The indirect effect of earthworms on methane flux in two temperate pasture soils

Methane, a potent greenhouse gas, is produced or consumed by microorganisms in soil, depending on soil conditions. In general, anaerobic conditions promote activity of methanogenic Archaea, whereas aerobic conditions promote methane consumption by methanotrophic bacteria. Earthworms may have an effect on methane flux in soil by altering soil pore space, aeration, moisture content, and organic carbon availability, all of which affect the activities of methanogens and methanotrophs. As the range of European and Asian earthworm species expands throughout the United States, their potential effects on methane flux may have global implications. My research focused on the effects of earthworms on methane flux in temperate pastures and on the potential mechanisms involved. In November, 2009, I established 24 experimental plots each at Peckham Farm (PF), Kingston, and at the W. Alton Jones (WAJ) campus of the University of Rhode Island, West Greenwich, both in southern Rhode Island. Three treatments (n=8) were implemented: (i) earthworms removed (D−), (ii) earthworms removed, counted and returned to the plot (D+), and (iii) undisturbed (U). Weekly methane flux measurements were made from April to November, 2010. Soil moisture and temperature were continually recorded. Methane flux measurements were also made three times over a 24-h period at each site to quantify diurnal patterns. I also conducted a second experiment to examine the relationship between earthworm population size and soil properties, and methane flux that did not disturb the soil. I determined methane flux in these supplemental plots at seven randomly selected locations in an area of the field adjacent to the manipulated plot experiment. Once gas sampling was completed, I sampled the soil under each chamber for pH, organic matter content, NH4 and NO3 concentrations, soil moisture, and soil temperature. Earthworms were counted, weighed and identified. I also examined the potential effects of earthworms on acetoclastic and hydrogenotrophic methanogenesis, and on methane oxidation, using cast, burrow, and bulk soil from soils inoculated with Anecic and epi-endogeic earthworms, the two most common earthworm ecological groups found at the research sites, and a control soil. Soil from bulk, burrow or casts was transferred to microcosms with conditions manipulated to favor only one of the three processes—methane oxidation, hydrogenotrophic methanogenesis, or acetoclastic methanogenesis. Microcosms were periodically sampled for methane concentration in the headspace to determine process rates. In the manipulated plots field experiment the majority of the differences were found at the W. Alton Jones site. This is in agreement with the results of the microcosm experiments, which suggest that earthworms may affect methane flux specifically by increasing acetoclastic methanogenesis and decreasing oxidation, and that effects may be site-specific. At PF, the decrease in methane oxidation as found in the anecic casts treatment in the microcosms may have occurred in the field but to a minimal degree, that which was not enough to cause a measurable difference. However, at WAJ, the combination of lower methane oxidation in anecic cast soil, paired with an increased acetoclastic methanogenesis in the epi-endogeic burrow soil, may drive differences in flux. (Abstract shortened by UMI.