| Abstract | An open laboratory sand column system was developed to study interactions between N₂-fixing pea,Pisum sativum L. and the nitrifying organisms, Nitrosomonas europaea and Nitrobacter winogradskyi or the denitrifier,Pseudomonas fluorescens in the presence of NH₄-ion or NO₃-ion, respectively. In columns watered continuously with 1 mM (NH₄)₂SO₄, nitrification began immediately after inoculation and NO₃-ion production was not inhibited by the plants. Above ambient levels of nitrous oxide were detected in all columns containing nitrifiers. In the presence of plants, nitrifiers were evenly distributed throughout the column; in their absence, they were found predominantly within the top 8 cm of the column. When ¹⁵N-enriched (NH₄)₂SO₄ was added to columns containing nitrifiers and plants,¹⁵N-enriched NO₃-ion was detected in the xylem sap. Nitrogenase activity (as indicated by acetylene reduction rates and atom %¹⁵N of shoot N) was inhibited more in the presence of nitrifiers and NH₄-ion than with NH₄-ion alone. In columns continuously watered with 2 mM KNO₃ and containing a denitrifier, low levels of denitrification were detected in columns maintained at 21% O₂ or at 5% O₂. Nitrate uptake was inhibited from days 26 to 31 in peas grown at 5% O₂. Denitrification may have been limited by available carbon in the root exudates or by competition with roots for NO₃-ion and rates were insufficient to counteract the inhibitory effects of NO₃-ion on symbiotic nitrogen fixation. |
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