PLANT PHYSIOLOGY , Vol 101, Issue 1 141-146, Copyright © 1993 by American Society of Plant Biologists
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ENVIRONMENTAL AND STRESS PHYSIOLOGY |
Contamination of Ammonium-Based Nutrient Solutions by Nitrifying Organisms and the Conversion of Ammonium to Nitrate
P. E. Padgett and R. T. Leonard
Department of Botany and Plant Sciences, University of California, Riverside, California 92521-0124
Conversion of ammonium to nitrate and contamination by nitrifying organisms
are often assumed not to be significant in ammonium-based nutrient
solutions. To assess this assumption, maize (Zea mays) and pea (Pisum
sativum) were grown under greenhouse conditions in aeroponic, hydroponic,
and sand-culture systems containing 2 mM ammonium chloride as the sole
nitrogen source and evaluated for the activity of contaminating nitrifying
organisms. In all three culture systems, root colonization by nitrifying
organisms was detected within 5 d, and nitrate was detected in the nutrient
solution within 10 d after seedling transfer. In sand culture, solution
nitrate concentration reached 0.35 mM by the end of the 17-d experiment.
Consistent with the microbial ammonium oxidation sequence, nitrite was
detected earlier than nitrate and remained at lower levels throughout the
experiment. Nitrate was found in significant quantities in root and shoot
tissues from seedlings grown in ammonium-based nutrient solutions in all of
the solution culture systems. Maize seedlings grown in an ammonium-based
hydroponic system contained nitrate concentrations at 40% of that found in
plants grown in nitrate-based solution. Determination of nitrate (or
nitrite) levels in the nutrient solution was the weakest indicator of the
activity of nitrifying organisms. A bioassay for the presence of nitrifying
organisms in combination with tissue analysis for nitrate was a better
indicator of microbial conversion of ammonium to nitrate in nutrient
solution culture. The results have implications for the use of
ammonium-based nutrient solutions to obtain plants suitable for research on
induction of nitrate uptake and reduction or for research using solution
culture to compare ammonium versus nitrate fertilization.
