How earthworms thrive and drive silicate rock weathering in an artificial organo-mineral system

Abstract

To slow the rise in atmospheric carbon dioxide concentrations, Enhanced Silicate Weathering is emerging as a potentially significant Carbon Dioxide Removal technology. However, the biotic controls on rock weathering are not well understood, particularly for key soil faunal groups such as earthworms. Earthworms have shown to possibly enhance weathering, highlighting their potential to be introduced in controlled or engineered settings, such as reactors, to increase carbon sequestration. Here, we determined the potential for earthworms to thrive and to increase weathering rates in an artificial organo-mineral system simulating a bioreactor. We used two earthworm species (Aporrectodea caliginosa [Savigny] and Allolobophora chlorotica [Savigny]) at four densities (10, 20, 25 and 30 earthworms kg⁻¹ organo-mineral mixture), four silicate rock types (two basanites, dunite and diabase) of two to three grain sizes (d50 between 0.026 and 1.536 mm), two sources of organic materials (straw and co-digestate), two amounts of biochar (0 and 100 g kg⁻¹ organo-mineral mixture) and/or enzyme additions (laccase, urease and carbonic anhydrase at 20, 177 and 1955 units kg⁻¹ organo-mineral mixture, respectively), three water irrigation rates (125, 250 and 375 mL day⁻¹ kg⁻¹ organo-mineral mixture) and three watering frequencies (one, two and five times day⁻¹). The experiment was conducted in eight rounds, each one lasting eight weeks, yielding data for a total of 323 experimental units. We measured earthworm survival and activity, as well as several commonly used weathering indicators in the organo-mineral mixture and in the leachate, as total alkalinity, inorganic carbon, pH, electrical conductivity and major cations. Using random forest regression, we found that earthworm survival and activity mainly depended on variables influencing the structure and drainage potential of the organo-mineral mixture, such as the presence of straw and increasing percentages of coarse grain sizes. Furthermore, we concluded that the effect of earthworms on weathering indicators depended on whether they survived or died by the end of the experimental period. Surviving earthworms had a neutral or negative effect on weathering indicators, likely because the experimental duration was too short to detect an increase in inorganic carbon, or because there was an increase in organic rather than inorganic carbon in the organo-mineral mixture. In contrast, dead earthworms enhanced almost all weathering indicators considered, suggesting that microbial processes associated with decomposing earthworm bodies may play a role in enhancing weathering. Our results also emphasize that the role of earthworms in Enhanced Silicate Weathering within bioreactors might be overestimated if weathering indicators exclusively rely on changes in mineralogy and ions release to quantify earthworm effects on carbon sequestration through weathering.