N₂O emissions from rice phyllosphere and rhizosphere when supplied with different nitrogen forms and light intensities

Objectives Paddy ecosystem is an important source of N₂O emission, this paper is aimed to clarify the effects of different nitrogen forms under different light intensities on N₂O emission from rice phyllosphere and rhizosphere and the mechanism of these effects.

Methods Hydroponic methods with rice were adopted in small incubators in which the light was controlled. The incubators were separated into inner and outer chambers, and the above-ground part of rice was completely sealed in the inner chamber and the roots in the outer chamber. The N₂O emissions from rice phyllosphere and rhizosphere were measured by gas chromatography method. It was firstly studied about the effects of three N forms (NO₃^–-N, NH₄NO₃-N, NH₄⁺-N) on N₂O emission of rice phyllosphere and rhizosphere under weak light (8:00-18:00, 4000 Lux) and same N applications (90 mg/L). Then the experiments were conducted to further study the effects of different N forms on N₂O emission from rice phyllosphere and rhizosphere under different light intensities (weak light, 8:00-18:00, 4000 Lux; 18:00-22:00, 0 Lux; strong light, 8:00-18:00, 8000 Lux, 18:00-22:00, 0 Lux and natural sunlight, respectively).

Results 1) Under NO₃^–-N, NH₄NO₃ and NH₄⁺-N with weak light, the mean rates of N₂O emission from phyllosphere and rhizosphere were 6.37, 5.03 and 0.46 μg/(pot·h), respectively, and 16.30, 15.71 and 1.31 μg/(pot·h), respectively, during rice tillering stage, and similar results were obtained during rice flowering, seeding and mature aging stages under the same conditions. 2) Under weak light, the N₂O emissions from rice phyllosphere treated with NO₃^–-N, NH₄NO₃ and NH₄⁺-N were 10.47, 3.70 and 0.26 μg/(pot·h), respectively, and 20.83, 10.82 and 2.08 μg/(pot·h) with strong light, respectively, during flowering and seeding stages. There were significant differences in N₂O emissions among the three N forms under these two light intensities, but no significant difference in N₂O emissions between NO₃^–-N and NH₄NO₃ under the sunlight. 3) Under NO₃^–-N, NH₄NO₃ and NH₄⁺-N with the weak light during flowering and seeding stages, the N₂O emissions of rice rhizosphere were 27.76, 5.19 and 0.30 μg/(pot·h), respectively. Under the strong light, the N₂O emission rates were 20.83, 10.82 and 2.08 μg/(pot·h), respectively, and 16.49, 20.21 and 1.74 μg/(pot·h), respectively, under the sunlight. The N₂O emission from rhizosphere was high with high light intensity under NH₄NO₃. No significant difference was observed in the N₂O emission between NO₃^–-N and NH₄NO₃ under sunlight, but did under weak light. 4) There was a significantly positive linear regression relationship between N₂O emission from rice phyllosphere( Y) and rhizosphere( X) ( Y = 1.963 + 0.444 X, R² = 0.661, P < 0.01).

Conclusions The N₂O emissions from rice phyllosphere and rhizosphere can be increased significantly by NO₃^–-N, followed by NH₄NO₃. The N₂O emissions is also increased with the increase of light intensity. The N₂O emissions of phyllosphere can reflect the N₂O emissions of rhizosphere. Therefore, while applying N fertilizers, ammonium fertilizers should be preferentially used and nitrate nitrogen and fertilizers containing nitrate nitrogen should be avoided.