Can earthworms increase inorganic carbon sequestration in an artificial environment?

Enhanced Silicate Weathering (ESW) has emerged as a promising Carbon Dioxide Removal (CDR) technology. However, it is still not clear which factors could maximize ESW rates. Previous studies have shown the potential of soil biota to amplify mineral weathering rates. Among the relevant soil biota are earthworms, which have been found to enhance nutrient release from soil minerals. This is indicative of their ability to increase weathering rates and further inorganic carbon (C) sequestration. Here we aim to accelerate ESW rates in a bio-reactor through earthworm activity. First, we identified the optimal conditions for earthworm survival and activity in an environment exclusively composed of ground silicate rocks and organic substrate. Second, we determined to what extent earthworms can enhance inorganic C sequestration in such a system. We carried out 5 rounds of 2-month experiments in a climate chamber at 25°C. The set-up of the experiments consisted of 200 columns, each topped by a sprinkler connected to an irrigation system, which allowed for different water irrigation rates and watering frequencies. The leachate of each column was collected in a jerrycan kept at 4˚C. Within this set-up, we used two endogeic earthworm species (Aporrectodea Caliginosa and Allobophora Chlorotica) at different densities (10, 20, and 30 earthworms kg^-1 soil), three types of rock flours (Basalt, Lava and Dunite) of two grain sizes (0.063 and 1.5 mm), one organic source (straw) and two water irrigation rates (125 and 250 ml day^-1 kg^-1 soil) at three watering frequencies (1, 2 and 5 times/day). Rock flours were used not only as single type or single size, but also as mixtures of types and/or sizes. At the end of each experiment, we measured earthworm survival and activity, and inorganic C sequestration rates by summing cumulative dissolved inorganic C in the leachate and the newly formed solid inorganic C content. We found no differences in survival and activity between the two earthworm species, but we did find an optimum for both parameters at a density of 10 earthworms kg^-1 soil. Earthworms showed a clear preference for a mixture of grain sizes compared to single size, and for single mineral type compared to a mixture of mineral types. The response of earthworm survival and activity to the two water irrigation rates was similar, but at a water irrigation rate of 250 ml day^-1 kg^-1 soil a frequency of 5 times/day resulted in higher activity. Preliminary results indicate that earthworms increase mineral weathering rates and thereby sequester inorganic C. We demonstrate that earthworms can thrive in a fully artificial environment designed to ESW rates, removing one hurdle for designing a bio-reactor aimed at optimizing carbon sequestration.