Evaluation of irrigated tree crops for land disposal of municipal effluent at Wodonga.  H.T.L. Stewart, P. Hopmans, D.W. Flinn, T.J. Hillman and J. Collopy.  April 1988  Tech. Rep No. 7, Albury-Wodonga Development Corporation, Albury, N.S.W.  28 pp. + Tables + App.

In the Murray-Darling Basin, irrigation of tree crops is being evaluated as a method of land disposal of municipal effluent. A 4-year study was carried out from 1980-84 at Wodonga. Seven tree species were established in a replicated experiment, and sprinkler-irrigated with effluent at a rate calculated each week from an assumed crop factor and on-site measurements of pan evaporation and rainfall. Annual irrigation ranged from 1191-1752 mm due to variations in climate. In the third and fourth years, around 15 MI ha^-1 yr^-1 of effluent was applied to land by irrigating the tree crops.

Effective weed control and frequent irrigation resulted in good survival of all species (range 86-100%) at 12 months. Growth varied significantly between species: by age 4 years, mean dominant height of flooded gum (Eucalyptus grandis), Sydney blue gum (E. saligna) and poplar clone 65/31 (Populus deltoides x P. nigra) ranged from 14.3-15.0 m compared with 6.6-9.8 m for river she-oak (Casuarina cunninghamiana), river red gum (E. camaldulensis), poplar clone 70/51 (P. deltoides) and radiata pine (Pinus radiata). The fast-growing eucalypts (flooded gum and Sydney blue gum) produced a mean annual volume increment of around 32 m^-1 ha^-1 yr over the 4-year period, nearly three- fold that of the other two native species, and twice the volume production of radiata pine. Mean annual increment of the semi-evergreen poplar clone 65/31 was double that of poplar clone 70/51 (18.9 vs 9.4 m³ ha^-1 yr ).

Total productivity was estimated by measuring biomass of each species inclusive of litter and roots to a soil depth of 80 cm. In the above-ground biomass, percentage leaf weight of the high-yielding species was small (8-9%) compared with the relatively slow-growing river she-oak (25%) and radiata pine (29%). Poplars had the greatest proportion of roots, consistent with these being the most wind-stable of the species planted. The annual biomass increment of irrigated flooded gum was double that of 5-year old plantations of this species on rainfed sites in New South Wales.

Accumulation of nutrients in the total biomass differed significantly between species and ranged from 34-54 g m^-2 (340-540 kg ha^-1) for nitrogen, 4.0-10.4 g m^-2 for phosphorus, 2.1-12.2 g m^-2 for sodium, 22-34 g m^-2 for potassium, 12-61 g m^-2 for calcium and 4.7-9.3 g m^-2 for magnesium. Two of the slower-growing species (river she-oak and river red gum), because of their relatively large crown and litter masses, accumulated more nitrogen, phosphorus, potassium and calcium than flooded gum or Sydney blue gum.

Chemical properties of soils (0-150 cm) were measured in 1980 and again in 1984. Irrigation significantly increased pH (by around 1 unit), but reduced salinity, throughout the profile. Levels of total phosphorus, and exchangeable sodium, calcium and magnesium were increased in the upper profile. Overall, soil chemical properties were not adversely affected by effluent irrigation.

Nutrient accumulation in soil occurred mainly in the 0-35 cm depth, coinciding with the main root zone of the trees. Renovation of the effluent was therefore estimated as the amount of each nutrient accumulated in the biomass (averaged over the seven species) plus soil (0-35 cm), expressed as a percentage of amount applied in irrigation over the 4 years; that is, nitrogen, 29%; phosphorus, 78%; sodium, 15%; potassium, 26%; calcium, 98%; and magnesium, 54%. In view of the successful tree establishment and fast growth under irrigation, together with considerable accumulation of nutrients applied in effluent and no adverse effects on soils, it is concluded that land disposal of municipal effluent by irrigating tree crops is feasible on the Albury-Wodonga floodplain.