DIFFERENCES IN CARBON AND NITROGEN STOCKS AND ISOTOPIC COMPOSITIONS REGARDING THE EXPOSURE TIME OF SOILS TO URBAN CONDITIONS: THE CASE OF STREET TREE-PIT SOILS FROM THE CITY OF PARIS

摘要: When "new" soils, mostly deprived of technogenic materials, are imported into urban from periurban areas, an important question is to understand how these soils will evolve when exposed to urban environmental conditions. The soils of street tree-pits in Paris could be an interesting subject for such investigations. Indeed, when the city of Paris plants a new tree, it removes all the previous soil from the pit and replaces it by a newly imported soil from peri-urban agricultural areas. Since there are no artificial nutrient inputs during the tree’s life, whilst its fallen leaves are removed from the soil, it can be expected that soil carbon and nitrogen stocks would decrease with time. Furthermore, due to specific urban isotopic signatures of atmospheric carbone and nitrogen (strongly negative d13C, strongly positive d15N), we can respectively expect a decrease and an increase of soil d13C and d15N with the aging of trees. The presentation will show the results of a campaign realized in the summer of 2011 in Paris, where the soils (at 10-30 cm and 30-40 cm depths) and leaves (4 on the lower canopy) of 79 silver lindens (Tilia tomentosa) were sampled. The sites were chosen according to three dbh (diameter at breast height) classes : 7-15 cm (n = 28) ; 32-43 cm (n = 29) ; 56-73 cm (n=22). The results will be discussed regarding our hypotheses and other upcoming analyses on soil microbial communities and their activities.