Abstract
At the panicle formation stage, denitrification N loss in the rhizosphere soil was investigated in a pot experiment. A solution of ammonium-15N was injected with a syringe at a soil depth of 50 mm to distinguish the N losses between the surface and the rhizosphere soil. Denitrification process in the pots with plants ((+)Plant) was compared with that in pots where plants were cut off at the base of the culm ((−)Plant) to evaluate the effect of plant growth on the denitrification N loss. About 50% of the applied 15N was absorbed by the plants during the first 2 d after fertilizer application (DAF) in the (+)Plant treatment. In the (−)Plant treatment, no 15N absorbed by the plants after plant cutting. The amount of exchangeable NH4 +-15N in the 1st soil layer (0∓20 mm) was negligible throughout the experimental period in both treatments. The amount of immobilized 15N in the (+)Plant treatment was larger than that in the (−)Plant treatment until 4 DAF. These results indicated that plant growth enhanced biological N fixation in soil during the first 4 DAF. The amount of denitrified 15N estimated in the (−)Plant treatment (62% of applied N at 2 DAF and 21% at 4 DAF) was larger than that in the (+)Plant treatment (2.1% at 2 DAF and 3.5% at 4 DAF). The rate of suppression of the denitrification N loss (RSDN) by plants was 57% at 2 DAF and 17% at 4 DAF. The higher RSDN at 2 DAF in this experiment was mainly due to differences between the two treatments in the amounts of 15N recovered by the plants and immobilized in soil. These results suggest that N absorption by the plants and N immobilization in soil reduced the denitrification N loss in the rhizosphere soil.