Productivity, essential oil yield, and heavy metal accumulation in lemon grass (Cymbopogon flexuosus) under varied wastewater–groundwater irrigation regimes

Abstract

Huge quantities of wastewater generated from municipalities need to be disposed off at regulated rates in non-edible crops viz. aromatic and medicinal plants to avoid food chain contamination and protecting the valuable natural resources. For finding optimum loading rates, an experiment was conducted in lysimeters during 2007–2010 on a sandy loam soil taking 5 irrigation depth:cumulative pan evaporation water regimes (irrigation at 0.6, 0.8, 1.0, 1.2 and 1.5 ID:CPE) of primary treated wastewater, groundwater and their conjunctive use and studied their effects on the herbage yield, essential oil yield, accumulation of heavy metals in lemon grass (Cymbopogon flexuosus). Averaged over water quality, herbage yield, dry biomass and essential oil yield varied from 10.11 to 13.68; 3.02 to 3.99 kg m−2 and 53.6 to 70.1 mL m−2 and were 43, 32 and 30% higher at 1.0 ID:CPE compared to 0.6 ID:CPE, respectively. The yields obtained at 1.0 and 1.2 ID:CPE were at par but significantly reduced with further wetter irrigation regime of 1.5 ID:CPE. Similar yields of lemon grass were obtained at various irrigation regimes of wastewater alone or in conjunction with groundwater and on an average were significantly (16%) higher than the sole use of groundwater. Concentrations of Cd, Cr, Ni and Pb in the herb ranged from 1.54 to 1.85, 3.27 to 4.04, 4.35 to 5.58 and 3.53 to 4.46 mg kg−1, respectively at different irrigation regimes. The accumulation of heavy metals was the maximum in wastewater irrigated lemon grass which got reduced with conjunctive mode and the least with groundwater irrigation. However, heavy metal concentrations in essential oil were not influenced by the water application rates and water quality. In essential oil, Cd was in traces whereas average Cr, Ni and Pb concentrations were 0.14, 0.10 and 0.04 ppm, respectively. Heavy metal concentration both in herb and essential oil were well below the critical or permissible limit. With wastewater irrigation, there was a significant improvement in soil fertility status. Heavy metals started accumulating in soil but were well below the threshold level to reduce the crop growth. The results demonstrated that lemon grass could be successfully grown using primary treated municipal wastewater alone or in conjunction with groundwater at 1.0–1.2 ID:CPE for achieving higher crop productivity without contamination of the end product – the essential oils.