Soil Nematodes as Indicators of Heavy Metal Pollution: A Meta-Analysis

Nematodes have been used as bioindicators of soil quality for more than 20 years, and have been shown to have good potential for assessing the impact of heavy metal pollution on soil. They provide information about the biological condition of soil and can reveal dysfunctions linked to the presence of contaminants. In the case of contamination by multiple pollutants, bioindicators can reveal synergistic toxic effects (or “cocktail effects”) on organisms living in soil. These impacts are not revealed by the individual measurement of each pollutant. As the effects of heavy metals on nematode communities are not fully known, identifying reliable nematode-based parameters is not straightforward. Currently, knowledge gaps limit the operational use of these types of indices by soil managers. In this study, we performed a meta-analysis on the results of 37 studies from different countries to reveal general trends regarding the effect of multiple types of heavy metal pollution on soil nematode communities and indices. Based on the contamination level of each metal and using known toxicological threshold values, we defined four contamination classes to categorize soil polluted by heavy metals: normal concentration (c0), low contamination (c1), high contamination (c2), and very high (c3) contamination. The most sensitive nematode parameters, showing a strong relationship with the level of soil pollution, were the structure footprint, community footprint, abundance per trophic group (plant feeders, bacterial feeders and omnivores/predators) and taxonomic richness: all these parameters decreased with increased contamination. Our findings showed that fungal-feeding nematodes were relatively insensitive to metal contamination of soil and actually had a higher abundance in the very high contamination class (c3).