The pH of volcanic grassland soils (3.8 and 4.9) was adjusted to 6.5 and 7.0 by liming with calcium carbonate, respectively. 1) Mineralization of C and N from the treated soils was accelerated after liming and the degree of mineralization during 20 days was larger in the soil with adequate application of lime (pH7.0) than in the soil adjusted to pH6.5. 2) Soil microbial biomass C decreased during the 2 day period after liming and then gradually reached the level of the control soil within the following 2 weeks. 3) Fungal biomass C which was determined by a microscopic method decreased significantly immediately after liming in spite of the constant level of fungal biomass C in the control soil. On the other hand, bacterial biomass C in the control soil increased from the early stage of incubation in treated and control soils. 4) On the basis of these results, it is concluded that (a) a considerable portion of fungal biomass was destroyed by liming and (b) soil respiration and nitrogen mineralization were promoted due to the subsequent decomposition of the cytoplasm and cell walls of the dead fungi by the bacteria.

View full abstract

The number of nodules formed by the soybean cv. Bonminori in combination with Bradyrhizobium japonicum strain A1017 was very low compared with other cultivars, Enrei, Nanbushirome, Okushirome and Suzukari. In the current experiment, attempts were made to determine the mechanism underlying low capacity of nodule development in the soybean cv. Bonminori which possesses the Rj gene. The attachment of the bacteria to the root hairs, curling of root hairs, infection thread formation and cortical cell divisions were investigated by comparing these characteristics in the soybean cv. Bonminori and Enrei in combination with the B. japonicum strains A1017 and IRj2101. 1. The number of nodules formed by Bonminori inoculated with the strain A1017 was less than two per plant, although the acetylene reduction activity of the individual nodules was almost identical with that of the control. 2. There was no difference in the attachment of the strain A1017 to root hairs between Bonminori andEnrei. 3. The frequencies of root hair curling and infection thread formation in Bonminori induced by the strain A1017 did not differ from those by the strain IRj2101. 4. The number of primary nodule meristems in the tap roots of Bonminori inoculated with the strain A1017 was lower than that of the strain IRj2101. 5. The results of the observations suggest that the low compatibility of Bonminori with the B. japonicum strain A1017 is due to an abnormality in the development at a relatively early stage of cell division prior to the formation of nodule meristems rather than to the inhibition of the attachment of the rhizobia to the root hair and induction of root hair curling.

View full abstract

Rice straw, partially decomposed straw, and cellulose were incorporated uniformly in the reduced layer of a paddy soil at 0.1% and 1% on a dry soil basis in a pot experiment. Populations of heat-resistant anaerobic N_2-fixing bacteria (HANB) were estimated by the anaerobic roll tube method using a modified nitrogen-free Clostridium medium. Diluted soil suspension was heated at 80℃ for 10min or 60℃ for 30min before inoculation to the medium. In the soil treated with straw at 1%, the number of well-grown colonies on the medium increased slightly compared with the control during the 20day period after incorporation. The well-grown colonies were likely to be mainly N_2-fixing Clostridium based on the ARA assay and microscopic observation of some isolates, medium components, and heat-treatment of the inocula. In the other treated soils, the populations of HANB were similar to those of the control. On the other hand, the populations of heterotrophic anaerobic bacteria in the soils treated with partially decomposed straw or cellulose at 1% increased to about three-fold of the value of the control. The increase in the populations of heterotrophic anaerobes in the soils was not due to an increase of the number of N_2-fixing Clostridium in the soils. These results suggest a possibility of minor-contribution of N_2-fixing Clostridium to the decomposition or utilization of incorporated cellulose or straw in the reduced layer of paddy soil.

View full abstract