Effects Of Nitrogen Forms On The Chemical Behavior Of Heavy Metals In The Soils Amended With Sewage Sludge And The Uptake By Plant

Land application, as a good way of sewage-sludge disposal, has been adopted in world-wide scale. However, it has also become an anxious problem that whether the heavy metals fixed temporarily in sewage sludge (SS) would be activated in the soils and absorbed by plants. The experiments of simulative incubation, adsorption/desorption and rhizobox for heavy metal uptake, as well as collection and detection of root exudates, were carried out to study the effects and mechanisms of nitrogen forms applied to the soils amended with sewage sludge on the transformation of heavy metals between fractions, the behavior of adsorption/desorption, the activation of heavy metals in the rhizosphere and the uptake by plant, which would provide agricultural production with the theoretical foundation how to take measures for reducing heavy metal uptake by plants to ensure the security of agricultural products. The results obtained from experiments are as follows:1. Effects of different N forms on the properties of soils and the fractions of heavy metals in the soils amended with sewage sludge: The results showed that the effect of dicyandiamide (DCD) on inhibition of N nitrification could last only for 4 weeks in the soils. Application of nitrate and sewage sludge would result in declining soil pH remarkably, which should be related to the nitrification of ammonium from mineralization of organic nitrogen in sewage sludge. However, in the treatments of both SS and SS+NH4+-N with addition of DCD, the changes of soil pH during 10 week's incubation tended to increasing in the anterior 4 weeks, then decreasing quickly. pH decline in the soil treated with NH4+-N was the biggest in all treatments, which should ascribed mainly to the ammonium nitrification. After applying sewage sludge, the mobile fraction of heavy metals was increased significantly with incubation period then reached a balance after 8 weeks. The NO3--N could promote further the heavy metal in sewage sludge transforming to the mobile fraction. Almost all of the heavy metals in the treatments of both SS and SS+NH/-N with addition ofDCD were passivated in the anterior 4 weeks. But after 4 weeks, the heavy metals were activated immediately again. After 8 week's incubation, the activity of Cu Zn Pb was largely enhanced in all treatments amended with sewage sludge, especially in the treatments of NH4+-N and NCV-N. There was a negative relation between soil pH and mobile fraction of heavy metals in all treatments, the mobile fraction of heavy metal in the soil was mainly controlled by soil pH. There existed a balance among of the mobile fraction, easily mobilizable fraction and occluded in Mn oxides fraction. The transformation between fractions was mainly restricted by soil pH. A lower pH is in favor of transforming easily mobilizable fraction and occluded in Mn oxides fraction into mobile fraction while the equilibrium directed to opposite when pH rised. As far as the organically bound fraction, occluded in poorly crystalline Fe oxides fraction, occluded in well crystalline Fe oxides fraction and residual fraction, they hardly changed in the course of 10 week's incubation, and kept relatively stable in the soils as they were difficult to be affected by the change of environmental conditions.2. Effects of different N forms on the behavior of Cu2+and Zn2+ adsorption-desorption in the soils amended with sewage sludge: The adsorption capacity of Cu2+ and Zn2+ in the Entisol was higher than in the semihydromorphic soil while the adsorption intensity in the semihydromorphic soil was higher than in the Entisol, which should be ascribed to the difference of main adsorption type between soils. Under the conditions of low initial concentration of adsorption solution (Cu-80ug ml-1 and Zn-60ug ml-1), the quantity of Cu2+ and Zn2+ adsorbed by the semihydromorphic soil was equal to or over that by the Entisol, but the adsorption saturation was much higher than that of Entisol. However, after initial concentration of adsorption solution higher than above value, the adsorption quantity in t...