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• Afforestation urban restoration sites had increased soil carbon storage compared to degraded urban soils. • High success afforestation sites had significantly larger carbon pools than low success afforestation and degraded sites. • Diversity treatments (6 versus 2 planted species) had no effect on soil carbon levels. • Inherent site characteristics had a strong influence on the success or failure of urban afforestation sites. There is great interest in the ability of afforestation programs to sequester carbon, improve soil health, and provide other ecological benefits to urban areas. However, the capacity of urban soils to support successful afforestation and sequester carbon is poorly understood. This study quantified soil carbon in a series of experimental restoration sites established between 2009 and 2011 as part of the MillionTreesNYC Afforestation Project in New York City. Soil cores (0–100 cm) were collected at 10 sites and analyzed for total carbon content. Data were analyzed with respect to depth (0–10, 10–30, 30–70, 70–90, 90–100 cm), high (six species) versus low (two species) diversity planting palettes, and afforestation success (high or low). Results were compared with data from regional reference forest, degraded urban sites in New York City, and disturbed and undisturbed sites in other cities. High success afforestation sites had significantly larger carbon pools than low success afforestation sites and degraded NYC sites. We suggest that these differences were created by interactions between initial site conditions that facilitated plant community establishment and growth, which in turn increased soil carbon accumulation. These initial site conditions include land use history that influences soil physical and chemical factors, as well as proximity to existing forest stands. Diversity treatments had no effect on soil carbon levels, but these may need a longer time period to emerge. These results suggest that afforestation may enhance the capacity of urban soils to store carbon compared to urban degraded soils, but that urban soil properties and site characteristics constrain this capacity. [ABSTRACT FROM AUTHOR]