EDGA amendment of slightly heavy metal loaded soil affects heavy metal solubility, crop growth and microbivorous nematodes but not bacteria and herbivorous nematodes

Phytoextraction of heavy metals is a promising technology to remediate slightly and moderately contaminated soils. To enhance crops' uptake of heavy metals, chelates such as EDGA are being tested as soil additives. Heavy metal loaded EDGA can affect soil organisms such as bacteria and nematodes in various ways: directly via the soil solution surrounding the organisms and indirectly by changing the approachability, amount and quality of specific food items for nematodes and bacteria. In a pot experiment with various crops growing in slightly polluted acid sandy soil (pH 4.5, 2 mg Cd and 200 mg Zn kg−1 soil), Cd and Zn loaded EDGA in the soil solution did not affect herbivorous nematodes but did strongly reduce the increase in bacterivorous nematodes. Moreover, while the crop-stimulated increase in numbers of bacterivorous nematodes dropped, the measured amounts and the growth of their food (bacteria) were not reduced. This differential effect of the EDGA addition occurred under moderate (grass) and strong (lupine and yellow mustard) stimulation of bacterivorous nematodes by the crops, and of moderate (grass, yellow mustard) and no (lupine) stimulation of herbivorous nematodes. We assume that EDGA addition did not increase the load of bacteria with adsorbed heavy metals. Probably the bacterivorous nematodes were inhibited to feed by the high concentration of heavy metal-complexed EDGA in the soil solution, also surrounding their prey (bacteria). Although EDGA addition to the soil stimulated uptake of heavy metals by the crops, it decreased heavy metal concentrations in the roots. Herbivorous nematodes were therefore not negatively affected by the EDGA addition to the soil. Fungivorous nematodes were negatively affected by EDGA addition, probably due to increased heavy metal concentrations in the fungal hyphae. Thus, EDGA can have significant side effects on the functioning of the soil organisms, and a thorough analysis of trophic relationships among soil organisms is needed to understand these effects.