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Effects of N source on proton excretion,ionic balance and growth ofAlnus glutinosa (L.) Gaertner: comparison of N2 fixation with single and mixed sources of NO3 and NH4
Authors:Sep R Troelstra  Kees van Dijk  Tjeerd Blacquière
Institution:(1) Department of Dune Research, Institute for Ecological Research, 3233 EG Oostvoorne, The Netherlands
Abstract:Summary Non-nodulatedalnus glutinosa plants were grown for 6 weeks in nutrient solutions using 3 combined-N treatments (NO3; NO3/NH4; and NH4) at a total N level of 4 meq.l–1, and growth was ccmpared with nodulated plants at zero N (N2 fixation). Of the combined-N sources, 100 per cent NH4 resulted in the highest dry matter yields when the solution pH was adjusted daily atc. 6. The dry matter yield was lowest with NO3.During the first 3 weeks, the yield of the N2-fixing plants was as high as that of the NH4 plants, but fell relatively behind during the second 3-week period. These effects could be attributed to higher initial N contents and higher shoot:root ratios, respectively, in the N2-fixing plants. Specific rates of N acquisition in the root were of a comparable order of magnitude for the combined-N and zero-N treatments.When NO3 was taken up, it was almost completely reduced in the roots. Regardless of N source there was a large excess of cations (C) relative to inorganic anions (A) in the plants, which was presumed to be balanced by an equivalent amount of organic anions (C-A). The relatively small differences in generation of organic anions for the various modes of N supply indicated the relative importance of the proton pump when NH4 or N2 was the N source. Proton or hydroxyl-ion effluxes, calculated on the basis of plant analyses, were generally in good agreement with measured excretion values. The acidity generation with N2 fixation amounted toc. 0.5 meq H+.mmol–1 Norg, which was distinctly higher than the range of 0.1–0.2 mentioned by Raven and Smith43 for dinitrogen-fixing plants.Without pH adjustment, specific rates of cation uptake and carboxylate generation were strongly depressed as the acidity increased, when NO3/NH4, NH4 and N2 were the N sources. Growth ofAlnus glutinosa appeared to be still normal at a pH ofc. 2.8. During the final 3 weeks, only the NH4 plants ceased growing at a pH of 2.6.
Keywords:Alnus glutinosa  Hydrogen/hydroxyl-ion efflux  Ionic balance  Ion uptake  N2 fixation  Nitrate reductase  Nitrogen sources  Rhizosphere pH
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