Biochar-biofertilizer combinations enhance growth and nutrient uptake in silver maple grown in an urban soil

Declining tree health status due to pollutant impacts and nutrient imbalance is widespread in urban forests; however, chemical fertilizer use is increasingly avoided to reduce eutrophication impacts. Biochar (pyrolyzed organic waste) has been advocated as an alternative soil amendment, but biochar alone generally reduces plant N availability. The combination of biochar and either organic forms of N or Plant Growth Promoting Microbes (PGPMs) as biofertilizers may address these challenges. We examined the effects of two wood biochar types with Bacillus velezensis and an inactivated yeast (IY) biofertilizer in a three-month factorial greenhouse experiment with Acer saccharinum L. (silver maple) saplings grown in a representative urban soil. All treatments combining biochars with biofertilizers significantly increased sapling growth, with up to a 91% increase in biomass relative to controls. Growth and physiological responses were closely related to nutrient uptake patterns, with nutrient vector analyses indicating that combined biochar and biofertilizer treatments effectively addressed nutrient limitations of both macronutrients (N, P, K, Mg, Ca), and micronutrients (B, Fe, Mn, Mo, Na, S, and Zn). Biochar-biofertilizer treatments also reduced foliar concentrations of Cu, suggesting potential to mitigate toxic metal impacts common in urban forestry. We conclude that selected combinations of biochar and biofertilizers have substantial promise to address common soil limitations to tree performance in urban settings.