Intermediate proportions of composted sewage sludge improve Cedrela fissilis seedling growth and quality

Abstract Substrate composition and irrigation management influence forest seedling quality, but the proportion of composted sewage sludge (CSS) suitable for Cedrela fissilis remains unclear. We evaluated seedlings grown in five substrate formulations containing 0, 25, 50, 75, or 100% CSS under three operational irrigation schedules in a randomized complete block design. Height and leaf number were assessed at four evaluation times, whereas final stem diameter, biomass, H/SD, and DQI were analyzed using factorial ANOVA. CSS proportion was additionally evaluated using linear and quadratic models. Substrate effects on leaf number became detectable from 40 days onward, whereas the effect on height was evident at 120 days; substrate formulation also strongly affected shoot, root, and total dry mass at harvest. Relative to the commercial-substrate reference, final height increased by 44.7, 30.1, 49.3, and 28.5% with 25, 50, 75, and 100% CSS, respectively; final leaf number increased by 22.8, 22.2, 28.9, and 12.9%. Total dry mass reached 3.31 and 3.32 g with 25 and 50% CSS, respectively, representing increases of approximately 201 and 202% relative to the commercial-substrate reference. DQI was significantly higher in the 25 and 50% CSS formulations than in the commercial-substrate reference. Quadratic models estimated response maxima at approximately 61% CSS for final height, 59% for final leaf number, and 52% for total dry mass. Irrigation schedule did not significantly affect final height or total dry mass but strongly affected final stem diameter, with greater diameter under WS3 than under WS1 and WS2. Irrigation schedule significantly affected root dry mass, with lower marginal values under the longer-pulse schedules, while the shift toward shoot-dominated biomass allocation under WS3 was driven mainly by the commercial-substrate reference; simple allocation effects were not detected within CSS-containing formulations. Thus, intermediate CSS proportions improved C. fissilis seedling performance under the conditions evaluated here. Operational application should therefore remain conditional on batch-specific chemical and microbiological characterization, compliance with the applicable regulatory or product-registration framework, and validation across additional CSS batches, seed sources, nursery environments, and post-planting conditions.