We investigated the changes in microbial biomass C and N, available soil N and neutral sugar composition induced by wheat straw and manure application in four types of central Hokkaido soils. 1) Microbial biomass C and N increased by organic material applications. Microbial biomass N / total N in wheat straw application soils showed a remarkable increase compared with the other two treatment soils. 2) There were significant positive linear correlations among available soil N, microbial biomass N and hot-water extractable C. Available soil N can be estimated by the value of hot-water extractable C. 3) Neutral sugar C increased by organic material applications, and especially, xylose C increased remarkedly by wheat straw application. Neutral sugar compositions were different among three treatments and four soil types. 4) There were significant positive linear correlations between available soil N, microbial biomass N and neutral sugar C. Moreover, microbial biomass N / total N showed a significant positive correlation with xylose C / mannose C. These results explain that the ratio of microbial biomass N in total N is affected by the neutral sugar composition in soil.

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We investigated the changes in microbial biomass C and N, available soil N, hot-water extractable C and neutral sugar composition induced by the application of rice straw compost and fertilizer for 29 years to alluvial upland soil in Saitama Prefecture. 1) Long-term application of rice straw compost increased total C and N, microbial biomass C and N, available soil N and hot-water extractable C. Particularly, remarkable increases of microbial biomass N and its ratio in total N were observed in non-mineral fertilizer N application plots where soil pH was high. In addition, there was a negative correlation between soil mineral N content and microbial biomass N. 2) There was a positive correlation between hot-water extractable C content and available soil N content. Except for non-lime plots, the increase in correlation coefficient was much higher. 3) Long-term application of rice straw compost increased the amount of neutral sugar C, particularly xylose C, which is believed to originate from plant material. The ratio of xylose/mannose was higher in rice straw compost application plots than in no-application plots, and lower in no-fertilizer and no-nitrogen plots in both rice straw compost application and no-application plots as compared to other treatment plots. 4) The amount of neutral sugar C was correlated positively with that of available soil N. Particularly, the amount of xylose C showed the highest correlation coefficient with available soil N. Thus, humus components derived from plant material in soil are important as substrates of soil microbial biomass contributing to the production process of available soil N.

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Characteristics of micro-topographic changes and soil physico-chemical properties accompanied with the formation of parallel cattle tracks were investigated in sloping pasture lands. 1) Where the grassland with a slope in the southeast direction became barren from cattle tracking, the surface soil was pushed to a lower position. In some instances, the soil accumulated in small mounds, leaving depressions on the uphill side. 2) The formation of mounds caused an increase in the height and steepness of the slope. This increase made it difficult for cattle to cross the mound. Thus, the cattle used only the existing tracks, thereby further exacerbating the problem. 3) The acreage ratios of the mounds, which were almost covered with heliophyte weeds, and cattle tracks, which were barren, ranged from 14 to 37% of the total pasture land area. This led to a lowering of the grazing capacity of the pasture land. 4) As the soil in the cattle tracks was compressed, coarse-size pore volume (CP) and easily-available water-holding pore volume (EA) of the soil in the cattle tracks decreased to less than those of the original slope. Thus, the permeability of soil in the cattle tracks was low, and the cattle tracks became waterways when it rained. This caused soil erosion. On the other hand, bulk density of the soil was less, and CP and EA of the soil at mounds were larger than those of the original slope. 5) Nitrogen content, which can be extracted with hot water, and the available phosphorus content of soil in the cattle tracks were higher than those of the soil mounds. 6) The soil physico-chemical properties mentioned above are related to the formation of cattle tracks.

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The results obtained were as follows. 1) Among 36 forest soil samples of four different soil types, the amounts of organic and inorganic soil components studied which are responsible for proton consumption were quite variable, even within those samples from the same soil type. 2) According to a cluster analysis using nine variates of Ex-Ca, Total-C, Pyr-Al, Pyr-Fe, Ox-Si, Am-Al, Am-Fe, Ex-Al and BS (base saturation degree), five clusters of soil samples were differentiated, where each bluster reflected the difference in the nature of parent materials and the value of BS. Namely, cluster 1 (C1) comprised those samples derived from basic and ultra-basic rocks, cluster 2 (C2) from volcanic ashes of mafic composition, cluster 3 (C3) from acidic rocks, cluster 4 (C4) from volcanic ashes of felsic nature, and cluster 5 (C5) comprised 3 other samples. C1 and C2 samples showed BS of more than 30%, while C3 and C4 less than that. 3) By the use of principal component analysis, components 1 and 2 extracted from the covariance matrix of four variates of Pyr-Al, Ox-Si, Am-Fe and BS, explained about 90% of the variability of the 36 soil samples' matrix. Moreover, the "potential buffering capacity" of the soils was evaluated by the sum of points obtained from the square-root of each percentage of total variance multipied by the characteristic value of components 1 and 2. The results showed that the decreasing order of points was as follows : C5>C2>C1>C4>C3 samples. 4) By the application of a multivariate regression analysis, the variate extracted to explain the amounts of proton consumption by soil samples in each cluster were as follows : C1, Total-C and Ox-Si ; C2, Ex-Cation; C3, Ex-Cation, Pyr-Al and Ox-Si ; C4, Pyr-Al ; and for all samples, Ex-Cation, Pyr-Al and Am-Fe. 5) Among the different organic and inorganic soil components, Ex-Cation showed the most reactive component during proton consumption. However, the stepwise acid neutralizing mechanism (YOSHIDA and KAWABATA 1986) seems to be less plausible, as shown by this study, and to be rather strongly affected by the amounts and composition of each soil component responsible for proton consumption.

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Chemical forms of soluble selenium were studied in the cultivated soils (n=48) collected from Kameoka Basin, Kyoto Prefecture. Soluble selenium was extracted with a 0.1 mol L^<-1> Na_2SO_4 solution (soil : solution=1: 10) in a boiling water bath for 30 min, and was fractionated into three fractions : organically-bound selenium and inorganic seleniums (IV) and (VI). 1) The mean content of total soluble selenium was 21.8 μg kg^<-1> with a range of 7.8-75.3 μg kg^<-1>, amounting to 7.6% of the total selenium content in the soils. 2) The mean contents of soluble inorganic seleniums (IV) and (VI) were 4.8 and 1.9μg kg^<-1>, respectively. On the other hand, the soluble organically-bound selenium content was 15.0 μg kg^<-1>, which amounts to 69% of the total soluble selenium. This indicates that organically-bound selenium is the predominant form in soluble selenium. 3) Soluble selenium increased with increasing pH, especially, the correlation coefficients between pH and selenium contents in the forms of organically-bound and inorganic selenium (IV) were 0.70 and 0.80, respectively. It is believed that organic matter containing selenium and selenium (IV) adsorbed in the soils were released as pH increased. 4) Although a positive correlation was found between selenium (VI) and pH, the amount of selenium (VI) showed a slight increase with increasing pH. Most inorganic selenium (VI) in the soil was considered to be leached out from the soil because the solubility of selenium (VI) compounds is generally greater than that of inorganic selenium (IV) compounds.

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Previously, we showed results that upland rice (Oryza sativa L.) had a higher response to organic nitrogen applied as rice bran than maize (Zea mays L.). We proposed three hypotheses for the results. Firstly, rice has a greater ability to compete with microorganisms for inorganic nitrogen. Secondly, rice is superior in the uptake of ammonium, amino acids and peptides. Thirdly, the mineralization in the rice root zone occurs more actively as compared to other crops. We conducted pot trials in which upland rice, maize and soybean (Glycine max Merr.) were cultivated with ^<15>N-labelled rice bran. The crop and soil were taken for N and ^<15>N analysis at three stages. The total nitrogen in the plants and inorganic nitrogen in the soils were determined by a calorimetric method, and followed by Kjeldahl digestion and KCl extraction, respectively. The ^<15>N was measured by the emission spectrometric method. The ^<15>N concentration of the rice plant was higher than other crops. This suggests that rice is more liable to take up nitrogen such as ammonium, amino acids and peptides rather than nitrate, which is accumulated in the soil and subsequently reduces the ^<15>N concentration in plants. The ^<15>N concentration of inorganic nitrogen in the cultivated soil was higher than that in the fallow. This suggests that the mineralization of organic nitrogen in accelerated by cultivation, but no differences among the crops studied were observed. Consequently, the second hypothesis is supported by these results, while the third in not.

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Previously, we showed that upland rice (Oryza sativa L.) had a higher response to organic nitrotgen applied as rice bran than maize (Zea mays L.). We proposed three hypotheses for the results. Firstly, rice has a greater ability to compete with microorganisms for inorganic nitrogen. Secondly, rice is superior in the uptake of ammonium, amino acids and peptides. Thirdly, the mineralization in rice root zone occurs more actively as compared to other crops. We investigated the protease activity in the soil to clarify how much the third hypothesis contributed to the nitrogen uptake of rice, We cultivated rice, maize and soybean (Glycine max Merr.) in small pots (5 × 5 × 5 cm) with Andosol in a glasshouse at the National Institute of Agro-Environment Sciences, Tsukuba, Japan, in 1995. We took the soils in rhizosphere and non-rhizosphere separately 26 and 34 d after sowing, and then measured the protease activity of these soils using casein and Z-phe-leu as the substrates. The soil protease activity of maize was higher than that of rice, and soybean was found to be the medium of these crops. These results show that the rhizosphere effect related to nitrogen mineralization such as protease activity does not specifically occur in rice cultivation soil. The other hypotheses are to be investigated to enable comprehension of the strong ability of nitrogen uptake by rice.

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It has been shown that the nodulation of soybeans is inhibited by the application of nitrogen fertilizers. However, the influence of the nitrogen fertilizers on the nitrogen-fixation activity (nodule activity : measured as acetylene-reduction activity) of soybean nodules is not always clear. Therefore, the influence of applied nitrogen on the activity of nodules was examined under several conditions including the amount of nitrogen fertilizer applied and the methods of fertilization. The results are summarized as follows. 1) When soybeans were grown in a split-root system, and fertilizer was applied to only one side, the decreases in nodule dry mass and nodule activity on the fertilized side were significant with increasing nitrogen application. The nonfertilized side of the root system also produced less nodule dry mass and had reduced nodule activity ; however, the decrease was not as drastic as that on the fertilized side. In addition, the nitrogen concentration of the leaves showed a negative linear correlation to the specific activity of the nodules on the total root system. 2) The concentration of nitrate nitrogen in the soybean roots was increased by increasing the amount of basal nitrogenous fertilizer ; however, this trend disappeared two weeks after the flowering stage. In this time, there was no correlation between the concentration of nitrate nitrogen of the roots and the nodule activity ; however, the nitrogen concentration of the leaves correlated negatively to the specific activity of the nodules. 3) The application of nitrogen to the plant leaves was found to decrease nodule activity. In view of the above results, it is suggested that the concentration of nitrate nitrogen to the roots does not affect the activity of soybean nodules.

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It is necessary to make nitrogen fertilization and nitrogen fixation compatible in order to obtain a large quantity of nitrogen absorption, which is a main contributor to the yield of soybean. However, it is known that the specific activity of soybean nodules is inversely proportional to the nitrogen concentration of the leaves. It is important to understand the control mechanism of nodule activity. Therefore, we examined the relationship between sugar content, as the photosynthetic product, and nodule activity in soybean. The results are summarized as follows. 1) When the nitrogen concentration of the leaves was increased by nitrogen fertilization, the photosynthetic ability of the lower node leaf of the main stem was extremely decreased and nodule activity of the soybean declined. 2) The relationship between leaf nitrogen concentration and root glucose concentration showed negative linear regression, but root glucose concentration and specific activity of the nodule showed positive linear regression. The following was estimated from above the results. Because the nitrogen concentration of the leaves was increased by nitrogen fertilization, the photosynthetic ability of the lower node leaf of the main stem was decreased. This decrement of photosynthetic ability caused the decrement of glucose concentration in the roots, resulting in a decline in nodule activity. It is believed that the mechanism of the feedback control system of nodule activity is managed by the host plant.

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