We studied the effect of irrigation intensity and soil temperature on the nitrate concentration in leaching water. Four lysimeters (0.4 × 2 m) were treated with two levels of irrigation intensity combined with two levels of soil temperature, respectively. Both irrigation intensity and soil temperature were changed monthly according to the climate in Miyazaki Prefecture, Japan. The experiment was carried out for 14 months. Four crops, "Benrina" (Brassica sp. ), spinach, potato and radish were cultivated in that order. The amount of leaching water was large in high irrigation treatments but small in high temperature treatments, caused by the large amount of evapotranspiration. Much nitrogen was uptaken by crops during low irrigation treatments and high temperature treatments, although the amount of nitrate leached was large during high irrigation treatments and low temperature treatments. Mean nitrate concentrations for the whole period of leaching water during low irrigation treatments were larger than those during high irrigation treatments. This result indicates that nitrate in leaching water was diluted by irrigation water in the fallow period, especially during high irrigation treatments.

View full abstract

Pot and field experiments were carried out in 1987 and 1988 at YanBan Agriculture College, JiLin Province, China, to examine the influence of temperature in the growing period and the manuring practice on the grain yield of paddy rice to establish the appropriate manuring practice to sustain the stable production of rice in an area of northeastern China. The temperature of water in the pot experiment was set at three levels: 15, 25 and 35℃. Four kinds of manuring practice were performed : chemical fertilizer, compost, chemical fertilizer plus compost and no fertilizer. The same amounts of N, P and K were applied in the fertilized treatment and were 120, 77 and 130 kg ha^<-1>, respectively, in the field experiment. In the pot experiment, the grain yield was almost the same at water temperatures of 25 and 35℃ but significantly decreased at 15℃. At 35℃, chemical fertilizer application gave the highest grain yield, whereas the grain yield was the highest in the compost application at 15℃. At 25℃, the grain yield was the highest in the chemical fertilizer plus compost application. In the field experiment, the influence of the manuring practice was examined. Temperature in the growing season was high in 1987 while low in 1988 as compared to the mean temperature. In 1987, the grain yield was in the order of chemical fertilizer, chemical fertilizer plus compost, compost and no fertilizer plots, having yields of 5.1, 4.9, 4.5 and 4.2 Mg ha^<-1>, respectively. The grain yield in 1988 was considerably decreased by the lower temperature during the growing season, and was in the order of compost, chemical fertilizer plus compost, no fertilizer and chemical fertilizer plots, having yields of 3.7, 3.3, 3.0 and 1.9 Mg ha^<-1>, respectively. The application of chemical fertilizer is appropriate to maintain a high grain yield in a higher temperature year and the application of compost is recommended to suppress the yield reduction in lower temperature years.

View full abstract

Soil erosion from fields was investigated in hilly lands in Shikoku where agricultural practices are becoming shorthanded. As the results, soil loss calculated for terraced paddy fields and upland fields was about 3 t ha^<-1> y^<-1> and 8 t ha^<-1> y^<-1>, respectively. Those values were the sum of soil loss of appearance (less than 1 t ha^<-1> y^<-1>) and average soil mass returned to the field. Large soil loss (about 90 t ha^<-1> y^<-1>) was calculated to occur within several years immediately after the abandonment of fields. Though soil loss did not largely increase in fields which changed to mowing grass fields (about 12 t ha^<-1> y^<-1>), there was a significant increase in mowing grass fields which change to wild fields (about 40 t ha^<-1> y^<-1>). Soil loss decreased in wild fields which had changed to be stable (about 10 t ha^<-1> y^<-1>). The soil losses measured in abandoned fields were larger than those observed in the frame plot measurement. Surface runoff intruding from zones above the fields was regarded as one of the main causes for soil erosion. To estimate the potential soil erosion on fields when they are abandoned, the Universal Soil Loss Equation (USLE) was revised by the addition of a term related to the erosion caused by intrusive surface water runoff. Using the new equation, the soil loss caused by abandonment was properly estimated.

View full abstract

1) The clay mineralogical composition of cultivated soils of Nagasaki Prefecture was grouped into the following 8 types according to previous analytical data : smectite, smectitic, chlorite/mica/vermiculite/intergrade (I), chlorite/mica/vermiculite/intergrade (II), kaolin mineral (I), kaolin mineral (II), allophanic and noncrystalline mineral-layersilicate minerals. 2) The mapping of these 8 clay mineralogical types was found to correspond well with the distribution of cultivated soils in Nagasaki Prefecture, suggesting the usefulness of the clay mineralogical map as soil information on a regional basis covering the whole prefecture. 3) The clay mineralogical map could be a good index for the evaluation of soil productivity, other than land suitability classification, from the close relationship between the clay mineralogical types and the cation exchange capacity and specific surface area of soils. 4) The clay mineralogical map was suggested to give available information on land-use planning, soil management, manuring practice, prevention of environmental pollution and prediction of landslide-prone areas, and some examples are shown.

View full abstract

The change of nitrate nitrogen concentration in the surface water of a paddy field irrigated with spring water flowing from the Kushibiki plateau in Saitama Prefecture was examined, and the δ^<15>N value was determined. 1) The average nitrate nitrogen concentration of irrigated spring water was 15.9 mg L^<-1>(range : 15.7-16.3 mg L^<-1>), and its corresponding δ^<15>N value was 8.5 %_0 (range : 8.3-9.1%_0). 2) The average nitrate nitrogen concentration and δ^<15>N value of the drainage water under the paddy field were 10.5 mg L^<-1> (range : 9.8-11.2 mg L^<-1>) and 10.4 %_0 (range: 9.83-14.11%_0), respectively. 3) It was estimated from the nitrate nitrogen concentration and δ^<15>N value that a major part of the nitrate nitrogen concentration in the irrigation water disappeared by denitrification in the paddy field.

View full abstract