Biochar Research: Charcoal Boosts Soil Microbial Respiration.

Researchers from U. of Wisconsin–Green Bay have published what must be the most extensive evaluation of microbial response to charcoal application in soils from managed temperate systems. Insights abound. Lacking are any insight into the proportion of the effect to allocate to the charcoal-C relative to the reactive effects of the ash component.

Our understanding of the mechanisms by which charcoal influences soil processes is limited primarily by a lack of information on how charcoal affects microbial biomass and activity. ... there is no currently accepted range of effective application levels, which is not surprising given that the effects of charcoal application will probably depend on how the microbial community interacts with both soil and charcoal properties.

One has to admire the clean lines, simple controls, and rugged build of the studied conditions. Charcoal was custom produced for uniformity, with 0, 1.0, 2.5, 5.0, and 10% by weight added to four classic soil types (Mollisol, Entisol, Spodosol, and Alfisol) in pint jars, moistened to 60% AWC, incubated at 25⁰C for 3 months. Burped and re-moistened as needed. Three replicates, sacrificed to analysis at 0, 6 weeks, and 12 weeks. Findings:

[microbial respiration levels were] significantly affected by charcoal application, soil type, and incubation duration. ... despite significant interaction effects, the differences in [substrate induced respiration] SIR and [basal respiration] BR were primarily driven by differences in soil type and charcoal application levels, respectively. ... As hypothesized, both SIR and BR increased significantly with increasing charcoal application in all soil types; however, the strong influence of soil type ... is highlighted by the fact that, in general, the larger the level of SIR in the unamended soil, the larger the level of SIR at the highest charcoal application levels.

Respiration is the cleanest proxy we have for measuring soil life. Increasing respiration accompanies increasing microbial biomass, and we soil scientists are not going to ignore a tool that enables us to increase and sustain higher levels of soil life. Several of us suspect the ability to goose soil microbial respiration to be a universal characteristic of biochar, but have so far only caught glimpses of this based on indirect observations. Two recent examples: Wardle et al 2008 showed that biochar increases the decomposition rate of soil humus in boreal forests on a long-term basis. Yoshizawa et al 2008 attributed a 33% reduction in time needed to produce finished compost to 1%v/v charcoal. Counter indications occur also, but my understanding is that the no-effect results have been on low organic matter, low nutrient, moisture deficient desert soils, thus entirely consistent with this new research.

I want to believe, I truly do. Unfortunately, this research does not establish as universal a relationship between charcoal and soil respiration that one might be tempted to read into it. Biochar includes material with low reactivity, and this material was apparently not a low reactivity charcoal. The biochar stock used was capable of moving the soil pH from 6.3 to 8.9 (Entisol w/ 10% char). Raising soil pH (and surely compost pH) boosts microbial respiration. This effect is masking microbial effects due solely to the addition of charcoal-C. Reactivity can happen when you use manure as a biochar feedstock. Reactivity increases with pyrolysis temperature. Reactivity increases with ash content. Because these researchers did not characterize reactivity we are left to wonder how much increased respiration was due to charcoal, and how much effect was due to the ash reactivity.

One could ask somewhat the same of the Wardle study, and of the Yoshizawa observations, although their biochar stocks looked to me to have low reactivity potential. Still, without the data, whose to say. I look forward to the day when biochar researchers make the simple experimental design adjustments that will control for reactive effects.

Laboratory protocols established (and regulated) for characterizing materials used in mining and agriculture should port easily for biochar characterization. Google "acid-base accounting" and "acid neutralizing potential ANP" for how this plays out in the mining industry. For the agriculture context, Google "Effective Neutralizing Power ENP", "Neutralizing Index NI" and "Effective Calcium Carbonate Equivalent ECCE".