I try to avoid politicized climate nutters even when they are posting about things that interest me, but it's not possible for me to avoid them completely.

Everyone on this list subscribes because he or she believes that biochar has the potential to ease the climate crisis.

Which is why I resigned from the list. It was mostly tedious and futile debates about politics. The referenced post was collected and reposted elsewhere or I would never have seen it.

But how great is that potential? It seems pretty clear that we will never be able to bury enough biochar each year to match the amount of fossil carbon being taken out of the earth. In any case, what would be the point? Instead of digging up coal and burning it we could burn the biochar instead.

Well, lots of carbon is already recaptured by living things in the soil and seas as well as chemical reactions with minerals. It is only the excess above that natural rate that increases atmospheric concentrations. The only way it makes sense to burn biochar is if it comes from unclean sources such as shredded tires or municipal wastes contaminated by various toxins. Otherwise, why make biochar in the first place?

We know that biochar increases the fungal and microbial content of the soil. Both contain carbon, and therefore represent an increase in the soil's carbon content. The fungi develop because the plants growing in the soil send sugars down their roots on which the fungi feed and, in return, the fungi release the nutrients locked up by the biochar and make it available to the plant. So the plants themselves sequester carbon. But how much? What we need to know is the weight of the carbon in the fungi and microbes in comparison with the weight of the biochar. Does anyone have a figure? And, as plants grow better on biochar soil, we could also include the carbon held by their bigger roots and, if they are semipermanent, locked up in their greater above-ground stems and foliage.

Also consider "root pruning". In general the plant you see above ground is mirrored below ground by its roots. If that plant is pruned, such as by grazers and browsers, it sloughs off some of its root mass. This is a variable and contingent process that can involve an initial recapture of nutrients stored in the roots by the plant to accelerate regrowth, but much is left behind. The plant then grows new roots as it grows above ground and has more leaf area to manufacture more flesh. Repeated cycles of growth, grazing and subsequent regrowth can be understood as a carbon pump, sucking carbon out of the air and storing it in the soil.

The use of biochar increases the amount of nitrogen-fixing bacteria in the soil. This makes it necessary for the farmer to apply less. So does the fact that the char will hold a lot of whatever external nitrogen is applied, whether in dung/slurry or in artificial fertiliser, preventing it being leached away and also being broken down into nitrous oxide, a powerful greenhouse gas. Nitrous oxide emissions currently account for approximately one third of GHG emissions from agriculture in Ireland. Biochar also reduces the release of methane from the soil and, because it holds phosphate ions, reduces the need for applications, thus saving the energy required by the mining, preparation and delivery process. In short, biochar both reduces greenhouse emissions from the land itself and from the agricultural chemical supply chain. There may also be other greenhouse emissions savings I've not considered.

Well, the use of biochar does seem to stimulate rhizobia, which do suck nitrogen out of the air, but the net effect depends on many other things as well. It's a bit misleading to say that "biochar increases the amount of nitrogen-fixing bacteria in the soil" since it is contingent. It is worth noting that the production of biochar releases energy and many gases, and that some of that can be used to manufacture nitrogen fertilizers. An ideal scenario would be to dope the biochar produced with nitrogen compounds that are coproduced, providing an efficient and parsimonious vehicle for fertilizer application.

We really need to be able to put figures on the tonnage of carbon likely to be taken out of the air by putting biochar into different soils in different parts of the world, and with different types of plants being grown on them. We need figures too for the emissions savings that biochar can achieve in different types of farming so that we can work out where, from a climate perspective, the limited amount of biochar likely to be available should best be applied.

It is an exaggeration to say that we need precise measurement of carbon sequestered in this way. It's of academic interest of course, but the need expressed is in service of bureaucratic schemes rather than any useful thing. The net amount of carbon in the soil varies with conditions. There are seasonal variations as well as longer term cycles. That's the carbon cycle. A net increase over time resulting from agronomic practice will be particular in every way: the location as well as the practices - which necessarily vary too - will yield variable results.

It would be better if bureaucrats and politicians kept their nasty noses out of the biochar tent. Every parcel of land is different, and varies over distances of less than a meter on each parcel. This is a basic tenet of precision agriculture, recognition of micro environments and appropriate treatment for each one. It is conceivable that at some point we will be able to gather and process real-time data on the carbon state of each bit, but not soon. This would enable the bureaucrats to exploit such information in some way - penalize or subsidize or whatever - but it serves no useful purpose other than enabling growers to adapt their management to conditions. Adding the meddling of bureaucrats to that decision process would skew decisions away from sensible choices for farming the land to senseless choices for farming the government.