Root Plasticity and Elemental Stoichiometry Are Associated with Competitive Shifts Between Azolla and Lemna Under Different Nitrogen Levels

Nitrogen (N) availability fundamentally shapes the community structure and competitive dynamics of floating macrophytes in paddy ecosystems. This study investigated the competitive interactions between Azolla and Lemna by applying a gradient of N concentrations (0–12 mg L−1) across two experimental periods (November–January and March–May). Our results demonstrate a clear divergence in resource-use strategies between the two species: Azolla exhibited stronger stoichiometric homeostasis and a more conservative growth profile, retaining a competitive advantage under N-limiting conditions. Conversely, Lemna displayed a more opportunistic strategy, gaining a competitive advantage in N-rich environments through greater morphological plasticity and luxury nutrient uptake. This nitrogen-driven shift in competitive balance was associated with differences in root traits and stoichiometric flexibility. Stoichiometrically, Lemna exhibited greater flexibility in nutrient balance, including higher phosphorus accumulation under N-rich conditions, which may support rapid biomass expansion. Differences between the two experimental periods were also associated with variation in trait expression, suggesting that temporal context influenced how the two species responded to N enrichment. These findings highlight the importance of nitrogen management in steering floating-plant communities in paddy ecosystems: low-N inputs may help maintain Azolla-dominated communities with biofertilizer potential, whereas high-N conditions may favor Lemna and its rapid nutrient uptake.