Retention Characteristics And Coupling Uptake Effect Simulation Of Nitrogen And Phosphorus In Headwater Streams Based On TASCC

Headwater streams are numerous and widely distributed,which play an important role in improving and regulating the water quality downstream of the basin.In order to reveal the kinetics characteristics and coupling uptake mechanism of nitrogen and phosphorus,ten field instantaneous pulse experiments including single nutrient and dual-nutrient addition were conducted in two headwater streams of Ershibu river in Hefei suburb,Chaohu Lake basin,from October 2017 to May 2018.NaCl and NaBr were selected as the conservative tracers;the reactive nutrients were KNO₃（or NH₄Cl）and KH₂PO₄.The dynamic nutrient spiraling metrics calculated by TASCC（Tracer Additions for Spiraling Curve Characterization）method were used to evaluate the retention potential and analyze the uptake kinetics of NO₃-N,NH₄-N and PO₄-P.The dynamic nutrient spiraling metrics from single nutrient and dual-nutrient addition were used to analyze the difference of nitrogen and phosphorus nutrient uptake rates between decoupling and coupling condition.The corresponding mechanism was revealed by simulating the kinetics of nitrogen and phosphorus coupling uptake.The main environmental factors affecting the coupling uptake of nitrogen and phosphorus were identified by correlation and regression analysis.The main results of this research were summed up as follows:（1）The results of dynamic nutrient spiraling metrics based on TASCC showed that the average S_w-amb of NO₃-N,NH₄-N and PO₄-P were 464.37m,395.72m and 303.22m,respectively,which were significantly lower than the total length of the test stream.It means that the stream has a certain degree of retention and purification ability for NO₃-N,NH₄-N and PO₄-P.（2）The uptake kinetics of NO₃-N,NH₄-N and PO₄-P were well simulated by the Michaelis–Menten model,and the average maximum uptake rate(U_max)of the corresponding nutrients were 0.16mg/（m²·s）,0.25mg/（m²·s）and 0.17 mg/（m²·s）,respectively.According to the partial least squares regression（PLSR）,the temperature and complexity of channel morphology are important factors affecting the retention process and potential of NO₃-N,NH₄-N and PO₄-P,while other factors have relatively small impact.（3）The maximum uptake rate(U_max)of NO₃-N and PO₄-P at the dual-nutrient addition were 15.38%⁸1.25%and15.38%¹27.78%higher than those obtained at the single nutrient addition,while the half-saturation constant（K_m）of NO₃-N and PO₄-P at the dual-nutrient addition were 15.38%⁴1.71%and 8.54%⁴0.43%lower than those obtained at the single nutrient addition,respectively.Similarly,the U_max of NH₄-N and PO₄-P increased by 34.38%⁸0.95%and 46.15%¹15.38%,but the corresponding K_m decreased by 8.23%⁴8.70%and 12.66%⁴0.63%at the dual-nutrient addition,respectively.Which implied that mutual promotion（i.e.,synergy effect）of nutrient uptake between NO₃-N（or NH₄-N）and PO₄-P.（4）The coupling uptake kinetics of NO₃-N（or NH₄-N）and PO₄-P was quantitatively simulated by the response surface model.The average U_max of NO₃-N and PO₄-P were 0.28 mg/（m²·s）and 0.39 mg/（m²·s）at the dual-nutrient addition,respectively.The average U_max of NH₄-N and PO₄-P were 0.44 mg/（m²·s）and 0.33mg/（m²·s）at the dual-nutrient addition,respectively.In addition,there were varying degrees of deviation in the fitting results of nutrient maximum uptake rate(U_max)between the M-M model and the response surface model.Moreover,all fitting results of U_max from the M-M model were consistently lower than those from the response surface model.（5）According to the correlation and regression analysis,the complexity of channel morphology is the key factor affecting the coupling uptake of nitrogen and phosphorus,followed by hydrological factors,and the influence of background concentration is relatively minimal.