Abstract
In 1992 and 1993, maize(Zea mays L. var. DK250), potato(Solanum tuberosum L. var. Toyoshiro), soybean(Glycine max Merr. var. Tachinagaha), sugar beet(Beta vulgaris L. var. Beetmonobar)and upland rice(Oryza sativa L. var. Toyohatamochi)were grown in the Andosol(41.7 g C kg^-1 soil, 3.4 g N kg^-1 soil)field at the National Institute of Agro-Environmental Sciences, Tsukuba, Japan. Organic nitrogen was supplied as a 4 : 1 mixture of rice bran and straw. The C/N ratio of this mixture was 19.9. Nitrogen uptake by upland rice, soybean and potato supplied with organic nitrogen was higher than in controls. However, the amount of inorganic nitrogen in the fallow plot applied with organic nitrogen was less than in the controls. These facts suggest that some crops have the ability for nitrogen uptake from organic nitrogen sources and not only from inorganic nitrogen. In 1992 and 1994, maize, soybean and upland rice were cultivated in Andosol pots(vol. 3.8 L)holding approximately 3 kg dry soil with the same organic nitrogen as in the field test. In the rice bran plot, upland rice took up more nitrogen than the other crops even with limited soil volume in pots. This result means that the nitrogen uptake response to organic nitrogen depends on the crop. Several hypotheses were proposed. First, high nitrogen uptake crops cas take up the ammonium, amino acids or relatively high molecules of organic nitrogen more preferentially than the low ones. Secondly, rice has stronger activity in competing with soil microorganisms than the other crops. Thirdly, they secrete organic substances that can support multiplication of microfauna, which results in rapid decomposition of organic matter. Fourthly, rice has superior K_m(Michaelis constant), V_<max>(maximum uptake velocity)and C_<min>(minimum concentration of nutrient) for nitrogen uptake. Studies to test these hypotheses are in progress.
