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Diffusional Constraints on Denitrification in Soil¹

ABSTRACT

A model for NO^–₃ reduction and diffusion in aggregated soils is presented and used in conjunction with a derived constant, the Thiele modulus, to examine the conditions which could contribute to a NO^–₃ diffusion limitation of denitrification. The Thiele modulus is a function of the anaerobic radius of an aggregate, the maximum rate of NO^–₃ reduction for a given soil, the K_m value for NO^–₃ reduction, and the intra-aggregate NO^–₃ diffusion coefficient. Results from this theoretical exercise suggested that the anaerobic radius is the most important factor in determining whether denitrification is limited by NO^–₃ diffusion. The model predicts that under anaerobic conditions, only soils with a mean aggregate radius greater than 2 mm will experience a NO^–₃ diffusion limitation. This limitation may not be effective in practice if the bulk NO^–₃ concentration is much greater (100 times) than the K_m for NO^–₃ reduction; this appears to often be the case in fertilized soils but may not be the case in soils of natural ecosystems. Under aerobic conditions, a diffusion limitation will exist in most aggregated soils, since only large aggregates have anaerobic microsites (i.e., large anaerobic radii) where denitrification can occur. A carbon limitation, through its lowering of the maximum denitrification rate, lessens the magnitude of any existing NO^–₃ diffusion limitation. Diffusive limitations were experimentally examined in two ways. First, the ratio of denitrification rates of anaerobic cores relative to anaerobic slurries were used to indicate the degree of any substrate supply limitation; the slurry rates were always greater suggesting either NO^–₃ or carbon was limiting. Second, soil cores were preincubated with either NO^–₃ or a diffusible carbon source (succinate) at 4°C to allow diffusion but to suppress biological responses. These results revealed that carbon, rather than NO^–₃, was limiting denitrification rates in this clay loam soil. These experimental results were in agreement with model predictions.

¹ Contribution from the Dep. of Crop & Soil Sciences and from Microbiology & Public Health, Michigan State Univ., East Lansing, MI 48824-1114. Published as Journal Article no. 11345 of the Michigan Agric. Exp. Stn. This work was supported by National Science Foundation Grant DEB-80-12168.

² Former Graduate Student and Professor of Soil Microbiology, respectively. Present address of the senior author is, Dep. of Soil Science, Oregon State Univ., Corvallis, OR 97331-2213.

Received for publication June 18, 1984. Accepted for publication December 27, 1984.

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