Water use, root activity and deep drainage within a perennial legume-grass pasture: A case study in southern inland Queensland, Australia

Abstract

Water use and depth of water extraction of leucaena (Leucaena leucocephala) and Rhodes grass (Chloris gayana) pasture, irrigated with desalinated coal seam water (a by-product of the coal seam gas industry), were monitored to provide background information on root activity, spatial and temporal water use and deep drainage over a 757-day period from August 2011 to August 2013. Methodology comprised measurement of soil water from surface to 4 m depth using 8 EnviroSCAN probes connected to dataloggers positioned within leucaena twin rows and within the Rhodes grass inter-row. Just over 581,000 individual moisture measurements were collated and are reported here. Water extraction (and by inference root activity) of leucaena and Rhodes grass showed marked seasonal fluctuation with deepest and highest water extraction occurring during the first growing season; water extraction was greatly diminished during the following drier and cooler seasons due to the negative influences of lower soil moisture contents, lower temperatures and increased defoliation on pasture growth. The highest values of deep drainage below 4 m depth occurred when high rainfall events corresponded with high soil water storage in the entire profile (0–4 m depth). Given that water usage by both leucaena and Rhodes grass was greatest in the upper layers of soil (<1.5 m), future research should focus on how the level of competitive interaction might be managed by choice of row spacing and frequency of irrigation. Further studies are needed, including: (a) physical sampling to determine the depth of active roots; (b) how defoliation affects rooting behaviours and water use of leucaena; and (c) modelling of the water and salt balances of leucaena and grass inter-row systems using data from this study, with various levels of irrigation, to investigate the risks of deep drainage over an extended climate sequence.

Keywords: Active rooting depth, agroforestry, Chloris gayana, Leucaena leucocephala, water extraction.

DOI: 10.17138/TGFT(4)129-138