Upon my recent second viewing of the “Kiss the Ground” documentary, I felt the need to share my thoughts on the film’s optimistic claims about Earth’s soils capacity to store carbon dioxide for climate stabilization.
The film, in its pursuit of a utopian “silver bullet” solution, overlooks significant details. A misleading graphic shown early in the film presents a simplified and false carbon cycle, omitting the return of CO2 from the soil to the atmosphere via microbial respiration — thus removing one half of the carbon cycle. Soil sequesters and stores atmospheric CO2, but it’s complicated.
Earth’s soil contains approximately 2,500 billion metric tons of C, almost twice the amount in the atmosphere and vegetation combined. Estimates of the global sequestration potential of agricultural soils range from 0.4-1.8 gigatons per year (Gt/yr). Global carbon dioxide emissions from fossil fuels and industry were approximately 37 GtCO₂ in 2021. You can easily see the significant gap in thes numbers. Approximately two-thirds of the total increase in atmospheric CO2 is a result of the burning of fossil fuels, with the remainder coming from Soil Organic Carbon (SOC) loss due to land use change (Lal 2004), such as the clearing of forests and the cultivation of land for food production.
How does soil store carbon?
Plants via photosynthesis absorb CO2 and then utilize water and sunlight to synthesize simple sugars made of C, H, O to form leaves, stems and roots. Plants send some of that carbon to the root system to feed the microbial communities. The invisible C sink in the soil contains many forms, but we mostly talk about the Organic (Soil Organic Matter-SOM) or the Inorganic (carbonates) forms. SOM is typically 1-5% of the total soil composition and contains 58% C, with the remaining consisting of other minerals. Microbes must utilize C in Soil Organic Matter as it is an inherent part of the carbon cycle and agricultural food production. The microbiology in the soil breaksdown the carbon compounds and uses them to metabolize and grow, respiring CO2 into the atmosphere. The majority of soil carbon sequestered in soils is recycled, not stored. There is great complexity in regards to how carbon functions underground.
Johannes Lehmann’s recent paper titled “The Contentious Nature of Soil Organic Matter” suggests that soil organic matter is a continuum of progressively decomposing compounds and that humic substances (the stable C form in soils) are not observed with modern analytical techniques.
To accumulate any carbon in the soil, the Net Primary Productivity (NPP), all the C fixed and stored in plants, has to exceed the rate of soil microbial respiration. The next step is stabilizing that carbon in the soil to protect it from that microbial activity. One key way to do that involves the formation of soil aggregates, which ties directly to the tenets of the soil health principles. Only when carbon can be stabilized for long periods can we increase the soi’s C stocks.
The limitations
With our modern farming techniques, we’ve drastically cut back the efficiency of the system for capturing carbon and for plants to work at their photosynthetic capacity. The C sequestration potential of soils should be a component of any climate mitigation strategy, but the storage potential in agricultural soils can be overestimated with respect to current scientific data. Most agricultural crops are carbon sinks for only a short period during the growing season, and a lot of that carbon is removed in the plants at harvest.
Even long-term studies — like the one conducted by Gregg Sanford, et. al. as part of the Wisconsin Integrated Cropping System Trial (WICST) — cast doubt on the net soil carbon benefits with various current agricultural management techniques (no-till, inclusion of perennial crops, manure applications and grass pasture systems). Their research highlights the limitations of soil carbon storage using current agricultural management as a pactical climate mitigation tool. Climate change is also making it harder for soils to naturally store carbon because warming soils have the potential to release more CO2. Periods of thawing of permafrost soils in the Arctic are also releasing more CO2.
There are so many different soil types and other abiotic variables that influence the C sequestration and storage capacity to draw simple conclusions. Getting a diversity of growers worldwide to adopt regenerative practices to increase soil C capacity will also provide a scaling challenge, and Earth’s soils do have a saturation point.
Realistic hope
Implementation of soil health practices (limiting or eliminating tillage, keeping diverse living roots in the ground for as long as possible and integrating livestock in specific situations) provide essential benefits for agronomic production and profitability. Enacting the aforementioned soil health principles will reduce the need for synthetic inputs, increase plant nutrient availability,promote microbial diversity, reduce erosion, improve water holding capacity and water quality, increase pest/disease resistance and increase yields. Reducing synthetically produced and land applied fertilizers will also reduce Greenhouse Gas (GHG) emissions (especially N2O and CH4). Through proper management of soil microbes in agricultural production systems, soils will be better suited to supporting C sequestration and stabilization (i.e., increasing the F:B ratio in soils). Fungi have the capacity to protect carbon from respiration by building their cell walls and expanding their hyphal networks and helping to build soil aggregates.
Perennial plants with deep root systems can also push carbon exudates deeper into the soil profile to reduce the amount of the carbon recycled into the atmosphere. Inoculated biochar is also a promising option to help increase the soil carbon storage capacity in certain circumstances and should be utilized on more agricultural lands.
Soil-based carbon torage can and should be a tool to help fight climate change, but likely cannot realistically take carbon out of the atmosphere as fast as we are currently adding it. In agriculture, we need to allow carbon to cycle to plant and soil health. It’s by putting that carbon back in the soil and letting the microbes to function optimally that we’ll see an increase in plant primary productivity, and ultimately regeneration of the landscape.
To halt global warming, a multiple tiered strategy to increase store carbon in worldwide soils must be coupled with drastic cuts in greenhouse gas emissions. Ultimately, the best way to combat climate change is to reduce fossil fuel consumption and utilize more renewable resources. Improving agronomic soil and plant productivity and subsequent yields for a growing population is essential to heal degraded agricultural land and farmer’s profits, but may not be the “silver bullet” to solve the Earth’s climate crisis.
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Adam Brown is Michigan Agriculture Environmental Assurance Program technician with the Leelanau Conservation District. He has a background in ecology and a B.S. from Western Michigan University in earth science with minors in environmental studies and biology. Prior to becoming a MAEAP technician, he and his wife Haley Breniser owned and managed a certified organic fruit and vegetable farm called Undertoe Farm in Kewadin. He has a passion for sustainable agriculture with a focus on soil health.
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