title: Cyclic variations in nitrogen uptake rate of soybean plants:  effects of pH and mixed nitrogen sources.
abstract: To determine if the daily pattern of NO3- and NH4+ uptake is affected by acidity or NO3- :  NH4+ ratio of the nutrient solution, non-nodulated soybean plants (Glycine max) were exposed for 21 days to replenished, complete nutrient solutions at pH 6.0, 5.5, 5.0, and 4.5 which contained either 1.0 mM NH4+, 1.0 mM NO3- [correction of NO3+], 0.67 mM NH4+ plus 0.33 mM NO3- (2:1 NH4+ : NO3-) [correction of (2:1 NH3+ : NO4-)], or 0.33 mM NH4+ plus 0.67 mM NO3- (1:2 NH4+ : NO3-).  Net uptake rates of NH4+ and NO3- were measured daily by ion chromatography as depletion from the replenished solutions.  When NH4+ and NO3- were supplied together, cumulative uptake of total nitrogen was not affected by pH or solution NH4+ : NO3- ratio.  The cumulative proportion of nitrogen absorbed as NH4+ decreased with increasing acidity; however, the proportional uptake of NH4+ and NO3- was not constant, but varied day-to-day.  This day-to-day variation in relative proportions of NH4+ and NO3- absorbed when NH4+ : NO3- ratio and pH of solution were constant indicates that the regulatory mechanism is not directly competitive.  Regardless of the effect of pH on cumulative uptake of NH4+, the specific nitrogen uptake rates from mixed and from individual NH4+ and NO3- sources oscillated between maxima and minima at each pH with average periodicities similar to the expected interval of leaf emergence.
authors.affiliation: Department of Soil Science, North Carolina State University, Raleigh.
authors.forename: C D
authors.initials: CD
authors.lastname: Raper
authors.forename: J K
authors.initials: JK
authors.lastname: Vessey
authors.forename: L T
authors.initials: LT
authors.lastname: Henry
authors.forename: S
authors.initials: S
authors.lastname: Chaillou
mesh_terms: D001692:Biological Transport; D005308:Fertilizers; D006863:Hydrogen-Ion Concentration; D018527:Hydroponics; D009566:Nitrates / Q000493:pharmacokinetics*; D009584:Nitrogen / Q000493:pharmacokinetics*; D018517:Plant Roots / Q000378:metabolism* / Q000502:physiology; D000644:Quaternary Ammonium Compounds / Q000493:pharmacokinetics*; D013025:Glycine max / Q000378:metabolism* / Q000502:physiology; D013997:Time Factors
chemical_list: D005308:Fertilizers; D009566:Nitrates; D000644:Quaternary Ammonium Compounds; D009584:Nitrogen