Spatial patterns of delta C-13 and delta N-15 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 delta N-15 and delta C-13 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 delta N-15 and delta C-13 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 delta N-15(soil) and delta N-15(grass) data varied between -1.5 parts per thousand to 11.7 parts per thousand and -4.0 parts per thousand to 16.2 parts per thousand, respectively. The delta C-13(soil) and delta C-13(grass) data varied between -30.7 parts per thousand to -12.0 parts per thousand and -33.8 parts per thousand to -19.9 parts per thousand, respectively. Despite the overlapping standard deviations, the average delta N-15(soil), delta N-15(grass), delta C-13(soil), delta C-13(grass) data showed a clear land use dependent pattern. The delta N-15(soil), values showed the highest continuity with land use. The agricultural areas showed the highest delta N-15(soil) data, which could be attributed to a more open N cycle and the loss of N-15 depleted N species or the use of N-15 enriched organic fertilizers. The wet grasslands of the nature reserves also showed higher delta N-15(soil) data, probably caused by enhanced denitrification losses. Urban greens showed the lowest delta N-15(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 delta N-15(soil) data. The delta N-15(grass) correspond very well to the delta N-15(soil) data, but the delta N-15(grass) data were on average 1.1 parts per thousand depleted and showed a more random distribution. The delta C-13(soil) data indicated the introduction of maize (C4 plant) in agriculture, causing an increase of the delta C-13(soil) values of the agricultural zone of the study area. The wet grasslands of the nature reserves showed the lowest C3-signals in delta C-13(soil). The harbor area and the historic city centre and some isolated industrial sites showed the highest delta C-13(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. (c) 2006 Elsevier Ltd. All rights reserved.