[Nitrate Uptake Kinetics and Correlation Analysis in an Agricultural Drainage Ditch]

To investigate the whole-reach nitrate (NO₃^--N) uptake dynamics in a headwater agricultural stream, we performed five pulse tracer additions of a reactive solute (as KNO₃) and a conservative solute (as NaBr) in an agricultural drainage ditch in Hefei district, Chaohu Lake basin, from October 2016 to April 2017. The TASCC (tracer additions for spiraling curve characterization) approach and Michaelis-Menten (M-M) method were applied for the simulation of NO₃^--N uptake dynamics. Results showed that the ambient areal rate of total NO₃^--N uptake U_amb varied from 11.40 to 69.13 μg ·(m² ·s)^-1 with an average of 34.45 μg ·(m² ·s)^-1, and the ambient uptake velocity V_f-amb averaged 0.24 mm ·s^-1 and varied from 0.07 to 0.43 mm ·s^-1 across three well-mixed sub-reaches in the study. The ambient uptake length S_w-amb averaged 199.06 m with a range from 92.51 to 405.74 m, which was much smaller than the length of the drainage ditch (about 2.5 km), suggesting that the agricultural drainage ditch had a high potential for NO₃^--N retention. Generally, the M-M model fit the NO₃^--N uptake dynamics well, and the maximum uptake U_max ranged from 158 to 1280 μg ·(m² ·s)^-1 with a mean of 631.13 μg ·(m² ·s)^-1. The half saturation constant K_m ranged from 0.16 to 5.52 mg ·L^-1 with a mean of 1.46 mg ·L^-1. According to correlation analysis, S_w-amb was negatively correlated with NO₃^--N_amb, and U_ambwas significantly positively correlated with NO₃^--N_amb, while other nutrient spiraling metrics were not correlated with the NO₃^--N ambient concentration. Hydrological conditions had no distinct effect on the NO₃^--N retention, but both the width variability Ф_w and variability in cross-sectional area Ф_A were significantly correlated with most of the nutrient spiraling metrics, indicating that geomorphic features in the drainage ditch evidently impacted NO₃^--N uptake.