Effect of Mycorrhizal Symbiosis on Biogeochemistry Changes of Alfalfa Rhizosphere under Presence of Lead Using Rhizobox System

ABSTRACT: Following the increasing expansion of industry sector and using chemical fertilizers and pesticides in agriculture sector there has been an increase in the amount of heavy metal and semimetal in soil which has turned into a serious problems and issue as those substances are risks to human and cattle health and endanger plant growth (reduction in biomass) with negative impacts on soil microorganisms. In order to assess the pollution dangers of a metal in a region, considering the direct and indirect role of rhizosphere microorganisms, understanding the biogenesis aspects of that metal (that is most depended to its chemical form in the rhizosphere than its total amount in soil) is a necessity. Studying the chemical forms of metals in rhizosphere is a great help in understanding the quality of metal absorption by the plant and its fate in rhizosphere. The main goal of this research is to study chemical transformation of lead in the rhizosphere soil of alfalfa as an accumulator plant, and the possibility of increasing the biomass of this plant by using microbial potentials and inoculation of local microorganisms (Mycorrhiza) fungus In this study, two samples of soil with high lead pollution and low lead pollution were taken from different places of Zanjan City. Alfalfa was planted in those soils with two treatments of Mycorrhiza inoculation and no Mycorrhiza inoculation (only containing local Mycorrhiza of the region soil) using Rhizobox system. Alfalfa which has ability to heavy metals absorption was planted in the Rhizobox containing soil treatments Samples were taken from rhizosphere soil in different distances from the root and the chemical forms of heavy elements in the soil samples were determined. the concentration of heavy metals was measured in roots and shoots of alfalfa . In addition, the percent of spore and colonization of treatments were studied. Results showed that rhizosphere fungi inoculation even in Pb pollution soils can Increase the alfalfa growth (shoots and roots). the concentration and absorption of lead in alfalfa root increased but in the shoots, the concentration and total absorption of lead decreased. This means that fungi inoculation significantly increased the weight of root in comparison to shoots. The colonization percent (coexistence) and spore percent in treatments with fungi inoculation was significantly more than treatments with no fungi inoculation. Reduction of soil pH due to fungi inoculation in different levels of pollution was observed. The chemical forms of lead in alfalfa rhizosphere were very different than the soil mass. The different levels of pollution had significant effects on carbonate, oxide, organic leads in the rhizosphere and bulk soil; The presence of Mycorrhiza affected significantly on labile, carbonate and oxide forms of lead in rhizospheric soil and in bulk soil, no significant effects were observed. Pollution treatment affected significantly on oxide and carbonate Pb in bulk soil and on oxide, carbonate and labile lead in rhizospheric soil. According to Pb accumulation in alfalfa roots this plant can be used for Pb polluted soils phytoremediation while its shoots can be use for animals feed safely.