Biochar and biosolids increase tree growth and improve soil quality for urban landscapes.

Urban soil quality is often degraded and a challenging substrate for trees. This study was conducted to assess the impacts of biochar (BC), biosolids (BS), wood chips (WC), compost (COM), aerated compost tea (ACT), and a nitrogen plus potassium fertilizer (NK) for improving three typical urban soils and tree sapling growth. Across the three soil types, the most significant changes in soil properties were observed with BS and BC. Biosolids decreased soil pH and increased available N, N mineralization, and microbial respiration. Biochar increased total organic C. Increases in microbial respiration were also observed with NK, COM, and WC in only the sand soil. Leachate concentrations of dissolved organic C were greater with BS and COM, but nitrate in leachates did not differ among the treatments. The greatest and most significant increases in and growth were found with BS and BC. Tree growth was modeled from plant-available N and microbial respiration. The N content in the treatments appeared to be a strong determinant of tree growth for all treatments except BC. Nitrogen fertilizer, COM, and WC are the most common urban soil amendments and mulches in use today. This study provides evidence that BS and BC are acceptable, and possibly preferred, alternatives for improving urban soil quality and tree growth.