The data includes soil microbial processes of carbon (C) and N in 10 afforestation sites in New York City as part of the MillionTreesNYC initiative (MTNYC) of the New York City Department of Parks and Recreation. Long-term research plots were established between 2009-2011 in municipal parks: Alley Pond, Canarsie, Ft. Totten, Clearview, Conference House, Clove Lakes, Marine Park (3 sites), and Pelham Bay. Sites were planted with low (two tree species) and high diversity (six tree species) treatments. More detailed description of sites and MTNYC project is provided by Downey et al. (2021). In 2018, 1 m soil cores were collected from plots at each site and analyzed for microbial biomass C and N, basal respiration, potential net N mineralization and nitrification, denitrification potential, soil inorganic N, and total soil N. Laboratory methods followed those used by Raciti et al. (2011a,b), and Groffman et al. (1999).