Soil dynamics of Cr(VI) and responses of Portulaca oleracea L. grown in a Cr(VI)-spiked soil under different nitrogen fertilization regimes

The reduction potential of the highly toxic Cr(VI) to the inert Cr(III) in an alkaline soil was studied during a 50-day experiment with Portulaca oleracea L. grown in pots. We aimed at assessing whether our test species can be a phytoremediation candidate for Cr(VI)-contaminated soils. We measured the Cr(VI) reduction rate in soil, determined the Cr(VI) and Cr(III) concentrations in aerial and root P. oleracea tissues, and calculated the transfer coefficient (TC = metal in plant over metal in soil) and the translocation factor (TF = metal in aerial biomass over metal in roots) in order to assess Cr(VI) uptake and distribution in plant tissues, while we also studied the effect of added nitrogen in the studied parameters. We added five different Cr(VI) levels (from the unamended T-0 to the treatment of T-4 = 150 mg Cr(VI) kg^-1 soil) and also had two N levels (equivalent to 0 and 200 kg ha^-1). The results indicated that Cr in plant tissues was mainly found in its reduced form (Cr(III)) and only a minor fraction of Cr was detected in its oxidized form (Cr(VI)), with only 1.04% of plant Cr being hexavalent at T-4 with no added N and 1.30% at T-4 with added N. The main remediation mechanism was found to be that of the naturally occurring Cr(VI) reduction that effectively produced Cr(III), followed by the uptake of Cr(VI) from our test plants (at T-4 with no N, 58% of soil added Cr(VI) was reduced and 0.1% absorbed, while at T-4 with added N, 63% was reduced and only 0.4% absorbed by plant). We also found that Cr(VI) in P. oleracea tissues was mainly found in roots and relatively low Cr(VI) concentrations were found in the above-ground tissues. We concluded that P. oleracea is a tolerant plant species, especially if assisted with a sufficient level of N fertilization, although it failed to approach the threshold of being categorized as an accumulator species. However, as this is a rather preliminary experiment, before reaching more conclusive suggestions about P. oleracea as a potential phytoremediation species, further investigation is necessary in order to verify the gained results with naturally contaminated soils with Cr under field conditions.