## 3 Results and Discussion
### Effect of Treated Sludge on Yield, Composition and Nutrient Uptake by White Clover  
The aboveground and total (aboveground plus root) biomass yields of white clover were significantly affected by treatment, in both soils. In the acid soil treatments, the highest total and aboveground biomass yields were observed in air-dried sludge treatment (AcD), the latter also being similar to that of treatments AcB and AcV (Figure 1a). Moreover, acid soil treatments with bentonite-, vermiculite- and biochar-treated sludge (AcB, AcV, AcBc) had similar aboveground and total biomass yields, which were higher than the control but also higher than the AcL treatment and inorganically-fertil treatments (AcIF, AcIFX). Specifically, aboveground biomass yields were significantly increased by 233, 150, 140 and 117% in AcD, AcV, AcB and AcBc treatments, respectively, compared to the control. The order of treatments regarding the root biomass yield was AcD > AcB > AcB > AcIFX > AcV > AcC > AcIF > AcL. The beneficial effect of treated sludge with the clay minerals or biochar on the yield of white clover was attributed to the improvement in the soil's physical and chemical properties due to the supply of soils with organic matter and nutrients, as was also reported in studies with crops other than white clover [45,46]. Moreover, the slight increase in the pH (>5.0) of acid soil upon the addition of the aforementioned treated sludge cannot be excluded as a beneficial factor, which resulted in the better growth and, consequently, the obtained (positive) response of white clover plants to the particular treatments. This can also justify the fact that no response of white clover was observed for the inorganically-fertil treatments (AcIF, AcIFX) (pH < 5.0), and also for the lined sludge treatment (AcL) (pH = 8.4). Furthermore, similar increases in the aboveground biomass of white clover, to those obtained in the present study, have been reported for plants grown in an acid soil of pH 6.1 under different rates of P-K fertilization [20]. The concentrations of macronutrients in the aboveground biomass of the plants grown in acid soil treatments ranged between 19.9 and 43.8, 2.2 and 3.7, 5.6 and 42.9, 6.4 and 30.5 and 2.0 and 3.3 g kg\({}^{-1}\) for N, P, K, Ca and Mg, respectively. Comparing these concentrations with those reported in the literature [19,47], it was evidenced that plant N, P, K and Mg contents were within the common range in all cases, whereas the Ca content was low in all cases, except for the lined sludge treatment (AcL). In each acid soil treatment, the uptake of N, P and K by white clover plants followed the same pattern as the aboveground biomass yield. Specifically, the uptake of these macronutrients by plants grown in the treatments with bentonite- (AcB), vermiculite- (AcV) or biochar (AcBc)-treated or untreated (AcD) sludge was significantly higher than all other treatments and the control (Figure 2a). This was attributed again to the pH effect, meaning that the initial strongly acidic soil pH slightly increased, establishing better growing conditions for white clover, while sludge supplied soil with nutrients [1]. In the case of the lined sludge treatment (AcL), the pH increase to the strongly alkaline range probably depressed P uptake leading to insufficient development of the roots (the lowest root biomass among all treatments) and inevitably creating an inability for nutrient absorption (e.g., N, K). Therefore, the lined sludge treatment on the acid soil (AcL) delivered a lower aboveground dry biomass yield by 32% compared to the control (Figure 1a). The latter agrees with white clover's sensitivity to P and K deficiency [12]. Although a higher uptake by plants grown in the inorganically-fertil treatments was expected compared with that observed, since readily available forms of macronutrients were applied, the uptake was similar to the control in all cases of the macronutrients, probably because of the low soil pH (Figure 2a).