Here are a few good articles on why all this soil-carbon-sequestration-by-changing-agricultural practices stuff should be met with skepticism, along with some notes from me: Quanta: A soil-science revolution upends plans to fight climate change – Humus is dead, but the climate community doesn’t really seem to know it? World Resources Institute: Regenerative Agriculture: Good for Soil Health, but Limited Potential to Mitigate Climate Change – Some good links to other resources in this one, as well as examples of papers making claims about the capacity of soils to offset emissions that just don’t make sense/aren’t defensible. MIT Technology Review: Why We Can’t Count on Carbon Sucking Farms to Slow Climate Change – This one has a good treatment of the variability problem (also a fun account of CarbonPlan taking down Climate Action Reserve— CarbonPlan is good at debunking BS climate solutions and CAR is good at developing them). CarbonPlan: Getting Soil Carbon Right – And here is their overview; covers some of the same territory as the above. Earther: Soil May Not Be Able to Keep Storing Carbon Dioxide as Emissions Increase – Summary of Nature paper casting doubt on another sanguine assumption in the soil carbon world. Anthropocene: The hype behind carbon farming comes down to earth – This one’s pretty succinct and covers a lot of problems with the soil carbon hype. CDR Primer: Soil Carbon Sequestration – A more technical overview that isn’t as negative but does introduce the issues and discusses the likely implementation costs (which was one of your original questions). Fuss et al.: Negative emissions—Part 2: Costs, potentials and side effects – Covers several negative emissions technologies (NETs), including soil carbon sequestration (SCS). They look at a bunch of other studies and land on 5GtCO2/yr as the annual C storage potential from SCS— far below the 31GtCO2/yr this Guardian article is talking about. Also, the authors note that permanence is a big issue with SCS. Paustian et al: Soil C Sequestration as a Biological Negative Emission Strategy – Also lands on 5GtCO2/yr “as an upper limit for global biophysical potential” of soil carbon sequestration, upping the number to 8GtCO2/yr in the best case scenario for development of current frontier technologies. This is all from my husband’s list of resources… there is more, if you want, but I figure this is a good start!