Soil microbial nitrogen cycling and nitrous oxide emissions from urban afforestation in the New York City Afforestation Project

The establishment of stands of trees in previously unforested areas (afforestation) is active in cities around the world. Given the complex and often degraded state of urban soils, there is great interest in soil biological processes that support plant growth but may also produce greenhouse gases in these new forests. We measured soil to atmosphere fluxes of nitrous oxide (N₂O) in order to determine how the presence/absence of shrubs and compost in urban afforestation site preparation affects the emission of this potent greenhouse gas. To complement the measurement of N₂O flux, microbial biomass carbon (C) and nitrogen (N), potential net N mineralization and nitrification, microbial respiration, and soil inorganic N were measured in experimentally afforested plots in New York City, USA. Results suggest that afforestation with shrubs and trees stimulates smaller fluxes of N₂O from soils than afforestation without shrubs and trees. The range of N₂O flux observed from recently afforested plots was -0.031-0.641 ng N cm^-2 h^-1. There were no significant differences in N₂O fluxes and microbial biomass C between sites with shrubs and/or one-time application of compost. The results suggest that afforestation efforts to create natural vegetation structure (i.e. canopy trees with understory plants) and foster a functional microbial community through additions of organic matter may not increase emissions of N₂O to the atmosphere. Rather, this method of afforestation site preparation may tighten C and N cycles and leave N₂O emissions in these urban ecosystems unchanged.