We conducted a field experiment in a paddy field in a cold region of Japan to elucidate the effects of crop rotation and rice straw application on methane (CH_4) and nitrous oxide (N_2O) emissions. Methane and N_2O fluxes were measured by the conventional closed-chamber method. The experimental field was divided into four plots: upland paddy with crop rotation (i) with and (ii) without rice straw application (BR and BN, respectively), and upland paddy in consecutive paddy rice cultivation (iii) with and (iv) without rice straw application (PR and PN, respectively). In the BR and BN plots, soybean was cultivated from 2006 to 2007 and then paddy rice was cultivated from 2008 to 2009. Paddy rice was cultivated consecutively in the PR and PN plots from 2006 to 2009. In 2006 and 2007, of the greenhouse gas emissions from the BR and BN plots, most was N_2O, whereas from the PR and PN plots most was CH_4. The combined global warming potential (GWP) of CH_4 and N_2O for each year was 176〜177 g CO_2-eq m^<-2> in the BR plot and 157〜207 g CO_2-eq m^<-2> in the BN plot, which corresponded to 15〜18 and 95〜186% of those of the PR and PN plots, respectively. In 2008 and 2009, of the greenhouse gas emissions, most was CH_4 for all plots. The combined GWPs for the BR and BN plots were 187〜477 and 59〜230 g CO_2-eq m^<-2>, respectively, which corresponded to 17〜62 and 91〜185% of those of the PR and PN plots, respectively. The total combined GWP of emissions from the BR plot for the four years was 1017 g CO_2-eq m^<-2>, which was 25% of that from the PR plot. On the other hand, the total combined GWP of CH_4 and N_2O emissions from the BN plot was 653 g CO_2-eq m^<-2>, which was 140% of that from the PN plot. These results indicate that crop rotation reduces GWP from upland-paddy fields, but is more effective if rice straw is incorporated in the soil. Further, the effect of rice straw incorporation is very effective at reducing GWP even with consecutive paddy cropping.

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We investigated which of two fertilization methods would be more suitable for organic rice cultivation when using cotton-mulch sheets in low-fertility paddy fields of southwestern Japan. The two methods were (i) application of fertilizer before puddling (fertilizer before puddling) and (ii) application of fertilizer before laying the cotton-mulch sheets (fertilizer before laying). With fertilizer before puddling, the measured soil inorganic nitrogen concentration was the same as in non-fertilized plots both before and after laying the cotton-mulch sheet, and the number of tillers that developed was also the same as in non-fertilized plots. With the fertilizer before laying method, soil inorganic nitrogen increased, the number of tillers and straw weight increased, and the yield of rice was greater. With topdressing at the panicle formation stage, the number of grains per unit area, thousand-kernel-weight, unhulled rice weight, and brown rice yield all increased. The rice growth and yield did not improve with intermediate topdressing (i.e., between planting and panicle formation). The recovery rate of nitrogen from fertilizer applied before laying was 35% and from the fertilizer applied in topdressing at the panicle formation stage was 58%. There was no apparent nitrogen recovery from the intermediate topdressing. These results indicate that intermediate topdressing is unnecessary for rice cultivation with cotton-mulch sheets. In terms of yield performance, lodging resistance, and taste when using fertilizer before laying in low-fertility paddy fields, we found that the optimum nitrogen application rate is 8g-Nm^<-2> followed by topdressing with 4g-Nm^<-2> or less at the panicle formation stage.

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At the Yamada Biomass Plant in Chiba Prefecture, Japan, the final product of a methane-fermented mixture of cow manure and vegetable matter is separated into liquid and solid (dehydrated cakes) components. The dehydrated cakes are carbonized at 230〜330℃ using super-heated vapor to create biochar. We tested the efficacy of this biochar as a phosphorus source. The biochar contained 74.1 g kg^<-1> total phosphate, 40.9 g kg^<-1> citric-acid-soluble phosphate, and 2.0 g kg^<-1> water-soluble phosphate. Citric-acid-soluble phosphate comprised 55.2% of the total phosphate. Water-soluble phosphate accounted for 4.9% of the citric-acid-soluble phosphate, which is one-twelfth of the value for superphosphate and eight times that of fused magnesium phosphate. We performed consecutive extractions of inorganic phosphate from biochar using, in order, water, 0.5 mol L^<-1> NaHCO_3, 0.1 mol L^<-1> NaOH, and 1 mol L^<-1> HCl. The biochar contained 1.7 g kg^<-1> water-extractable phosphate, 13.7 g kg^<-1> NaHCO_3-extractable phosphate, 17.1 g kg^<-1> NaOH-extractable phosphate, and 42.2 g kg^<-1> HCl-extractable phosphate. The available phosphate (sum of water-extracted phosphate and NaHCO_3-extracted phosphate) accounted for 21% of the total extracted phosphate. We tested the efficacy of biochar as a phosphorus fertilizer by planting komatsuna (Brassica napa) in pots filled with either Andosol or Fluvisol soil. Biochar improved the growth of komatsuna in both types of soil. The efficacy of biochar as a phosphorus fertilizer was the same as that of inorganic phosphate fertilizers (superphosphate, multi-phosphate, and fused magnesium phosphate).

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To support the cultivation of winter wheat in a heavy clay upland field previously used for paddy rice, we investigated the effect of deep placement of calcium cyanamide before sowing as compared with the conventional method of basal application of fertilizer to the plow layer and five topdressing applications during crop development to a total rate of 20g-Nm^<-2>. A mixture of calcium cyanamide and urea was applied 10cm below the wheat seeds. In an incubation experiment (25℃, 60% WHC), most of the ammonium nitrogen applied to the soil had changed to nitrate nitrogen by 3 days after ammonium sulfate addition. In contrast, the ammonium nitrogen remained even 20 days after calcium cyanamide addition. In the deep-placement plots with 20 and 15g-Nm^<-2>, the number of panicles per plant was equal to that in the topdressed plot. The amount of nitrogen uptake by wheat in the deep-placement plot with 20g-Nm^<-2> was not significantly different from that in the topdressed plot and the deep-placement plot with 15g-Nm^<-2>. The recovery rate of basal nitrogen in the deep placement plots was significantly higher than that in the topdressed plot. Wheat yield in the deep placement plot with 20g-Nm^<-2> was not significantly different from that in the topdressed plots and deep placement plots with 15g-Nm^<-2>. The crude protein content of grain in both of the deep-placement plots and the topdressed plot was within the appropriate range for noodle production. Deep placement of calcium cyanamide was effective for winter wheat cultivation in a heavy clay upland field converted from paddy production, and it appears that for deep placement, an application rate of 15g-Nm^<-2> is just as effective as 20g-Nm^<-2>.

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