Emergent Plants Improve Nitrogen Uptake Rates by Regulating the Activity of Nitrogen Assimilation Enzymes

Effectively utilizing aquatic plants to absorb nitrogen from water bodies and convert it into organic nitrogen via nitrogen assimilation enzyme activity reduces water nitrogen concentrations. This serves as a critical strategy for mitigating agricultural non-point source pollution in the Yellow River Basin However, emergent plants' rate and mechanism of uptake of different forms of nitrogen remain unclear. This study determined the nitrogen uptake rates, nitrogen assimilation activities, root properties, and photosynthetic parameters of four emergent plants, <i>Phragmites australis</i>, <i>Typha orientalis</i>, <i>Scirpus validus</i>, and <i>Lythrum salicaria</i>, under five NH₄⁺/NO₃^- ratios (9:1, 7:3, 5:5, 3:7, and 1:9) using ¹⁵N hydroponic simulations. The results demonstrated that both the form of nitrogen and the plant species significantly influenced the nitrogen uptake rates of emergent plants. In water bodies with varying NH₄⁺/NO₃^- ratios, <i>P. australis</i> and <i>T. orientalis</i> exhibited significantly higher inorganic nitrogen uptake rates than <i>S. validus</i> and <i>L. salicaria</i>, increasing by 11.83-114.69% and 14.07-130.46%, respectively. When the ratio of NH₄⁺/NO₃^- in the water body was 9:1, the uptake rate of inorganic nitrogen by <i>P. australis</i> reached its peak, which was 729.20 μg·N·g^-1·h^-1 DW (Dry Weight). When the ratio of NH₄⁺/NO₃^- was 5:5, the uptake rate of <i>T. orientalis</i> was the highest, reaching 763.71 μg·N·g^-1·h^-1 DW. The plants' preferences for different forms of nitrogen exhibited significant environmental plasticity. At an NH₄⁺/NO₃^- ratio of 5:5, <i>P. australis</i> and <i>T. orientalis</i> preferred NO₃^--N, whereas <i>S. validus</i> and <i>L. salicaria</i> favored NH₄⁺-N. The uptake rate of NH₄⁺-N by the four plants was significantly positively correlated with glutamine synthetase and glutamate synthase activities, while the uptake rate of NO₃^--N was significantly positively correlated with NR activity. These findings indicate that the nitrogen uptake and assimilation processes of these four plant species involve synergistic mechanisms of environmental adaptation and physiological regulation, enabling more effective utilization of different nitrogen forms in water. Additionally, the uptake rate of NH₄⁺-N by <i>P. australis</i> and <i>T. orientalis</i> was significantly positively correlated with glutamate dehydrogenase (GDH), suggesting that they are better adapted to eutrophication via the GDH pathway. The specific root surface area plays a crucial role in regulating the nitrogen uptake rates of plants. The amount of nitrogen uptake exerted the greatest total impact on the nitrogen uptake rate, followed by root traits and nitrogen assimilation enzymes. Therefore, there were significant interspecific differences in the uptake rates of and physiological response mechanisms of emergent plants to various nitrogen forms. It is recommended to prioritize the use of highly adaptable emergent plants such as <i>P. australis</i> and <i>T. orientalis</i> in the Yellow River irrigation area.