Soil water status dominates growth and nitrogen acquisition strategy of Carex thunbergii in response to nitrogen and water additions.

Purpose: Subtropical lake riparian ecosystems experience seasonal water table fluctuations and increased nitrogen (N) deposition with changing N composition. However, the interplay of soil water content (SWC), N deposition chemical composition, and their interaction on plant growth through regulating N acquisition remains poorly understood. Methods: In this controlled experiment with a sedge species Carex thunbergii, we investigated two treatment factors: (1) SWC at 100%, 60%, and 30% of field capacity, combined with (2) N treatments with NH₄⁺:NO₃⁻ ratios of 1:3, 2:2, and 3:1. Treatment verifications were conducted using ¹⁵N isotope tracer (¹⁵NH₄NO₃ and NH₄¹⁵NO₃) technology. Results: Results showed a notable trend of reduced biomass of C. thunbergii with increasing NH₄⁺: NO₃⁻ ratio, especially under high SWC conditions. This negative effect of a high NH₄⁺: NO₃⁻ ratio on plant biomass accumulation also aligned with reduced N use efficiency (NUE). Conversely, C. thunbergii exhibited accelerated N uptake with increasing SWC, with the most pronounced response observed in the treatment of NH₄⁺: NO₃⁻ ratio of 3:1. Principal component analyses provided evidence for SWC-dominated functional coordination between plant below- and aboveground parts in mediating plant N acquisition, while correlation analyses revealed that NUE mainly contributed to belowground productivity of C. thunbergii. Conclusions: Our findings suggest that manipulating water table (as a proxy of SWC) and managing soil NH₄⁺: NO₃⁻ ratios could optimize the productivity of this sedge species in riparian ecosystem. The coordination of leaf-root trait highlights the necessity to integrate above- and belowground traits for a comprehensive understanding of plant N acquisition strategies. Understanding plant N acquisition and use efficiency may help us better predict the potential impacts of future climate change components on ecosystem functions. [ABSTRACT FROM AUTHOR]