Growth, Root Plasticity, and Nitrogen Allocation of Bolboschoenus planiculmis Under Soda Saline–Alkaline Stress

Bolboschoenus planiculmis is a dominant clonal sedge in saline–alkaline wetlands of the Songnen Plain, and its corms provide important food resources for migratory waterbirds. However, its growth mechanism under saline alkali conditions is still unclear. We combined a field survey with a greenhouse experiment that decoupled saline–alkaline stress into two associated constraints, physiological drought and nitrogen deficiency. Six treatments were established: control, nitrogen deficiency, PEG-induced physiological drought, their combination, moderate saline–alkaline stress, and severe saline–alkaline stress. In the field, soil electrical conductivity and pH increased synchronously, whereas inorganic nitrogen availability was heterogeneous. In the greenhouse, compared with the CK, moderate and severe saline–alkaline stress reduced total biomass by approximately 47% and 51%, respectively, and inhibited leaf growth, root expansion, and corm biomass. It also increased SOD and CAT activities, and H2O2 and MDA accumulated. 15N tracing further showed reduced nitrogen enrichment and lower nitrogen input into corms under saline–alkaline stress, despite partial maintenance of corm allocation. Integrated trait analyses suggested that growth limitation was associated with oxidative damage, restricted morphological development, defense costs, and reduced nitrogen acquisition. These findings provide a plant-level perspective that may inform future assessments of belowground food-resource conditions in migratory waterbird stopover habitats.