Annual changes in the gross rates of nitrogen transformations in a converted field and yield of soybean
1997
Nishio, T. (Hokuriku National Agricultural Experiment Station, Joetsu, Niigata (Japan)) | Toriyama, K. | Sekiya, H. | Kogano, K.
We determined the annual changes in the gross rates of nitrogen transformations using the 15N-ammonium isotope dilution method for soils where a rice paddy was converted to an upland field. In the converted field, we raised soybean and examined the relationship between growth and yield of soybean and the rates of N transformations. In consequence, the annual changes in the microbial activities of N transformation caused by conversion of the field were revealed. We discussed the environmental factors controlling the changes of N transformations and the effects of N transformations on the growth and yield of soybean. The results obtained were as follows: 1) Both the rates of N mineralization and immobilization decreased in the first year after field conversion. Although the mineralization rate increased after the second year, the immobilization rate continued to decrease year by year. The nitrification rate increased with the passage of time. The decrease in immobilization rate could be attributed to the decrease in moisture content of the soil and to competition for ammonium between heterotrophs and nitrifiers. 2) The yield of soybean increased for three consecutive years after field conversion. This increase could be attributed to the increase in the supply of soil nitrogen during the initial and middle stages of the growth of soybean, because an increase in the number of pods and grains per pod resulted in the increased yield. 3) the amount of weeds growing in the converted field increased year by year. The mixing of the weeds at the time of plowing would result in enhancement of the initial growth of soybean, because the weeds mixed into the soil enhance the absorption of N by soybean during the initial growth stage
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