Microbial consumption of atmospheric isoprene in a temperate forest soil

Abstract

Isoprene (2-methyl-1,3 butadiene) is a low-molecular-weight hydrocarbon emitted in large quantities to the atmosphere by vegetation and plays a large role in regulating atmospheric chemistry. Until now, the atmosphere has been considered the only significant sink for isoprene. However, in this study we performed both in situ and in vitro experiments with soil from a temperate forest near Ithaca, N.Y., that indicate that the soil provides a sink for atmospheric isoprene and that the consumption of isoprene is carried out by microorganisms. Consumption occurred rapidly in field chambers (672.60 +/- 30.12 to 2,718.36 +/- 86.40 pmol gdw day) (gdw is grams [dry weight] of soil; values are means +/- standard deviations). Subsequent laboratory experiments confirmed that isoprene loss was due to biological processes: consumption was stopped by autoclaving the soil; consumption rates increased with repeated exposure to isoprene; and consumption showed a temperature response consistent with biological activity (with an optimum temperature of 30 degrees C). Isoprene consumption was diminished under low oxygen conditions (120 +/- 7.44 versus 528.36 +/- 7.68 pmol gdw day under ambient O(2) concentrations) and showed a strong relationship with soil moisture. Isoprene-degrading microorganisms were isolated from the site, and abundance was calculated as 5.8 x 10 +/- 3.2 x 10 cells gdw. Our results indicate that soil may provide a significant biological sink for atmospheric isoprene.

FIG. 1

Time courses of isoprene consumption in 30-g samples of fresh soil, autoclaved (sterile) soil, leaf litter, and blank (no soil) incubations. Error bars represent ±1 standard deviation from the means of triplicate samples.

FIG. 2

(A) Time courses of isoprene consumption in whole-soil cores exposed to different initial isoprene headspace concentrations. (B) Enrichment of soil microorganisms with increasing isoprene concentrations resulting in an acceleration of the rate of isoprene consumption.

FIG. 3

Time courses of isoprene consumption in duplicate 30-g soil samples from 0- to 3-cm (A) and 15- to 18-cm (B) layers of a temperate forest soil incubated at different temperatures.

FIG. 4

Isoprene consumption rate versus temperature. An exponential (n = 4) function is fit to the data from 5°C to 30° C shown in Fig. 3A. The curve fit to all data points merely illustrates the increase in isoprene consumption rates between 5°C and 30°C, followed by a rapid decrease at higher temperatures.

FIG. 5

Time courses of isoprene consumption in duplicate 30-g surface (0- to 3-cm) soil samples from a temperate forest incubated in the laboratory with different soil moisture contents.