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Nitrate Reduction and Nitrite Utilization by Nitrifiers in an Unsaturated Hanford Sandy Loam¹

ABSTRACT

In order to discern any relationship between added nitrate and microbial growth in a field plot, a Hanford sandy loam was infiltrated for 5 weeks at a rate of 5 cm day^–1 with a solution containing 100 ppm -N and 1.25 meq/liter Cl^– as KNO₃ and CaCl₂ respectively. Nitrate-N, NO₂^–-N and Cl^– concentrations in soil solution were determined and corresponding counts of nitrate reducers, denitrifiers, and NH₄⁺- and NO₂^–-oxidizer populations were made.

Ratios of measured to applied concentrations for both NO₃^–-N and Cl^– were nearly identical in all solution samples taken from the unsaturated soil profile, i.e., nitrate reduction was not pronounced and NO₂^–-N was less than 1 ppm.

Nitrate reducer populations (6x10⁴–6x10⁵/g soil) were always larger than those of denitrifiers (3x10³–3x10⁴/g) and varied little with time and depth. Ammonium oxidizers (10¹–10⁵/g) generally decreased with depth. In contrast, NO₂^– oxidizers (10¹–3x10⁶/g) varied markedly with time at all depths. An increase in numbers of NO₂^– oxidizers, after 2 to 3 weeks, at a depth of 60 cm, is attributed to NO₂^– produced by reduction of NO₃^– at anoxic microsites within the profile. Evidently, a cyclic oxidation and reduction of nitrite and nitrate, occurring simultaneously, is responsible for growth of nitrite oxidizers in the profile.

Key Words: nitrification • microbial ecology

¹ Contribution from the College of Natural Resources, University of California, Berkeley. Supported by NSF-RANN Grant No. GI 34733X, Nitrate in Effluents from Irrigated Lands, and the Kearney Foundation of Soil Science.

² Assistant Research Biochemist, Staff Research Associate, Assistant Research Chemist, and Professor of Soil Biology, respectively. Department of Soils and Plant Nutrition, Univ. of Calif., Berkeley, CA 94720.

Received for publication April 25, 1974.