Short rotation forestry for land treatment of effluent: A lysimeter study

Land treatment of wastewater using short rotation forestry (SRF) has potential as a sustainable method for disposal of dairy-farm effluent. We compared 3 SRF species, 2 evergreen species of eucalypts (Eucalyptus nitens, E. saligna) and a deciduous willow (Salix kinuyanagi), in the land treatment of dairy-farm effluent. The trees were grown in lysimeters (1.8 m diameter, 1.0 m depth), and a bare soil treatment was used as a control. The application of dairy-farm oxidation-pond effluent totalled 218 g N/lysimeter (equivalent to 870 kg N/ha) over 2 irrigation seasons (December 1995-June 1996 and September 1996-April 1997). Effluent was applied weekly in summer at a rate of 18.9 mm/week. No effluent was applied during the winter period. The evapotranspiration (ET) rates of the trees, and the volumes and nitrogen contents of the leachates are compared for a winter period (4 weeks) and a summer period (5 weeks). The biomass accumulation and the uptake of nitrogen by the 3 tree species were also investigated. The SRF trees improved the renovation levels of dairy-farm effluent and produced biomass suitable for energy conversion. Of the 3 tree species, only the S. kinuyanagi treatments maintained leachate nitrate concentrations below the New Zealand drinking water standard of 11.3 mg NO₃/N/L throughout both the winter and summer periods. The E. nitens treatment produced significantly more oven-dry biomass (19.1 kg/tree) than the E. saligna trees (9.7 kg/tree) (P = 0.05). The S. kinuyanagi treatment had intermediate production (13.3 kg/tree) and was not significantly different from the other 2 tree species (P = 0.05). The nutrient accumulation was not significantly different among the species (P = 0.05). S. kinuyanagi was considered the best overall performer for the land treatment of dairy-farm effluent, based on the concentrations of leachate moving beyond the root-zone.