Carbon dioxide and methane fluxes from experimental salt marsh soil microcosms containing artificial or natural fiddler crab burrows with varying degrees of crude oil contamination

A series of soil microcosm experiments were designed to test the impacts of fiddler crab burrows and oil contamination on greenhouse gas emissions from salt marsh soil. Soil and fiddler crabs were collected from the salt marsh near the Louisiana Universities Marine Consortium (LUMCON) DeFelice Marine Center (29.254474, -90.661858) on 2018-07-09. Experiments were carried out in July and August 2018 and May 2019. Soil microcosms consisted of PVC pipes filled with 30 cm of marsh soil. In an initial experiment, there were 3 crab treatments (no burrow, artificial burrow, and crab burrow) and 4 oil treatments (no oil, low oil, medium oil, and high oil) that were applied in a factorial manner for a total of 12 unique treatment combinations (n = 4 each). Microcosms were incubated for 5 days with treatments applied prior to measurements. Carbon dioxide and methane fluxes were measured using an ultraportable greenhouse gas analyzer (UGGA, Los Gatos Research). Crab burrow depths and the mass of excavated material were also measured. A second experiment provided a more detailed look at crab burrows with 4 non-burrow control microcosms and 20 fiddler crab burrows. Following 5-day incubations, gas fluxes were measured both before and after the crab burrows were plugged with a rubber stopper to calculate the burrow-specific gas flux. A final experiment was conducted with artificial burrows excavated with a small spatula. Burrows had the same size but different depths to determine whether the mass of excavated material or the burrow depth was more important in determining greenhouse gas fluxes. Repeated measurements were made after artificial burrowing activity ceased to determine how long the greenhouse gas fluxes remained elevated following burrow excavation.