Plant nitrogen-acquisition in response to waterfowl grazing along a hydrological gradient

Abstract

Background and aims

Both waterfowl grazing and water table fluctuation can alter plant and edaphic conditions and may affect plant nutrient acquisition strategies interactively in wetland ecosystems. However, our understanding of their combined effect on nitrogen (N) acquisition and associated functional traits remains limited.

Methods

We conducted an in situ study by measuring the responses of a set of traits of roots (i.e. specific root length [SRL], root diameter [RD], specific root area [SRA], root tissue density [RTD]) and leaves (i.e. specific leaf area [SLA], leaf dry matter content [LDMC]) of Carex thunbergii from paired waterfowl grazed and ungrazed plots. Plots were arranged along a hydrological gradient in a subtropical riparian wetland, and we measured plant N uptake rates using ¹⁵N isotope tracer technology to assess impacts of these treatments.

Results

Waterfowl grazing increased plant N uptake rates by inducing acquisitive strategies (e.g. higher SLA, SRA, and SRL) at deep water table. However, under shallower water tables (wetter soils), C. thunbergii shifted to more conservative N-uptake strategy with low N-absorption capacity under grazing. At sites with deep water tables, grazing separated N acquisition traits into acquisitive and conservative strategies in grazed and ungrazed plots, respectively. The usual coordination of above- and belowground traits was decoupled at shallower water tables, likely due to the combined stress of grazing and waterlogging. Redundancy analysis (RDA) indicated that the plant’s N uptake pattern was most strongly affected by soil properties (especially the soil NH₄⁺/NO₃⁻ ratio) at sites with shallower water tables compared with sites with deep water table by RD.

Conclusions

The functional trait-mediated plant N acquisition strategy that we observed under waterfowl grazing might be mitigated by altered edaphic conditions in wetland systems.