Our literature review, encompassing a range of scientific fields including soil biogeochemistry, microbiology, geology, archaeology, and in situ observations, conclusively demonstrates millennia-scale biomass persistence. This review illuminates subterranean wood injection (SWI) as a novel and cutting-edge carbon sequestration technique, offering unparalleled efficiency and climate resilience benefits.

Key Takeaways:

• Mummified wood, characterized by minimal degradation of original tissues, arises from inhibition of wood-destroying microbes, reduced oxygen availability, and the absence of harsh chemical or physical conditions.

• Mummification is more likely in environments such as deep submersion in water, burial in impermeable sediments, arid regions, or areas with low temperatures.

• Examples of mummified wood, ranging from 12,000 to 66,000,000 years old, have been discovered across various regions and climates, including Canada, the USA, Europe, and East Asia.

Our Conclusions:

• Natural conditions that suppress microbial activity and limit oxygen availability can lead to the complete preservation of wood for periods ranging from millennia to megaannum. These conditions, which occur sporadically in nature across different climates, are consistently present deep underground, below 5 meters. Therefore, by injecting biomass into apertures at these depths, we can replicate the conditions that have resulted in these mummified wood samples, thus promoting remarkably long-term preservation of biomass and carbon sequestration within its tissues.

Key Takeaways:

• Since 1960, various theories on soil organic matter persistence have emerged, none of which are mutually exclusive. In fact, they can often be reconciled, suggesting that multiple processes contribute to persistence in natural soils. Factors that aid in the persistence of soil organic matter include:

• Large, complex molecules that enhance chemical recalcitrance.

• High carbon-to-nitrogen (C:N) ratios that restrict microbial activity due to limited substrate stoichiometry.

• Organic matter occlusion, which limits the spatial accessibility of organic matter, necessary substrates, and microbial populations.

• Organic matter with low energetic yield, reducing microbial enzymatic activity and imposing energetic constraints.

Our Conclusions:

• Our process, which utilizes large, lignin-rich biomass particles with high C:N ratios and injects them into apertures that foster organic matter occlusion and spatial obstruction in anoxic conditions with low energetic yield, effectively amplifies natural processes that promote persistence. This leads to the long-term sequestration of carbon.

Key Takeaways:

• Depth contributes significantly to carbon persistence, with a remarkable 1,729 gigatons of carbon sequestered below 20 cm, influenced by both abiotic and biotic environmental factors. Key soil structure elements and conditions that enhance this persistence with increasing depth include:

• Increased compaction, which limits the movement of gases, substrates, nutrients, and microbes.

• A higher presence of carbon-sequestering soil aggregates, facilitated by elevated levels of Calcium and Magnesium.

• A decrease in soil Carbon (C), Nitrogen (N), and Phosphorus (P) with depth, which are vital for the composition, population, and metabolic activity of microbial communities.

• Decrease in temperature constraining microbial respiration

• This crucial soil structure and conditions at depth lead to a drastic decline in microbial diversity and activity—decreasing by factors of 100 and 300, respectively. This is often coupled with an adapted "starvation response" in microbial activity at these depths, predominantly involving anabolic processes that do not produce greenhouse gasses (GHGs).

Our Conclusions:

• Even in soils shallower than our sequestration target depth, the observed trends with increasing depth strongly suggest enhanced carbon persistence and reduced decomposition as depth increases.

Key Takeaways:

• Oxygen limitations significantly underlie the restricted decomposition in subsoils, due to the forced reliance of microbes on alternative electron acceptors such as Fe(III). These alternatives are thermodynamically less favorable, thus inhibiting decomposition and favoring preservation under anaerobic conditions.

Our Conclusions:

• The entirely anoxic conditions in our sequestration aperture mean that the energy required for microbial decomposition is insufficient. This leads to the long-term persistence of injected biomass.

Key Takeaways:

• In unstructured soils, anoxic conditions—regardless of the presence of alternative electron acceptors—uniquely promote the preservation of phenolic carbon, diverging from the typical linear increase in dissolved organic carbon (DOC) with depth. This suggests the simultaneous action of enzymatic and energetic preservation mechanisms on soil carbon under reducing conditions.

Our Conclusions:

• The preservation of soil carbon occurs even in the absence of the compact and occlusive conditions typically associated with carbon preservation, due to anoxic conditions imposing enzymatic and energetic limitations. Therefore, our injection aperture is exceptionally conducive to carbon persistence and decomposition inhibition. It leverages not only the anoxic conditions, which result in enzymatic and energetic limitations on decomposition, but also occlusive conditions that further promote persistence.

Key Takeaways:

• Deep soil horizons (at depths ≥0.3 meters) are generally regarded as biologically inactive, with residence times that increase with depth, often reaching millennia.

• Despite an increase in CO2 levels with depth, atmospheric flux decreases and does not significantly affect surface fluxes. This suggests that CO2 in the subsoil is not rapidly transported to the surface; it may instead stay trapped in soil pores, solution, or be consumed by subsoil autotrophs.

• In several subsoils, CH4 generated by methanogens tends to be consumed by methanotrophs in oxic areas before it can reach the surface. This indicates that CH4 dynamics in soil have a limited impact on carbon sequestration outcomes.

Our Conclusions:

• Even when considering "deep soil horizons" between 0.3 and 1 meter—which are substantially shallower than our sequestration aperture—GHG migration is extremely slow. Due to the soil structure and the presence of oxidizing microbes, this does not result in any significant GHG flux.

Key Takeaways:

• Soil CH4 consumption predominantly occurs within the 0-0.4 meter soil horizon. Beyond 0.4 meters depth, CH4 flux was negligible throughout the observation period.

• Soil CH4 production results from two simultaneous processes: production by methanogens and consumption by methanotrophs. The balance between these processes depends on soil oxygen availability, with higher oxygen levels favoring consumption.

Our Conclusions:

• Given that our sequestration apertures are located well below any oxygen-rich (oxic) regions, any CH4 produced must pass through these methanotroph-rich areas. These regions are more biologically active and are able to consume CH4 before it reaches the atmosphere, effectively preventing atmospheric CH4 flux from any CH4 production that may occur.

Key Takeaways:

• In wetlands, which have the highest CH4 flux levels, 60-90% of the produced CH4 is reoxidized in aerobic zones before reaching the atmosphere.

• Both methanogens and methanotrophs can sustain their populations under unfavorable conditions, maintaining a correlated microbial density.

• Aboveground plant biomass stimulates methane emissions into the atmosphere, as roots provide a pathway for upward methane migration.

• High clay content and soil compaction significantly reduce CH4 emissions by trapping CH4 bubbles in the soil.

• Lower soil temperatures (below 30 degrees Celsius) decrease CH4 production by reducing methanogenic activity, whereas methanotrophs are less temperature-sensitive, showing consistent activity from -1 to 30 degrees Celsius.

Our Conclusions:

• In the specific land type and environmental conditions present on our injection sites, any produced CH4 will be substantially hindered from atmospheric emission. This is due to environmental temperatures that favor methanotrophic activity and inhibit methanogenic activity, thereby promoting CH4 oxidation and reducing CH4 production. Additionally, high clay content and soil compaction present at depth will further impede CH4 migration.