Contribution of nitrogen absorption by rice plants and nitrogen immobilization enhanced by plant growth to the reduction of nitrogen loss through denitrification in rhizosphere soil

At the panicle formation stage, denitrification N loss in the rhizosphere soil was investigated in a pot experiment, A solution of ammonium-N-15 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 N-15 was absorbed by the plants during the first 2d after fertilizer application (DAF) in the (+)Plant treatment. In the (-)Plant treatment, no N-15 absorbed by the plants after plant cutting. The amount of exchangeable NH4+-N-15 in the 1st soil layer (0-20 mm) was negligible throughout the experimental period in both treatments. The amount of immobilized N-15 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 N-15 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 N-15 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.