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Nitrogen Transformations in Soil as Related to Structure, Moisture, and Oxygen Diffusion Rate¹

ABSTRACT

The aeration status of a Brookston clay loam was varied by varying aggregate size, moisture content, and moisture tension. Moisture tensions ranged from saturation to about field capacity. Determinations for pH, NH₄⁺, NO₃^- and oxygen diffusion rate were made periodically during 2 months' incubation at 35°C. Oxygen diffusion rate was determined amperometrically, using platinum electrodes 4 mm. long by 0.8 mm. in diameter. Average rates associated with aggregate size and moisture content ranged from 3 to 43 g. O₂ x 10^-8 cm.^-2 minute^-1.

Fluctuations in oxygen diffusion rate with time paralleled changes in concentrations of NH₄⁺ and NO₃^- and appeared to reflect changes in metabolic demand for oxygen by components of the microbial population other than the nitrifiers.

Large discrepancies between NH₄⁺ disappearance and NO₃^- accumulation made it necessary to distinguish between nitrification as the biological oxidation of reduced forms of nitrogen and net nitrification as the observed accumulation of nitrate.

Net nitrification varied with apparent delay period and rate of NH₄⁺ oxidation, and with losses of nitrified N. Rates of ammonium oxidation were related directly to the general level of oxygen supply as measured grossly with the platinum electrode. Apparent delay periods were influenced primarily by anaerobic metabolism in microenvironments too small to be differentiated by the electrodes which were used. Mineral N deficits appeared to be due primarily to enzymatic denitrification at oxygen diffusion rates less than 20; at higher levels of oxygen supply, chemical instability of NO₂^- appeared to become increasingly important because of an accompanying decrease in pH.

¹ Authorized for publication by the Director as Journal Article No. 3135 of the Michigan Agr. Exp. Sta., East Lansing. Part of a thesis submitted by the senior author in partial fulfillment of the requirements for the Master's degree, and supported by the Regional Technical Committee NC-17, "Soil Organic Matter and Structure." Presented before Div. III, Soil Sci. Soc. Am., Chicago, Ill., Dec. 5, 1960.

² Graduate Research Assistant, Associate Professor, and Professor of Soil Science, respectively.

Received for publication March 21, 1963. Accepted for publication May 25, 1963.