Date of Award:


Document Type:


Degree Name:

Master of Science (MS)


Plants, Soils, and Climate


R. L. Smith


Investigations were carried out to study the effect of alternate wetting and drying cycles on the mineralization, nitrification, and denitrification of soil nitrogen in two soils, in the presence or absence of added ammonium and nitrite. Soils were analyzed at the end of each drying cycle for total inorganic nitrogen, ammonium, and nitrite nitrogen. Gaseous products, i.e., ammonia, and the oxides of nitrogen (NO and NO2) were collected in the course of drying for determination.

The drying cycles accelerated the rate of nitrogen mineralization in both soils and further increased the oxidation of applied and available ammonium. The rate of ammonium transformation was faster in Yolo clay loam soil than in Miami silt loam.

A faster rate of nitrification was also observed in Yolo clay loam. This may be because of a higher pH of the Yolo clay loam soil. The successive drying cycles had a pronounced effect on the nitrate formation in both soils.

The extent of nitrite decomposition was inversely related to soil pH. The amount of nitrogen oxides (NO and NO2) evolved were inversely related to soil pH, but significant amounts of NO and N02 were evolved from even the slightly alkaline Yolo clay loam. The loss of nitrogen oxides increased with increased number of drying cycles and, further, the losses of nitrogen increased with increase in nitrite level. The majority of the nitrogen losses from these soils may be due to the direct decomposition of nitrite and possibly slight losses due to an interaction of nitrite and ammonium ions during the course of drying the soil.

A considerable amount of nitrogen remained as undetected in treatments receiving either ammonium or nitrite or both. These losses of nitrogen apparently are other than by the release of NO, NO2, and NH3. The undetected nitrogen loss from the soil system may be either in the form of N2O or N2.

No volatile losses of ammonia were recorded from the acid Miami silt loam soil. The greatest amount of applied ammonium nitrogen volatilized as ammonia in Yolo clay loam during the initial drying cycles. The rate of ammonia volatilization increased with increase in applied ammonium concentration.

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