Soil carbon and carbon/nitrogen ratio change under tree canopy, tall grass, and turf grass areas of urban green space

Soils in urban green spaces are an important carbon (C) store, but urban soils with a high carbon to nitrogen (C/N) ratio can also buffer N eutrophication from fertilizer use or atmospheric deposition. The influence of vegetation management practices on soil C cycling and C/N ratios in urban green spaces is largely unknown. In 2013, we collected replicate (n = 3) soil samples from tree canopy, tall grass, and short turf grass areas (n = 3) at four random plot locations (n = 4) established in 13 golf courses (n = 13). At each sample point, soil was separated into 0- to 0.1-, 0.1- to 0.2-, and 0.2- to 0.3-m depths (total n = 1404). Linear mixed models investigated the relationships between soil properties, vegetation attributes, and green space age. Tree canopy soil was less compacted (1.07 g cm^-3) than grassy areas (1.32 g cm^-3). Similarly, tree canopy soil had mean C/N ratios of 17.2, as compared with between 14.2 and 15.3 in grassy areas. Soil properties in tree canopy areas were best explained by tree basal area and understory vegetation volume. Soil C/N increased with increasing understory vegetation, and the difference in soil C/N between tree canopy and short turf grass areas increased over time. The soil properties in tree canopy areas of urban green space mean they can increasingly buffer the localized use of N fertilizers and atmospheric N deposition. Managers of urban green spaces concerned about N pollution of groundwater and waterways could consider planting trees in suitable topographic locations and promoting understory vegetation and surface litter accumulation.