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Communication Dans Un Congrès Année : 2017

Potential use of dam sediment for soil construction in urban greening: Agronomic fertility and soil structuration

Résumé

Sediments are natural materials coming mainly from watershed soils and rocks erosion. They are composed of elementary mineral and organic particles. Each year in France, several hundred thousand cubic meters of sediments are dredged from EDF hydraulic power installations to ensure their correct operation, and returned to the river to respect the sedimentary continuity. In some cases, sediments may not be returned to water. In order to find a way to reuse the dredged sediments and to preserve natural soil resources, a research program to use sediments, and particularly fine sediments (< 2 mm) as fertile constructed soils in urban green spaces has been developed. Soils built from sediments could be at the same time a way to restore soils functions and ecosystem services in urban areas and a way to prevent natural soil resource destruction and scarcity. Then the potential fertility of such sediments, and particularly their ability to form aggregates is studied. The objective of the present study is to assess the agronomic fertility of dam sediments located in different geological environments by (i) following the early pedogenesis of constructed soil from sediment mixed or not with exogenous organic material and (ii) understanding the involved aggregation dynamics, leading to soil structuration. Four sediments contrasted in texture (from sandy loam to silty loam), mineral composition and initial organic matter content (from 29.1 to 224.3 g kg-1) were studied and compared to an agricultural loam soil used as control. The sediments were dredged from hydropower plant reservoirs, air-dried and sieved with a 40 mm screen to eliminate the coarse organic debris and stones. An in situ experiment started in July 2015 where the four sediments and the control soil were put into 50 individual 350-L containers, sown with ray grass (Lolium perenne) and placed under natural conditions over a 3-yr period. The sediments and the control soil were used alone or mixed with 40% v/v of a green waste compost. The hydraulic properties (water content and soil matric potential) of the substrates were continuously monitored using dataloggers. Moreover, after 6 (April 2016), 12 (October 2016) and 18 (April 2017) months, we measured the evolution of substrates chemical properties (pH, CEC, OM content, nutrients contents), biological properties (C-mineralization potential, microbial C biomass, plant biomass production) and physical properties (aggregate stability, bulk density, hydraulic conductivity at saturation (Ks), porosity, available water). The aggregate stability was assessed according to Le Bissonnais (1996) to distinguish three breakdown mechanisms: slaking, mechanical breakdown and microcracking and calculate the mean weight diameter (MWD (mm)) index. Studied sediments presented good initial agronomic properties that allowed plant growth. After 12 months we observed contrasted cumulated plant biomass production, depending on the sediment and on the compost addition, from 522 g DM m-2 to 1637 g DM m-2. These results are mainly explained by the different physical properties of the constructed soils and more precisely by the low aggregate stability (MWD Fourvel G. - 121 - under 1 mm whereas a stable aggregate MWD is above 2 mm) of certain sediments leading to slaking crusts and poor hydrodynamic behavior. The addition of compost in sediment changed the soil structure organization by slightly increasing the soil total porosity by 2.5 % to 13 % v/v, increasing the macroporosity (34 % v/v on average) and decreasing the microporosity (10 % v/v on average). From a chemical point of view, the addition of compost leads to a lower aerial biomass production than for pure sediments after 12 months (from 17% to 89%). These results can be mainly explained by the low nitrogen content of the mixed sediment-compost soil after 6 months (from 2.34 to 16.9 mg N kg-1) and after 12 months (from 0.2 to 11.16 mg N kg-1), leading to nitrogen organization by the microbial communities to the detriment of plant nutrition. Nevertheless, the development of roots is important for all the modalities, which suggests a plant growth potential for next months. In conclusion, the four studied sediments have contrasted aggregation and plant biomass production capacities but all of them showed a potential use for urban greening of high interest. Results on sediment aggregation showed that some sediments are already mature with strong aggregates that resist to external aggressions and that other sediments with lower maturity should be improved with treatment, such as high organic matter content material addition. In situ experiment gave interesting results about aggregation dynamics but the main factors favoring sediment structuration (addition of a high organic matter content material, wetting-drying cycles and microbial community dynamics) need to be investigated during a controlled conditions experiment to understand more accurately the aggregation processes in such constructed soils. In parallel, the remaining 18 months of the in situ experiments should confirm over time physical properties improvement or conservation and they will allow to assess the impact of the structuration on the hydrodynamic behavior of constructed soils (soil water retention and aeration, and soil water drainage).
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Dates et versions

hal-02516107 , version 1 (23-03-2020)

Identifiants

  • HAL Id : hal-02516107 , version 1

Citer

Gaëtan Fourvel, Laure Vidal Beaudet, Agathe Le Bocq, Violaine V. Brochier, Patrice Cannavo. Potential use of dam sediment for soil construction in urban greening: Agronomic fertility and soil structuration. SUITMA 9 The 9th International conference of Soils of Urban, Industrial, Traffic, Mining and military Areas, May 2017, Moscou, Russia. pp.120-121. ⟨hal-02516107⟩
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