Johannes Lehmann has a paper pending in the journal Frontiers in Ecology and the Environment that covers some of the issues I've been fretting about with biochar.

Nutrients are retained and remain plant available in soil mainly by adsorption mechanisms to minerals and organic matter. While we are usually unable to change the mineralogy of a given soil, we can change the amount of soil organic matter. Typically, the ability of soils to retain cations in an exchangeable and thus plant-available form (called cation exchange capacity – CEC) increases in proportion to the amounts of soil organic matter and this also holds for bio-char. Bio-char, however, has an even greater ability than other soil organic matter to adsorb cations per unit carbon (Sombroek et al. 1993) due to greater surface area, greater negative surface charge and greater charge density (Liang et al. 2006). In contrast to other organic matter in soil, bio-char also appears to be able to strongly adsorb phosphate even though it is an anion (Figure 4), but the mechanism is not entirely clear. These properties make bio-char a unique substance to retain exchangeable and therefore plant-available nutrients in the soil, improve crop yields while decreasing environmental pollution by nutrients.

If these suspicions are confirmed by more research they provide something unique about char, as opposed to lime or ashes in combination with SOM amendment. PH is raised and CEC improves with both approaches, but char does it better, and even has some mysterious ability to adsorb phosphates.

Cation exchange capacity of soil organic matter is typically absent at very low pH but increases with higher pH and bio-char is no exception. However, the point where the CEC of bio-char is zero (called point of zero net charge – PZNC) depends on the temperature at which the bio-char is produced and consequently, the potential CEC (standardized for pH 7) increases with higher temperature (Figure 5). Also pH and surface area of fresh bio-char appear to increase with higher production temperature at the same time as the carbon yield decreases (Figure 5), making an optimum of 450-550°C likely. It should be noted, however that bio-char properties significantly change during exposure in the environment, and interactions between production procedure and environmental effects have to date not been investigated.

I'm not sure that the word optimum is appropriate. It might be from the perspective of a commercial producer who wants to have more product to sell, even if it's just through some carbon subsidy hustle, but from a grower's perspective better material would be preferred. I can imagine a time when there are grades of char on offer, the more effective sort being more expensive since less is produced from equivalent feedstock, but cheaper to apply since less is required for equivalent agronomic benefit.

The effects of bio-char on biological processes (Lehmann and Rondon 2006) or water relationships in soil are much less explored but could potentially lead to significant returns. For example biological fixation of atmospheric nitrogen by common beans was found to be enhanced by bio-char additions to a highly weathered savanna soil most likely through greater micronutrient availability (Rondon et al. 2007). Higher bacterial growth rates with bio-char were explained by a better attachment and possible physical protection of microorganisms in the pore structure (Pietikäinen et al. 2000). Similar explanations were put forward for observations of greater infection by mycorrhizal fungi (Saito and Marumoto 2002). A greater surface area (Liang et al. 2006) is likely to result in greater water holding capacity but has not yet been investigated. Such observations merit further research to underpin the proposed mechanisms.

These are among the most important attributes of char, if confirmed, but they are the least studied because we are gripped by climate hysteria. Much attention is given to sequestration potential to the detriment of progress. Many disciplines are being warped and degraded by this phenomenon. There will be an accounting one day.

[B]io-char is not inert and will eventually decompose and release carbon dioxide. However, the time scale over which this will occur is very long compared to other organic carbon forms in soil and to uncharred organic additions (Baldock and Smernik 2002). And the total amount of carbon that can be stored are not limited by soil properties such as clay content and mineralogy as typically found for other soil organic matter. One still has to consider that a portion of bio-char can be mineralized very rapidly. The magnitude of this mineralization has to be better understood and opportunities exist to reduce but not avoid those losses. Preliminary results indicate that bio-char bio-energy not only leads to a net sequestration of carbon dioxide, but that the presence of bio-char in soil may decrease emissions of two even more potent greenhouse gases, nitrous oxide and methane. In greenhouse experiments, nitrous oxide emissions were reduced by 80% and methane emissions were completely suppressed with bio-char additions of 20 g kg-1 to a forage grass stand (Rondon et al. 2005). The reason for the reductions of methane and nitrous oxide emissions are presently not clear. Lower nitrification could be one reason for lower nitrous oxide emissions possibly due to lower mineralization as a result of for example a higher C-to-N ratio or lower carbon quality. However, in forest soils, additions of biochar were recently found to increase nitrogen mineralization due to adsorption and therefore inactivation of secondary plant compounds that would normally decrease microbial activity (DeLuca et al. 2006). The effects of bio-char on the soil nitrogen cycle and the associated emissions of greenhouse gases clearly require more attention.

Some of these benefits are available using any method that adjusts PH and increases SOM. That emissions are reduced is true but I think not the most important point. The same mechanisms, perhaps adsorbtion, that retain nitrogen so that less is leached away or below the root zone also reduces bacterial decomposition and the resultant emissions. More is available for plants so less needs to be added in the first place. Less is applied, less leaches into surface and ground waters, yet plants still thrive. Even if we had no concerns for climate change these would be very important points.

Lehmann discusses many of these issues in the paper. It is worth your attention. It isn't just another char booster article, he's actually thinking about the subject. In the section titled Pitfalls and impediments to successful adoption he voices some concerns including one I've fretted about.

If not incorporated into soil, bio-char may be prone to erosion. Eventual burial in river or ocean sediments may increase the mean residence time of the bio-char in the environment, but also jeopardizes any intended soil improvement and may even increase net losses of adsorbed nutrients. Suitable technology for soil injection or incorporation have not been developed yet.

I'm not sure that injection or incorporation are either necessary or desirable, but there do seem to be constraints on when and how char dust is applied in order to avoid loss. The mesh size of the char - how finely it is ground - also seems to be a consideration. All of this will vary with the site. Some lands will be better suited than others.