Soil depth, rather than hydrological gradient, dominates uptake of water and nitrogen by Carex thunbergii in a wetland ecosystem

Plant acquisition of nitrogen (N) and water is a dominant aspect shaping plant productivity. Plants can achieve adequate uptake levels by modifying root architecture. Resource acquisition, however, is often merely inferred from the distribution of root biomass rather than being actually measured. We used ²H and ¹⁵N (in the form of NO₃^-) stable isotope labeling approach to measure the contribution of different soil depths to the water and N uptake of a sedge species Carex thunbergii across three groundwater table levels in a subtropical riparian wetland system, China. Twenty hours after labelling, the isotopes ²H and ¹⁵N were traced in the transpiration water and leaves, respectively. Concurrently, we measured the vertical patterns of available N and water, as well as absorptive root traits at different soil depths. With increasing water table level, C. thunbergii exhibited a dimorphic root pattern, one being spongy, acquisitive, shallow roots and the other dense, conservative deep roots. Most soil N and absorptive roots (either measured as root length density [RLD], or absorptive root biomass [ARB]) were both concentrated in the topsoil, with more than 85 % of absorptive roots occurring at upper 10 cm depth. We found that water uptake was more sensitive to increased water table depth, lower ARB, and RLD than was N uptake. Also soil depth (F = 61.85, p < 0.001), rather than the hydrological gradient (F = 16.85, p < 0.001), determined the plant water and N acquisition patterns. Water and N uptake patterns were well correlated with each other when data from all soil depths were combined, and more than 50 % of the variation in uptake of N was explained by water uptake. These patterns were themselves correlated with ARB, RLD, and N availability with increasing water table, respectively, suggesting that edaphic factors might override functional traits in controlling resource use under conditions of saturated soils in this wetland ecosystem. These results indicate that C. thunbergii has distinct depth-specific adaptations in terms of water and N acquisition that were independent from the effects of water table.