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Effects of N source on proton excretion, ionic balance and growth of Alnus glutinosa (L.) Gaertner: comparison of N₂ fixation with single and mixed sources of NO₃ and NH₄

SEP R. TROELSTRA, KEES VAN DIJK and TJEERD BLACQUIÈRE
Plant and Soil
Vol. 84, No. 3 (1985), pp. 361-385
Published by: Springer
Stable URL: http://www.jstor.org/stable/42934779
Page Count: 25
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Effects of N source on proton excretion, ionic balance and growth of Alnus glutinosa (L.) Gaertner: comparison of N₂ fixation with single and mixed sources of NO₃ and NH₄
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Abstract

Non-nodulated Alnus 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 4meq.l⁻¹, and growth was compared 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 at c. 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 to c. 0.5 meqH⁺. 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 source. Growth of Alnus glutinosa appeared to be still normal at a pH of c. 2.8. During the final 3 weeks, only the NH₄ plants ceased growing at a pH of 2.6.

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