Planar optodes reveal spatiotemporal heterogeneity of oxic and pH microenvironments driven by dung beetle activity in soil

Dung beetles (Coleoptera, Scarabaeidae) contribute to soil biogeochemical cycling via dung burial and soil mixing, yet little is known about their impact on soil biogeochemistry at the microscale. We employed planar optode imaging to simultaneously resolve oxygen and pH gradients in soil bioturbated by the tunneling species Onthophagus nuchicornis (Linnaeus, 1758). Using a “soil sandwich” setup, we monitored spatial and temporal changes in the soil microenvironments across a vertical plane over 96 h. Beetles generated a heterogeneous network of tunnels and dung balls, leading to steep oxygen and pH gradients and an 8-fold increase in the 2D oxic-anoxic interface zones. Buried dung balls exhibited persistent anoxia, confirmed via microsensor profiling, with more than 75 % of the volume remaining anoxic for over 45 h. Oxygen depletion was coupled to a rise in pH extending millimeters beyond the anoxic zone. Using fluorescent dye-labeled nanoparticles we were also able to track dung movement under waterlogged conditions, demonstrating continued, albeit reduced, beetle activity under anoxia. The combined effects on oxygen, pH, and the organic matter redistribution enhance microbial habitat heterogeneity and are expected to favor the coupling between aerobic and anaerobic processes, such as nitrification and denitrification. Finally, we obtained quantitative estimates of soil displacement and dung removal, providing direct metrics of the ecosystem services delivered by dung beetles, including enhanced soil porosity and organic matter burial. The novel methodological approach described here provides mechanistic insights into the microscale processes underlying dung beetle-mediated soil modification, sustaining their role as soil ecosystem engineers.