Mohamed A. Khalil

Analysis of separate and combined effects of heavy metals on the growth of Aporrectodea caliginosa (Oligochaeta; Annelida), using the toxic unit approach

Abstract The assessment of polluted soils is often made difficult by the presence of a mixture of toxicants. In this study we try to analyse the effects of mixtures of three heavy metals, cadmium, copper and zinc, on the growth of earthworms, Aporrectodea caliginosa. Laboratory toxicity tests were conducted in which worms were exposed, over a period of 6 weeks, to sulphate salts of metals mixed into a natural soil. EC50s and EC10s were estimated for each metal separately following a log-logistic curve fitting of growth versus exposure concentration. Cadmium was the most toxic metal followed by copper and zinc; relative toxicities on a molar basis were approximately Cd:Cu:Zn = 1:2:10. Based on single metal EC50s, a mixture of one-third of the concentrations of the three metals was prepared to cover the whole effect range and the effects of the mixture were evaluated against the toxic units derived from the separate metal tests. The EC50 of the mixture was not significantly different from unity (1.33 toxic units). It may be concluded that the effects of mixtures of metals on the growth of earthworms can be described by relative addition of concentrations, although there was a tendency for antagonism.

Effects of the herbicide atrazine on Entomobrya musatica (Collembola) in field and laboratory experiments

Abstract Field and laboratory toxicity tests were carried out, to evaluate the effects of atrazine on population abundance, distribution pattern, fecundity and instar duration of Entomobrya musatica (Collembola). In field experiments atrazine treatments at doses of 1.6 kg a.i. ha−1, 4.0 kg a.i. ha−1 and 6.3 kg a.i. ha−1 were adopted and Collembola densities were estimated four times during 1 year after treatment. A significant effect on the abundance of Collembola in the 4.0 kg a.i. ha−1 treated plot was observed. However, at a dose of 1.6 kg a.i. ha−1, intensive vertical migration was recorded. In the plot treated with 6.3 kg a.i. ha−1, population abundance was severely decreased. In an atrazine-free plot, E. musatica showed a high tendency of less frequent spatial aggregation. The application of atrazine had a marked influence on the aggregation indices. Laboratory toxicity tests using different atrazine concentrations (0.58, 1.46 and 2.33 mg a.i. g−1 fresh weight of food) were conducted and observations on moulting, growth and egg production were made for a period of 2 months. A slight effect, at the dose of 0.58 mg a.i. g−1 fresh weight, on fecundity was observed but no significant effect of that dose on instar duration period could be detected. Fecundity and instar duration period were clearly affected as the atrazine dose was increased to 1.46 mg a.i. g−1. Moreover, at the dose 2.33 mg a.i. g−1, a marked negative effect on egg production and the duration length of instars was shown. Extrapolation of data between field populations and laboratory conditions are discussed. It is concluded that the atrazine treatment of 4.0 kg a.i. ha−1 has an adverse effect on the abundance and the development of the test species.

Effects of fungicides and biofungicides on population density and community structure of soil oribatid mites.

To compare the side-effects of chemical versus biofungicides on non-target organisms in agricultural soil, a study of population structure, spatial distribution and fecundity of oribatid mites, a diverse and species-rich group of microarthropods indicative of decomposer activity in soil was done. Plots laid out in agricultural fields of a research station in Egypt, were cultivated with cucumber and treated with two chemical fungicides: Ridomil Plus 50% wp (active ingredients=metalaxyl and copper oxychloride) and Dithane M-45 (active ingredient=mancozeb), and two biofungicides: Plant Guard (containing the antagonistic fungus Trichoderma harzianum) and Polyversum (containing the fungi-parasitic oomycete Pythium oligandrum). All treatments were done using both low-volume and high-volume spraying techniques to check whether any effects were dependent on the method of application. Oribatid mite communities were assessed from soil core samples collected during the growing season. Total abundance of oribatids was not different across the plots, but some species decreased in number, while one species increased. Species diversity and community equitability decreased with the application of chemical and biofungicides especially when using high-volume spraying. In control plots most oribatid species showed a significant degree of aggregation, which tended to decrease under fungicide treatment. Ridomil Plus, Plant Guard and Polyversum had a negative effect on the gravid/ungravid ratio of some species. Egg number averaged over the whole adult population was not directly related to the application of chemical and biofungicides but it showed a species-specific relationship with population density. In general biofungicides had a smaller effect on population size and community structure of oribatid mite species than chemical fungicides. The results indicate that biofungicides may be the preferred option when aiming to prevent side-effects on sensitive groups among the species-rich soil detritivore community.