Effect of potassium nutrition of rice on rhizosphere redox status

With four soils differing in K supplying power and with four rice cultivars (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 substances 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. The results show that K application on K-deficient soils reduced the content of active reducing substances and ferrous iron in the soil, raised the soil redox potential in the rhizosphere, increased the Eh value of rice roots and lowered the content of iron in the rice plants. These effects of K varied with different rice cultivars. When no K fertilizer was applied, active reducing substances and ferrous iron in rhizosphere soils were decreased more by the rice cultivars absorbing K strongly (e.g. Shanyou 64) than by cultivars absorbing K weakly (e.g. Zhongguo 91). Therefore, the diminution of the toxic substances by K application in the weakly K-absorbing cultivars was more significant. The observation of a rhizobox separated by a nylon screen showed that appreciably more iron oxides, compared with the control, were deposited at or adjacent to the root surfaces of the rice plant supplied with K fertilizer, fully demonstrating the relationship between K nutrition and the total oxidizing power of rice plants. According to the distribution of active reducing substances and ferrous iron, the oxidizing range of the rice root extended in K application treatment a few centimeters away from the root plane. K application to rice affected the soil redox status in rhizosphere in many ways. The main effect was an increase of the oxidizing power of the rice root. As a result, the value of soil Eh was increased, the contents of active reducing substances and ferrous iron were lowered, as well as the number of oxygen consuming microorganisms.