Role Of Phosphate Forms On Mobilization And Accumulation Of Cd Bound To Iron Oxides By Roots Of Chinese Flowering Cabbage(Brassica Parachinensis L.)

Iron oxide is an important component of the soil and has a high ability to adsorb heavy metals. Crops make some adaptation for variation of phosphates, which not only influences mobilization of phosphates in the rhizosphere soil but may affect the bioavailability of Cd bound to iron oxides. New zealand 701 yaliuyecaixinwang, a high Cd accumulation cultivar of Chinese Flowering Cabbage( Brassica parachinensis L.), was employed as a tested crop.Quartz sand with 0.4 mg·kg-1 Cd bound to iron oxides was employed as substrate in the experiment. Fe-P, Al-P, Ca-P and KH2PO4 respectively were employed as the only phosphorus supply. The study aimed to clarify difference in uptake and mobilization of Cd bound iron oxides and variable forms of phosphate. The research aimed to provide a theoretical basis of utilizing fertilizer management to reduce Cd in crops.1)Goethite, lepidocrocite and magnetic hematite were iron oxides commonly in the soil, which were employed as research object. X-ray diffraction, field emission scanning electron microscope and specific surface area analysis technique were used in qualitative analysis and surface characteristics of three iron oxides before and after Cd adsorption. Results showed three synthetic iron oxides respectively were identified as goethite, lepidocrocite and magnetic hematite, and Cd adsorption experiment didn’t change its crystal structure.2) The results in the Cd absorption experiment among three iron oxides showed goethite had the largest amount of EDTA extractable Cd and total Cd, which followed by lepidocrocite and magnetic hematite was the least. In order to ensure the Cd content of quartz sand in the pot experiment was 0.4 mg·kg-1, 0.06 g Goethite, 1.77 g Lepidocrocite and 2.50 g Magnetic hematite relatively were needed to add in 500 g sand. Therefore, lepidocrocite was regarded as an ideal experimental material for its relatively high absorption of Cd and operability.3)Accession of K2HPO4 concentration in the rhizosphere could accelerate the growth of crops, enhance P accumulation in different parts of crops and reduce Cd uptake by the roots. Numbers of Free P in the rhizosphere might combine with Cd bound lepidocrocite in form of more stable Cd3(PO4)2 or ternary complex(lepidocrocite-phosphorus-cadmium), which reduced the proportion of free Cd and P and uptake of Cd bound lepidocrocite by plants.4)The order of utilization of sparingly soluble phosphate by Chinese Flowering Cabbages was followed as: Fe-P > Al-P > Ca-P. Compared with Al-P and Ca-P, different parts of crops in Fe-P had higher biomass, P accumulation and amount of bioavailable phosphorus in rhizosphere. Cd difference among treatments was consistent with P. While mobilizing sparingly soluble phosphate, crops released Cd from lepidocrocite. Root exudate and P released in the same time might reduce the formation of ternary complex(lepidocrocite-phosphorus-cadmium) and enhance Cd mobilization.5)With accession of K2HPO4 concentration in the rhizosphere, species and amount of root exudate became higher. Species and amount of root exudate under different sparingly soluble phosphate treatments followed the order: Fe-P >Al-P > Ca-P. Species of organic acid under different treatments were the most, and proportion of organic acids in total root exudate was more than 40%. Root exudate had a close relationship with mobilization and uptake of P and Cd bound lepidocrocite by Chinese Flowering Cabbages.Therefore, K2HPO4 would be recommended in Cd lightly contaminated farmland as phosphate fertilizer, because it reduced Cd accumulation while promoting the growth of plants.In case of accumulation of heavy metals in crops, Fe-P should be avoided as possible.