Differential responses of nitrifying and denitrifying microbial communities to EDTA remediation in heavy metal-contaminated versus non-contaminated soils

Heavy metals (HM) are among the most important pollutants in soils worldwide. Soil washing with ethylenediaminetetraacetate (EDTA) is an effective method for removing Pb, Zn, and Cd from contaminated soil. This study evaluated the impact of EDTA remediation on N-cycling microbial community through a mesocosm experiment. Remediated and untreated (original) soils with varying HM levels were studied, including two heavily contaminated soils from Austria and Slovenia differing in pH, and additionally non-contaminated soil (CONT) with low natural geogenic HM concentrations. Influence of plant addition ( Lolium perenne ) and soil inoculum (grassland rhizosphere) on soil microbial community was examined in combination with remediation. EDTA remediation significantly reduced soil Pb, Cd, and Zn content. However, the process led to a decline in microbial community abundances in contaminated soils, especially from the Slovenian site, likely due to the release of high amounts of HM during EDTA washing. In contrast, no such effect was observed in non-contaminated soil. Site specific changes in N cycling gene abundances after EDTA treatment were observed in both contaminated soils. Increase of pH as a result of remediation coincided with increased abundance of bacterial over archaeal ammonia oxidizers. Decreased nosZI / nosZII and ( nirK + nirS )/( nosZI + nosZII ) ratios in soils after remediation process indicate lower genetic potential for N 2 O emissions in remediated than original soil while no such shift was observed for control soil. Notably, the addition of soil inoculum negatively impacted microbial community abundance regardless of soil type. Overall, our results highlight that although EDTA remediation poses challenges for microbial survival, combining remediation with plant-based strategies can foster ecosystem recovery and enhance the long-term sustainability of remediated soils. • EDTA washing did not affect total bacteria or archaea abundances in non-contaminated control soil, but altered them in contaminated soils. • Microbial recovery was driven by remediation-induced soil properties changes rather than metal release during remediation process. • Plants promoted microbial recovery, while inoculation with semi-natural grassland rhizosphere soil inhibited it.