To determine if long-term equilibration may alleviate molybdenum toxicity, earthworms, enchytraeids, collembolans and four plant species were exposed to three soils freshly spiked with Na₂MoO₄.2H₂O and equilibrated for 6 or 11 months in the field with free drainage. Total Mo concentrations in soil decreased by leaching, most (up to 98%) in sandy soil and less (54–62%) in silty and clayey soils. Changes in residual Mo toxicity with time were inconclusive in sandy soil. In the other two soils, toxicity of residual total Mo was significantly reduced after 11 months equilibration with a median 5.5-fold increase in ED50s. Mo fixation in soil, i.e. the decrease of soil solution Mo concentrations at equivalent residual total soil Mo, was maximally a factor of 2.1 only. This experiment shows natural attenuation of molybdate ecotoxicity under field conditions is related to leaching of excess Mo and other ions as well as to slow ageing reactions.

The herbicide terbuthylazine is widely used within the EU; however, its frequent detection in surface and groundwater, together with its intrinsic toxicological properties, may pose a risk both for human and environmental health. Organic amendments have recently been proposed as a possible herbicide sorbent in soil, in order to limit herbicide movement from soil to water. The environmental fate of terbuthylazine depends not only in its mobility but also in its persistence. The latter is directly dependent on microbial degradation. For this reason, the effects of pine and oak residues on terbuthylazine soil microbial community functioning and on the potential of this community for terbuthylazine degradation were studied. For this purpose, degradation kinetics, soil dehydrogenase activity and the number of live bacteria were assessed in a clay loam soil treated with terbuthylazine and either amended with pine or oak wood or unamended (sterilised and non-sterilised). At day 65, 85 % of the herbicide applied still persisted in the sterile soil, 73 % in the pine-amended one and 63 % in the oak-amended and unamended ones. Pine residues increased the sorption of terbuthylazine to soil and hampered microbial degradation owing to its high terbuthylazine sorption capacity and a decrease in the bioavailability of the herbicide. On the contrary, in the presence of oak residues, the herbicide sorption did not increase significantly. The overall results confirm the active role of the soil microbial community in terbuthylazine degradation in amended and unamended soils and in a liquid enrichment culture performed using an aliquot of the same soil as the inoculum. In this clay loam soil, in the absence of amendments, the herbicide was found to be quite persistent (t ₁/₂ > 95 days), while in the enrichment culture, the same natural soil bacterial community was able to halve terbuthylazine in 24 days. The high terbuthylazine persistence in this soil was presumably ascribable to its texture and in particular to the mineralogy of the clay fraction.

Bioremediation strategies, including biostimulation, exogenous bioaugmentation and autochthonous bioaugmentation, were evaluated to determine their ability to degrade petroleum hydrocarbons in two recently polluted agricultural soils, one with a clayey texture and a silty loam soil. It was hypothesized in this work that the bioavailability of the pollutant may depend on the soil type, which would determine the biodegradation rate and the correct methodology to be used. The soils were artificially contaminated with diesel fuel, and several soil–water suspension batch microcosm experiments were conducted to observe the bioremediation process. The inocula used in the experiments included an autochthonous soil consortium and an exogenous consortium that had been acclimated to diesel consumption. The clayey soil desorbed diesel quickly, while the silty soil, with a higher organic content, did not. Hydrocarbon availability was limited in the latter case. After 48 h of treatment, the diesel removal efficiency in the clayey soil was clearly higher than that in the silty soil. However, after 11 days, the efficiencies were similar, and more than 95% of the diesel was biodegraded in most experiments. According to the efficiency and bioavailability analyses, the best methodology to bioremediate the silty soil was biostimulation with the native consortium. In contrast, bioaugmentation with a combination of native and exogenous consortia was chosen to treat the clayey soil. The results of this study suggest that when pollutants are easily available, bioaugmentation can successfully remediate the pollution. However, when availability is limited, biostimulation can be more efficient.