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The Effect of Roots on Soil Denitrification¹

ABSTRACT

The effects of plant roots on denitrification were studied using the acetylene inhibition method, both with short-term anaerobic assays for potential denitrifying activity and with intact plants in soil. Experiments with anaerobic assays demonstrated that denitrification activity was greater in the rhizosphere of both pot and field grown plants. A split-plate technique showed that potential denitrifying activity decreased rapidly in the first few millimeters away from roots. These effects were shown to be due, at least in part, to differences in available organic matter.

A new method was developed for measuring short-term denitrification rates of soils with intact plants in an aerobic atmosphere. The soil conditions in these assays were comparable to field conditions. When soil NO₃^- concentrations were high, the results confirmed prevailing opinion that denitrification rates are increased in the rhizosphere. In contrast, the planted soils denitrified at significantly lower rates than unplanted soils when NO₃^- concentrations were low. Thus, denitrification may sometimes be reduced in the rhizosphere. These experiments also indicated that increasing NO₃^- concentration increased the ratio N₂O/(N₂ + N₂O), but that the roots had no consistent effect on this ratio.

¹ Contribution from the Dep. of Crop and Soil Sci. and the Dep. of Microbiology and Public Health, Michigan State Univ., East Lansing, MI 48824. Journal Article no. 8654 of the Mich. Agric. Expt. Stn. The research was supported by NSF Grant DEB 77-19273 and Regional Res. Proj. NE-39.

² Former Graduate Research Assistant and Professor, respectively. Senior author is currently Assistant Professor of Soil Microbiology, Dep. of Agronomy, Univ. of Kentucky, Lexington, KY 40546.

Received for publication July 31, 1978. Accepted for publication June 5, 1979.

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