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Nitrate in Unsaturated Zone of an Alluvial Soil in Relation to Fertilizer Nitrogen Rate and Irrigation Level¹

ABSTRACT

Soil samples to the 15-m depth beneath asparagus (Asparagus officinalis L.) and celery (Apium graveolens L.) were taken to determine the NO₃^– concentration in the solution of the unsaturated zone and to estimate the soil N balance. Transit time for water to move to the 15-m depth was calculated from drainage volumes and volumetric water contents. Nitrogen balance was calculated from data for N input, N removal in harvested crops, and water records for the calculated transit time.

The NO₃^– concentration in the unsaturated zone increased with increase in N rate but was inversely related to the leaching volume. Denitrification was assumed to be the cause of high N losses with high irrigation levels. However, the rather high loss of 67.7% in the celery experiment, which received large amounts of chicken manure, was assumed to be a result of both net immobilization and denitrification. A combination of high losses of N, which ranged from 18.3 to 67.7%, and large amounts of excess N, in the NO₃^– form in the soil could have been one cause of the low efficiency of recycling N by these crops. The data suggest that high rates of N combined with high levels of water use are conducive to denitrification in relatively permeable soils.

Key Words: nitrate and salts in drainage water • recycling of N by crops • soil N balance • N use • water use • net immobilization • denitrification

¹ Contribution of the Dep. of Soil Science and Agricultural Engineering with the Dep. of Plant Sciences, Univ. of California, Riverside, cooperating. Financial support from the Kearney Foundation of Soil Science and Water Quality Office of the Environmental Protection Agency, through Grant No. 16060 DOE, is gratefully acknowledged.

² Postdoctoral Fellow and Professor in Soil Science, Dep. of Soil Sci. and Agr. Eng., and Research Specialist, Dep. of Plant Sci., respectively, Univ. of California, Riverside 92502. Senior author is now Visiting Assistant Professor, Dep. of Crop and Soil Sci., Michigan State Univ., East Lansing 48823.

Received for publication February 10, 1972.