alkaline soil treatments were either within or below usual ranges [19,47] in all cases and ranged between 11.4 and 20.3, 9.5 and 27.4, 281 and 465, 71 and 85 and 29.5 and 39.4 mg kg\({}^{-1}\), for Cu, Zn, Fe, Mn and B, respectively. In agreement with the aforementioned findings of the current study concerning plant parameters, other researchers [7, 48, 49] also reported no clear effect of sewage sludge on plant composition. However, similar beneficial results have been reported for nutrient uptake by various crops, such as grasses, legumes, wheat and sugar beet, after their fertilization with sewage sludge in the form of raw, dewatered, composted, limed etc. [7, 48, 50]. Although AMF root colonization was the highest (51%) in inorganically-P-K-fertilized treatment of the acid soil (AcIFX) (Figure 4a), this had no effect on the yield and nutrient uptake of plants. As is shown in Figure 1a, a lower aboveground biomass was observed in that particular treatment compared with the treatments with sludge, except for the limed sludge (AcL) treatment, which was also similar to the control. It has been suggested that when P availability increases, the relationship between the clover host and the AMF changes  
Figure 3: Uptake of micronutrients by white clover plants, grown in **(a)** acid soil treatments and **(b)** alkaline soil treatments. Treatment of the acid and alkaline soil, respectively, with: AcB and AlB: bentonite-treated sludge; AcV and AIV: vermiculite-treated sludge; AcB and AlB: biochar-treated sludge; AcL and AIL: limed sludge; AcD and AID: air-dried sludge; AcIF and AIIF: N-P-K inorganic fertilization; AcIFX and AIIFX: P-K inorganic fertilization; AcC and AIC: control. Different letters, within each soil and property, indicate significant differences among means, using the LSD test, at \(p\leq 0.05\). from symbiosis to parasitism [51]. The AMF colonization ranged between 32 and 36% among all other treatments with no significant differences, including the control (Figure 4a). The alkaline soil offered better conditions for AMF colonization (41-59%), possibly due to lower P availability compared to acid soil, as this was deduced from P concentration ranges in the aboveground biomass of white clover (2.2-3.7 and 1.7-2.2 g kg\({}^{-1}\), for acid and alkaline soil treatments, respectively). Nevertheless, no significant differences were observed among treatments as far as AMF root colonization is concerned (Figure 4b). According to the literature, the highest mycorrhizal fungal root colonization (60%) was reported when biochar was added along with AMF, and P fertilization was absent [15]. Although white clover is highly mycotrophic and AMF colonization is expected to promote plant growth and increase P absorption [13], the results of this study were rather poor or inconclusive, probably because the available P of all treatments of both soils (Table 3) was well above the lower limit of 10 mg kg\({}^{-1}\) of the P sufficiency range [52]. Furthermore, no visible nodulation on the roots of white clover was achieved by indigenous rhizobia; thus, no determination or analysis were conducted. This was attributed to the absence of white clover-specific rhizobia strains (e.g., _Rhizobium leguminosarum bv. trifolia_) in both original soils that could nodulate plant roots, rather than potential sludge-induced stress [53]. Figure 4: Percentages of AMF root colonization of white clover plants, grown in (**a**) acid soil treatments and (**b**) alkaline soil treatments. Treatment of the acid and alkaline soil, respectively, with: AcB and AIB: bentonite-treated sludge; AcV and AIV: vermiculite-treated sludge; AcB and AIBc: biochar-treated sludge; AcL and AIL: limed sludge; AcD and AID: air-dried sludge; AcIF and AIIF: N-P-K inorganic fertilization; AcIFX and AIIFX: P-K inorganic fertilization; AcC and AIC: control. Different letters, within each soil, indicate significant differences among means, using the LSD test, at \(p\leq 0.05\).