Summary
Soil Carbon Sequestration means increasing the carbon content of (usually) agricultural lands by changing soil structure to store more biomass, or implementing practices to preserve existing soil carbon content. Soil carbon may be mineralized carbonate, free organic molecules such as humic acids, and in biomass such as soil bacteria, fungi, and root mass.
History
In his book “Regenesis” (2022), George Monbiot reports that soil erosion is highest in many of the world’s poorest countries, caused by tropical weather extremes and sometimes desperation-driven bad farm practices. In a number of tropical nations, severe erosion affects 70% of the arable land. Worldwide, increasing use of non-organic fertilizer chemicals has led to widespread mineral leaching as excess fertilizer solubilizes minerals and transports them with groundwater and surface water. More intensive livestock grazing - techniques devised in industrial nations - are also diminishing soil quality. Crops have shifted from high regional diversity worldwide, to fewer “standard” commodities: maize, rice, wheat, and soybeans - and intensive grazing is partly a response to more intensive farming with grain-supplemented livestock feed. Taken together, these factors mean that worldwide agricultural soil quality has diminished in many respects over the past century.
2022 - Carbon180.org calls for "Soil Carbon Moonshot - agricultural lands in the world are an immense area, critical to human survival and also critical to sequestration of carbon. These existing lands could be sequestering 5 Gt/year and the soils could simultaneously become more productive and more resilient to climate extremes. Yet there are many questions about how to achieve this and quantify it, and there is virtually no government funding. The Soil Carbon Moonshot plan is for the USDA to spend $2.3 billion over five years to do fundamental research, develop MRV practices, gather and manage data, develop soil management practices, and conduct soil carbon sequestration demonstration trials.
The 2023 USDA budget request includes a variety of climate-related expenditures, including “$21 million to support key climate priorities within the Natural Resources Conservation Service (NRCS), including establishing a soil health monitoring network that will include a network of soil sampling sites, integrating soil carbon monitoring into the conservation planning process, and efforts to increase the internal capacity of NRCS staff regarding key soil carbon and climate smart activities.” This is approximately 4% of the Carbon180 Soil Carbon Moonshot level of effort.
Further citation of unknown factors and the need for research https://doi.org/10.1038/s41467-020-18887-7
A reverse perspective; carbon emissions from land-use changes in China, can be deconstructed to predict how land use changes can also lead to carbon sequestration
Forms and Variations
From “Grassland soil carbon sequestration: Current understanding, challenges, and solutions”, Yongfei Bai and M. Francesca Cotrufo, Science, 377 (6606), • DOI: 10.1126/science.abo2380 (2022) - A major element of SCS is improvement of grasslands, which cover about 40% of Earth’s land surface, excluding Greenland and Antarctica. In grassland biomes, about 50-60% of the biomass is below ground and subsurface biomass production has a disproportionate effect on SOC stabilization. Two forms of SOC are particulate organic matter (POM) - fragmented plant and bacterial mass of high molecular mass; and mineral-associated organic matter (MAOM) - lower molecular mass organic compounds tightly bound to soil minerals and resident for decades to centuries. Grassland topsoil SOC is 50 - 75% MAOM. The MAOM saturation level is about 50 g C/kg.
Factors that affect grassland SOC positively and negatively:
light grazing -5%
moderate grazing -10%
heavy grazing -20% +/-5%
adding nitrogen fertilizers +5% - +10%
adding organic matter +30%
implementing rotational grazing in lieu of continuous +25%
Sequestration rates observed from improved soil management practices (Mg/ha yr) and study durations:
grazing changes 0.25 - observed over 38 years
ecosystem diversity improvements 0.5 - 22 years
fertilization 0.6 - 27 years
legume sowing 0.7 - 8 years
grass sowing 3.0 - 10 years
transitioning from agriculture to grassland 0.9 - 19 years
These management practices targets are cited despite a great lack of research; it is known that subsurface fungal biomass can have substantially greater carbon sequestration potential than soil bacteria, both in terms of quantity and permanence and that the greatest current soil fungi observations are correlated to about 1000 mm per year of rainfall and diverse grasslands ecosystems, but how to best create and support such systems, and how to promote greater sequestration in sub-optimal conditions are largely unknown.
SCS may be considered a subset of “regenerative agriculture”
Adaptive Multi-paddock (AMP) grazing plans - high stock density, short duration rotational grazing - evaluated in the southeastern USA in the past 20 years show increased soil carbon and nitrogen retention, for unknown reasons. Carbon sequestration of 0.3 to 1.6 Gt CO2 equivalent per year is projected for improved grazing land management alone. (https://doi.org/10.1016/j.jenvman.2021.112409)
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Notable Projects
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Further Learning
This Is CDR E02 - Rock Dust
CDR Horizons E05 - Harnessing Crops for Durable CDR
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