Biodiversity of ciliated protozoa in soil ecosystems and assessment of their potential as bio-indicators of soil quality.

Abstract

The wide range of goods and services provided by terrestrial ecosystems are dependent on soil function. There is an increasing requirement for policy to monitor soils to improve agricultural productivity and protect soil quality. Soil biota, including protists, have fundamental roles in the services provided by soils, and have been considered as potential effective indicators to assess soil quality. Ciliated protozoa are abundant phagotrophic microorganisms in soil and play important roles in food webs by controlling smaller microorganisms and recycling organic matter. They can also be affected by soil properties, presenting a potential for their use as bio- indicators of soil quality. In the present study, a comparison of the abundance and species richness of soil ciliates and soil properties in the natural and agricultural soils was carried out in order to establish the relationships between soil ciliates and soil properties, and to investigate the potential of soil ciliates as bio-indicators of soil quality. Both field and pot based experiments were used and a wide range of methods was employed to determine soil properties. The abundance and species richness of soil ciliates were determined after 4 days and 10 days of incubation using the non-flooded petri dish method. The results showed that there were significant differences in soil properties amongst the soils investigated and there was a significantly higher abundance of soil ciliates in the natural soils. The seasonal variation in soil ciliate abundance was also clearly seen in the natural habitat. However, there were no significant differences in the total species richness of soil ciliates amongst the three ecosystems investigated. Three potential new species of ciliates were recorded, and one of them is fully described here at morphological and molecular levels. Soil ciliate abundance and total species richness of soil ciliates in the three ecosystems together had positive correlations with soil moisture, soil organic matter, available concentrations of phosphorus, ammonium, the available concentrations of Cu, Ni and Zn, total microbial activity and activity of β-glycosidase, but a negative correlation between ciliate abundance and soil pH was found. No correlations between the abundance of soil ciliates and the available concentrations of K and S and activity of acid-phosphomonoesterase in the three ecosystems together were found. Interestingly, strong negative correlations between ciliate species richness and available concentrations of measured trace metals were shown in the natural soil only. Copper at the highest treatment concentration (960 mg kg-1) decreased significantly the total microbial activity, but stimulated the abundance of soil ciliates after 4 days of incubation due to increased numbers of two species, Homalogastra setosa and Chilodonella uncinata. Similarly, insecticide cypermethrin increased the abundance of soil ciliates, but Homalogastra setosa started to be limited at concentration of 320 mg kg-1. This species was also affected by glyphosate concentrations of 16.82 and 33.6 mg kg-1. The structure of the soil ciliate community changed at the highest concentrations of all pollutants i.e., Cu, cypermethrin and glyphosate at 15 days after application. Overall, this research shows that the abundance and species richness of soil ciliates had strong correlations with soil properties. Hence, they have been shown to be potential bio-indicators for the presence of nutrients, soil pH, soil texture, and contamination from trace metals and pesticides.