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Ozone and Soil Moisture Deficit Effects on Nitrogen Metabolism of Soybean¹

Ozone can interfere with nitrogen metabolism of soybean [Glycine max (L.) Merr.], but the effect of O₃ on nitrogen metabolism of field-grown soybean has received only limited attention. A two-year field study was conducted to determine the effects of soil moisture deficit and O₃ on nitrogen metabolism of ‘Davis’ soybean. Two soil moisture regimes, well-watered and water-stressed (WS), were established using different irrigation frequencies. Ozone treatments ranged from 0.025 to 0.107 µL L^–1 (7-h day^–1 seasonal means) and were charcoal filtered, nonfiltered, and two (1983) or four (1984) levels of 0₃ addition to nonfiltered ambient air entering open-top field chambers. Growing conditions were quite different between the two years. The 1983 season was dry and hot throughout vegetative growth. In contrast, 1984 was wetter and cooler, with no requirement for irrigation during vegetative growth. Effects due to WS were more pronounced than those due to O₃. Nitrogen fixation, estimated by the acetylene reduction (AR) technique, was decreased by both O₃ and WS treatments in both years. As O₃ concentration increased, there was a decrease in total AR activity and a more rapid decline of activity during reproductive growth. Water deficit decreased AR activity significantly both years. The activity of nitrate reductase (NR) was also decreased by WS treatments. Ozone effects on NR activity varied between years. Rates of N accumulation were generally lower in higher O₃ and WS treatments. The partitioning of N among various plant parts and concentrations of N within plant parts were altered in WS treatments during all stages of growth and to a far lesser extent by O₃ treatments during reproductive growth. While there were no significant O₃ x water regime interactions, the results demonstrate that soil water status has a pronounced effect on plant response to O₃.

Key Words: Water stress • Dinitrogen fixation • Nitrate reductase activity • Nitrogen partitioning • Nitrogen accumulation • Glycine max (L.) Merr.

² Research assistant, and plant physiologist, Dep. Crop Science; plant pathologist, USDA-ARS and Dep. Plant Pathology; and plant physiologist, USDA-ARS and Dep. of Botany, North Carolina State Univ. Current address of senior author: Forest Science Dep., Texas A&M Univ., College Station, TX 77843.

Received for publication September 5, 1986.