Effect of rice plant growth on denitrification in rhizosphere soil

The effect of rice plant growth on the loss of basal nitrogen (RT) through denitrification in the rhizosphere of subsurface soil was investigated by the N-15 balance method. Labeled N-15 was applied to the deep soil layer to distinguish between the N losses in the surface and subsurface soils. Denitrification in pots with whole plants (Control) was compared with that in pots with plants cut off at the base of the culm (Pcut) to evaluate the effect of plant growth on denitrification. The upward movement of the applied N-15 from the deep soil was negligible. Thus, the amount of unrecovered N-15 was equal to the amount of N lost through denitrification in the rhizosphere of the subsurface soil (20-150 mm soil depth). In the Control treatment, values of redox potential at 50 and 90 mm soil depths were negative throughout the experimental period. Therefore, it was assumed that the redox potential could not have been the limiting factor for the N loss through denitrification in this experiment. The alpha-naphthylamine-oxidizing activity of roots decreased drastically after the cutting treatment. The estimated amount of denitrified N-15 in the rhizosphere of the subsurface soil was significantly higher in the Pcut treatment than in the Control one at 30 and 40 d after transplanting (DAT), while it was comparable in the two treatments at 52 and 64 DAT. Since a greater amount of N-15 loss was found to occur when there was no absorption of N-15 by the plants, the absorption of N-15 by plants may have contributed to the suppression of denitrification. The amount of immobilized N-15 in the Control treatment was larger than that of the Pcut treatment throughout the experiment. N immobilization might have also contributed to the suppression of denitrification in the rhizosphere of the subsurface soil.