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Differences in Ethylene Production among Three Horizons of a Florida Spodosol

Dep. of Soil Science

R. H. Biggs

Dep. of Fruit Crops, Univ. of Florida, Gainesville, FL 32611

* Corresponding author.

ABSTRACT

Ethylene is a potent plant growth regulator that can inhibit root function at µL L^–1 levels. The potential for microbial production of C₂H₄ is greatest for wet soils. Since citrus plantings in Florida are expanding into areas dominated by seasonally wet soils, the C₂H₄-producing potential of such a soil was examined in two laboratory studies, using soil samples from the Ap, E (A2), and Bh horizons of a sandy, siliceous, hyperthermic Alfic Arenic Haplaquod (Oldsmar series). In one experiment, concentrations of C₂H₄, O₂, and CO₂ were monitored for 21 d in the headspace of closed-system soil samples that were incubated at 27 °C, at either field-capacity or saturation water content. A second study investigated the effects of soil pretreatment (air drying and autoclaving before incubation) on the cumulative evolution of C₂H₄ from incubated soil samples. When incubated aerobically at field-capacity water content, no measurable C₂H₄ was produced from the Ap and E horizons, but low (<1.6 ng g^–1) levels from the Bh horizon. For incubation at saturation water content and anaerobic or near-anaerobic conditions, the ranking for C₂H₄ production was Bh = Ap >> E (maximum C₂H₄ production < 0.4 ng g^–1 soil). Decreases in the levels of O₂ and increases in CO₂ during incubation were ranked Ap > Bh >> E, with the lowest concentration of O₂ at 4% and the highest concentration of CO₂ at 10% in the Ap at field capacity. Neither sulfide production nor significant changes in pH were detected during incubation. Autoclaving three times prior to incubation reduced the production of C₂H₄ in the Bh material at field capacity but did not eliminate it. In the autoclaved Ap and E materials, the rate of C₂H₄ production at saturation increased and then decreased, suggesting a sudden increase in microbial growth.

Contribution from the Inst. of Food and Agric. Science, Univ. of Florida, Florida Agric. Exp. Stn. Journal Series no. 9077.

Received for publication June 20, 1988.