Different Effects of Nitrate and Ammonium Forms of Nitrogen on Growth and Photosynthesis of Slash Pine Seedlings

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Abstract
Young slash pine (Pinus elliottii Engelm.) which received 250 lb of nitrogen per acre over 5 weeks showed greater dry matter production when nitrogen was in the ammonium form than when it was in the nitrate form. Over a range of pH from about 5.5 to 7.0 dry matter production of ammonium treated plants was greatest and dry matter production of control plants was least. There was evidence that dry matter production of ammonium treated plants would be less than that of nitrate treated plants below a pH of about 5.0. Differences in the ratio of needle area to plant dry weight did not explain the differences in response to the two N sources, but small differences in distribution of dry matter to roots and shoots were noted. In an experiment in which 400 lb nitrogen per acre were used, depressed growth of nitrate treated plants was associated with a low photosynthetic rate, and stimulated growth of ammonium treated plants was associated with a high photosynthetic rate. Differences between photosynthetic rate of ammonium treated and control plants depended on foliage temperature and there was evidence that dry weight growth response to nitrogen source similarly depended on temperature. All the data were consistent with the view that nitrate and ammonium N sources affected growth differently by influencing the net assimilation rate of seedlings.