Mechanism Of Iron Plaque Effect On Phosphorus And Lead Transformation And Its Bioavailability

Constructed wetland was widely used in treating domestic sewage and controllingagricultural non-point source pollution. Wetland plant is one of the most essentialcomponents in constructed wetland system. The oxygen from atmosphere andphotosynthesis was transported to roots through aerenchyma. Micro-oxidation zoneformed in rhizosphere, leading to iron plaque formation on plant root surfaces. Formationof iron plaque not only affects the migration, transformation and absorption of mineralelements in rhizosphere, but also their utilization by plants. Therefore, it is very importantto study the effects of iron plaque on migration and bioavailability of phosphorus and leadin order to understand the mechanism of purifying water by wetland plants.The difference of iron plaque and characteristics of phosphorus adsorption wereinvestigated through sampling wetland plants under natural condition to determine ironplaque effects on root activity and growth of adventitious roots, and to examine the effectof lead and iron on soil microbial community structure. The formation of iron plaque onroot surface and its effects on utilization of phosphorus and lead by Typha latifolia andIris pseudacorus were studied under solution culture and pot experiments. The resultsshowed that the amount of iron plaque was affected by the addition of iron and lead, andthen influenced the utilization of phosphorus and lead by plants. The mechanism of ironplaque effect on bioavailability of phosphorus and lead by plants was discussed. The mainfindings are as follows:(1) The amount of iron plaque on the root surfaces varied greatly in various wetlandniches and species. For the Cyperaceae family plants, the amount of iron plaque followedthe order of Scirpus triangulates L.＞Scirpus triqueter L.＞Cyperus alternifolium L. ssp.Flabelliformis(Rottb.) Kuk. and was 6.5 times in Scirpus triangulates L.'s than Cyperusalternifolium L. ssp. Flabelliformis(Rottb.) Kuk.'s.(2) Formation and accumulation of iron plaques were inhibited by lead, butpromoted by iron. Solution culture experiment showed that higher concentration of lead caused more loss of iron plaque, and the loss of iron plaques was more when iron plaqueswere abundance. After iron was added to lead contaminated soil, the amount of ironplaque on the roots of broad-leaf cattail and yellow flag was increased with the increase ofiron addition. The amount of iron plaque on roots of broad-leaf cattail was decreased withaddition of lead, but that on root of yellow flag was not obviously influenced by theaddition of lead.(3) In solution culture experiment, the fresh weight of new adventitious roots inbroad-leaf cattail was decreased with the increase of ferrous concentration from 0 to 100mg/L, and then was increased after ferrous concentration was beyond 100 mg/L. Therewas no obvious correlation between fresh weight of adventitious roots in yellow flag andferrous concentration in solution, but a significantly negative correlation between rootactivity of broad-leaf cattail and amounts of iron plaque with a correlation coefficient of0.861. The root activity of yellow flag ranged from 2.36 to 5.48μg/g·h in this experiment.(4) The response of microbial community structure in the soil planted broad-leafcattail with the addition of iron and lead were as follows: the microbial biomasscharacterized by phospholipid fatty acid in the rhizosphere was more than that in the bulksoil. Shannon index(H) in the bulk soil was increased with lead of 500 mg Fe/kg level.The H index in the rhizosphere in treatments with 500 and 1000 mg/kg lead wassignificantly higher than that of low lead addition. Based on cluster analysis, themicrobiology community was mainly affected by lead nitrate in bulk soil, but by ferroussulfate in the rhizosphere.(5) There was significantly positive correlation between amounts of phosphorusadsorption and iron plaque on the roots surface of wetland plant planted in bothno-polluted soil and lead contaminated soil. The correlation between lead adsorption andiron plaque depended on the amounts of iron plaque.(6) Solution culture experiment showed that the small amounts of iron plaque formedon the roots surface of broad-leaf cattail would promote the phosphorus utilization,whereas, the inhibition was observed when the amount of iron plaque was 30 g/kg, exceptthe treatment of 200 mg Fe/L. Phosphorus content in over-ground parts of yellow flag wasdecreased with the formation of iron plaque. The formation of iron plaque on the roots of broad-leaf cattail could increase the content of lead in roots and over-ground at thetreatment of 250 mg Pb/L in solution. Content of lead in roots of yellow flag wasimproved by iron plaque in 10 mg Pb/L solution, and formation of iron plaque with thetreatment of ferrous less than 100 mg/L increase the content of lead in roots. Once theferrous concentration was beyond 100 mg/L, iron plaque would decrease the content oflead in roots. When the ratio of iron to lead added was 1, accumulation of phosphorus andlead in over-ground of broad-leaf cattail was more than that with other treatments, andlead content in over-ground parts of yellow flag was promoted by iron plaque. Theaccumulation of phosphorus and lead in yellow flag with the soil treatment of 100 mg/kgiron was more than that of 0 and 500 mg/kg iron, and amount of phosphorus and lead inroots was higher than that in over-ground parts.