Hysteresis in nitrate uptake

Accurately quantifying nutrient removal and retention in streams is crucial for the proper management of watersheds as anthropogenic nutrient inputs continue to increase. The Tracer Additions for Spiraling Curve Characterization (TASCC) method is commonly used to estimate the ambient uptake rates of nutrients, such as nitrate (NO3). However, uptake dynamics along the concentration gradient from TASCC additions are often dependent on whether nutrient concentrations are increasing or decreasing, exhibiting hysteresis. The hysteresis in uptake relative to nutrient concentration affects the TASCC method`s ability to obtain accurate estimates of ambient uptake metrics. What properties of the stream that are influencing the hysteresis is unclear, though exchange with subsurface flowpaths, such as the hyporheic zone is suspected. We conducted TASCC additions of NO3 at 5 headwater mountain streams whose hyporheic extent and exchange rates have been measured by 3D geophysical imaging and estimated from solute transport models to see if the hyporheic zone affects hysteresis in NO3 uptake dynamics. We present a single metric for quantifying hysteresis in uptake, M_H, that describes both its magnitude and direction. M_H was positive (clockwise in direction) at all of the streams with measurable uptake. However, M_H was not related to the extent of the hyporheic zone as measured by 3D geophysical imaging or its size and exchange rate estimated by solute transport models. Describing hysteresis in nutrient uptake with M_H will provide a more comprehensive understanding of nutrient uptake dynamics and allow for more accurate estimates of nutrient processing in streams.