Well-decomposed peat materials are called “Muck” or “Sapric peat.” Ash content in peat materials is often used to distinguish the decomposition degree of peat materials worldwide. However, in the case of Japan, ash content does not always reflect the decomposition degree of peat materials. This is because alluvial or volcanic deposition frequently causes a high content of minerals in peat materials. Thus, Japanese peat materials tend to show high ash content, irrespective of the decomposition degree of organic materials. Hence, to design the new classification of peat materials reflecting characteristics and formation process of Japanese peat materials, we reviewed the definition and criteria of peat materials both within and outside Japan. Furthermore, cluster analysis using fiber and ash content relating to peat decomposition degree was conducted to distinguish the types of peat materials distributed on alluvial lowlands in Hokkaido, Japan. The discrimination criteria for each cluster were extracted by the decision tree algorithm. The suggested categories were as follows; fibric peat material (mineral depositic): fiber content ≥25 vol％ and ash content ≥25 wt％, fibric peat material: fiber content ≥25 vol％ and ash content <25 wt％, non-fibric peat material (mineral deposit): fiber content <25 vol％ and ash content ≥45 wt％, non-fibric peat material: ash content <25 vol％ and ash content <45 wt％. This classification system enabled us to explain that the high ash content in peat materials is either by the decomposition of organic materials or the deposition of exogenous mineral materials.
In Japan, attempts have been made on rice and various vegetables to replace chemical fertilizers with methane fermentation digestates. These reports often show successful replacement of chemical fertilizers by digestates in terms of crop yields. In contrast, only a limited number of reports are available that considers the replacement effects on nutrient concentration/content of the crops and the fertilizer efficiency of digestates. This study addressed the latter issue by conducting field experiments for two years on autumn–winter radish, especially using analysis of variance with contrast. Nitrogen fertilization of 50％ and above the standard fertilization scheme following the fertilization guideline was replaced by a digestate containing approximately 2,000 mg L−1 of water–soluble ammonium nitrogen. Compared with the plots that received standard fertilization, radish in the plots applied with the digestate was grown with a limited phosphate supply attributable to low phosphate concentration in the digestate. Radish yield was lower for the group replaced with the digestate than the group fertilized solely by chemical fertilizers in the first year. In contrast, no statistical yield difference was observed between the groups in the second year. The fertilizer efficacy of the digestate for two years was estimated to be approximately 50％ compared to the standard fertilization scheme that was set as 100％. The plots where all nitrogen was basal dressed, irrespective of whether it was applied as chemical fertilizers or the digestate, contained lower amounts of soil inorganic nitrogen after harvest than those applied with nitrogen both as basal and top dressings. In the scene of fertilization by digestates, application at once as the basal dressing was considered a feasible choice from the perspective of nitrogen leaching after harvest.
Variable-rate nitrogen application (VNA) based on nitrogen fertility has been introduced to upland fields in Hokkaido. This technique has been considered effective in fields where nitrogen fertility positively correlates with the variation in crop growth estimated from remote sensing, such as satellite images. This study estimated the surface soil humus contents (SSHCs) as an index of nitrogen fertility and aboveground growth of sugar beet and potato from satellite images in 123 fields in andosol areas of Tokachi region, Hokkaido. Based on the relationship between the above-mentioned soil and crop data, we investigated the number of effective VNA fields. Two satellite images were used for the analysis, one in April 2016 when the surface soil was visible and another in July 2016 (a year of heavy rain) when the crop grew abundantly. From the image in April, we found the highest coefficient of determination (R2=0.71) for the red wavelength in the correlation between image data and the measured value of SSHCs. Therefore, the SSHCs in the surveyed fields were estimated using the regression equation. Sugar beet and potato growth were evaluated by the normalized difference vegetation index (NDVI) of image data in July. The estimated SSHCs and NDVI were resampled by a 10 m grid, and then their relationship for each field was evaluated by linear regression. Resultantly, SSHCs were positively correlated with NDVI at a 1％ level in 20 fields (34.5％) for sugar beet and 31 fields (47.7％) for potato. However, those fields with high SSHCs, which have a negative correlation could have been affected by poor drainage due to heavy rainfall or low pH. In conclusion, those fields with a positive correlation between SSHCs and NDVI in the year of heavy rainfall are effective for VNA introduction in the study area.
We investigated the growth of soybean (Glycine max (L.) Merr.) under different cultivation temperature conditions, and the inoculation effect of useful bradyrhizobial strain and the community structure of infected soybean-nodulating bradyrhizobia to elucidate the effects of climate change on crop production due to global warming. “Orihime,” “Bonminori,” and “Fukuyutaka” were used as the test soybean cultivars, and Bradyrhizobium diazoefficiens USDA110T was used as the test strain. The soybean was grown in a temperature-gradient chamber with low-temperature zone, middle-temperature zone, and high-temperature zone. The results of cultivation test under different temperature conditions showed that inoculation with USDA110 significantly increased plant height, number of nodes, stem and leaf dry weight, the number of pods, pod dry weight, total plant dry weight, and number of nodules. However, all investigation items, except for the number of nodules were significantly reduced with increasing cultivation temperature. Correlation coefficients were significantly positive in many combinations, including the relationship between the number of nodules and stem and leaf dry weight. The occupancy of USDA110 was dominated by inoculated USDA110 in all cultivation temperature zones. However, the occupancy tended to decrease with an increase in cultivation temperature when comparing low- and middle-temperature zones and low- and high-temperature zones. These results suggest that the decreased number of nodules under high-temperature conditions inhibited soybean growth, reducing the growth and yield of soybean. However, the reduction of soybean productivity due to the increase in cultivation temperature may be reduced by ensuring the number of nodules and occupancy of B. diazoefficiens USDA110T．