Green roof substrate physical properties differ between standard laboratory tests due to differences in compaction.

Green roofs are expanding internationally due to the well documented benefits they provide for buildings and cities. This requires transferable knowledge of the technological aspects influencing green roof design, particularly substrate properties. However, this is made difficult due to differences in substrate testing methods referred to in green roof guidelines and standards. Therefore, we tested a green roof substrate using laboratory-based methods from European (FLL), North American (ASTM) and Australian (AS) green roof guidelines and standards to determine how these methods vary in characterising substrate physical properties (bulk density, water permeability and water holding capacity at field capacity (WHC)). Further, we compared the results from the laboratory-based methods with measures of bulk density and WHC in green roof platforms to determine whether standard methods accurately represent substrate properties in-situ. Results from the standard test methods varied due to differences in sample compaction. The standard test methods that employ Proctor hammer compaction (FLL and ASTM) had greater bulk density (at field capacity and dry) and lower water permeability than Australian standard methods that employ free-fall compaction. WHC did not differ among the standard methods. The Australian standard method better reflected bulk density at field capacity and WHC of the substrate under in-situ green roof conditions. For mineral based substrates, our results suggest that for the FLL and ASTM testing methods, a single Proctor hammer drop will produce a degree of sample compaction equivalent to the free-fall method (AS) and be more representative of bulk density in-situ. Subtle changes in testing procedures would allow for more direct comparison of substrate properties between standard methods and help enable the international transfer of knowledge for substrate design.

Biochar made from low density wood has greater plant available water than biochar made from high density wood.

Soil water limitations often restrict plant growth in unirrigated agricultural, forestry and urban systems. Biochar amendment to soils can increase water retention, but not all of this additional water is necessarily available to plants. Differences in the effectiveness of biochar in ameliorating soil water limitations may be a result of differences in feedstock cell structure. Previous research has shown that feedstock cell structure influences the pore structure of biochar and consequently the volume available for water storage. The availability of this water for plant uptake will be determined by biochar pore diameters, given its role in determining capillary forces which plants must overcome to access pore water. Therefore, we hypothesized that differences in hardwood feedstock cell structure would result in differences in the plant available water holding capacity of biochar. Before pyrolysis, we measured the wood morphology of 18 Eucalyptus species on three replicates of equal age on a gradient of wood density (572-960 kg m-3). Wood samples were then pyrolysed (550 °C) and the resulting biochars were sieved and their particle size distribution was standardised before their physical properties, including water holding capacity, plant available water and bulk density were measured. Our results show that biochar made from lower density eucalypt wood had up to 35% greater water holding capacity and up to 45% greater plant available water than biochar made from higher density eucalypt wood. Further, feedstock wood density related well to fibre cell wall thickness and fibre lumen diameter. Therefore, wood density could be used as a proxy for wood cell structure, which can in turn be used to predict plant available water in biochar. The simple measure of feedstock wood density can inform feedstock choices for producing biochars with greater plant available water, optimal for the use as soil amendment in water limited environments.