Good question, I think the carbon sequestered needs to be measured against what is contributed by the char making processes. Maybe, assumption only, because of high temperature use, the materials are well carbonized with limited release into the atmosphere.

I agree with Patrick S. Michael

For subsistence farmers in the tropics, making biochar from crop residues is a lot of work and would delay the farm operations.

Artur Braun Thank you very much for the link to the very informative video on how Holzkohle was produced.

Thank you all for your answers. My question was driven by the need to verify the real efficacy of biochar making from biomass. Within the agronomy, soil science and engineering community I found a lot of enthusiasm and papers describing the benefits of biochar on soil health and fertility. Engineers have been designing systems to harness the syngas during the conversion process and produce electricity. However, I have not found articles that do a little bit of accounting to evaluate the feasibility of biochar making that many folks are now producing (on a small scale) even in their backyards. I do not disagree with Artur Braun 's recommendation of leaving woody biomass decompose naturally yet, I remain interested to learn more about the energy and carbon balances of the process.

The carbon stabilized in the form of biochar is not easily decayed. As such once it applied to soil its carbon remains there for all purposes.

While a plant residue carbon is largely decayed and emitted carbon is returned back into the atmosphere or air within a year of so.

Compost carbon can takes years and biochar decades to be recycled back into the atmosphere.

According to Puget Lal 2005 no till agriculture can lead to sequester 330 kg C per ha from there long term field studies in a clay loam soil. This type of sequestration could neutralize about 10% of current fossil fuel emissions.

The application of animal raw manure for maize production would give a similar benefit.

When this compared to cover crops the increased Carbon sequestration can be 2 to 4 times that rate of sequestration.

If we look at compost the sequestration is 4 to 8 times the no till base or manure baseline.

The application of just 5,000 kg/ha of biochar can lead to sequestration 10 to 20 times the rate of no till or raw manure.

The carbon production by plants in the form of residue straw is about 10 tons per person globally. What this shows is there is no issue of lack of carbon sources for all the global needs for biochar.

The demonstration of Indian Black Soils show conclusively that the worst soils on the planet acid oxisols and ultisols can be bioremediated using biochar giving an improved soil condition that last hundreds and even thousands of years.

While the background oxisol shows about 7,000 kg/ha C in a fully developed Indian Black Soil the rate can exceed 300,000 kg/ha C.

Acid prrone soils predominate ina bout half of all global soils.

The ability of a energy system using the carbon as both biochar and energy resource is an approach that will make renewable energy from photosynthesis with an ability to avoid the issues with the fossil fuel economy and issues.

The advantage of this advancement is the ability to regenerate the worst of the tropical soils produce renewable energy economically and to provide a neutralization of the excess emissions from the fossil fuel economy and its legacy effects.

Some of the potential issues with biochar is the lack of current infrastructure for it. In addition biochar is phenomonal in the acid soil condition but since it is alkalinized soil the results can be some issues with micronutrient deficiencies.

Sooner of later the non renewable fossil fuel economy will need to converted into renewable resources. The biochar energy economy is a feasible approach to produce economic energy which is renewable and to produce by product which will mitigate the fossil fuel economy and its legacy. In addition the ability to remediate acid soil tropics which are highly constrain can allow the effective productive use of tropical environments which are constrained by poor soil and acidity but are abundant in solar and water resources.

Based on what I have read and heard from colleagues, biochar application improves crop yield in some studies but not in others. So I think it is not yet definite that biochar application is beneficial to crop production. It depends on a lot of factors like soil type, soil fertility status, rate of application, and kind of crop.

I do not have hands on knowledge on biochar production. But I feel that it has more benefits than its drawbacks though its production is not energy-friendly. It fixes a huge amount of carbon for a long period.

Luz there is no doubt that the reaction to biochar is related to soil type and the best reaction in poor acid soils oxisols and ultisols. My own research and experience confirms this.

The concerns I have with biochar studies is that they usually lack the key controls, including for pH and nutrients. For these reasons the inferences are based on unspecified 'biochar' effects within the study that cannot be interpreted more widely. As a reviewer I consistently recommend these manuscripts be rejected.

I agree with Dr. Paul Reed Hepperly that the best reaction of biochar is in highly weathered soils such as Oxisols and Ultisols because of the nature of their chemical and mineralogical properties. I also appreciate the insightful comment shared by Prof. Paul Milham .

I feel obliged to add that biochar may also contain silicates that may impact on the chemistry, at least in the short term. This topic is ripe for investigation.

I heard about, that you need a certain time until you can add charcoal to to foodchain. It first absorbs the microorganisms and reduces the production of the soil. After some time there are enough mircoorganisms back and then there is more production than before.

So you can't burn charcual and add it instant to the productionsystem. You first have to mix it with compost for some month and after that you can add it.

According to ETH Zurich, some of the drawbacks of biochar:

1) crop yield may decline due to sorption of water and nutrients by the biochar

2) reduced efficacy of pesticides due to sorption by the biochar

3) some biochars are a source of contaminants like heavy metals etc.

4) the fine ash in biochar poses health risk (as dust)

5) the increase in CEC depends on types of soil

Source: www.ethz.ch

Biochar is considered very fundamental in successful energy transition or the transition to low carbon development (LCD) if appropriately harnessed. This has been due to its proven extensive benefits, cutting across environment, agriculture and health according to researches.

On the environmental bit, Carbon Sequestration or Carbon Carbon and Storage (CCS) issue as raised in question is a business model that is applicable in ensuring the carbon negativity as a commendable opportunity area from the carbon neutrality of biomass techniques. This is ultimately an improved greenhouse gas mitigation strategy that is perceived to be necessary in view of the damages already done by the fossils usage. Hence, a climate change improved mitigation strategy as well.

However, I think the drawbacks of Biochar production and utilization revolves round the sustainability of biomass harnessing more broadly. Which is the biomass extraction and the necessary measures to be in place in its regeneration at a faster pace in making the system truly renewable. Also, in a more specific terms, Biochar if not managed well may lead to environmental pollution, specifically to air due to ash release, to land surface and water bodies as a contaminant.

Crop yield may decline due to sorption of water and nutrients by the biochar.

Reduced efficacy of pesticides due to sorption by the biochar.

Some biochars are a source of contaminants like heavy metals etc.