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Chemical Transformations of Urea-Nitrogen and Movement of Nitrogen in a Shortgrass Prairie Soil¹

ABSTRACT

This study, conducted on the Central Plains Experimental Range during the summer of 1979, was to (i) quantify the temporal dynamics of the transformations of nitrogen (N) in a simulated urine spot, (ii) determine the effect of urea hydrolysis on pH, and (iii) determine the depth of movement of nitrate by leaching. Miniplots of blue grama were established, and open-ended aluminum cylinders 10 cm in diameter and 40 cm in length were embedded in each plot. Simulated urine was applied to two-thirds of the plots at a rate of up to 45 g N m^–2. The remaining plots were controls.

After the simulated urine application, only 6% of the added N remained as urea at 2 days; no urea-N remained at 4 days. Maximum accumulation of ammonium-N (39.9 g N m^–2) was simultaneous with the disappearance of urea. Elevated levels of nitrate were first detected at day 12 when 6.7 g N m^–2 had accumulated. No measurable nitrite accumulated during the experiment.

Through the first 4 days, 83% of the (urea + ammonium)-N in the profile was in the top 15cm of the soil. With the rains near the end of the study, nitrification proceeded, and nitrate was leached through the profile to below the depth of the cores.

Simulated urine significantly affected soil pH to 15 cm. From an initial pH near 6.0, urea hydrolysis increased soil pH from 0.6 to 1.3 units. As nitrification prodeeded after day 40, the pH decreased to below control levels.

¹ This research was supported by NSF Grant no. DEB 7906009.

² Ph.D. Candidate in Range Science Dep., Colorado State Univ.; and Associate Professor, Range Science Dep., and Research Ecologist, Natural Resource Ecology Lab., Colorado State Univ., Ft. Collins CO 80523.

Received for publication January 22, 1981. Accepted for publication May 22, 1981.