The Response of Some Ecologically Distinct Plant Species to Nitrate- and to Ammonium-Nitrogen

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One of the differences between acidic and neutral or alkaline mineral soils is that in acidic soils the predominant form in which nitrogen is available to plants through the microbial mineralization of organic soil matter is, in general, the ammonium ion, and in neutral or alkaline soils it is nitrate. The reason for this is that the activity of the nitrifying bacteria Nitrosomonas and Nitrobacter is reduced in acidic mineral soils which have a pH value of 5 0 and below and consequently (in some acidic soils) little or no nitrification takes place. The question arises as to what effect, if any, the form of nitrogen in the soil has on the growth and distribution of plants in the field. This paper reports the results of a laboratory experiment in which the responses of seedlings, grown in solution culture whose source of nitrogen was either the nitrate or the ammonium form, were measured. Five species were chosen because they were naturally restricted to acidic or to calcareous soils, or else showed wider edaphic tolerance. The nutrient solutions used were maintained at pH values of 4-2, 5-8 and 7-2, representing mean values for acidic, intermediate and circum-neutral soil types. The aim of the experiment was to test the hypothesis that some calcifuge species grow better when nitrogen is available in the ammonium form, that some calcicoles grow better when the nitrate form is available and that widely tolerant species respond equally well to either form of nitrogen. The hypothesis was formulated from (i) the theoretical consideration that, nitrogen being the nutrient element of which the greatest number of atoms is taken up from the soil (Viets Jnr 1965), its form (ionic charge and oxidation state) must have an effect on the metabolism of the plant and therefore on growth and competitive ability, and (ii) several sets of experimental evidence: the oft-quoted statement that nitrogen in acidic soils occurs predominantly in the ammonium form and in calcareous soils in the nitrate form (Jackson 1967); the mass data for crop plants showing differential response to the form of nitrogen supplied in culture media (Hewitt 1966); and the ecological implications of the work of Evers (1964) and Bogner (1968). Evers and Bogner showed differential responses of naturally occurring species in sand and solution culture. Bogner, using a range of pH values (3 3, 6O0, 8 0) and nitrogen concentrations of 98 ppm (7 x I0'3 M),