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RESEARCH: HYDROPONICS |
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Nitrogen Dynamics in ALS:
Growing Wheat with 80% Ammonium
Dawn Muhlestein, T. Hooton, J. Norton, and B. Bugbee
Recycled nitrogen in Advanced Life Support (ALS) will be predominately
NH4+. Conversion of NH4+ to NO3- in bioreactors can be difficult.
Nitrogen is the only nutrient absorbed by plants as a cation (NH4+) or
an anion (NO3-). High ratios of NH4+/NO3- are considered toxic for
three reasons: (1) Excess acidification of the rhizosphere (2) Induced
Ca2+, K+, and Mg2+ deficiencies and (3) Root carbon skeleton
deficiencies.
Koenig and Pan (1996) reported that increased NH4+ supply increased
yield in soil with supplemental Cl-. The Cl- also increased calcium
uptake. This may be due to improved charge balance facilitating
increased uptake of Ca2+. However, it is not
clear if other anions (e.g. SO4-) might substitute for Cl-.
Nitrifying microorganisms convert NH4+ to NO3-. Padgett and Leonard
(1993) reported significant nitrification in NH4+-based hydroponic
systems. However, Allison and Prosser (1993) found that nitrifying
bacteria occur optimally within pH 7.0-8.5 in liquid media, and
hydroponic solutions are typically controlled between pH 5 and 6.
Surface attached nitrifiers can maintain activities at lower pH than
suspended cells. Root surfaces in hydroponics could provide the
surface necessary for nitrification to occur at lower pH's.
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 a 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+. Increased NH4+ ratio also increased protein content in
seeds. 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.
REFERENCES
Allison, S.M. and J.I. Prosser. 1993. Ammonia oxidation at low pH by
attached populations of nitrifying bacteria. Soil Biology and
Biochemistry 25:935-941.
Hart, S.C., J.M. Stark, E.A. Davidson, and M.K. Firestone. 1994.
Nitrogen mineralization, immobilization, and nitrification. In:
Methods of Soil Analysis Part 2-Microbiological and Biochemical
Properties. R.W. Weaver, J.S. Angle, and P.S. Bottomley (eds). Soil
Science Soc. Of America, Madison, Wisconsin.
Koenig, R.T. and W.L. Pan. 1996. Chloride enhancement of wheat
responses to ammonium nutrition. Soil Science Society of America
Journal 60:498-505.
Padgett, P.E. and R.T. Leonard. 1993. Contamination of ammonium-based
nutrient solutions by nitrifying organisms and the conversion of
ammonium to nitrate. Plant Physiology 101:141-146. |
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