Enhancement of neem growth and wood quality via humic acid and seaweed extract application under treated wastewater irrigation in arid environments
Wastewater-irrigated forestry is an important strategy for sustainable water resource management, biomass production, and ecosystem restoration in arid and semi-arid regions. A field experiment was conducted from September 2023 to June 2025 in Balana, Nasr El-Nuba District, Aswan Governorate, Egypt, to investigate the effectiveness of humic acid (HA) and seaweed extract (SWE) as biostimulants on the growth performance, physiological responses, heavy metal accumulation, and wood properties of Azadirachta indica irrigated with treated wastewater. The study followed a completely randomized block design with five treatments: control, HA at 1 and 2 mL L−1, and SWE at 1 and 2 g L−1. Measurements were recorded at 9 and 21 months after planting. Biostimulant application significantly enhanced vegetative growth, chlorophyll content, and wood quality compared with untreated plants. Seaweed extract at 2 g L−1 produced the greatest improvements in stem length by 7.7 and 11.3%, stem diameter by 35.4 and 10.7%, and chlorophyll content by 13.6 and 14.75 compared to control at 9 and 21 months, respectively. Also, it increased wood density by 10%, and reduced moisture by 9%. Whereas humic acid at 2 mL L−1 resulted in higher cellulose, hemicellulose, and holocellulose contents by 4, 6.4 and 10.2%, while reducing lignin and ash by 12.1 and 9.5% compared to control. Heavy metals (Pb, Cd, Cu, Fe, and Zn) accumulated in all plant organs, with accumulation patterns varying according to the element and biostimulant treatment. Most metals were preferentially retained in the roots, whereas Fe accumulated predominantly in the leaves, indicating different uptake and sequestration strategies. Despite continuous irrigation with treated wastewater, soil heavy metal concentrations declined over the experimental period, demonstrating the capacity of neem trees to remediate metals from contaminated soils. These findings demonstrate that integrating treated wastewater irrigation with biostimulant application can simultaneously improve tree productivity, wood quality, and environmental remediation. Seaweed extract at 2 g L−1 is recommended for maximizing tree growth and wood physical properties, while humic acid at 2 mL L−1 is preferable for improving wood chemical quality, highlighting the potential of this integrated approach for sustainable woody plantations in water-limited regions.