Abstract
PurposeUrban greening is politically fostered as an adaptation strategy to climate change. Therefore, the demand for fertile planting substrates increases. Such substrates are usually mixed from mined geogenic resources but should rather be produced from recycled materials. Furthermore, their hydraulic properties should be designed according to their application, e.g., by optimizing the mixing ratio of their components. Therefore, this study introduces an approach to investigate the water retention curves (WRC) of soil-like substrates as a function of the mixing ratio of two recycled components: exemplarily for green waste compost (GWC) and ground bricks (GB) in the fraction of sand.Materials and methodsSeven mixing ratios for GWC and GB, 0/100, 18/82, 28/72, 37/63, 47/53, 68/32, and 100/0 have been packed to mixture-specific densities using a newly constructed packing device. The packing density resulted from applying six strokes with a constant momentum of 5.62 × 10−3 N s m−2 that was chosen according to the German green roof guideline. Thus, a standardized compaction was assured. The WRCs were measured using the simplified evaporation method in five replicates for each of the seven mixtures. A set of water retention models was parameterized and analyzed in regard to their suitability to represent the full range of binary mixtures.Results and discussionThe newly constructed packing device enables to pack cylinders reproducibly. The densities in the cylinders for the mixtures varied from 0.64 g cm−3 (GWC/GB = 100/0) to 1.35 g cm−3 (GWC/GB = 0/100) with a coefficient of variation less than 1.3%. The simplified evaporation method delivered homogeneous results for all five replicates of the investigated mixtures. The WRC of the seven mixtures is the result of a complex combination of the pore systems of GWC and GB. The multi-modal water retention models of Peters, Durner, and Iden are principally suitable to describe soil-like substrates that are rich in organic matter. The models PDI (van Genuchten) and PDI (Fredlund–Xing) best described the WRCs for the full range of mixing ratios according to the quality criterion RMSE.ConclusionsThe study delivers a template how to prepare and analyze soil-like substrates regarding their WRCs using the simplified evaporation method. Complemented by total porosity and measurements at pF > 4, it is a suitable method to gain high-resolution WRCs of soil-like substrates. Available water retention models are capable to describe the hydraulic behavior of binary mixtures over the full mixing ratio. Therefore, it would be possible to model the WRC of binary mixtures as a function of their mixing ratio.
Highlights
Fertile soil-like substrates, especially their water storage and supply function, play a key role for the establishment of urbanResponsible editor: Kye-Hoon John KimJ Soils Sediments (2021) 21:2118–2129 infrastructure are often made of mined parent materials, such as natural soil material, peat, lava, and pumice, destroying rural ecosystems and landscapes (Fig. 1) (Flores-Ramirez et al 2018).A promising alternative is the use of soil-like substrates that are upcycled from suitable waste material (Caron et al 2015a), generated in the cities (Nehls et al 2015)
With the aim to study the water retention curves of binary soil-like substrates as functions of their mixing ratio, this study introduces a method that allows to measure the water retention curves of a soil-like substrate formulated as a binary mixture from ground bricks (GB) and green waste compost (GWC) for broad range of mixing ratios in a comparably and reproducible way
The standardized procedure for packing of cylinders as it was developed in this study enables to find a mixture-specific density, operationally and in a highly reproducible way
Summary
A promising alternative is the use of soil-like substrates that are upcycled from suitable waste material (Caron et al 2015a), generated in the cities (Nehls et al 2015). This idea follows the concept of circular cities (Pearlmutter et al 2019). There is a need to systematically explore and predict in what way mixing ratios influence resulting properties of soil-like substrates regarding their ecosystem functions. For profound understanding of how the mixing ratio influences the hydraulic performance of a planting substrate under different conditions, it is crucial to determine full water retention curves (WRCs) for a range of mixtures
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