Water flux and biomass production of native plants at different substrate compactions for landfill phytocaps in Southeast Queensland, Australia

Abstract

Phytocaps for landfills and degraded sites have multiple benefits provided by the established vegetation, including reducing substrate moisture content and drainage through evapotranspiration. One of the factors affecting plant development measured by biomass production is substrate compaction. However, the relationship between drainage and biomass production related to substrate compaction is largely unknown. In an outdoor pot experiment, four native plant species: Eucalyptus terreticornis, Acacia concurrens, Allocasuarina littoralis, and Themeda triandra, grown in coal overburden at three different relative compactions (RC) levels (64%, 77% and 87% of the maximum compaction) in Southeast Queensland, Australia, over seven months. Evapotranspiration rate was measured by change in pot weight. Drainage for each pot was collected in a container after every irrigation and rainfall. Biomass of plants were harvested for dry shoot and root weight measurement. Additionally, root length density was obtained by a root scanner. Our statistical model predicted that 69.8% RC for maximum weekly ET. Overall, 77% RC treatment resulted in higher total evapotranspiration (1.5%), more significant shoot and root biomass production (17.2% and 11.8%, respectively), higher root length density (33.4%) and lower total drainage (20%) compared to 64% and 87% RC treatments. Native woody species had a higher rate of evapotranspiration and a lower rate of drainage than the native grass. Reduction in drainage was correlated with increase in evapotranspiration rate for all plant species, as the soil moisture content decreases through evapotranspiration. These findings support the notion of maximising evapotranspiration to minimise drainage, a key objective for phytocap design. From biomass production, root biomass closely correlates to a reduction in drainage. Multiple lines of evidence from this study indicate that 77% RC treatment maximises evapotranspiration and plant development and minimises deep drainage for phytocap applications, even though harsh microclimate in this experiment may have constrained plant development and affected the water flux.