One bit of conventional farmer wisdom is that you can't build soil organic matter on farmed land, that it is like adding brush to a brush fire. More organic matter will just decompose more vigorously, sending the vast majority of the carbon back to the air as CO2 and CH4. It may be true, you can't win, you can't retain and build up soil organic matter on farmed land, but you can have a warmer fire, which helps some, and there is an ash residual in the form of humus, which does last longer and build up.

But I think that the conventional farmer wisdom assumes that the land will be tilled and left fallow between crops. Non tilled land that is sown with cover crops, intercrops and micro crops can make progress on increasing soil organic matter percentages.

You can build soil organic matter in pastures and prairies since they aren't plowed and don't have fallow periods. Something is always growing unless the weather is too foul (cold, dry etc.) and there's a cap on the soil - think of it as the bark on a tree - that helps protect the flesh. There's still a hot brush fire raging, but you are adding brush at a fast rate, and some of it is being added at depth where the fire burns more slowly. That's how the deep rich soils of molisols were built over the eons before farmers began plowing them up. Even better, in grazed pastures the forage is quickly returned to the soil as manure and urine so that the vast majority of organic matter stays in the field, and the warmth of the brush fire accelerates new growth.

This might be another place where biochar can help since it doesn't seem to decompose in the organic matter brush fire, or if it does then it's at a very slow rate. No one is quite sure just how long it lasts or under what conditions it breaks down if it does so. Study continues. Adding char to farmed soil at a significant rate, say 10% by volume, would be a one off vast improvement in soil carbon that would last quite a long time. There would be some initial shock as the char settles in, adsorbing cations like mad until saturated, and a grower would need to anticipate this and compensate for it by adding cations in sufficient quantities to normalize the soil, but once done the soil would be much better, renovated to some extent, healing the harms of farming in the past in some part. It would also absorb water, which could also be an issue depending on when the char was added.

Some might argue that you can just haul in organic matter in large quantities every year. You can use the nutrients locked in the organic matter, and they are freed by decomposition and mineralization. The brush fire isn't a bug, it's a feature. The ash residual does slowly build up the soil so all you have to do is scour the neighborhood for loose organic matter and haul it to your fields for a couple of decades. This works, sort of, but if all fields were managed this way then there wouldn't be any loose organic matter for anyone to improve their fields with. In the end you have to grow your own or else you are just displacing the deficit, robbing Peter to pay Paul.

That's another way that biochar can help since it is made from woody materials that are often burned since they decompose so slowly. Rather than just burning them they can be pyrolyzed, leaving a percentage of mass as a char residual, while also producing some useful gases that if captured can be used for a variety of purposes ranging from liquid fuels to fertilizers. Orchard and vineyard prunings, some nut shells and stones, forest slash and other woody urban and industrial wastes can be chared and improve farmland.

An increasing amount of such woody materials are burned as fuel for power generation or just heat. Char critics have argued that this is a better use from an energy and emissions perspective since more energy is released to serve human purposes, displacing a larger amount of fossil fuel that would otherwise be used, and so is a more useful climate hack. That's not what char is about. Sure, climate nutters seized on the concept and inflated it grotesquely, and opportunists tried to get policies enacted that would pay rents for carbon sequestered, but that's a misuse of the idea. It's not about emissions reduction or carbon sequestration though that is a secondary effect of char use. Limiting the evaluation of char utility to such narrow issues misses the main benefits. You have to look at the whole system.

To increase soil organic matter on the planet you can't just haul it in from elsewhere, you have to increase productivity, you have to grow more. Char isn't organic matter though it is carbon and does some of the tasks that make organic matter valuable in soil, and so can help achieve the end objective of being more productive. This wouldn't be truly beneficial if the char wasn't durable, it would just be another shell game where a limited amount of material is shuffled around to make some places look good while others are impoverished: it's the Potemkin villages hack in farmer drag.

IMV it is the durability that is the chief virtue of char. All of the other claims for it's mystical magical properties seem to be dissolving. It's just an organic matter substitute that lasts longer, though that is indeed a significant virtue since it supports long term development of soil communities which in turn keep nutrients and moisture in play rather than all just leaking away into the air and seas, leaving nothing but dirt rather than true soil.