Early Growth Responses of Raphanus sativus and Triticum aestivum to Primary-Sludge Biochar Applied as an Acidity Amendment in Two Acidic Cambisols

Primary-sludge biochar applied at liming-equivalent rates produced clear but soil- and species-dependent early responses in two acidic Cambisols. While germination metrics captured only part of the amendment effects, elongation-based indicators—particularly NRE and NSE—were more sensitive to subtle constraints on early development, revealing shifts in radicle and shoot growth that were not detectable through germination alone. This partial decoupling between emergence and elongation highlights the value of integrating multiple bioassay endpoints when screening biochars derived from industrial residues, especially in soils with different buffering capacities and ionic environments. The 170-day incubation period substantially attenuated short-lived inhibitory effects typically associated with soluble or volatile fractions, suggesting that the observed responses reflect more stabilized amendment–soil interactions rather than transient shock effects. Comparison with a dolomite reference indicated that the sludge-derived biochar contributed to acidity amelioration under the controlled conditions of this study, although responses varied markedly with soil background, dose, and species sensitivity. As an exploratory assessment, these findings suggest that combined germination and elongation metrics provide a useful framework for identifying soil-specific dose ranges that minimize early phytotoxicity risks while supporting circular strategies for sludge valorization. Further research involving longer growth periods, additional soil types, and field conditions is needed to refine safe and effective application guidelines.