Nutrient Sinks in a Constructed Melaleuca Wetland Receiving Secondary Treated Effluent

Abstract

This study examined N, P and K partitioning in the sinks of a 130 m2 constructed Melaleuca wetland after receiving secondary treated sewage effluent for 21 months. The sinks examined were: 1) biomass, which was further partitioned into the harvestable above ground portion and the roots; 2) sediment; 3) gravel and; 4) the clay base. Gravel was the major nutrient storage sink, however this was a function of the high gravel particle mass (525 kg m−2) rather than high nutrient concentrations. M. alternifolia trees had the highest biomass due to high growth rates, higher planting densities, and low litterfall. M. quinquenervia trees were severely attacked by a sap sucking Heteropteron (Eucerocoris. suspectus) which stunted growth, but resulted in a cumulative litterfall mass three times that of the M. alternifolia trees. The sediment sink was strongly influenced by litterfall, with the sediment sink in the M. quinquenervia terraces storing more than twice the nutrients in the sediment sink of the M. alternifolia terraces. Because of their higher grwoth rates and above ground biomass fraction, and their potential to produce tea tree oil, M. alternifolia is most suitable for constructed wetlands incorporating a harvesting regime. Because of their high rate of transfer from the biomass to sediment sink via litterfall, M. quinquenervia is more suited to non-harvested constructed wetlands.