Root‐zone acidity affects relative uptake of nitrate and ammonium from mixed nitrogen sources

Abstract

Soybean plants (Glycine max [L.] Merr. cv Ransom) were grown for 21 days on 4 sources of N (1.0 mM NO^‐ ₃, 0.67 mM NO^‐ ₃ plus 0.33 mM NH⁺ ₄, 0.33 mM NO^‐ ₃ plus 0.67 mM NH⁺ ₄, and 1.0 mM NH⁺ ₄) in hydroponic culture with the acidity of the nutrient solution controlled at pH 6.0, 5.5, 5.0, and 4.5. Dry matter and total N accumulation of the plants was not significantly affected by N‐source at any of the pH levels except for decreases in these parameters in plants supplied solely with NH⁺ ₄ at pH 4.5. Shoot‐to‐root ratios increased in plants which had an increased proporiton of NH⁺ ₄‐N in their nutrient solutions at all levels of root‐zone pH. Uptake of NO^‐ ₃ and NH⁺ ₄ was monitored daily by ion chromatography as depletion of these ions from the replenished hydroponic solutions. At all pH levels the proportion of either ion that was absorbed increased as the ratio of that ion increased in the nutrient solution. In plants which were supplied with sources of NO^‐ ₃ plus NH⁺ ₄, NH⁺ ₄ was absorbed at a ratio of 2:1 over NO^‐ ₃ at pH 6.0. As the pH of the root‐zone declined, however, NH⁺ ₄ uptake decreased and NO^‐ ₃ uptake increased. Thus, the NH⁺ ₄ to NO^‐ ₃ uptake ratio declined with decreases in root‐zone pH. The data indicate a negative effect of declining root‐zone pH on NH⁺ ₄ uptake and supports a hypothesis that the inhibition of growth of plants dependent on NH⁺ ₄‐N at low pH is due to a decline in NH⁺ ₄ uptake and a consequential limitation of growth by N stress.