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

Land treatment of wastewater using short rotation forestry (SRF) has potent ial as a sustainable method for disposal of dairy-farm effluent. We compare d 3 SRF species, 2 evergreen species of eucalypts (Eucalyptus nitens, E. sa ligna) 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 applicatio n of dairy-farm oxidation-pond effluent totalled 218 g N/lysimeter (equival ent 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 eva potranspiration (ET) rates of the trees, and the volumes and nitrogen conte nts of the leachates are compared for a winter period (4 weeks) and a summe r 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 pro duced biomass suitable for energy conversion. Of the 3 tree species, only t he S. kinuyanagi treatments maintained leachate nitrate concentrations belo w the New Zealand drinking water standard of 11.3 mg NO3--N/L throughout bo th the winter and summer periods. The E. nitens treatment produced signific antly more oven-dry biomass (19.1 kg/tree) than the E. saligna trees (9.7 k g/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 s pecies (P = 0.05). The nutrient accumulation was not significantly differen t among the species (P = 0.05). S. kinuyanagi was considered the best overa ll performer for the land treatment of dairy-farm effluent, based on the co ncentrations of leachate moving beyond the root-zone.