Green roof ageing or Isolatic Technosol’s pedogenesis?

Abstract

Purpose Green roofs (GR) offer a way to improve several ecosystem services in cities. However, the performances of GR are basically considered as steady over time whereas they are living media subject to ageing that are rarely managed by their owners. This study transposes a pedological approach to evaluate changes in GR physical structure and chemical composition over time. Materials and methods A full-scale experimental plot with various vegetation cover was studied. A dedicated sampling strategy was implemented to monitor substrate’s evolution over 4 years. Then, physical and chemical characterisation (carbon and nitrogen contents, particle size distribution, porosity, soil water retention) was conducted and compared to results on the original substrate. Results and discussion The upper layer of the substrate (0 to 5 cm depth) contained a large amount of fine and short roots whereas the root density was much smaller in the lower layer of the substrate (5 to 10 cm depth). There was a global drop of the organic carbon content from 5 % in the initial substrate to 2 % in the 4-year-old substrate. On the contrary, the nitrogen concentration has increased by 0.4 % during the same period. The mesoporosity decreased drastically from 0.11 to 0.02 cm3 cm−3. On the whole substrate, the <2-mm particles fraction was smaller after 4 years (12.5 %) than in the initial substrate (18.2 %) which was especially obvious in the upper horizon (9.5 %). Additionally, the monitored properties also varied significantly as a function of soil cover (sedum, moss and bare soil). Evidences of an early pedogenesis were highlighted such as poral evolution and fine particles eluviation. Conclusions: In conclusion, the study demonstrated the effects of time, climate and vegetation on physical and chemical properties of green roof substrate. They are not only classified as Isolatic Technosols due to their composition and implementation; they also exhibit one of the major characteristic of young Technosols: a fast and intense pedogenesis.