Elevated Nitrate Preference Over Ammonium in Aquatic Plants by Nitrogen Loadings in a City River

Abstract Aquatic plants are the dominant biota in retaining nitrogen (N) pollutants discharged into river ecosystems, but the sensitivity of plant N pools and N assimilation to riverine N loadings, which is essential for evaluating river biogeochemistry and functions, remains poorly understood. Here, concentrations and stable isotopes of N in river water and aquatic plants were measured along the Nanming River in Guiyang of southwestern China. Total N (TN) contents of aquatic plants increased, but the increments decreased with river N concentrations. The δ 15 N of plant TN decreased with sewage N loadings along the river, which revealed a distinct shift of plant N preference from ammonium to nitrate. Higher levels of nitrate reductase activity in plants at the downstream river section supported the elevated nitrate preference and assimilation, which might explain the decreased nitrate concentrations observed in the river water of the downstream section. These findings contributed to understanding riverine N biogeochemical processes and their response to N loadings, which can help trace sewage N pollution and its ecological effects on river ecosystems.