Land disposal of municipal effluents: importance of choosing agroforestry systems

Selection of an appropriate agroforestry system will not only extend the life of an effluent irrigated system but it also allows the disposal of wastewater in an environmentally-friendly manner. To test this hypothesis, plantations of seven agroforestry (AF) systems were raised in a two hectare experimental site at Yeppoon (Central Queensland) in June 2002. The AF systems included three species, viz Pangola grass (Digitaria decumbens; P), Ma bamboo (Dendrocalamus latiflorus; B) and Flooded gum (Eucalyptus grandis; E) that have shallow, medium and deep root system, respectively. The raised AF systems were either monoculture or in various combinations of the species and resulted in seven cropping systems (P, B, E, PB, PE, BE, PBE). The seven agroforestry systems were irrigated at 180 mm/ha/y as determined by the MEDLI (Model for Effluent Disposal using Land Irrigation). Managing the irrigation rate was found crucial for environmental sustainability of land disposal of effluents. Soil moisture monitored over two years revealed that E. grandis plots were driest but at par with Ma Bamboo + E. grandis plots closely followed by Pangola plots and other agroforestry systems. Root densities in the first 30 cm under all agroforestry systems were similar, but in 30-60cm, Pangola systems had the highest root densities. In 60-100 cm depth, eucalypt based systems produced greatest density of roots. Hence in short-rotation, agroforestry systems may perform at par with conventionally favoured monoculture tree plantations. The systems may be preferred for their additional commercial crops.