Absorption of atmospheric NO2 by plants and soils

Tomato, sunflower and com plants were grown in culture solution containing three different concentrations of ¹⁵N-Iabelled KNO₃ (260 ppm N, 105 ppm N, and 26 ppm N) as a nitrogen nutrient, and fumigated with 0.3 ppm NO₂ for 2 weeks during their vegetative stages. The amount of NO₂ nitrogen absorbed into the plants was estimated by “difference method” and “¹⁵N method.” “¹⁵N method” was found to give more probable values than “difference method.” According to “¹⁵N method,” the nitrogen derived from NO₃ was about 16% (tomato), 22% (sunflower), and 14% (com) of the increased amount of total nitrogen in the whole plants in the 105 ppm N plot, and these percentages increased in the 26 ppm N plot. Difference in nitrogen concentration of the culture solution resulted in big change in the dryweight increase of the tomato and sunflower plants, but the absorption rate of NO₂ nitrogen based on the dry weight changed slightly. The absorption rate of NO₂ nitrogen was around 0.8 mg (gDW)^-1 day^-1 in tomato and sunflower plants, and 0.3 mg (gDW)^-1 day^-1 in com plant. Leaves were found to be an active sink of NO₂ and the nitrogen of NO₂ seemed to be rapidly transformed into compounds of high molecules in the leaf cells.     

Combined effects of soil waterlogging and compaction on rice (Oryza sativa L.) growth, soil aeration, soil N transformations and 15N discrimination

 The combined effects of soil compaction and soil waterlogging on the growth of two rice cultivars (Oryza sativa L., cultivars Kanto 168 and Koshihikari) and soil N transformations were studied in pots. Although waterlogging eliminated initial differences in mechanical resistance between compacted and loose soils, Kanto 168 and Koshihikari roots had, respectively, less biomass and a lower porosity if soil was compacted prior to waterlogging. The cause for this was probably established before waterlogging. Redox values showed that upland soils were well aerated. Loose waterlogged soils contained oxic sites, but compacted waterlogged soils did not. Potential denitrification was stimulated by waterlogging and, to a larger extent, by plant presence. Waterlogging lowered potential nitrifying capacities, by competition between plants and micro-organisms for NH₄ ⁺ rather than by oxygen shortage. Compaction prior to waterlogging benefited the potential nitrifying capacity of soils with either cultivar and the potential denitrifying capacity for soils with Koshihikari. Compaction had no effect on nitrification or denitrification in upland soils. N recoveries were low, especially in pots without plants, as a result from sampling strategy and N loss. On day 42/43 after potting, total δ¹⁵N values of waterlogged pots were positive, whereas after 22 days all pots had negative total δ¹⁵N values. Final δ¹⁵N values of plant parts from waterlogged and upland soils were positive and negative, respectively. Although the δ¹⁵N values generally accorded well with the other results, they did not support higher N losses from compacted waterlogged soils than from loose waterlogged soils with plants, as suggested by potential denitrifying activities. Received: 4 February 2000