Nitrogen and Phosphorus Stoichiometry of Bolboschoenus planiculmis Plants in Soda–Alkali Wetlands Undergoing Agricultural Drainage Water Input in a Semi-Arid Region

In semi-arid regions, wetlands often face water scarcity, salinity, and alkalinity stresses. Agricultural drainage water has been used to restore degraded wetlands, but it alters water quality and plant growth and resource distribution. Nitrogen (N) and phosphorus (P) stoichiometry reflect plant resource strategies. In China's Songnen Plain, <i>Bolboschoenus planiculmis</i>, a key plant in soda-alkali wetlands and food for the rare white crane (<i>Grus leucogeranus</i>), is impacted by agricultural water input. However, the N and P stoichiometry in <i>B. planiculmis</i> and the influencing water variables remain unclear. This study analyzed N and P contents in <i>B. planiculmis</i> leaves, stems, tubers, and roots, and water variables. Results showed that leaf N content was highest, while tuber P content exceeded that of other organs. Leaf nitrogen to phosphorus (N:P) ratio was highest, and tuber's was the lowest. N and P contents in plants were positively correlated, except between roots and stems. Redundancy analysis (RDA) revealed water temperature (T), oxidation-reduction potential (ORP), N contents, and water depth (WD) as key factors influencing N and P stoichiometry. Structural equation models (SEMs) indicated water T negatively affected plant N, while water nitrate nitrogen positively affected it. Water P content directly influenced leaf and stem P, and ammonium nitrogen affected aboveground P accumulation. Water T and WD directly impacted N:P ratios. These findings show that while agricultural drainage water alleviated aridification and salinization in degraded soda-alkali wetlands, exogenous N and P inputs significantly affected vegetation's N and P utilization strategies.