Differences in Ethylene Production among Three Horizons of a Florida Spodosol

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.