Deciduous vs. Coniferous Biomass from Pruning Waste in Composting: Divergent Pathways of Nutrient Release and Heavy Metal Accumulation

The sustainable management of urban pruning residues is increasingly important for promoting circular bioeconomy practices and reducing green waste disposal. This study evaluated the composting potential of pruning biomass from 18 urban tree species (11 deciduous and 7 coniferous) and assessed the effects of biomass type and feedstock ratio on compost quality. Plant residues were composted with fresh cow manure using two compost ratios: V1 (700 g manure + 100 g plant biomass) and V2 (700 g manure + 700 g plant biomass). Compost quality was evaluated through pH, macroelement (N, P, K, Ca, Mg) and total microelement (Mn, Cu, and Zn) and heavy metal (Pb, Cd, and Ni) concentrations. The composting ratio was the primary factor controlling compost properties. V1 produced alkaline composts (pH 7.96–9.42) with higher nutrient concentrations, whereas V2 generated composts with pH values closer to neutrality (7.39–9.44). Increasing the proportion of plant biomass reduced macronutrient concentrations, with total N ranging from 2.23% in V1 to 0.72% in V2, and potassium decreasing by 50–80% relative to V1. In contrast, micronutrient concentrations increased in V2, reaching 1142.7 mg·kg−1 Mn and 481.1 mg·kg−1 Zn. Total heavy metal concentrations also increased with greater biomass incorporation, although values generally remained low; the highest Ni concentration (15.77 mg·kg−1) was recorded in Paulownia tomentosa compost. Species-specific responses were observed for nutrient release and metal accumulation, highlighting differences between deciduous and coniferous feedstocks. The findings demonstrate that compost quality is primarily determined by the manure-to-biomass ratio, while tree species influence nutrient dynamics and trace element accumulation. Composts with higher manure proportions are suitable for nutrient enrichment and acidic soil amelioration, whereas biomass-rich composts provide enhanced micronutrient contents but require monitoring of heavy metal accumulation.