Comparison of Absorption Rates between Ammonium and Nitrate Nitrogen in Plants

Absorption rares of ammonium ion(NH4+) and nitrate ion(NO3-) for 24 hours were compared using two absorption solutions, which were a single salt solution of ammonium nitrate (NH4NO3) and a complete nutrient solution containing NH4NO3. Test plants were Oryza sativa (Rice), Hordeum vulgare (Barley), Lactuca sativa (Lettuce), Cucumis sativus (Cucumber), Daucus carota (Carrot), Brassica pekinesis (Chinese cabbage), Spinacia oleracea (Spinach) and Raphanus sativus (Radish). From the absorption characteristics of NH4+ and NO3- between a single salt solution of NH4NO3 and a complete nutrient solution containing NH4NO3, the test plants were classified into goup 1(rice,barley and lettuce), which absorbed NH4+ more rapidly than NO3- in both absorption solutions, group 2(cucumber and carrot), which absorbed NH4+ slightly more than NO3- in the single salt solution of NH4NO3, the tendency of which was reversed in the complete nutrient solution containing NH4NO3, and goup 3(Chinese cabbage,spinach and radish), which absorbed NO3- clealy more than NH4+ in the complete nutrient solution, whereas the absorption of NH4+ or NO3- was almost equal in the single salt solution of NH4+NO3-. The above classisication of plants could be explained by the balance of a repressive or competitive characteristics of NH4+ absorption mainly as sociated with a capacity for absorption of calcium ion(Ca2+) and magnesium ion(Mg2+), and the relative root affinity to NO3- that can be evaluated by the relative absorption of NO3- to mono-phosphate ion(H2PO4-) in plant roots. The group 1 plants are the so-called acid tolerant plants, which appeared to be tolerant to NH4+, and showed a marked pH decreased during the 24 hours of the absorption experiments in those plants. By contrast, the group 3 plant seemed to prefer NO3- and divalent alkaline earth cations to the other nutrients in the complete nutrient solution containing NH4No3, and the pH decrease during each absorption experiment was small.