Soil Carbon and Carbon/Nitrogen Ratio Change under Tree Canopy, Tall Grass, and Turf Grass Areas of Urban Green Space

Abstract

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. Core Ideas We sampled soil under tree canopy, tall grass, and short grass in 13 urban greenspaces. Tree canopy soil had significantly greater soil C density and C/N ratios. Soil C/N ratios increased significantly with increasing green space age. Soil C and C/N were best modeled by tree basal area and understory vegetation. Tree canopy areas can be managed to buffer nitrogen eutrophication.

Vegetation Type and Age Drive Changes in Soil Properties, Nitrogen, and Carbon Sequestration in Urban Parks under Cold Climate

“…Our second hypothesis, predicting that park age affects the biochemistry of the soil, was also supported. In particular, young park soils contained lower concentrations of OM, tot-C, and tot-N. Our results are also in agreement with previous findings that the accumulation of C in park soils diminishes with soil depth (Golubiewski, 2006;Raciti et al, 2011;Edmondson et al, 2014a,b;Bae and Ryu, 2015;Livesley et al, 2016). However, as the stratification of these soil variables did not relate to park age, our third hypothesis is not supported.…”

“…For example, Bae and Ryu (2015) observed that mixed forests had the highest SOC stocks, followed by evergreen and broadleaf forests, and lawn in an urban park in Seoul, Republic of Korea. Similarly, in a study of 13 urban green spaces (golf courses) in Melbourne, Australia, Livesley et al (2016) reported that tree-canopy areas had higher concentrations of C and N than nearby grassy areas. These studies suggest that woody plants can enhance C sequestration in urban soils.…”

“…Like in most ecosystems, C and N also accumulate in urban soils (Pouyat et al, 2006(Pouyat et al, , 2010 and the accumulation rate reflects the vegetation and its management in the greenspace (see Raciti et al, 2012;Edmondson et al, 2014a,b;Livesley et al, 2016). However, the extent to which plant functional groups can modify urban soils and their C and N stocks is far less known.…”

“…However, when subjected to minor disturbances, e.g., construction of residential gardens or urban green spaces, urban soils can sequester even more carbon than native soils (Pouyat et al, 2002Qian and Follett, 2002;Kaye et al, 2005;Frank et al, 2006;Golubiewski, 2006;Raciti et al, 2011;Edmondson et al, 2014a). This high C and N storage is typical of intensively managed turf grass (i.e., lawn), but soils under urban trees also store C and other soil elements (Edmondson et al, 2014a,b;Bae and Ryu, 2015;Livesley et al, 2016). Indeed, theory predicts and evidence shows that the type and biomass of natural and semi-natural vegetation control soil formation, its OM content, plus the composition and activity of the below-ground food web (Wardle, 2002;Ponge, 2003;Bardgett and Wardle, 2010).…”

“…Little information is available on the impacts of vegetation or park age, other than for grasses/lawns, on C and N sequestration in urban greenspace soils. Some observational studies suggest that soil C content is not directly correlated with tree age in public parks (Edmondson et al, 2014b) or park age itself (Livesley et al, 2016), emphasizing the need for rigorous, well-designed studies of both vegetation and the age of urban greenspace on soil properties.…”