Nitrogen Addition Interacts With Soil Water Status to Determine Plant Nitrogen Acquisition in a Subtropical Wetland

Subtropical lake-wetlands are subjected to seasonal water table fluctuations (WTFs) and increasing nitrogen (N) deposition, with shifts in the relative availability of ammonium (NH₄ ⁺) and nitrate (NO₃ ^-). However, how N chemical composition, soil water status, and their interactions affect plant N uptake remains poorly understood. We conducted an in situ experiment with Carex thunbergii to test the combined effects of (1) soil water content (SWC; 30% and 60% field capacity; 90% excluded due to limitations in ¹⁵N labeling) and (2) four N addition treatments (NH₄ ⁺:NO₃ ^- ratios in 1:3, 2:2, 3:1, plus a no-N control). Plant biomass production and preferential N uptake form were assessed using ¹⁵N isotope tracers (¹⁵NH₄NO₃ and NH₄ ¹⁵NO₃). At 30% SWC, increasing the proportion of NH₄ ⁺ in N additions induced a preference shift in C. thunbergii from NO₃ ^- to NH₄ ⁺, a pattern that persisted even 1 year after N inputs ceased. Meanwhile, C. thunbergii consistently favored NH₄ ⁺ over NO₃ ^- at 60% SWC. However, the shift in uptake strategy did not consistently enhance biomass production; optimal plant growth occurred only under higher soil moisture (60% SWC) with a balanced NH₄ ⁺:NO₃ ^- ratio of 2:2. Regression analyses further revealed that the preferential uptake of NH₄ ⁺, rather than NO₃ ⁺, was linked to increased belowground biomass and a higher root-shoot ratio. These findings suggest that a shift in the N chemical composition can rapidly regulate plant N acquisition strategies in a soil water-dependent manner, with potential consequences for wetland nutrient cycling and plant performance.