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Some aspects of the nitrogen cycle in soil of the Douglas-fir forest Garm, Richard


Studies were carried out on nitrifying capacities of duff mull and raw humus soils of the Douglas-fir forest. For this purpose, perfusion apparatuses were set up according to the technique described by I.J. Audus (8) with certain modifications which seemed to improve considerably the application of this apparatus for soil studies. Certain changes were introduced into the method as given by H. Lees and J.H. Quastell (54). The quantitative estimations of ammonia and nitrates were made by colorimetric method using phenoldisulphonic and NaOH-EDTA solution for nitrates and Nessler's reagent for ammonium determinations. Soils used for this experiment were tested for total organic matter and total nitrogen. From the results obtained, the most striking difference between the raw humus (mor) and duff mull was observed in the total absence of nitrification in all samples of mor and the comparatively vigorous nitrification in all samples of mull. There was further confirmation that the acidic condition of mor humus alone is not the limiting factor in nitrification in such forest soils. In duff mull, nitrification occurred over a wide range of pH. In raw humus, after adjusting pH to 6.5 and 7.0 and inoculating with actively nitrifying garden soil, no nitrification was observed. It was of interest to note that nitrification occurred only when mor soil was subjected to complete drying or steam sterilization before being inoculated. This phenomenon might indicate the presence of some inhibitory action against nitrifying organisms. This inhibitory effect is typical of raw humus (mor) but lacking when it is sterilized by steam. In all leachates of raw humus some ammonium was always detected. As far as could be determined, nitrifying duff mull soils have failed to show any significant seasonal variation. It was difficult to establish any correlation between total nitrogen of soils, their carbon/nitrogen ratio, and their capacity for nitrification. It is understood that nitrates, are not the only source of nitrogen for the metabolism of forest trees; nevertheless, nitrates should be regarded as an important ecological factor in the evaluation of forest sites.

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