IMMOBILIZATION, MINERALIZATION AND AVAILA-BILITY TO RICE PLANTS OF NITROGEN DERIVED FROM HETEROTROPHIC NITROGEN FIXATION IN FLOODED SOIL

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The distribution of nitrogen immobilized by heterotrophic N_2-fixing microorganisms in anaerobic soils amended with glucose among various components of the soil-N was determined by a ^<15>N tracer method, and the immobilization and mineralization processes were compared with those from ammonium and native soil nitrogen. The availability of biologically fixed-N to wetland rice plants was also investigated. About 66% of the nitrogen originated from N_2 and 72% of that from NH_4^+ was converted to an amino acid-N fraction after 4 days. At 20 days, 7% of the fixed-N and 13% of the applied-N was mineralized. By drying the soils, most of the mineralized-N was reimmobilized to an amino acid-N fraction. These results suggest that biologically fixed-N undergoes transformation processes almost similar to those of immobilized fertilizer-N. The mineralization rates were 23.4% for the fixed-N and 4.6% for the native soil-N after 16 days. The rice plants had absorbed 34% of the fixed-N and 8% of the soil-N in 42 days. In the newly immobilized-N from N_2, nearly 68% of that taken up by the plants was derived from the amino acid-N fraction. It is concluded that the fixed-N remained as readily decomposable compounds, which were susceptible to microbial attack, making biologically fixed-N more available to the plants than native soil-N.
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