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 hours. Beetles generated a heterogeneous network of tunnels and dung balls, leading to steep oxygen and pH gradients and an eight-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 hours. 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 micro-scale processes underlying dung beetle-mediated soil modification, sustaining their role as soil ecosystem engineers. • Planar optodes revealed oxygen and pH gradients in soil bioturbated by dung beetles • Transport of dung into the soil generated anoxic and alkaline hotspots • Beetle activity on dung and soil was observed over 96 hours • Oxygen depletion inside the dung ball was confirmed via microsensors • Dung burial in anoxic soil was tracked using fluorescent nanoparticles