This study investigated the relationship between tillage method and crack generation in heavy clay soil fields cultivated with paddy rice on reclaimed land in Hachirogata, and analyzed the effect of crack size on plant water uptake and the quality and quantity of rice yield. Cracks in puddled fields appeared in 20 days after midseason drainage, and thereafter propagated rapidly and continuously until the end of August. The puddled fields contained a greater number of cracks than a non-puddled field. Stomatal apertures and transpiration of the rice flag leaves were higher in the non-puddled field with fewer cracks than in plants in the puddled fields. The rice roots on the field surface were frequently severed by the large cracks. The dry weight of the rice roots extending from the plants was 10-37% lower in the puddled fields than in the non-puddled field. Plant uptake of soil water and irrigation water were lower in the puddled field with large cracks than in that with small cracks. The number of panicles and the amount of unhulled rice did not differ among the three plots. The percentage of ripened grains and the thousand-grain weight were lower in the puddle field with large cracks. The rice yield decreased in order of the non-puddled field, the puddled field with small cracks, and the puddled field with large cracks. The rice yield in the puddle field with large cracks was 21% lower than that in the non-puddled field. The percentage of whole grains in the puddled field with large cracks was notably lower than that in the puddled field with small cracks and the non-puddled field. Moreover, the percentage of white as well as cracked rice kernels was remarkably higher in the puddled field with large cracks. These results suggest that the presence of large cracks in a puddled field decreases both plant water uptake and the quality and quantity of rice yield.

View full abstract

This study was conducted to clarify the variation of nitrate concentration of spring water and to estimate the discharge of nitrate from the double-cropped paddy fields into shallow groundwater in Doki-river alluvial fan in Kagawa by means of analysis of chemical properties of Desui-spring water, irrigation-canal water, and river water. The results were: 1) It was estimated that nitrate nitrogen was stored in soil water after the fertilizer input for the second crop and was flown into the shallow groundwater mainly at the first stage of irrigation of paddy fields. 2) Nitrate nitrogen outflow from double-cropped paddy fields to shallow groundwater was estimated to be 34.9 kgha^<-1>, which corresponded to 10.3% of total nitrogen input. 3) The concentrations of annual-mean NO_<3^->N, SO_<4^->S and Cl- of spring water increased with the increase of distance from the fan apex. Especially Cl- concentration showed the highest rate of increase within them.

View full abstract

Field experiments were conducted to evaluate the effects of nitrogen and silica on the yields of rice plants under insufficient light condition. Results obtained were as follows: 1. A linear negative relationship was observed between sunshine irradiation periods and the rice yield ratios of Si-application plot to no Si-application plot in rice straw application experiment. The rice yield ratios were 113, 106, and 101 for the year of shorter, average, and longer sunshine irradiation periods (average irradiation time; 5.5, 7.0, and 8.1h d^<-1>), respectively. This result indicates that Si application is more effective for the rice yield in relatively shorter sunshine irradiation than in longer irradiation. 2. The amounts of SiO_2 and N uptake by rice plants and the rice yields were higher in the year of longer sunshine irradiation (1996) than in the shorter year (1995), regardless of soil type (significant at 5% level). 3. The recoveries of topdressed N by rice plants in the panicle formation stage were 57.9, 40.0, 34.8, and 31.4% under irradiation conditions of 545, 325, 208, and 146 MJ, respectively. 4. Apparent photosynthetic rate of flag leaf was significantly higher in Si application plot than in no Si application plot. In addition, brown rice and dry matter productions in the heading to maturity stages of rice were higher in Si application plot than in no Si application plot.

View full abstract

This study was conducted to clarify a change of shallow groundwater utilization after the establishment of large scale irrigation system built in 1975 in the Doki-river alluvial fan, Kagawa Prefecture. As the shallow groundwater is used for the irrigation of rice cropping from springs named as "Desui", the water level and discharge of springs and water balance in a research field were surveyed in a severe drought year, 1994 and in a normal-weather year, 1995. The results were: 1) Most of the springs are distributed in the wash trench of alluvial fan. 2) Water level of springs kept high from June to October and low from November to May relating to the supply of irrigation water. 3) Water discharge from springs increased in the irrigation period, and decreased or disappeared in the non-irrigation period. The flow velocity of the shallow-groundwater was calculated to be 11m d^<-1>. 4) The mean discharge of spring water per unit area of research field was estimated at 6.1mm d^<-1>, from which the repeated use ratio of spring water to the percolation water was calculated at 54% in the drought summer 1994. The ratio in the summer of normal-weather 1995 became as large as 96%, calculated from mean discharge of spring water 13.3mm d^<-1>. 5) The repeated use ratio of channel water to the percolation water was 31% in the summer 1994 and 12% in the summer 1995, whish showed a decrease compared with the value of summer 1956. 6) These results could be considered that the shallow ground water utilization through springs increased after the construction of large scale irrigation system.

View full abstract