Crop Physiology Laboratory:  Research

AMMONIUM / NITRATE
NUTRITION

Dawn Muhlestein: slbdn@cc.usu.edu
Bruce Bugbee: bugbee@cc.usu.edu

Nitrogen Dynamics in ALS: Growing Wheat with 80% Ammonium

Dawn J. Muhlestein, Tom Hooten, Jeanette M. Norton, and Bruce Bugbee Utah State University Crop Physiology Laboratory

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.