This study was conducted on a model made under the following conditions. Groundwater level was set at approximately one meter. Plow layer and plowsole were closed system percolation and made into a reducing layer. Subsoil above the groundwater level was open system percolation and made into an oxidation layer. Subsoil below the ground water level was closed system percolation and was made into a reducing layer. This model was used to evaluate eluviation and illuviation of soluble ions in percolating water. Findings are listed as follows.
1.Iron ion in the percolating water were found in the reducing layer, but the concentration of the iron ion was reduced in the oxidation layer. Manganese ion was found in both the oxidationlayer and the reducing layer.
2. In the closed system percolation concentration of iron ion in percolating water can affected concentrations of calcium, magnesium, sodium and potassium ion. when concentration of iron ion is higher, the ion cconcentrattion of calcium, magnesium, sodium and potassiumin start to increase.
3. Nitrogen ion concentration in the form of ammonium ion was high in the plow layer and plowsole, and the concentration was considerably lower in the subsoil. However, nitrate ion concentration was low in plow layer and plowsole, and increased sharply in the open system percolation (oxidation layer) in upper subsoil, and lowered considerably in the closed system percolation (reducing layer) of the lower subsoil. As the above results demonstrate that percolation systems were found to be an important factor for eluviation and illuviation of soluble ions.
The nitrogen outflow model in agricultural area has been constructed. This model is composed by three land use elements ; field, forest and paddy field, and the livestock. Field and forest area, and livestock exist in upland part. Paddy fields exist in lowland part. Water flows naturally from upland to lowland. Then nitrogen caused from the field, forest and livestock in upland area flows into lowland. If the water is irrigated to paddy fields in lowland, nitrogen will be removed in this process by denitrification under anaerobic condition and uptake by rice plants. This kind of nitrogen removal function by paddy fields is investigated by many researchers. The author represented this function by the equation (1)and attached this function to the model of nitrogen outflow in agricultural area. Calculated results by this model are shown in Table 1,Fig. 3 and Fig. 4. In the case in which 100 kg/ha nitrogen outflows from the field, the concentration of outflow water from this watershed will attain to 10.3 mg/l when the percentage of the field area is 70%. However concentration will be decreased to 5.7 mg/l, if 50% of water is irrigated to paddy. Nitrogen removal by paddy fields has a large effect on the concentration of outflow water. Calculated values will be examined by the measured field data in next paper.
Nitrogen concentration of stream waters are investigated in 13 small watersheds. Table 1 shows the land use and pig density of each watersheds. NO3-N concentrations are quite different in watersheds, maximum value is 19.7 mg/Z, and minimum value is 0.7 mg/Z (Table 2). It depends on land use and especially on pig density (Fig. 3). NO3-N concentrations measured in ricegrowing season are lower than the values measured in non-ricegrowing season. During rice-growing season the water flowed out of upland area is almost irrigated and flooded within paddy fields in lowland area. Then nitrate caused from chemical fertilizer and pig slurry in upland is removed by both denitrification and aqua-plant uptake in the paddy. The nitrogen outflow model with nitrogen removal function of paddy fields were proposed in previous paper. Measured values are compared with the values calculated by this model. For non-ricegrowing season (Fig. 5), and for rice-growing season (Fig. 7), the calculated values are nearly equal to measured values. Large difference is appeared in watershed L, where lots of pigs were fed in the past. It means that large amount of nitrogen flowed out of the pig farms in the past remains in the subsoil and groundwater.
It is very important to elucidate the mechanism or mysteries in soil freezing and thawing processes, such as the actual situation of freezing or thawing fringe, the effect of initial water content on the maximum frozen depth, the role of each terms of heat balance under different conditions or different periods etc., for the correct understanding and prediction of freezing and thawing phenomena, the prevention of freeze injury, the use of freezing method, and the explication and control of mass transfer in soils as an environmental problem. However it is difficult to be practiced only by measurement method. Herein a numerical experiment method was made by the coupled heat and water transfer model to simulate both the freezing and thawing processes of soil under in situ boundary conditions for a long period and large scale. The dealing method of the ice formation and ice thawing plays a very important role in the simulation, because it affects the convergence and precision principally. Therefore a new method following the mechanism closely (without using any unreasonable or unrealistic assumption) was presented here, which made us possible to simulate especially the thawing process as well as the freezing process more sensitively and reasonably than any previous approach. The frozen or thawed depth was determined due to the definition of frozen soil but not the 0℃ line. All the numerical experiment results of freezing and thawing penetration, frost heave, temperature, liquid water content, ice content and soil particle content profiles at different time responded the mechanism of freezing and thawing processes of soil very well.
We studied the mechanisms of changes in the microstructure of anaerobic rice paddy soil when it is converted to upland field crops or restored from upland field to a rice paddy, by comparing the paddy soils with a model substance consisting of smectite-Fe oxide complexes. For the paddy soil (Epiaquepts), the sediment volume (SV) of the soil decreased when the matric potential of the soil was lower than —1.5 MPa and increased again with the flooding following drying. The amount of reduced iron with flooding suggested that the increase in the SV depended on the history of soil reduction and not on the state of reduction. The effect of the reduction of iron oxide on the SV was evaluated using the smectite-Fe oxide complex. The SV of the smectite with iron oxide decreased more than that of the smectite without iron oxide when the matric potential of the sample was lower than -1.5 MPa. When samples were reduced by adding sodium ascorbate, only the SV of the smectite that contained iron oxide increased. SEM micrographs showed that layersilicates were not aligned in smectite containing 0.085 kg kg-1 iron oxide, and it seemed that iron oxides bound to the layersilicates randomly. We concluded that the aggregation of layersilicates in the presence of iron oxide with drying was one of the factors that decrease the SV, and that the decreased volume could be restored by the reductive dissolution of the iron oxides in the soil.