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Chapter 8, Soil Carbon Sequestration and Biochar[1], is 50 pages in length and full of details. I will not do it justice in this summary.
Degraded soils have the most potential for restoration through Soil Organic Carbon Management. After a soil saturates with C, SCS needs to continue. Agricultural and grasslands have a feasible technical potential of ~ 2-5 Gt Co2eq per year on a global scale. The economic potential is at $100 per t CO2eq.
There is ~ 2480 Gt of SOC held in the top 30 cm of soil, and ~5,500 Gt CO2eq stored at depths of 1m. The material balance is 220 Gt CO2 eq flowing into the soils, while 220 Gt Co2eq is flowing back to the atmosphere from heterotropic respiration.
SOC management can reduce CO2, N2O, and methane emissions, and therefore act as a greenhouse removal technology.
The UN Framework Convention on Climate Change has an aim to globally increase soil C by 0.4% each year. This translates to 10.2 Gt CO2eq per year sequestered in the top 30 cm of soil.
No tillage can have a significant impact on CO2eq sequestration. 351 studies showed that 16.9 t CO2eq per ha could be sequestered over 10 years.
Arid regions could benefit the most from SOS. Practices like residue management could increase SOC by 194 to 2160 kg CO2eq per ha per year. Compost is an excellent choice as it degrades over decades.
Ledo et al., showed that annual crops, grasslands, and forests to perennial crops could increase SOC by 11 to 20%. Agroforestry can have a significant impact with 26% increase in SOC at 0-15 cm, 40% increase at 0-30 cm and 34% increase at 0-100 cm. With that said, agricultural land become degraded due to erosion, which can be reversed by returning the land to perennial forests and grasslands.
We must be careful when restoring wetlands because an increase in C uptake can be countered by methane emissions.
Biochar has a global mitigation potential between 0.5 and 11.9 Gt CO2eq per year. but the "Overall feasible technical potential for biochar is 2.6 Gt CO2eq per year." It retards the conversion of soil C to CO2. For this reason and more, it is considered a GHG removal technology.
"The costs of Biochar (15-120 $ per t CO2eq) are in a wide range but are comparable to the costs for implementing BECCS (15-400 $ per t CO2eq). In contrast, DAC (25 -1000 $ per t CO2eq) and EW (15 - 3460 $ per ton CO2eq) have a much higher costs and AR(0-50 $ per ton CO2eq) is lower." SCR can deliver CO2 removal at negative costs.
Once again, my notes have a lot of details that are not included here, and the 50-page chapter is packed with details. In a nutshell, SCS can work well with other technologies to enhance soil nutrients, water retention, soil C, etc.
References:
[1] Greenhouse Gas Removal Technologies. (2022). United Kingdom: Royal Society of Chemistry.