Phytoremediation and rhizosphere manipulation using different amendments
In two pot experiments using two different crop ryegrass (Lolium perenne) and two flax (Linum usitatissimum) varieties Viola and Elise, ryegrass decreased in the pool of heavy metals compared with bare soil using EDTA as extractant. NH4+ decreased the soil pH, increased EDTA-extractable Zn and increased the Zn uptake. Lime addition increased the pH and depressed Zn uptake. The pool of extractable EDTA was not changed by growing both of the flax varieties. Lime increased EDTA-extractable Cu and Pb significantly, but decreased the Zn, and pH increased in this order NH4+NH4++lime>NH4+>NO3-. Ammonium decreased the pH more than other treatments. In agar using Bromocresol purple indicator NH4+ increased the pH in the rhizosphere of different plants. With two different initial pH treatments (7 and 3.2) the NH4_ decreased the pH in the rhizosphere at high initial pH 7 and maintained the low pH at initial pH 3.2 to 4 against the buffer capacity. At different initial pH 4, 5, 6, 7 and 8 the ammonium decreased the high pH and maintained the low pH, but NO3- had no effect on the pH. Ammonium increased the toxicity of Zn due to pH decreases. There was no effect of both nitrogen sources NH4+ or NO3- on rhizosphere pH when applied as a foliar application. These indicated that the NH4+ can decrease the pH in the rhizosphere of plants and could play an important role in manipulation of the rhizosphere bioavailability of heavy metals. Toxicity of the three metals is Cu>Pb>Zn in this order and the crops tolerance is following this order pea>flax>barley. An agar-Hoagland nutrient solution contaminated with two soils, sewage treated soil (SBS) and galena soil (G), was used with flax as a test crop. The ammonium treatment lowered the pH in both soils, but with galena treated greater than SBS soil, this is attributed to the buffering capacity of the SBS soil. Averaged over all the concentrations the NF4+ treatments resulted in higher Zn shoot content that NO3- treatment, while in Cu shoot content nitrate was more than ammonium. The transfer factor of lead with ammonium treatment was greater than nitrate treatments at the 0.1 and 0.25% galena and the transfer factor of the Zn and Pb more than Cu in all treatments. At high initial pH 8 and high concentration of Zn and Cu barley grew well and this is attributed to immobilisation of Zn and Cu compared with low pH 5 and 6.5 where the barley plant did not survive. Ammonium lowered the high pH 8 and caused lower biomass production of barley than nitrate.