Soil Sci.Plant Nutr. 45-3 ,599-608 (1999)
K.Kakuda, H.Ando and M.Harayama
The effect of rice plant growth on the loss of basal nitrogen
(N) through
denitrification in the rhizosphere of subsurface soil
was investigated by
the 15N balance method. Labeled 15N 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 15N from the deep soil was negligible. Thus,
the amount of
unrecovered 15N 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
denitrification N loss in this experiment. The α-naphthylamine-oxidizing
ac
tivity of roots decreased drastically after the cutting
treatment. The
estimated amount of denitrified 15N in the rhizosphere
of the subsurface
soil was significantly higher in the Pcut treatment than
in the Control one
at 30 and 40 days after transplanting (DAT), while it
was comparable in the
two treatments at 52, and 64 DAT. Since a greater amount
of 15N loss was
found to occur when there was no absorption of 15N by
the plants, the
absorption of 15N by plants may have contributed to the
suppression of
denitrification. The amount of immobilized 15N 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.