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

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.

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 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.

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.