Carbon Dioxide Fluxes Related to Structural Changes in Earthworm-Worked Soil by Computed Tomography (CT) Scanning.

Earthworms accelerate decomposition and nutrient mineralization, which may stimulate carbon dioxide (CO₂) fluxes from earthworm-worked soils. In particular, earthworm burrows are a favorable habitat for aerobic microbes that produce CO₂; as well, they constitute macropores that facilitate gas diffusion from deeper soil layers to the atmosphere. The objective of this study was to relate CO₂ fluxes to soil structural changes resulting from earthworm activities, quantified from computed tomography (CT) scan data. A total of 12 intact soil columns contained in PVC cylinders (15 cm diam., 15 cm height minus 2 cm of soil removed from top) and composed of sandy-loam soil from a cultivated corn agroecosystem were subjected to CT scanning to map the indigenous porosity. On Day 0 of the experiment, four replicate cores received 3 adult Lumbricus terrestris, 5-6 adult Aporrectodea turgida or no earthworms. At 0, 5, 8, 15, and 25 days following earthworm introduction, the columns were capped at both ends for 1 hour. At each time, gas samples were taken from the headspace through a septum in one of the caps 0, 30, and 60 min after capping. Classical procedures were followed for gas measurement and flux estimation. After removal of earthworms from the cylinders by formalin extraction, the 12 soil columns were CT scanned again. Using the two CT scan datasets, the difference in ‘free space’ in soil columns from beginning to end of the experiment was estimated for different thresholds susceptible to characterize macropores, including earthworm burrows. The strongest correlations were obtained with gas data collected 5 and 8 days after introduction of earthworms, that is, when earthworms were ‘establishing’ in soil columns and their burrowing activity was more intense. We conclude that CT scanning technology is appropriate for studying earthworm activity in situ – particularly for quantifying space occupancy by earthworms – and show that soil structural changes induced by earthworms are correlated with soil CO₂ fluxes.