Eucalypt trees may provide a more effective vegetation cover for wastewater irrigation disposal systems compared to pastures or herbaceous cover crops; due to their larger standing biomass, reported larger water use, high water use efficiency and salinity tolerance. Use of trees may reduce the amount of area dedicated to water disposal as larger water use means less area is required compared to crops. Trees may also provide alternative commodities, making it an attractive option for processing industries with suitable liquid waste streams.
This study focused on a gelatine production facility site in southeast Queensland, Australia where a waste stream with a mean EC of 6dS m-1 and sodium absorption ration (SAR) of 36 was applied to existing pastures. The aim of the production facility was to select the best vegetation system to extend on the site. A trial site composed of a selection of five vegetation covers; two species of eucalypt tree, E. tereticornis and E. moluccana planted at two separate densities, 1250 and 2500stems ha-1, and a Chloris gayana (Rhodes grass) pasture, was established on a dermosol soil and irrigated from early 2001. The individual water and salt balance components of these vegetation treatments were identified and quantified between April 2002 and January 2005. Above ground biomass production, water use, drainage, leaching rates; and soil and biomass salt accumulation was analysed for each treatment. This study hypothesised that a Eucalypt tree plantation would be the better vegetation cover as it would reduce the amount of area required for sustainable gelatine wastewater disposal compared to the Chloris gayana pasture; due to expected higher biomass production, water use, salinity tolerance and salt uptake.
At any single point in time the tree plantations maintained a greater biomass on site; however, the pasture exceeded the plantations in cumulative above ground biomass production over time. Vegetation growth peaked in all treatments following the first year of monitored irrigation. Annual production in the pasture plots reached 45t ha-1 in the first year but was only one third of this value in subsequent years. The plantations exhibited extremely fast growth and rates of biomass accumulation compared to the pasture, but after the first year of monitored irrigation they recorded a consistent decline in biomass, eventually leading to tree death. The removal of 2.5t ha-1 of salt in the pasture biomass contrasted with the return of up to 1.1t ha-1 to the soil system through leaf fall and decay in the plantations.
The water balance showed significant variations in total drainage between the treatments. Drainage volume and standing biomass were inversely proportional, reflecting the vegetation influences on the root zone moisture contents via the water loss mechanisms of evapotranspiration. With no water limitations, potential water loss was twice under the high density plantation compared to the pasture. This resulted in crop factors (a coefficient allowing comparison of treatment evapotranspiration with a generic reference crop) of up to 2.8 in the summer season, compared to 0.9 in the pasture. Data also showed drainage events and the effective leaching of salts was highly dependent on rainfall events rather than on the large volume of irrigation water applied to meet the leaching requirement. Rainfall and the leaching fraction maintained the soil salt content within a range reported to be healthy for the species studied, however salt gradually accumulated at approximately 0.2dS m-1 y-1 in the root zone of the treatments. The exchangeable sodium percentage (ESP) also increased to 25% in the plantation plots and 18% in the pasture plots; resulting in decreased hydraulic conductivities and a harmful environment for optimum plant growth.
A steady state of salt concentration was not achieved under any of the plantation covers in this study. The pasture treatment appeared to approach a level of equilibrium, which may have eventually resulted in a sustainable rate of input and output; although with significantly reduced biomass production rates due to the effects of a more saline soil profile. This study concluded that while trees have a higher water use, potentially reducing the disposal area required, a harvested Chloris gayana based pasture is more likely to result in a sustainable system than eucalypt plantations for the irrigation disposal of saline, sodic gelatine production wastewater at a site in southeast Queensland. Vegetation for use in saline/sodic wastewater disposal systems should be chosen based not only on potential water use, but also on salt dynamics, salinity tolerance, flexibility in growth performance and ease of removing biomass.