Effects of Different Amounts of Inorganic Nitrogen and Phosphate Buffer Extractable Nitrogen on Nitrogen Use Efficiency in Cultivated Andosols

The purpose of this study was to clarify the influence of the amounts of inorganic nitrogen and degradable organic nitrogen on nitrogen use efficiency of chemical fertilizer. In this experiment, Chinese cabbage (Brassica rapa) and carrot (Daucus carota) were grown in Andosol soils, and the absorption of nitrogen by the plants was investigated using 15N tracer techniques. Positive correlations between chemical fertilizer 15N and the amount of 15N absorbed were observed in Chinese cabbage but not in carrot. 14N was absorbed together with 15N in Chinese cabbage ; however, this relation was not shown in carrot plants. Nitrogen use efficiency of chemical fertilizer were 53-88% and 4-57% for Chinese cabbage and carrot, respectively, showing wide range of fluctuation in both crops. There was no relationship between the nitrogen use efficiency of chemical fertilizer and the amounts of inorganic nitrogen or phosphate buffer exchangeable organic nitrogen in the soil. It became clear from these findings that the amount of inorganic nitrogen and phosphate buffer exchangeable organic nitrogen cannot serve as a predictor of the nitrogen use efficiency of chemical fertilizer.