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Nitrogen Cycling in a Multiple-Crop Vegetable Production System

USDA-Agricultural Research Service, Southeast Watershed Res. Lab., P.O. Box 946, Tifton, GA 31793;

Doyle Smittle

Dep. of Hortic., Univ. of Georgia, Tifton, GA 31793.

^* Corresponding author.

ABSTRACT

Denitrification losses of N and crop uptake of N were measured in intensive vegetable crop production systems on a Bonifay sand soil (loamy, siliceous, thermic, grossarenic Plinthic Paleudults) in the Georgia Coastal Plain. Nitrogen uptake efficiencies for total aboveground biomass averaged 59% of N applied to crops, but only 28% of fertilizer N was removed in crop harvest. Over the course of the study, a total of 388 kg ha^–1 of N fertilizer was applied and a total of 90 kg ha^–1 was removed in crop harvest. Most of the remainder of the fertilizer N (about 290 kg ha^–1) was available for leaching as nitrate (NO₃). Even though N fertilization rates were high, denitrification rates were low, <2 kg ha^–1 yr^–1. Additions of nitrate and glucose to undisturbed soil cores showed that denitrification was limited by C availability and by lack of anaerobic conditions in the field. In the first year of the study, denitrification rates before fertilizer addition and irrigation were higher than after. The method of fertilizer application (broadcast vs. applied in irrigation water) had no effect on denitrification rates or crop N uptake.

Key Words: Denitrification • Sand soils • Water filled pore space • Irrigation • Fertilizer • Fertigation • Water quality

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