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.Fine sediment accumulates upstream of hydroelectric dams. To ensure that dams can operate properly, part of the sediment has to be dredged and land managed. In parallel, using topsoil from agricultural parcels for urban greening or land restoration is currently controversial because arable surface areas are decreasing. An alternative idea for protecting these natural resources consists in reusing fine dredged sediment to construct multifunctional soils. This agronomic use is only possible if sediment can provide acceptable physical and chemical properties for plant growth.
Materials and methodsFour dredged sediments with contrasted initial agronomic properties and one control soil were mixed or not with green waste compost (40% v/v) and used to construct triplicate 30-cm depth soils in lysimetric containers (1.11?×?0.71 m). The 30 constructed soils were exposed to the in situ conditions and sown with ryegrass (Lolium perenne). The evolution of soil chemical and physical properties and plant development were studied every 6 months for 18 months.
Results and discussionAbove- and below-ground biomass production of the constructed soils contrasted according to the sediment properties and to compost addition. A statistical approach identified eight soil parameters linked to biomass production. Among these parameters, soil structure, quantified from aggregate stability, played a fundamental role. A focus on physical properties confirmed that some sediments were only partially adapted to ryegrass support. Compost addition improved sediment physical properties over time, but caused temporary N deficiency during the first months after installation which limited shoot biomass production. Exogenous plant species developed on the constructed Technosols, especially on the soils where the lack of structure and N deficiency had the strongest effect.
ConclusionsAll sediments were suitable for plant growth over the 18 months of the study. A few soil properties emerged as markers of the fertility of sediment-made Technosols. Among them, the soil structure was one of the most determining parameters. It can be assessed by measuring aggregate stability, macroporosity, the crustability index, and bulk density, while available nutrients (N, P, K) and pH seem sufficient to assess chemical fertility. The balance between the properties of the sediment-made Technosols and the needs of the plants seems to be an essential lever for the establishment of functional soil-plant systems for urban greening or for ecological restoration.
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