The effect of substrate compaction on plant water use and the implications for phytocap design specifications

Abstract

There is little experimental evidence to guide the design of substrate compaction for the optimal plant water use performance of a landfill phytocap. A glasshouse study was undertaken to address this with aim to evaluate the effect of substrate compaction on the water use of a phytocap plant community. Four levels of relative compaction (RC), i.e. the ratio of dry bulk density over the standard maximum dry bulk density, (72%, 77%, 82%, and 87%) were considered. The native tree and grass species selected were typical of an Australian phytocap plant community: Themeda triandra, Microlaena stipoides, Eucalyptus camaldulensis, Eucalyptus cladocalyx, Acacia mearnsii and Allocasuarina verticillata. Plant water use was measured by weight as the difference between planted and unplanted cores over 5 drying periods occurring through the first 6 months of plant establishment traversing winter, spring and summer. Plant water use was optimal for all species at low-intermediate RC (72%, 77% and 82%), and all species except Themeda triandra, were most negatively impacted by the highest RC of 87%. The best linear model based on Akaike’s Information Criterion included a second-order term for the continuous fixed factor ‘RC’ and the categorical fixed factor ‘species’. This model showed plant water use to be optimum at a RC of 76.5% and highlighted a wide range of RC’s (70–83%), for which plant water use is not <90% of this optimum. It also highlighted increasing plant water use-sensitivity to RC’s beyond these ranges, with a RC > 86% and a RC < 67% leading to reductions in plant water use of 20% or more. Substrate specifications are recommended to optimize phytocap plant water use within achievable RC ranges. These can be generalized beyond application to a single species or substrate texture to inform the design and quality assurance of substrate placement for future landfill phytocaps.