Spatial patterns of δ 13C and δ 15N in the urban topsoil of Gent, Belgium

Abstract

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 δ 15N and δ 13C signatures in soil and plants could be reflected in patterns of functional urban land use. The study area (81.5 km 2) was the city of Gent in Belgium. In the study area 200 sample locations were identified and the δ 15N and δ 13C 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 δ 15N soil and δ 15N grass data varied between −1.5‰ to 11.7‰ and −4.0‰ to 16.2‰, respectively. The δ 13C soil and δ 13C grass data varied between −30.7‰ to −12.0‰ and −33.8‰ to −19.9‰, respectively. Despite the overlapping standard deviations, the average δ 15N soil, δ 15N grass, δ 13C soil, δ 13C grass data showed a clear land use dependent pattern. The δ 15N soil values showed the highest continuity with land use. The agricultural areas showed the highest δ 15N soil 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 δ 15N soil data, probably caused by enhanced denitrification losses. Urban greens showed the lowest δ 15N soil data, which could be explained by the absence of fertilizer input and the presence of N 2-fixing species (clover). The historic city centre and the residential areas showed average δ 15N soil data. The δ 15N grass correspond very well to the δ 15N soil data, but the δ 15N grass data were on average 1.1‰ depleted and showed a more random distribution. The δ 13C soil data indicated the introduction of maize (C4 plant) in agriculture, causing an increase of the δ 13C soil values of the agricultural zone of the study area. The wet grasslands of the nature reserves showed the lowest C3-signals in δ 13C soil. The harbor area and the historic city centre and some isolated industrial sites showed the highest δ 13C soil 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.