Study On Cd And P Fractionation And Bioavailability In Farmland Soil Of The Loess Plateau

12 soil samples of the 0-20cm layer were collected from north to south on the Loess Plateau, carrying on laboratory analysis and pot experiment to study the soil Cadmium and phosphorus fractionation, their factors affecting, Cd and P distribution in wheat, the effect of adding Cd on the growth, photosynthesis parameter and fluorescence parameters of wheat, preliminary research Cd and P bioavailability. The main results showed as follows:1. Cd and P fractionation in farmland soil of the Loess PlateauThe different fractionation of P in soil distributed in an order of succession: HCl-P>Residual-P>NaHCO3-Po>NaHCO3-Pi>NaOH-Po>NaOH-Pi>H2O-P, the content of HCl-P and residual-P were the more, both accounted for 89.33% of soil total P content. The overall distribution of soil P content on the Loess Plateau was increasing from north to south.The different fractionation Cd content in soil increased from north to south varying degrees on the Loess Plateau. Planting wheat effected soil Cd distribution to a certain extent. After planting wheat the total Cd content decreased obviously, Cd fractionation changed from bound to organic matter and bound to Fe-Mn oxide Cd into bound to carbonate, bound to Fe-Mn oxide and exchangeable Cd. The result indicated planting wheat increased soil Cd activity and soil potential hazard.2. The distribution of P and Cd fractionation in the soil-wheat system after adding CdIn wheat Jointing stage, compared with the control, the P content of canopy and root decreased after adding Cd. Cd inhibited the P content and accumulated amount of canopy and root obviously. In control the Cd content in wheat was: root>canopy, after adding Cd it was canopy>root. The mechanism still needed to be further studied. The Cd accumulation amount in wheat was both canopy>root in control and adding Cd.Adding Cd maked Cd fractionation change obviously, the main performance was the content of exchangeable Cd and percentage increasing.3. The effect of adding Cd on biological characteristics, photosynthetic parameters and fluorescence Parameters of wheatAdding Cd significantly inhibited wheat growth (P<0.01), compared with control the main performance were green leaves missing, plant dwarfing, biomass decreasing, the number of tillers reducing etc.Adding Cd significantly decreased the value of SPAD (P<0.01), with the growth stage promoting inhibition gradually strengthen. The effect of adding Cd on photosynthetic rate (Pn),Intercellular CO2 concentration (Ci),stomatal conductance(Gs) and transpiration (Tr) lacked regularity in the wheat tillering, till the jointing stage, Pn, Ci, Gs and Tr were inhibited obviously after adding Cd. The result indicated the effect of adding Cd on photosynthetic parameters varied from growth stages of wheat and soil types. With the growth stage promoting inhibition gradually strengthen in all.Adding Cd maked the maximal fluorescence value obviously low (Fm), Cd showed significantly inhibition on Fm. In the wheat tiller and pre-jointing stage the minimal fluorescence(F0) value decreased obviously after adding Cd. Till jointing late inhibition gradually was weakened. The result indicated effect of Cd on Fm and F0 haved a gradual weakening trend with promoting the growth period. Cd inhibited Fv/Fm and Fv/F0 in the jointing late.4. Cd,P fractionation, physical and chemical properties of soil and bioavailability in farmland soil of the Loess PlateauThe significant correlations were existed between soil total N, total P and different forms of P, the extremely significant correlations were existed between soil total N and NaOH-Pi, NaOH-Po, HCL-P and total P. C/N ratio, pH, the sand and total P, different forms of P were negative correlation. Except NaOH-Pi and NaOH-Po, other forms of P, total P and clay had a certain degree of negative correlation. Except NaOH-Po and Residual-P, available P and other forms of P, total P were extremely significant correlations. The result of multiple and stepwise regression analysis between different forms P and available P indicate that H2O-P >NaOH-Pi>NaHCO3-Po>NaOH-Po>Residual-P>HCl-P>NaHCO3-Pi。The H2O-P was the largest contribution to available P among different forms P; the H2O-P had the highest bioavailability.The significant correlations were existed between soil total N, available P and Cd bound to carbonate, The extremely significant correlations were existed between soil total N, available P and Cd bound to Fe-Mn oxide and total Cd, Silt and Cd Fe-Mn oxide were significantly positive correlated, and the sand, clay and Cd bound to carbonate, Cd bound to Fe-Mn oxide, total Cd were negative correlation. Correlation analysis showed that the accumulation amount of Cd in wheat canopy and Cd of various forms, the total Cd content were no significant correlation. Accumulation amount of Cd in root, the total accumulation amount of wheat Cd and soil total Cd, Cd of bound to Fe-Mn oxide were significantly correlated, Regression analysis showed on the Loess Plateau mainly farmland soils Fe-Mn oxide Cd had the higher bioavailability.