Ecotoxicological effects of citrus processing waste on earthworms, Lumbricus terrestris L.

Land disposal of agro-processing wastes containing potentially toxic compounds may lead to adverse ecotoxicological effects. Compared to agrochemicals, limited data are available on the ecotoxicological effects of biowastes such as those from citrus fruit processing. The objectives of the current study were:(1) to determine the effects of varying concentrations of citrus waste extract on adult mortality, fecundity and avoidance behaviour of the earthworm species Lumbricus terrestris under laboratory conditions; and (2) to determine earthworm abundance on contaminated and uncontaminated soils under field conditions. The laboratory experiment was a short-term acute toxicity test and it showed that the concentration of citrus extract had a significant (p < 0.05) negative effect on mortality, fecundity and avoidance behaviour. Exponential relationships (p < 0.001) were observed between citrus extract concentration and earthworm mortality (r2 = 0.98-0.99) and fecundity (r2 = 0.98), respectively. Tests showed that citrus waste-induced avoidance behaviour in earthworms changed by 44% to 100% compared to the control. Mean effect concentration (EC50) showed that earthworm avoidance behaviour (47%) was more sensitive to citrus waste concentrations than for number of cocoons (91%) and live earthworm counts (83%). Soil dumping of citrus waste significantly increased the concentration of mineral N, available N and exchange Ca, Mg and K but significantly reduced pH relative to the uncontaminated control. The shifts in chemical properties were about 1-2 orders of magnitude for P and Ca, double for K and approximately 30-60% for pH, N and Mg. Field data showed that the contaminated site had 80%significantly lower earthworm abundance than the uncontaminated control. Overall, the current study revealed that citrus waste has adverse ecotoxicological effects, which threatens the soil biota. Further studies should focus on elucidating the exact mechanisms and exploring the pesticidal effects of citrus extract solutions, particularly on meso- and macro-invertebrates. Investigations are also required to determine the persistence and mobility of the waste in soil and how to restore contaminated soils.