Four-year-old tea plants were transferred from a field to hydroponics for 16-week culturing. The nitrogen in the culture solution was varied; no N (NO), 50 mg N L^<-1> ammonium nitrate (AN-50), 100 mg N L^<-1> ammonium nitrate (AN-100), 500 mg N L^<-1> ammonium nitrate (AN-500), 500 mg N L^<-1> ammonium sulfate (A-500) and 100 mg N L^<-1> nitric acid (N-100). At harvest, samples from new shoot, mature leaves, white roots and brown roots were obtained, freeze dried and ground. The weight increase of tea plants after nitrogen treatment was largest in AN-50, AN-100 and AN-500, followed by N-100. The content of nitrate was also highest in AN-100 and AN-500. Significant nitrate in AN-50 and N-100 treatment was observed only in the white roots, while an obvious increase of nitrate was observed in the young shoots and mature leaves of plants that received AN-100 treatment, and more in those that were treated with AN-500. The content of oxalic acid was highest in mature leaves, and increased with the increase of nitrate in the culture solution. If nitrate reduction is related with oxalic acid synthesis, the major site of nitrate reduction is roots when the administration of nitrate is small while being mature leaves when nitrate addition is large. Polyamines (putrescine, cadaverine, spermine, spermidine) were present in tea organs. Putrescine content was large in roots and increased by the treatment of higher ammonium concentrations. Spermidine content was high in young shoots, and also increased with the treatment of higher ammonium nitrate concentrations.

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We reported previously a mechanism of nitrogen release from resin-coated urea based on the differences in vapor pressure between inside and outside of the coated granules. In the case of coated compound fertilizers, however, the proposed mechanism did not explain fully the differences in release rates of individual nutrients, nitrogen, phosphate and potassium. The release rates of nutrients from 4 resin-coated compound fertilizers were determined periodically in water at 30℃. Ammonium, nitrate and chloride were released earlier in all samples. Phosphate and potassium release were different with the samples. The release of sulfate was the latest in all samples. The constituent compounds in the samples were determined quantitatively by using the X-ray powder diffraction method. Based on the quantities of the salts and double salts, and their estimated solubilities, we concluded that release rates were proportional to the solubilities. The most soluble compound in the granules governed the initial permeation of water vapor into the granules and released the nutrients earlier. Accordingly, release from the less soluble compounds was delayed.

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In 1995, the denitrification profile through soil percolation of nitrate introduced by groundwater irrigation was studied in a rice paddy surrounded by a large area of upland fields. The denitrifying bacterial population was found to be higher in the suface layer of 0 to 2 cm than in the deeper plow layers during the entire submerged period from April to August. Denitrifying activity, as measured by the acetylene blockage method, was also found to be higher in the surface layer than in deeper layers at all times. Denitrification proceeded immediately after the start of incubation, with no supply of organic matter, suggesting that denitrification in the investigated paddy was not restricted by the depletion of organic matter during the submerged period and that denitrification can occur in the paddy as soon as nitrate is introduced by groundwater irrigation. In the interior of the paddy, nitrate in percolating water was nearly absent by 3 cm of depth and did not reach to the roots in the deeper plow layer throughout most of the submerged period. This is considered to be the reason the rice plants grew normally, with no damage caused by excess nitrogen uptake. In an exception to this, 90 d after submersion of the paddy, nitrate removal at the site near the water inlet was incomplete. Nevertheless, as this period is rather brief, the rice plants did not absorb excessive nitrogen and thus suffered no damage. In conclusion, the locus of denitrification is a notable factor in the actual application of nitrate removal in rice paddies, with the elimination of nitrate in the shallow plow layer being necessary.

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In a small river basin of the Mossi Plateau, Burkina Faso, the toposequences of soils along three transects were described and selected soil samples analyzed in order to determine the pedogeomorphic conditions for land use. The soils on each landform unit were classified and their suitability for farming examined. On a relic glacis, lithic leptosols with a continuous petroferric horizon at several centimeters below the surface are distributed. The land is not used for farming as the effective soil depth is very shallow. Various soil types on each glacis of the three transects were investigated. Ferric luvisols with a sandy loamy or sandy clay loamy subsoil are distributed in the upper reaches of a basin belonging to a tributary of the White Volta River. Although having high water permeability (10^<-3> cm s^<-1> through the profile) and a deep effective soil depth (more than 100 cm), the soil is moderately acid and poor in natural fertility. At a site in the middle reaches of the basin, two soil types, chromic cambisols and ferric luvisols, are dominant and display an irregular distribution. Although they are more fertile than the ferric luvisols examined from the upper reaches of the basin, they are less water-permeable and often shallower in effective soil depth due to the existence of either a compact silty or petroferric horizon. Within a riverain glacis of the White Volta River, there are eutric leptosols with a gravelly subsoil. This soil type is least favorable in terms of both physical and chemical characteristics. On a riverain plain, eutric cambisols occur and are characterized by a silty clay loamy texture at the surface. This soil type, with a deep effective soil depth and high water retentivity, is considered to be the most fertile of all soils distributed in the survey area and therefore the most suitable for farming.

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In a small river basin of the Mossi Plateau, Burkina Faso, the distribution of degraded bare ground identified on aerial photographs was examined in relation to pedo-geomorphic conditions, especially for soil texture and the estimated rate of annual soil loss by the universal soil loss equation (USLE). On a glacis located in the upper reaches of a basin belonging to a tributary of the White Volta River, sandy soils are distributed. Here, the occupation rate of degraded bare ground was 2.1% in 1955 and 2.7% in 1988, while on a riverain plain where silt-rich soils are distributed, it was 1.7% in 1955 and 21.9% in 1988. The land of the latter landform unit seems to be less intensively used than the former. Thus, it is suggested that the riverain plain is more susceptible to degradation than the upper glacis. The degraded bare ground, which has been levelled and covered with a thin surface crust, has a finer topsoil or a finer subsoil outcropping due to soil erosion. The bare soil profiles are dry and very hard (31-34 mm using a Yamanaka-type soil hardness meter) even in the rainy season. This condition must contribute substantially to degradation as revegetation or plant recovery does not occur. The erodibility factor (K) of the topsoil of the study area ranges between 0.38-0.55 in US customary units, and is very high on the riverain plain. The estimated annual soil loss from the riverain plain is, however, quite low (3 t ha^<-1>) due to the gentle slope (0.25%). This indicates that the high occupation rate of degraded bare ground on the riverain plain cannot be explained by the annual soil loss rate. The very low occupation rate at the glacis of the upper reaches may be explained by a low soil loss rate and the fragile consistency of sandy soil, of which the surface crust is easily destroyed. On the other hand, the high occupation rate on the riverain plain can be explained by the presence of a silt-rich topsoil which becomes very hard when dry. This condition may often result in abandonment of cultivated land.

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In order to understand the effect of groundwater level on methane emission from submerged paddy soil during the fallow season, methane flux was measured during the cropping season of 1996 at the lysimeter of Sandy soil, Loam soil, Clay soil and Peat soil, of which the groundwater level was kept at 0 or -30 cm during the fallow seasons from 1985 to 1995 and at -1.2 m in 1995-96. Methane fluxes at -30 cm plots were higher than that at the 0 cm plot, especially in Peat soil and Sandy soil. From these soils, methane emission from -30 cm plots started even in the early period of submergence. Cumulative amounts of emitted methane from -30 cm plots during the cropping season were 1.8, 1.2, 1.2 and 3.5 times higher than those from the 0 cm plots of Sandy soil, Loam soil, Clay soil and Peat soil, respectively. The amounts of free iron oxide in -30 cm plots tended to be smaller than those in 0 cm plots, particularly in Peat soil, which would be one of the reasons for higher methane emission from -30 cm plots.

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A ^<15>N-field experiment was conducted on poorly drained alluvial paddy soil in 1996 and 1997 to investigate the decomposition pattern of rice straw and recovery rate of nitrogen released from it by rice plants in no-tillage transplanting cultivation as compared with tillage cultivation. 1) Decomposition rates of the rice straw from fall to transplanting time in the no-tillage plot tended to be lower than that in the tilled plot, whereas those from transplanting to harvesting time showed a reverse tendency. 2) Nitrogen concentration of the rice straw applied in the fall changed a little or was unchanged till the middle of May both for tilled and no-tillage plots. However, after that, it increased significantly till the middle of August, and this increment was higher in the no-tillage plot. On the other hand, the changes in C/N ratio in the rice straw showed a reverse tendency. 3) The changes of apparent nitrogen remaining in the rice straw were almost the same between no-tillage and tilled plots till the middle of March. However, after that, nitrogen content tended to be increased by the immobilization of mineral nitrogen in the soil, and this increment was higher in the no-tillage plot. 4) Recovery rates of nitrogen released from the rice straw were 3-4% at the maximum tillering stage and 9-10% at harvesting time both in tilled and no-tillage plots.

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The number of sulfate reducers and methanogens in paddy field soils was enumerated by the role tube and most probable number methods, respectively, and then sulfate reducers and methanogens were isolated from the enrichment cultures. The syntrophic relationship between the sulfate reducers and methanogens was examined. 1) Both bacterial numbers were 10^5 g^<-1> dry soil in paddy field soil. No significant difference in the number of both bacteria was detected between fertilizer treatment conditions. 2) A sulfate reducer, strain SK3-4, was motile, Gram-negative and had vibrio-shaped cells. Acetate and sulfide were the only end products detected during growth with lactate in the presence of sulfate. 3) A H_2-utilizing methanogen, strain MK3-6, had rod-shaped cells. H_2 and formate were the only electron donors utilized for methanogenesis. An acetate-utilizing methanogen, strain MSK3-1, had coccoid cells. Most of the cells grew as aggregates. The strain was able to use acetate, H_2+CO_2, methanol and methylamines as substrates for methanogenesis. 4) Strain SK3-4 was able to grow in a sulfate-free medium when it was cultured in association with strain MK3-6 and strain MSK3-1. Sulfate reducers seem to be involved in methanogenesis as hydrogen-producing acetogenic bacteria in syntrophic association with hydrogen-utilizing methanogens under sulfate-free conditions. 5) When paddy field soils were incubated anaerobically with the addition of lactate, the lactate was completely degraded to CH_4 and CO_2. The bacterial number of sulfate reducers stayed at the 10^5 level. The sulfate reducers seem to oxidize lactate to acetate in syntrophic association with methanogens. When sulfate and lactate were added to the soils, lactate was oxidized to acetate and methanogenesis was inhibited.

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In order to investigate the influence of iron and aluminium oxide on microbial biomass in volcanic ash soils, we examined the relationships between the amounts of soil microbial biomass, evolved carbon dioxides and free iron and aluminium oxides. Eight soils were treated with H_2O_2 to remove organic matter, sterilized, inoculated with nonsterile soil, and incubated for 98 days with or without the addition of rice straw. The results obtained are as follows. The amount of soil organic C was decreased, while the amounts of free iron and aluminium oxides were increased by treatment with H_2O_2. The amount of "stable" microbial biomass held in the soil at the latter period of incubation was also decreased. Therefore, it was suggested that soil organic matter could increase the amount of stable microbial biomass. Furthermore, within H_2O_2-treated soils, a negative correlation was obtained between the amount of evolved carbon dioxide and the amount of stable microbial biomass and the amount of free aluminium oxides, suggesting that free aluminium oxides retard the decomposition of organic matter by soil microorganisms and reduce the amount of stable microbial biomass.

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Using data accumulated in the past, I estimated the amount of phosphorus load in Hachirou-gata Lake by measuring phosphorus concentration and calculating the flow rate of rivers which poured into the lake during 1990 and 1991. Results obtained are follows: 1) The total amount of phosphorus loaded in to Hachirou-gata Lake was estimated to be about 136 and 156 t in 1990 and 1991, respectively. 2) The amount of phosphorus load derived from phosphate-rich spring water was estimated at about 30-40 t y^<-1>. 3) If we use phosphate-rich spring water as a nutrient resource for rice plants, about 25% of the phosphorus load that entered Hachirou-gata Lake can be reduced.

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