Effects of water-nitrogen interaction on resource use efficiency and adaptive strategies in the key growth period of <i>Carex schmidtii</i>

Abstract Climate change and intensified agricultural activities are altering hydrological regimes and nitrogen inputs in wetland ecosystems, yet the mechanisms by which dominant wetland plants coordinate their responses to water-nitrogen interactions across growth stages remain insufficiently understood. This study investigated the dominant species, Carex schmidtii, utilizing a controlled experiment with three water levels (−10 cm, 0 cm and 10 cm) and three nitrogen additions (0, 60 and 120 kg N hm⁻² a⁻¹). Results revealed that C. schmidtii displayed pronounced stage-dependent plasticity to water-nitrogen interactions. Water availability emerged as the key factor determining growth and resource allocation, whereas nitrogen effects were strongly modulated by water conditions. During the vegetative growth stage, water limitation suppressed plant growth regardless of nitrogen addition, with plant height (ZG) in the WLN0 group decreased by 22.16% reltive to the W0N0, and this growth inhibition persisted even under high nitrogen inputs, indicating that nitrogen could not compensate for water deficiency. At low water level, water use efficiency (WUE) and nitrogen use efficiency (NUE) reached highest values and displayed positive correlation, indicating efficient resource utilization. During the reproductive growth stage, nitrogen played a stronger role in promoting structural developement and maintaining functional stability. Nitrogen additions alleviated NUE reduction under low water, whereas high water induced an escape response marked by reduced WUE (−29.10%) and elevated NUE (+119%). These findings highlight contrasting coordination strategies across growth stages and provide new insight into the adaptive mechanisms of dominant wetland plants under shifting hydrological and nutrient regimes.