Composted sewage sludge with sugarcane bagasse as a commercial substrate for Eucalyptus urograndis seedling production

Sewage sludge can be used as a source of organic matter and nutrients, whereas sugarcane bagasse can be used as a decompaction material; by composting a mixture of the two, a low-cost substrate for forest nurseries can be obtained. This research investigated the use of composted sewage sludge with sugarcane bagasse (CSB) as a commercial substrate in nurseries to grow seedlings of the hybrid clone Eucalyptus urograndis. Several CSB treatments were evaluated in comparison with a control (no P addition) and a commercial substrate (CS). Before composting, CSB was conditioned with P to increase its final concentration: CSB+1.5, 3.0, and 4.5% triple superphosphate (TP) or reactive phosphate (RP). After 120 d, the Eucalyptus response to all eight substrates was assessed by: i) plant morphological traits (H, height; D, diameter; SB, shoot biomass; RB, root biomass; TB, total dry biomass; GCI, green color intensity; and root system quality) and ii) chemical parameters of shoots and roots. Significant differences among treatments were ascertained using an ANOVA, and variability was interpreted using principal factor analysis (PFA). The treatment with CSB+3% TP (TP3.0) exhibited statistically (p < 0.05) higher performance in regards to morphological parameters (H, D, SB, TB) and the nutrient contents of shoots and roots (N, P, Ca, Na, Mn, Zn, and Cu) than the other treatments and control. The results suggest that B and K could play a fundamental role in both the observed variability and the improved plant performance in the TP3.0 substrate. PFA also showed i) the key role of OM as the primary source/sink of some pivotal macronutrients/heavy metals and ii) the existence of important antagonistic/synergistic effects between elements as a primary driver affecting the concentration/behavior of elements in the shoot/root system. Overall, the research demonstrated that with an addition of only 3.0% TP, the CSB performance was better than the most commonly used and widespread commercial substrate in industrial forest nurseries.