Seasonal variations in coupled nitrogen and phosphorus uptake across various NO3¯-N/NH4+-N addition ratios in a vegetated stream: The effect of hydromorphology

Abstract

In-stream nutrient uptake is essential in regulating nutrient transport in fluvial ecosystems, yet knowledge of interactions between multiple nutrient uptake still needs to be improved. To reveal the characteristics of interactions between N and P uptake and their seasonal variability, we conducted a series of short-term (i.e., minutes to hours) nutrient addition experiments, including individual nitrogen (N) and phosphorus (P) and combined N plus P additions, in a vegetated forested stream during two contrasting seasons (i.e., the late spring and autumn). Moreover, we explored the responsiveness of N or P uptake rates to their concentrations under the conditions of varying ratios of nitrate-N (NO3¯-N) to ammonium-N (NH4+-N) in all N additions. We found a significant interaction between N and P retention and uptake in the stream where it was P-limited. Across all experiments, the responsiveness of NH4+-N uptake to increased P concentrations increased with decreasing ratios of NO3¯-N/NH4+-N additions in late spring or autumn. In contrast, NO3¯-N uptake showed an opposite pattern. Partial least squares regression results suggested that the hydromorphology (e.g., discharge, channel heterogeneity, aquatic macrophytes) was the main driver and powerfully impacted the whole-reach N and P uptake and their interactions.