In order to evaluate the effects of Si supply on the amelioration of Al injury of rice plants (Koshihikari), plants were water-cultured for 3 d with Al (0.8 mmol L^<-1>) and low molecular Si (3.0 mmol L^<-1>) or high molecular Si (3.0 mmol L^<-1> low molecular Si+21.7 mmol L^<-1> high molecular Si) in 0.5 mmol L^<-1> Ca(NO_3)_2 solution. The distribution of aluminum in roots was measured, and root-tip tissues were observed microscopically after staining with hematoxylin, fluorescein diacetate-propidium iodide and neutral red. The results were as follows. 1) The root elongation relative to the control plants (without Al and Si) decreased with aluminum treatment. Aluminum injury appeared 24 h after Al treatment in elongation parts of the roots and Al had entered the epidermis and outer layers of the cortex, resulting in a decrease in the physiological activity of these cells. The supply of Si ameliorated Al injury, and no damage was observed even at 72 h after Al treatment in the root tissues, especially in the Al plus high molecular Si treatment. 2) The monomer Al concentration in culture solution decreased slightly with Si addition. Total Al content in shoots and roots was higher in the Al plus low molecular Si treatment and lower in the Al plus high molecular Si treatment than in the only Al treatment. Thirty-Fifty percent of the total Al amount in roots was present in the apoplastic fraction and 1-2% in the symplastic fraction. The Al amount in each fraction was markedly decreased in the Al plus high molecular Si treatment. These results indicate that the ameliorative effect of Si on Al injury is attributed mainly to changes in the chemical forms of aluminum ions in the culture solution and plants due to the formation of aluminum silicate complexes which is less toxic than monomer aluminum.

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Direct and indirect (=through decomposition) uptakes of free amino acids (FAA) by rice and tomato plants were investigated by using ^<13>C- and ^<15>N-labeled aspartic and glutamic acids, serine, leusine and ammonium as tracers. 1) One week after the surface application of amino acid-N or NH_4-N to paddy soil, the amounts of ammonium remaining in the soil, assimilated ammonium, denitrificated ammonium and amounts taken up by plants were similar. 2) From 5.5 to 7.7% of the FAA applied was absorbed directly by rice plants, and from 42.5 to 47.2% of that was indirectly absorbed as ammonium after decomposition. It is suggested that the FAA degraded to ammonium around 2 or 3 d and the 1-^<13>C absorption rates of the FAA (RCH(NH_2)^<13>COOH) were high in proportion to the number of carbon atoms of the R side-chain. 3) The absorption rate of N derived from the FAA by tomato plants was lower than that by rice plants, namely, from 0.4 to 1.9% in direct-uptake and from 16.0 to 29.8% in indirect-uptake. Percentage of direct-uptake of the FAA in upland soil was much lower than that in the paddy field.

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The effects of a controlled availability fertilizer and a nitrification inhibitor (AM: 2-amino-4- chloro-6-methyl pyrimidine) on the mitigation of nitrous oxide emission from soil were surveyed by a pot experiment. Urea coated with a polymer of olefin, a chemical fertilizer amended with AM and urea were compared. In addition, the change of ammonium, nitrite and nitrate in the soil was monitored by laboratory experiment. In the pot experiment, sweet corn was cultivated in a 0.05 m^2 pot. Nitrous oxide flux reached peaks at 25, 31 and 52 d after fertilizer application in the pots applied with urea, coated urea and chemical fertilizer amended with AM respectively. The order of total amount of emitted N_2O through 80 d was urea>coated urea (without sweet corn)> coated urea>chemical fertilizer amended with AM>no N. The amounts were different with a 99% level of significance. In the laboratory, Low-humic Andosol applied with urea, a controlled availability fertilizer or a fertilizer applied with nitrification inhibitor was incubated at a 80% field capacity moisture content. Nitrous oxide flux synchronized with nitrification activity. AM reduced N_2O emission when compared with urea. It was concluded that AM reduced the ratio of nitrous oxide production through nitrification whether the plant grew or not. It is believed that the reduction of nitrous oxide emission was caused by the decrease of nitrite concentration in the soil. Controlled availability fertilizer decreased nitrous oxide emission from the soil where sweet corn grew. Without plants, the effects of the controlled availability fertilizer on reducing nitrous oxide were limited.

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The acidity of Japanese cultivated Andosols and significance of exchange acidity y_1 in their classification were examined with special reference to soil colloidal composition: allophanic Andosols and non-allophanic Andosols. The mean values of the soil pH for topsoil and subsoil in allophanic Andosols were 6.0 ± 0.6 and 5.9 ± 0.7, respectively, and for topsoil and subsoil in non-allophanic Andosols, 5.8 ± 0.6 and 5.5 ± 0.8, respectively. The soil pH of topsoil for both types of Andosols was higher than that for subsoil values, ranging from 0.1 to 0.3. Out of 610 cultivated allophanic Andosols, 28 soils with an exchange acidity of y_1≧6 were found. On the other hand, out of 317 cultivated non-allophanic Andosols, 239 soils with an exchange acidity of y_1 < 6 were found. Compared with the database of The Soil Information System, the ratio of strong acid soil to non-cultivated Andosols in each prefecture was relatively high, ranging from 3 to 93%. The liming of the plow layer soil was done well for both allophanic Andosols and non-allophanic Andosols. The 292 cultivated Andosols were divided into two groups by exchange acidity y_1 in the subsurface soil following The Classification of Cultivated Soils (The 3rd approximation): Group 1 (y_1 < 5) and Group 2 (y_1 ≧ 5). In Group 1, there were 242 soils: 202 soils, or 83%, were allophanic Andosols; and 40 soils, or 17%, were non-allophanic Andosols. In Group 2, there were 50 soils: 21 soils, or 42%, were allophanic Andosols; and 29 soils, or 58%, were non-allophanic Andosols. The exchange acidity y_1 of the cultivated Andosols was not closely related to the clay mineral composition. We conclude here that a criterion for non-allophanic Andosols in cultivated soils should reflect soil colloidal compositions using analytical date for amorphous materials.

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We examined the effect of three tropical grasses (Brachiaria decumbense, B. humidicola and Melinzis minzutiflora) on nitrification in soils. These three grasses were grown in pots filled with loamy Andisol for 2 weeks and then ammonium-N was applied to the soil as ammonium sulfate. After ammonium-N applications, the number of ammonium-oxidizing bacteria in soils increased more than ten times for two grasses compared from B. humidicola. But in the case of B. humidicola, the number of ammonium-oxidizing bacteria did not increase and kept the same value for 27 d. Three grasses were grown in 1/5000 a Wagner pots for 4 weeks and then ammonium-N was applied to the soils for the determination of nitrous oxide emissions. From one day after the applications of ammonium-N, emission of nitrous oxide from the soils in which B. decumbense and M. minutiflora were grown was observed and continued more than 4 d. However, there was little emission of nitrous oxide from the soil in which B. humidicola was grown. Moreover, after determination of nitrous oxide emissions, plant roots were removed from the soil and ammonium-N was reapplied to the soils. Only in the soil in which B. humidicola was grown, was nitrification delayed. From these data, it seems likely that the tropical grass, B. humidicola, inhibits nitrification in soil; in particular, it suppresses the multiplication of ammonium-oxidizing bacteria in the soil as well as the emission of nitrous oxide to the atmosphere.

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We compared the spectroscopic and physicochemical properties of the humic acids isolated from charred grassland plants after HNO_3 treatment (C-HA) and from the A horizon of Kuro- bokudo (S-HA). 1) The shapes of the absorption curves in the UV and visible regions of the C-HA and S-HA were featureless, and their humic acids belonged to the A-type. 2) The carbon and hydrogen contents of C-HA were almost the same as those of S-HA, while the nitrogen content of C-HA was much higher than that of S-HA and the oxygen content of C-HA was lower than that of S-HA. The atomic [O]/[C] and [O]/[H] ratios of C-HA were lower than those of S-HA. These analytical data of C-HA, except for the nitrogen content, fell within the ranges of those of A-type soil humic acids reported previously. 3) The ^<13>C-NMR spectra of C-HA and S-HA were similar to each other, and the values of their carbon species were in the order of aromatic-C ≫ carbonyl- C > carbohydrate-C≒aliphatic-C. The values of the aromatic-C and carbonyl-C were lower in C-HA than in S-HA. The reverse was true for carbohydrate-C and aliphatic-C. 4) The FT-IR spectrum of C-HA resembled that of S-HA. 5) The XRD patterns of C-HA and S-HA indicated that these humic acids had graphite-like structures.

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It has been pointed out that a conventional method for collecting soil solution using ceramic porous cups causes C0_2 degassing in the container (flask), which results in an increase in the pH (HCO_3-4+H^+ CO_2 ↑ +H_2O) of the sample. Thus, in order to collect soil solution samples that are little subjected to CO_2 degassing, we improved the conventional method, based on the method of Suarez, by setting a tube with a low volume (30 mL) between a porous cup and a flask to flush several volumes of soil solution from the tube to the flask. Using this setup, we collected soil solution samples twice a month from June 1997 to May 1998 in a Japanese cedar (Cryptomeria japonica) forest located in Gunma Prefecture, and compared the pH of the samples collected in the tube (pH(tube)) with that of samples in the flask (pH(flask)) using 22 samples. In addition, we compared the pH(tube) with the pH calculated (pH(cal)) when the solution was assumed to be in equilibrium with CO_2 partial pressure in the field. The results obtained were follows: 1) The pH(flask) was always higher than the pH(tube). The difference between the two was 0.29 pH units in average with a maximum value of 0.92. This was thought to be caused by CO_2 degassing in the flask because the concentration of dissolved inorganic carbon in the flask sample was always lower than that of the tube sample. This clearly shows that the improved method can suppress the increase in pH caused by CO_2 degassing. 2) However, the pH(tube) was higher than the pH(cal). This implies that CO_2 degassing also occurred in the tube samples. The difference between the two was larger in deeper soil horizons with higher CO_2 partial pressures. This corresponded to the difference between the pH(flask) and the pH(tube) at depths of 50 and 100 cm. 3) It was estimated that the percentage of CO_2 loss during tube sample collection was about 4% and that the resulting pH increase was as low as 0.02 pH units. This result suggests that CO_2 degassing of the tube samples mainly occurred during the processes of removing the tube sample from the collector and/or measuring the pH of the sample.

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The rooting ability of seedlings and early growth of rice plants with and without silica application were estimated. The pH value of silica used was about 5, indicating there was no effect of silica application on the mineralization of soil organic nitrogen. The results obtained were as follows: 1. The dry weight of seedlings and ratio of dry weight to plant height of seedlings were higher in plots with silica application than in those without silica application. These results were attributed to an increased photosynthetic rate caused by a high stomatal aperture and high light extinction coefficient plot with silica application. 2. A larger number of new roots was observed in silica-applied rice seedlings which had a higher amount of TAC than rice seedlings without silica application. Furthermore, the root activity of the rice seedlings, as indicated by the α-α naphtylamine method, was higher in silica application plots than when silica was not applied. Silica application was more effective on plant dry matter than tiller production of rice plants.

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Methane is one of the greenhouse gasses, having a global warming effect approximately 10 times stronger than CO_2. It is important to identify sources of methane emission and sinks of methane consumption, and to understand their processes. We measured the annual methane emission from a dam-lake and methane consumption by forest soil surrounding the dam-lake, at Morai, Hokkaido, Japan. Methane was continuously emitted from the surface of dam-lake. The methane emission rate increased with the increase of air temperature, from 0.022 mg C m^<-2> h^<-1> in spring to 0.431 mg C m-2 h-1 in fall, followed by a flash of 0.922 mg C m^<-2> h^<-1> in the ice-melting season. The emission rate was significantly correlated with the dissolved methane concentration in the surface water of the dam-lake (r=0.98, n=11). The annual amount of methane emitted from the surface of the dam-lake was estimated as 0.4 Mg C. Methane emission associated with water-discharge from the dam was estimated as 0.43 Mg C by calculation from the data of amount of water discharged from the dam-lake and the difference in concentration between the lake-water and discharged water. Consequently, the total amount of methane emission was 0.83 Mg C, in which 75% occurred during the period of waterdischarge from the dam. Forest soil surrounding the dam-lake always took up methane, even in the snow-covered season. The methane uptake rate increased as soil temperature increased, and there was a decrease of methane concentration in the surface soil, ranging from 0 to 0.1 mg C m-2 h-L Methane uptake during the snow-covered season accounted for 18% of the annual uptake. Taking the watershed area (2050 ha) and dam area (63 ha) into consideration, methane uptake by forest soil surrounding the dam-lake was 10 times larger than the methane emission from the dam-lake. However, the methane uptake rate was about 4 times smaller than the methane emission rate. Therefore, the methane emitted from the dam-lake was not directly consumed by the forest soil surrounding the dam-lake.

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