Spatial patterns of δ13C and δ15N in the urban topsoil of Gent, Belgium

Urban ecosystems are characterized by a diverse land use pattern, strongly influenced by anthropogenic activities. The aim of this study was to assess whether the spatial distribution of δ15N and δ13C signatures in soil and plants could be reflected in patterns of functional urban land use. The study area (81.5 km2) was the city of Gent in Belgium. In the study area 200 sample locations were identified and the δ15N and δ13C signature of soil (0–5 cm) and grass samples was measured. The spatial structure of the isotopic distribution has been investigated using simple kriging with land use as complete categorical secondary information. The δ15Nsoil and δ15Ngrass data varied between −1.5‰ to 11.7‰ and −4.0‰ to 16.2‰, respectively. The δ13Csoil and δ13Cgrass data varied between −30.7‰ to −12.0‰ and −33.8‰ to −19.9‰, respectively. Despite the overlapping standard deviations, the average δ15Nsoil, δ15Ngrass, δ13Csoil, δ13Cgrass data showed a clear land use dependent pattern. The δ15Nsoil values showed the highest continuity with land use. The agricultural areas showed the highest δ15Nsoil data, which could be attributed to a more open N cycle and the loss of 15N depleted N species or the use of 15N enriched organic fertilizers. The wet grasslands of the nature reserves also showed higher δ15Nsoil data, probably caused by enhanced denitrification losses. Urban greens showed the lowest δ15Nsoil data, which could be explained by the absence of fertilizer input and the presence of N2-fixing species (clover). The historic city centre and the residential areas showed average δ15Nsoil data. The δ15Ngrass correspond very well to the δ15Nsoil data, but the δ15Ngrass data were on average 1.1‰ depleted and showed a more random distribution. The δ13Csoil data indicated the introduction of maize (C4 plant) in agriculture, causing an increase of the δ13Csoil values of the agricultural zone of the study area. The wet grasslands of the nature reserves showed the lowest C3-signals in δ13Csoil. The harbor area and the historic city centre and some isolated industrial sites showed the highest δ13Csoil data. These high values can be explained through the introduction of carbonate-rich materials from, e.g. constructions material, metallurgic activities, use of carbonate rich soils for the construction works and bulk C deposition (e.g. coal dust). It could be concluded that the stable isotopic composition of the topsoil and grass showed a moderate to strong relationship with land use of the studied urban ecosystem.