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.

中文翻译：

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在两种组分的混合比范围内研究人造土壤样基质的保水功能

目的

从政治上讲，城市绿化是适应气候变化的战略。因此，对肥沃的种植基质的需求增加。此类基质通常是从开采的地质资源中混合而成的，而应该由回收材料制成。此外，应根据其应用设计其水力特性，例如，通过优化其组分的混合比。因此，本研究引入了一种方法来研究类土壤基质的保水曲线（WRC）与两种循环利用成分的混合比的关系：示例性地用于绿色废物堆肥（GWC）和碎砖（GB）沙子

材料和方法

使用新型包装设备将GWC和GB的七个混合比例，0 / 100、18 / 82、28 / 72、37 / 63、47 / 53、68 / 32和100/0填充为特定混合物的密度。堆积密度是根据德国绿色屋顶指南选择的六个冲程以恒定的动量5.62×10 ^-3  N s m ^-2产生的。因此，确保了标准化的压实。使用简化的蒸发方法，对七种混合物中的每种混合物进行五次重复测量，以测量WRC。参数化了一组保水模型，并就它们的适用性进行了分析，以代表所有范围的二元混合物。

结果和讨论

新构造的包装设备能够可重复地包装钢瓶。混合物在圆柱体中的密度从0.64 g cm ^-3（GWC / GB = 100/0）到1.35 g cm ^-3（GWC / GB = 0/100）不等，变异系数小于1.3％。对于所研究混合物的所有五个重复样品，简化的蒸发方法均提供了均一的结果。七种混合物的WRC是GWC和GB孔隙系统复杂结合的结果。Peters，Durner和Iden的多模式保水模型主要适用于描述富含有机物的类土壤基质。PDI（van Genuchten）和PDI（Fredlund–Xing）模型根据质量标准RMSE最好地描述了整个混合比范围内的WRC。

结论

这项研究提供了一个模板，该模板如何使用简化的蒸发方法制备和分析有关其WRC的类土壤基质。通过总孔隙率和pF> 4的测量来补充，这是一种获得土壤样基质高分辨率WRC的合适方法。可用的保水模型能够描述二元混合物在整个混合比下的水力行为。因此，有可能将二元混合物的WRC建模为其混合比的函数。