Characteristics of extractable soil organic nitrogen determine using various chemical solutions and its significance for nitrogen uptake by crops

More than 90% of the nitrogen (N) in soil is bound in the form of organic N (N org ) compounds. Within the Norg in soil, the available N org is considered to be a fraction of easily mineralizable N org This N org can be estimated based on the amount of released inorganic N when a soil is incubated under paddy and / or upland conditions at an appropriate temperature. However, incubation methods are time-consuming several methods of chemical extraction have thus been developed as substitutes for the incubation methods in many countries. In Japan, an extraction method using neutral phosphate buffer was proposed by Higuchi (Jpn. J. Soil Sci., 52, 481-489, 1981) and has been favorably evaluated throughout the country. We compared Higuchi's method with other methods of extraction, such as by 0.01 M CaCl 2 solution, electro-ultrafiltration (EUF), 0.05 M HCl, etcand summarized the common characteristics of extractable N org as follows: 1 ) similar C/N ratio; 2 ) the primary constituent may consist of protein-like N compounds; 3) the amino acid composition remains very similar, irrespective of soil types, kind of extractants, and organic materials added; 4) the extractable N org in the soil may be uniform in its molecular weight; 5) these compounds are derived from the cell wall components of microorganisms, and 6) the extractable N org 5 adsorbed on clay surfaces in soil, probably through binding to aluminum and iron, which protects it against microbial attacks, and this extractable N org would be an important constituent of available N org in soil. In addition, a recent study by Yamagata et al. (Plant Nutrient Acquisition-New Perspectives-, Springer Verlag, Tokyo, 2001) demonstrated that plant growth and N uptake were enhanced when organic materials as N sources were applied to a soil, indicating that the amount of available N org which increased by the application of organic materials may be taken up by certain species of crops before mineralization. If this phenomenon were to be confirmed more widely, it might be useful to examme the dynamics of available N org in the plant rhizosphere.