13C and 15N natural abundances of urban soils and herbaceous vegetation in Karlsruhe, Germany

We undertook what we believe to be a unique survey of the natural abundances of C-13 and N-15 in urban soils and plants in Karlsruhe (Germany), a European city of average size. We found broad patterns of these abundances in both soils and plants, which reflected geology and land use. In contrast with studies on smaller areas (showing the direct effect of human activities), our study first determined the extent to which the abundances correlated with land use or underlying geology and then assessed how we could further test such relationships. The spatial pattern of delta(13)C in surface soil correlated with that of the underlying parent material; construction activities superimposed a secondary signal. Maize cultivation was a source of less negative soil delta(13)C, whereas the C-3 vegetation is a source of more negative soil delta(13)C. There was a footprint of less negative plant delta(13)C in the industrial and port areas; plant delta(13)C downwind of the city was less negative than upwind, which might relate to atmospheric pollution from the port area or to differences in soil properties. There was no significant effect of wind direction or geology on soil or plant delta(15)N, which was correlated mainly with land use. The largest soil delta(15)N was under agriculture and the smallest under woodland. The abundance of N-15 in inner-urban soil and plants was intermediate between those of agriculture and forests. This study represents a major advance in the use of stable isotope geochemistry in understanding urban environments.