Purpose Urban 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 methods Seven 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 discussion The 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. Conclusions The 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.

Purpose Topsoil and peat are often taken from intact rural ecosystems to supply the urban demand for fertile soils and soil-like substrates. One way of reducing this exploitation is to recycle suitable urban wastes to produce Technosols and technogenic soil-like substrates. In this study, we investigate the role earthworms can play in impacting the hydraulic properties of such a soil-like substrate. Materials and methods In a 4-month microcosm experiment, the influence of the earthworm species D. veneta on the hydraulic properties of brick-compost mixture was examined. Of the ten boxes filled with ca. 11 dm3 of ground bricks (0.7 cm3 cm−3) and green waste compost (0.3 cm3 cm−3), five contained earthworms (W-boxes) and the remaining five were used as controls (C-boxes). The substrate was periodically irrigated and the weight of the boxes and of the drained water was monitored. At the same time, images were taken from the front of the boxes to quantify the activity of the earthworms by image analysis and soil aggregation was studied with micrographs. Before and after the experiment, water retention curves were determined from disturbed samples of the substrate using the simplified evaporation method. Results and discussion After 6 weeks, differences between the C- and the W-boxes were evident. Micrographs showed brick-compost aggregates only for the substrates processed by earthworms. The earthworm activity leads to reduced evaporation and an increased water content in the respective microcosms. The effect persists even after disturbing the substrate. The proportion of plant-available soil water is about 0.02 cm3 cm−3 higher for the substrate processed by earthworms (0.250 ± 0.009 cm3 cm−3) compared with the control (0.230 ± 0.008 cm3 cm−3). Conclusions This study shows that earthworms are capable of ingesting and processing crushed bricks together with compost. The earthworms produced aggregates which persisted after disturbance and had a positive influence on the water retention capacity of such a soil-like substrate constructed from waste.

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Abstract. Constructed Technosols are important means to substitute natural soil material such as peat and geogenic material to be used in urban green infrastructure. One of the most important features of such soils is related to the water cycle and can be described by the soil water retention curve (WRC). The WRC depends on the composition of the constructed Technosols e.g. their components and their mixing ratio. The diversity of possible components and the infinite number of mixing ratios practically prohibit the experimental identification of the optimal composition regarding the targeted soil functions. In this study we propose a compositional model for predicting the WRC of any binary mixture based on the measured WRCs of it’s two pure components only (basic scheme) or with one additional mixture (extended scheme). The model is developed from existing methods for estimating the porosity in binary mixtures. The compositional model approach was tested for four data sets of measured WRCs for different binary mixtures taken from the literature. To assess the suitability of these mixtures for typical urban applications, the distribution of water and air in 50 cm high containers filled with the mixtures was predicted under hydrostatic conditions. The difference between the maxima of the pore-size distributions ∆PSDmax of the components indicates the applicability of the compositional approach. For binary mixtures with small ∆PSDmax, the water content deviations between the predicted and the measured WRCs range from 0.004 to 0.039 m3 m−3. For mixtures with a large ∆PSDmax, the compositional model is not applicable. The knowledge of the WRC of any mixing ratio enables the quick choice of a composition, which suits the targeted application.