Is Nitrate Necessary to Biological Life Support?

Urea is 85% of the recycled nitrogen in a life support system. Urea is quickly converted to NH₄⁺ but nitrification to NO₃⁻ is difficult. Supplying NH₄⁺ directly to plants eliminates the need for a nitrifying bioreactor. Most 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 (NH₄)₂SO₄, NH₄CI, 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. Increasing calcium and potassium supply in the nutrient solution appears to be critical to ameliorating the detrimental effects of NH₄⁺. Seed protein concentration was increased from 17 to 22% at the highest NH₄⁺ level. These studies indicate that it may be possible to eliminate the need to recycle N as NO₃⁻ in regenerative life support systems.