The Inhibitory Effects Of Soil CD On Rice And Environmental Safety Threshold Of Rice Origin Under Different Soil Types

The farmland soil environment was worsened increasingly because of industrial waste and the irrationality use of chemical fertilizer and pesticide. Increasing attentions has been paid to the effects of soil heavy metal on rice growth and the environmental safety threshold. Rice is one of China’s major food crops, as China’s population growing and the quality of life constantly improving, the requirements of rice quantity and quality will increase gradually. Previous studies showed that there were significant impacts of soil texture on the toxicity of heavy metal (Cd), which in turn affected the influences of soil Cd on rice growth and the critical value of environmental safety. The purposes of this study were to determine:(1) the effects of soil Cd on rice photosynthetic characteristics of flag leaf and yield component,(2) soil Cd accumulation characteristics in rice aboveground organs,(3) the changes of exogenous Cd, and (4) the cumulative characteristics of soil Cd in rice grain and the safety threshold of rice farmland under different types of soil.1. Pot experiments were conducted to investigate the effects of soil Cd on the flag leaf photosynthetic characteristics and yield components of rice in two soil types (alluvial soil and paddy soil). Compared to the control, results showed that the net photosynthesis rate (Pn) of the rice flag leaf was increased by6.4%in the3mg/kg Cd treatment in the alluvial soil, but was significantly reduced by13.8%at the7mg/kg Cd level in the paddy soil. The Pn, Gs, Tr, OPSⅡ, SPAD value of rice were slightly lower in the treatments with low Cd concentrations (1-3mg/kg) relative to the control but, were significantly reduced in the treatments with high Cd concentrations (5-7mg/kg). The inhibitory effects of Cd on the Pn, Gs, Tr, Ci, ΦPSⅡ, SPAD value were significantly greater in the paddy soil than those in the alluvial soil.2. Pot experiments were conducted to investigate the accumulation characteristics of soil Cd in aboveground organs of hybrid Indica rice in two typical soils in2008. The results showed that there were significant differences in the concentrations of Cd among different aboveground organs, following the order:plant stem> leaf> grain (P<0.05). The inhibitory effects of the heavy metal on plant height and single panicle weight were significantly greater in the paddy soil than those in the alluvial soil. In addition, the inhibitory intensity increased with the growing concentration of the heavy metals. In comparison to the control, the single panicle weight showed the largest decline by28.4%while the concentrations of added Cd reached to7mg/kg. The bioavailability of the heavy metals exhibited significant differences between the two soil types, the concentrations of Cd in various rice organs were significantly higher in the paddy soil than those in the alluvial soil (P<0.05).3. Cd contents of various forms were significantly different in the same type soil. The content of Cd forms basically showed:Carbonate-bound>Fe-Mn oxide bound> exchangeable bound> residual bound> organic matter bound in different Cd levels under the alluvial soil, while Fe-Mn oxide bound> exchangeable bound> Carbonate-bound> organic matter bound in the paddy soil, the residual bound trend was inconsistent. Exchangeable bound content was much lower in alluvial soil than that in paddy soil, but Carbonate-bound and Fe-Mn oxide bound contents were much higher than those in pappy soil.4. Pot experiments were conducted to investigate the accumulation characteristics of heavy metals (Cd) in rice grain and the environmental safety threshold. The results showed that the content of heavy metal in the rice grain increased basically with the growing concentration of the heavy metal. The cumulative characteristics of Cd in rice grain indicated significant differences between the two types of soil. The contents of Cd in rice grain were significantly higher in the paddy soil than in the alluvial soil. Basing on food safety and the cumulative characteristics of heavy metal in rice grain, we could calculate the safety threshold of rice farmland (Alluvial soil and Paddy soil):Cd:1.63and0.74mg/kg, which were higher than national standards (HJ332-2006).