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Effect of pH on the Rate of Gaseous Products of Denitrification in a Silt Loam Soil¹

ABSTRACT

To examine the effect of soil pH on the rate and products of denitrification, a silt loam soil (Typic Argiudoll) was used that had been collected from a field experiment maintained at various pH levels for 18 years. The soil pH ranged from 4.6 to 6.9 and was nearly uniform in organic C (1.15 ± 0.05%) and texture. Incubation was performed under He in all-glass systems that were sealed throughout the incubation period. The soils were amended with 100 µg/g of NO^-₃-N before incubation. Denitrification products (N₂O, NO, and N₂) and CO₂ were determined by gas chromatography using an ultrasonic detector.

It appears that the rate of organic C mineralization rather than pH controls the rate of denitrification in C-limited systems. At 100 µg NO^-₃-N added per gram of soil, the overall rates of denitrification correlated with the rates of C mineralization (CO₂ evolution) but were not consistently related to pH or to total organic C.

Soil pH did affect the products of denitrification. Nitrous oxide comprised 83% of the gaseous-N products for pH 4.6 and 5.4 when 90% of the NO^-₃ had been denitrified. The N₂O was then rapidly reduced to N₂. The highest amount of NO was observed at pH 4.6 and 5.4. At pH 6.0 the rate of production of N₂O exceeded the rate of reduction only for the first 2 d of incubation. However, N₂O was the dominant product until 83% of the NO^-₃ had been denitrified. At pH 6.9, N₂ was the dominant product after the 1st d of incubation.

¹ Research supported by the College of Agricultural and Life Sciences, Univ. of Wisconsin-Madison, and by National Science Foundation grant DEB 7817404.

² Research Associate and Professor, Dep. of Soil Science, Univ. of Wisconsin-Madison, respectively. Present address of Senior Author is Southern Weed Science Lab., USDA-ARS, Stoneville, MS 38776.

Received for publication July 23, 1981. Accepted for publication July 6, 1982.

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