Urban afforestation and Nitrogen cycling in New York City parklands

Urban forests provide invaluable ecosystems services that regulate climate, provide habitat and improve human well-being. Worldwide, cities are promoting urban afforestation to enhance these ecosystem services that are essential to ecological resilience and urban sustainability. Urban forests are supported by soil processes mediated by microbial communities that transform organic matter into critical nutrients for plant uptake. Nitrogen (N) is a limiting nutrient in forest ecosystems, but its role in nutrient cycling processes in urban afforestation projects is poorly understood. We analyzed the impact of afforestation success and tree species diversity on microbially-mediated carbon (C) and N cycle processes in 10 experimental afforestation sites established in municipal parklands (2009-2011) as part of the MillionTreesNYC initiative. In 2018, soil cores were collected at each site up to 1 m for measurements of microbial biomass C and N content and respiration, potential net N mineralization and nitrification, denitrification potential, soil inorganic N, and total soil N. After nine years from initial planting, afforestation success varied markedly, with some sites having developed a closed canopy and leaf litter layer (high afforestation success) and some that did not (low afforestation success). We found that C and N cycling was higher in sites with high afforestation success. The difference in N cycling between high and low afforestation success appears to be driven by soil physical (e.g. texture, water availability) and chemical (e.g. pH, base cation exchange) properties that influence plant community establishment and development. These findings suggest that plant-soil feedbacks play a critical role in the success of afforestation in urban forests.