Plant growth and nutrient accumulation in Melaleuca quinquenervia and Cymbopogon citratus treating high strength sewage effluent in constructed wetland systems with biochar media

In constructed wetlands for wastewater treatment, plants contribute to nutrient removal. However, plant species differ in their ability to tolerate high nutrient loads and accumulate nutrients in biomass. Growth and nutrient accumulation were investigated in two subtropical/tropical plants: melaleuca trees (Melaleuca quinquenervia) and lemongrass (Cymbopogon citratus). These were planted in 240 L mesocosms containing sand media and biochar (0 to 25% by volume). The mesocosms were loaded with tertiary effluent for 4 months, secondary clarified wastewater (SCW) for 8 months and finally septage for 9 months, a total cumulative load of 1060gN m -2 and 320gP m -2. Plant growth, including stem height and girth for melaleuca and leaf regrowth for lemongrass were monitored. Growth rates were higher (0.48 cm/day stem height; 2.22 cm/day leaf regrowth) when irrigated with septage, compared to SCW (0.32 cm/day stem height; 1.14 cm/day leaf regrowth). Plants were harvested at the end of the experiment and the biomass and nutrient content in each part determined. Both melaleuca and lemongrass had very high N and P tissue content: melaleuca leaves 23 mg/gN, 2 mg/gP; roots15mg/gN, 2.1 mg/ gP; lemongrass leaves 18 mg/gN,2.2gP; roots 9.5 mg/gN, 2 mg/gP. Annual biomass and nutrient accumulation per plant was 486 g; 5.72 gN; 0.89 gP for melaleuca trees and 180 g; 2.54 gN; 0.39gP for lemongrass, a total nutrient accumulation of 33 g m -2 N and 5 g m -2 P/annum. Plant uptake accounted for 8.6%TN and 4.5%TP of load retained. Biochar amendments did not enhance growth rate nor increase N and P biomass. We postulate that effluent nutrient loads provided adequate nutrients for plant growth even in pure sand media.