Interspecific competition alters carbon investment responses of Bolboschoenus planiculmis to nitrogen enrichment from agricultural drainage

Agricultural drainage carrying significant amounts of nitrogen (N) from agricultural applications poses a significant threat to plant growth, composition, and structure in wetland ecosystems. However, how interspecific competition influences plants' responses to agricultural drainage-induced N enrichment has not yet been fully understood. We grew Bolboschoenus planiculmis, a key food source for the Siberian Crane (Grus leucogeranus) at stopover wetlands in northern China, with either Suaeda glauca or Phragmites australis across five mixture ratios under N enrichment (0 g vs. 0.5 g N per 10 kg soil). We investigated the relative competitiveness of B. planiculmis and determined non-structural carbohydrates (NSC), nitrogen (N), and phosphorus (P) concentrations in aboveground and belowground tissues. We observed that N enrichment, compared to the absence of N-addition, enhanced the competitive ability of B. planiculmis when grown with P. australis but reduced it when co-cultivated with S. glauca. N enrichment also decreased NSC concentrations in aboveground tissues and P concentrations in belowground tissues of B. planiculmis. Moreover, under N enrichment, the root:shoot (R/S) NSC ratio of B. planiculmis significantly increased when co-grown with S. glauca, whereas co-cultivation with P. australis resulted in an approximately two-fold higher R/S NSC ratio than when grown with S. glauca. Additionally, NSC concentrations decreased in the aboveground tissues of B. planiculmis with increasing proportion of S. glauca but not with P. australis. These findings highlight that N enrichment from agricultural drainage can shift interspecific competition among wetland plants, particularly disadvantaging sensitive species such as B. planiculmis, with implications for conserving and restoring crane stopover habitats.