Stable isotope analyses of nitrogen source and preference for ammonium versus nitrate of riparian plants during the plant growing season in Taihu Lake Basin.

Plants are vital components of the nitrogen (N) cycling in the riparian zones. Understanding of N uptake strategies of riparian plants, including N sources and preference in N forms (ammonium (NH4+) vs. nitrate (NO3-)), is essential to advance our knowledge on the role that plants play in regulating nutrient biogeochemical cyclings in the riparian areas. In this study, stable N isotopes (δ15N) of three riparian plants, including Acorus calamus, Canna indica and Phragmites australis, and the δ15N of NH4+ and NO3- in different sources were measured during the plant growing season (June-September) in the Taihu Lake Basin. The dissolved inorganic N (DIN) from river water, groundwater, rainwater and soil were considered as the major N sources for plants in the riparian ecosystem. Our results indicated that soil was the largest source for plant N nutrition, with significantly different (P < 0.05) contributions from soil observed among plant species (80.5 ± 4.1, 73.9 ± 2.8 and 58.7 ± 6.1% for A. calamus, C. indica, and P. australis, respectively). Meanwhile, complex water networks, shallow water tables, and high DIN content in rainwater lead to nonignorable N contributions from river water, groundwater and rainwater to plants. Groundwater contributed more percentage of N to P. australis (12.8 ± 3.2%) than A. calamus (6.1 ± 1.9%) and C. indica (8.0 ± 1.5%), which is likely attributed to the deeper roots of P. australis. All plants showed similar N preference for NO3- during the growing season. External environmental conditions and plant characteristics and adaption to more abundant soil NO3- content are possible explanations. Our research could provide important information for vegetation selections during the process of riparian ecological restoration. Reasonable choice of vegetation is essential to plant growth and water quality management, especially in agricultural watersheds where N concentrations are relatively high in agricultural runoff due to the wide uses of N fertilizers.