Differences in ethylene production among three horizons of a Florida spodosol

Ethylene is a potent plant growth regulator that can inhibit root function at microL 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 degrees 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 very much greater than 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 very much greater than 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.