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RESEARCH: HYDROPONICS |
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Nitrogen
Dynamics in Advanced Life Support
Dawn Muhlestein, T. Hooten, J. Norton, and
B. Bugbee
- American Society for Gravitational and
Space Biology
Nov. 1999; Seattle, WA
ABSTRACT
Conversion of
NH4+ to NO3- in advance life support systems can be difficult. The
ability to supply
NH4+ directly to the plants can eliminate the need
for a nitrifying bioreactor. Many plant physiology textbooks indicate
that NH4+ is toxic to plants, but we now know that this may not be
true if pH is rigorously controlled. However the long term effects of
high NH4+/NO3- uptake ratios are poorly understood. In four studies,
two cultivars of wheat were grown to maturity with
NH4+/NO3- ratios
from 0 to 0.85 in recirculating hydroponic solution. In the third and
fourth studies,
NH4+ was supplied as either (NH4)2SO4, NH4Cl or both.
Contrary to conventional wisdom, there was no beneficial effect of
supplying 25% of the N as
NH4+ compared to nitrate control. The high
NH4+ treatment (85%
NH4+) reduced seed yield by 20% in the first two
studies, but yield was not reduced in the third and fourth studies.
Chloride and sulfate were equally effective as counterbalancing ions
for NH4+. Nitrification potential was measured in the fourth study to
estimate NH4+ conversion to
NO3-. Potential nitrification could
account for a maximum of only 0.2% of N in plants taken up over the
entire life cycle. Studies are currently being conducted using
inoculation and at pH 5.8 and 7.0 to quantify the potential for
nitrification in
NH4+-based hydroponic solutions.
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