Growth, morphology, and nitrogen uptake adaptivity of Phragmites karka in response to nitrogen forms and oxygen availability

The responses of Phragmites karka to different N-forms and O₂ supply were investigated. Fourteen-day old plants were selected and assigned randomly to six experimental treatments (n = 10), with three N-forms (NH₄⁺, NO₃^-, NH₄NO₃) at equilibrium concentration (500 µM N) and two O₂ conditions (normoxia and hypoxia). All plants were cultivated in greenhouse conditions for 77 days. Under normoxia, NH₄⁺-fed plants had significantly higher dry mass production and NH₄⁺ removal capacity than NO₃^--fed plants. In addition, the NH₄⁺- and NH₄NO₃-fed plants had significantly higher root dry mass and nitrogen use efficiency (NUE) compared to the NO₃^--fed plants. This suggests NH₄⁺ preference under normoxia. Under hypoxia, the plants had reduced growth, biomass production, N removal capacity, and NUE. The plants also modulated root traits by reducing root dry mass, root length and root diameter. The plants had no significant difference in dry mass across N-forms under hypoxia. However, the NH₄⁺-fed plants increased their shoot:root ratios, while the NO₃^--fed plants retained their shoot:root ratios under hypoxia. Moreover, the NO₃^--fed plants had slighter changes in root size and biomass, NH₄⁺ removal capacity, and NUE, suggesting better stability under hypoxia compared to the NH₄⁺-fed plants. The different response patterns between N-form are probably due to the energy preservation of roots associated with N uptake and assimilation. It appears that, P. karka utilizes both NH₄⁺ and NO₃^-, but that it prefer NH₄⁺ when sufficient O₂ is available. In addition, this species can tolerate prolonged hypoxia and adaptation patterns appear to be N-form dependent.