EFFECT OF POTASSIUM NUTRITION OF RICE ON RHIZOSPHERE REDOX STATUS

With four soils differing in K supplying power and with four rice cult ivars (Oryza sativa L.) differing in K uptake kinetic parameters, the relationship between K fertilizer application and soil redox status in rhizosphere and; the distribution of ferrous iron and other toxic sub stances on the root surface and in the rhizosphere; and the effect of K supply on uptake of reduced iron by rice plants have been studied. T he results show that K application on K-deficient soils reduced the co ntent of active reducing substances and ferrous iron in the soil, rais ed the soil redox potential in the rhizosphere, increased the Eh value of rice roots and lowered the content of iron in the rice plants. The se effects of K varied with different rice cultivars. When no K fertil izer was applied, active reducing substances and ferrous iron in rhizo sphere soils were decreased more by the rice cultivars absorbing K str ongly (e.g. Shanyou 64) than by cultivars absorbing K weakly (e.g. Zho ngguo 91). Therefore, the diminution of the toxic substances by K appl ication in the weakly K-absorbing cultivars was more significant. The observation of a rhizobox separated by a nylon screen showed that appr eciably more iron oxides, compared with the control, were deposited at or adjacent to the root surfaces of the rice plant supplied with K fe rtilizer, fully demonstrating the relationship between K nutrition and the total oxidizing power of rice plants. According to the distributi on of active reducing substances and ferrous iron, the oxidizing range of the rice root extended in K application treatment a few centimeter s away from the root plane. K application to rice affected the soil re dox status in rhizosphere in many ways. The main effect was an increas e of the oxidizing power of the rice root. As a result, the value of s oil Eh was increased, the contents of active reducing substances and f errous iron were lowered, as well as the number of oxygen consuming mi croorganisms.