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Environmental Effects on Nitrogen Dynamics in Soybean under Conservation and Clean Tillage Systems

USDA-ARS, P.O. Box 555, Watkinsville, GA 30677
Agronomy Dep., Univ. of Georgia, Athens, GA 30602
USDA-ARS, P.O. Box 555, Watkinsville, GA 30677
USDA-ARS, ARS, P.O. Box 87, Byron, GA 31008

^* Corresponding author.

Soybean [Glycine max (L.) Merr.) obtains required N from the soil via root uptake of mineralized organic matter and fertilizer, and from the air by symbiotic gaseous N₂ fixation and ammonia (NH₃) absorption. The purpose of this study was to evaluate environmental and tillage effects on dynamic N transport under conservation (NT) and clean tillage (CT) management systems. Soil, plant, and microclimate measurements were made concurrently with estimates of gaseous N₂ (using ¹⁵N₂ techniques) and NH₃ uptake from the atmosphere. Soybean growth and N uptake characteristics were similar in both tillage systems, except during a 14-d drought period, which caused water stress in the CT system, but not the NT system. Nitrogen fixation decreased in the CT system to less than half that of the NT system fixation rate, resulting in a seasonal reduction of 29% in the CT system. Net NH₃ transport varied from net absorption by the canopy during periods of adequate soil water to net evolution by the CT system during the drought-stress period. Integration of the net NH_3M flux density curves indicated that approximately 3% of the NT system and less than 1% of the CT system plant N was derived from the atmosphere. Also during the drought-stress period, there was a net loss of plant N to the soil in the CT system indicated by soil and plant mass balance. Insect infestation was much greater in the NT system, reflecting insects' preference for a higher N crop, resulting in a significant N removal from the system. This study showed that soybean in these systems decreased soil N by about the amount of harvested N yield.

Received for publication August 5, 1988.

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