Irrigation with industrial effluent leads to mortality of coppice growth in Eucalyptus

Land disposal of effluent can be an effective and productive method of dealing with wastewater. One approach has been to use fast-growing trees in plantations to transpire large amounts of water and immobilise chemicals. The productivity of these plantations can be enhanced, and hence the rate of water use and chemical immobilisation increased, by felling trees at an early age and regenerating the plantation using coppice growth. Trials were carried out to evaluate the ability of coppice systems to dispose of effluent produced by a food processing factory in south-eastern Queensland. Plantations of Eucalyptus moluccana and E. tereticornis were irrigated with effluent from an early age-once the seedlings were clearly established. Both species grew well and reached a height of about 8 m within 33 months. At this age selected plots of trees of both species were felled and both readily produced coppice. Irrigation of the coppiced and uncut trees continued. After a further 8 months, coppice growth of both species was failing. By contrast, uncut trees remained healthy. The main causes of the failure of coppiced trees appeared to be a combination of acid soils, high soil Al concentrations and high foliar Mn concentrations. The acidification appears to have been induced by nitrification of the ammonium-N applied in the effluent irrigation, while the high Mn reflects both the low soil pH and the reducing conditions induced by waterlogging with high BOD effluent. At this time the uncut trees showed no clear visual signs of stress. There was less waterlogging in the plots with uncut trees and these had lower foliar concentrations of potentially toxic elements. The use of coppice growth in the context of wastewater disposal could potentially remove greater quantities of water and nutrients than established trees. Results here, however, suggest that the coppice growth was more susceptible to unfavourable environmental conditions than normal tree growth and this should be taken into account when choosing a management regime