In order to clarify the major factors controlling nitrous oxide (N_2O) emission from two rice paddy fields under the continuation of no-tilled direct seeding cultivation (ND) more than seven years and one rice paddy field under conventionally tilled transplanting cultivation (TT), we measured N_2O fluxes from these three rice paddy fields with gleyed gray lowland paddy soils in Okayama Prefectural General Agriculture Center, by using a closed chamber method in duplicate once a week or per two weeks through the year for 2-5 years. Coated urea at a rate of 8-11g N m^<-2> was mainly applied to the ND plot and TT plot. Rice straw was spread on the surface of the ND plot after harvest, and incorporated into the soil of the TT plot in winter. Consequently, the following results were obtained. 1) The emission of N_2O was not observed during the flooded period, but observed during the no-flooded and fallow season, from all the three rice paddy fields. The N_2O fluxes were higher in the ND plot than in the TT plot. 2) In the ND plot, some peaks of N_2O flux were observed mainly in May and/or early June, in a period from the basal dressing of nitrogen fertilizer to the submergence, through nitrification and/or denitrification of the applied nitrogen. 3) In the ND plot, some high peaks of N_2O flux were observed from November to February in the fallow season, when the soils quickly became anaerobic from aerobic conditions due to heavy rainfall, after nitrate concentration in the plowed-soil solution greatly increased for a while in dry conditions with very little precipitation. 4) These results indicate that the nitrate which was produced by the mineralization of the accumulated organic matter on the surface soil in the ND plot maintained a high concentration in soil solution due to dry conditions with less precipitation. Then, after heavy rainfall, the nitrate in the plowed soil solution was reduced by the fast reduction of soils through denitrification to produce N_2O, which was finally released to the atmosphere. 5) In contrast, very small peaks of N_2O flux in the fallow season were found in the TT plot, possibly because no organic matter was accumulated in the surface soil of the TT plot. This N_2O flux was possibly caused by diffusion of soil gas through the plowed layer from the subsoil layer with high N_2O concentration.

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We investigated that the effect of desalinization by using guinea grass in accumulated chemical fertilizer soils. Each 1/2000 a pot was filled with about 10kg of air-dried soil and chemical fertilizer. The soil was fertilized at the rate of 20kg N 10a^<-1>, 40kg P_2O_5 10a^<-1>, 20kg K_2O_10a^<-1>. This rate of fertilizer is treatment 1. Two and four times that amount of fertilizer were called treatment 2 (40kg N 10a^<-1>, 80kg P_2O_5 10a^<-1>, 40kg K_2O 10a^<-1>) and 4 (80kg N 10a^<-1>, 160kg P_2O_5 10a^<-1>, 80kg K_2O 10a^<-1>). The pot experiment was conducted at the green house under high temperature and evaporation. The seeds of guinea grass from Yukijirusi Syubyou were sown in each pot. Sampling (plant and soils) was done 0 to 9 weeks after sowing. Inorganic nitrogen, available phosphorous, water soluble bases (potassium, calcium and magnesium) in surface soil (depth 0-15cm) of 3 treatments decreased with guinea grass over the 9 weeks. Nitrogen, phosphorous and base uptake by guinea grass was highest in treatment 4. In treatment 4 (4 times the amount of fertilizer), guinea grass growth (shoot and root) was higher than in other treatments. C/N of guinea grass in treatment 1, 2 and 4 were about 20 in 9 weeks; however, this value was 8-12 over 6 weeks. It is possible to utilize guinea grass in accumulated fertilizer soil of green house, green manure and pasture plants after harvesting

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Percentage of useful rhizobium inoculated on the seed coat was known to be low because of their low competition against indigenous rhizobia. This object was to clarify the effect of inoculation method and inoculum density of Bradyrhizobium japonicum USDA110 on production of soybeans (Glycine max L. Merr.). Five experimental plots were designed: no inoculation (NI), seed coating inoculation with density of 10^5 cells seed^<-1> (SI5) and 10^<-7> cells seed^<-1> (SI7), and previous inoculation with density of 1.7×10^3 cells g^<-1> soil (PI7) and 1.7×10^5 cells g^<-1> soil (PI9). The PI plot was plowed after being spread with BM2 soaked rhizobium culture. Consequently, in SI5, SI7, and PI9 plots the percentage of serotype USDA110 was significantly high, and it was assumed that the fixed nitrogen was distributed in large amount to pod and seed, and as a result the yield (g m^<-2>) significantly increased. It was thought that the density of 10^5 cells seed^<-1> was more effective at seed inoculation because there were no effects from the increase of inoculum density. From result for PI9 plot, it was thought the previous inoculation using BM2 (1.7×10^5 cells seed^<-1>) was effective in competition with indigenous rhizobia on nodulation.

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Chemical analysis and pot experiments using Komatsuna (Brassica campestris L.) plants were conducted to investigate relationships between N availability and the N forms of 30 animal manure composts (16 from cattle manure, 5 from pig manure, 9 from poultry manure). Values of EC, total C, total N, water-soluble C, water-soluble N and NH_4-N were highly correlated with each other. On the other hand, NO_3-N was independent of them. Germination rate correlated with EC and water-soluble NH_4-N. In the first pot experiment, 10 or 20g of the compost was applied to low-N sandy soil in a 1-L pot. N uptake by the plants from the compost N were 12% of the applied total N on average, and N uptake was irrespective of the source animals. N uptake was explained using a multi-linear regression equation generated from NO_3-N and the water-soluble N other than NO_3-N (γ=0.63). It was indicated that NO_3-N is related not only to compost maturity, but also to N availability. In the second trial, the composts were applied to high-N upland soil at the same dose as for the first trial. Nitrogen uptake from the compost-amended upland soil could be estimated from the regression equation obtained from the first trial, even though significant interactions between compost and soil were indicated.

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Information on temporal change and spatial variability of gas behavior in the surface soil is important to estimate the total gas efflux from an agricultural field to the atmosphere accurately. The objectives of this study were to clarify the relationships between the cultivation type and the temporal change and spatial variability of the CO_2 efflux from the agricultural fields and the effect of the gas diffusion movement and the soil physical properties on the gas efflux. The CO_2 efflux from the soil surface was measured by a closed chamber method at four agricultural fields in Japan, two bared fields after plowing, one dent-corn field and one pasture. The measurements of the gas efflux from the soil surface were carried out simultaneously at 10 points along a straight line with 3m intervals in each of the four fields. The gas diffusion fluxes through the surface soil were also estimated from the vertical gradients of gas concentrations and gas diffusion coefficient measured at the same 10 points where the gas efflux was measured. Both the average values and the coefficients of variance for the measured CO_2 efflux showed a tendency to decrease with time after plowing except in the pasture. This tendency is considered to be by the non-uniform activation of soil microorganisms caused by soil disturbance. Contrarily, those two values for the pasture increased with time and it could be explained in terms of the non-uniform growth of plant roots. In pasture, the measured gas efflux from the soil surface was larger than the estimated values of the gas diffusion flux through the surface soil. It could be explained by the gas generation by the roots. In the dent-corn field, on the other hand, the diffusion gas flux was larger than the gas efflux.

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[in Japanese]

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