Assessing the ecological quality and reuse potential of bioremediated hydrocarbon-contaminated soils

Bioremediation is increasingly recognized as a sustainable strategy for improving the environmental quality and supporting the beneficial reuse of hydrocarbon-contaminated soils. By preserving soil agronomic properties, this technique transforms bioremediated soils otherwise classified as waste into potential resources within circular-economy frameworks. However, current compliance criteria are primarily based on chemical thresholds, which do not account for ecological functionality or reuse potential in green infrastructure. In this study, we propose and preliminarily evaluate a multidisciplinary framework integrating chemical parameters, plant bioassays, and soil fauna avoidance tests to assess the ecological compatibility and potential reuse of 13 hydrocarbon-contaminated soils after bioremediation in two treatment plants in Northern Italy. All treated soils met the regulatory chemical limits established for reuse; however, the proposed framework goes beyond compliance by supporting potential reuse decisions based on ecological functionality. The framework was applied through a sequential three-phase approach: (i) phytotoxicity screening based on the germination index (GI), applying an “OR” criterion (GI > 90% in at least one test species, Lepidium sativum or Cucumis sativus); (ii) avoidance assays with Eisenia fetida and Folsomia candida based on Net Response (%); and (iii) assignment to ecological quality classes. In addition, we propose a list of potential green infrastructure applications associated with each ecological quality class. Most bioremediated soils showed no phytotoxic effects and were classified as high ecological quality. Multivariate analysis indicated that environmental variability was structured by texture, nutrient content, and pH rather than residual hydrocarbon concentrations. Hydrocarbon levels alone did not explain biological responses: E. fetida avoidance was associated with nutrient gradients and texture, while C. sativus germination was related to texture components. Microbial community composition, assessed through 16S rRNA sequencing, was strongly associated with nutrient gradients, reflecting integrated soil conditions rather than residual contamination. Overall, the results suggest that bioremediation may contribute to the improvement of soil ecological functionality beyond contaminant reduction. The proposed framework provides an ecological interpretation based on ecotoxicological endpoints and may support evidence-based reuse decisions aligned with the functional capacity of treated soils.