There's a semi-interesting conversation afoot on one of the biochar lists, a good thing since it had degenerated into fairly uninformed political posturing recently and I was about to depart to avoid a wasteful time sink.

It began with a seemingly innocent but actually heavily loaded question: How can biochar be used to reduce nitrate leaching in crop production? What are the methods and mechanisms? The short answer is that no one really knows much. But, there are all sorts of tantalizing possibilities that are difficult to neatly encapsulate since the N cycle in soil is so complex. N exists in many forms and is in constant flux as soil organisms convert it from one form to another, and back. To talk about nitrate you must talk about N in all of its forms and all of the participants and parameters that affect the complex multi-path cycle.

Against my better judgment I replied:

In general, increasing the c/n ratio in soils reduces nitrate leaching and nitrous oxide emissions. There are claims that biochar is effective this way. Woolf, 2006 says that more study is needed for biochar since the effect has not been adequately demonstrated.

IMV biochar can be understood by analogy to existing methods that seek to manage soil nitrogen.

Increasing the ability of soil to hold water will reduce travel through the soil column and nitrate leaching below the root zone into aquifers or into surface flows. Biochar is reported to be effective in this way.

Anion repulsion will be reduced to the extent that PH is adjusted higher by the ash component of char which tends to be alkaline. Wood ash and lime do this as well. Most soil particles have negative charges.

I've read claims that biochar pores can retain nitrogen until released by plant root enzymes, but specifics were lacking. The Eprida process precipitates ammonium bicarbonate into biochar pores, which makes me wonder if it is ammonium rather than nitrate that is held by biochar, and they have opposite charges. Even so, if ammonium is held then soil bacteria can't nitrify it, so it can't leach or be denitrified.

Nitrate leaching also varies with the type of crop since nitrogen uptake varies. Fruits, for example, take up much less than grains. Matching nitrogen application rates and timing to the crop and its growth stage, increasing soil organic matter and carbon in general, and reducing anion repulsion will reduce leaching, reduce input costs, reduce emissions and increase net returns.

I expected to be ignored or scorned for neeping about tech stuff rather than bloviating about politics but it turns out that there are tech geeks lurking on the list who piped up. One IBI staff member confirmed part of the comment, that it is ammonium rather than nitrate that is directly retained. A practitioner (one of the rarer breeds that has a Phd as well as dirt under his nails) asked:

If there is relationship between biochar and nitrate retention do you not think that the synergism between organic material and charcoal needs to be considered as primary rather than focus on single factor? Soil is complex so why do we need to simplify?

So, I ventured another reply:

Aye, it is so. N cycling from all sources - organic and mineral - is usefully viewed as an integrated complex process where changes in any parameter affect all others. Urea applied by a grower and urea from organic matter is the same. In every event soil microorganisms convert them to gases and minerals in a multistep process where the waste compounds of one are the food of another, some of which are used by plants, but in the end it all goes back to the atmosphere in one form or another and the cycle repeats. The process can be speeded or slowed, diverted to one path or another, but it can't be stopped or avoided since it is how life rolls.

It goes both ways, which interests you (and me too!), since there are microorganisms (some variety, but rhizobia matter greatly) that draw nitrogen from the atmosphere as well as emit it back. This process can be affected by agronomic practices too.

There are more subtle issues as well since some soil organisms, such as VAM, may not be principals in the nitrogen cycle so much as expediters that are active in nutrient transport, in effect extending the root system of plants into deep and distant places to mine for minerals. It's an economy as well as an ecology with a trade system that can be helped or harmed.

Biochar is reputed to have affects on all of these processes, but I suspect that my muzziness about the specifics is shared by even the most informed investigators. In all other processes you can't optimize everything. There are trade offs. That implies that the agronomic practices used will vary by application, some favoring one optimization and some another depending on objectives, initial conditions, and relative costs.

My hope is that I'll get a lesson, that investigators who haven't published yet or practitioners who never do publish will share insights supported by data.

Update: Dr. Franco Berruti, a chemical engineer answers more precisely.

The long-term improvement in soil fertility arises from the fact that the biomass thermal cracking process (pyrolysis) generates stable compounds consisting of single and condensed ring aromatic carbon with a high surface area per unit mass and high surface charge (Lehmann, 2007). This leads to an increase in surface oxidation and cation exchange capacity (CEC) (Liang et al., 2006), which intensifies over time (Cheng et al., 2008; Cheng et al., 2006) and can lead to greater nutrient retention in “aged” as opposed to “fresh” biochar. The resulting high CEC captures positively charged plant nutrients like NH4+, K+, Ca2+ and Mg2+ which are retained on the biochar surface and not lost through volatilization (NH4+ -> NH3) or leaching (K+, Ca2+, Mg2+)(Glaser et al., 2000).

Surface charge also contributes to anion exchange capacity, important for the adsorption of negatively charged ions like NO3- , H2PO4- and HPO42- that are susceptible to fixation and leaching. The binding of NH4+ to the biochar surface is of particular interest because this is can slow the rate of nitrification (NH4+ -> NO3- ) and hence the loss of N2O and N2 via denitrification. Thus, biochar-amended soils may require less nitrogen fertilizer to achieve target crop yields, leading to, for instance, less contamination of ground water by nitrates (Almasri and Kaluarachchi 2004; Elmi et al. 2005) and less production of the greenhouse gas N2O (Almaraz, Smith et al. 2009a).

Soil-applied biochar probably favours the growth of microorganisms (Warnock et al, 2007), However, much of this remains to be demonstrated under field conditions, and most testing of biochar application to soil has been carried out in tropical regions where soil fertility is very low.

N.B. I am adding links to the referenced papers as I find them.

Update:

This page at Biochar Fund liats many resources and has links to many research papers.

IBI has an engine to Search IBI's RefShare Database for Peer-Reviewed Articles Relevant to Biochar, Black Carbon, and Terra Preta Research.