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 s oil was investigated in a pot experiment, A solution of ammonium-N-15 was i njected 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 t he 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 p lant growth on the denitrification N loss. About 50% of the applied N-15 wa s 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 pe riod in both treatments. The amount of immobilized N-15 in the (+)Plant tre atment was larger than that in the (-)Plant treatment until 4 DAF, These re sults indicated that plant growth enhanced biological N fixation in soil du ring the first 4 DAF, The amount of denitrified N-15 estimated in the (-)Pl ant 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 mainl y due to differences between the two treatments in the amounts of N-15 reco vered by the plants and immobilized in soil. These results suggest that N a bsorption by the plants and N immobilization in soil reduced the denitrific ation N loss in the rhizosphere soil.