Biochar – The state of the science so far


Biochar is still only partially understood but has received strong support from some quarters for its ability to combine a means for carbon sequestration with a way to improve soil health. Matthew da Silva reviewed the CSIRO’s scientific tests.


The term biochar is misleading “because there is no such thing as a biochar per se ”, says Dr Evelyn Krull, senior research scientist, CSIRO Land and Water. The word should be used in the plural, she says.

“We probably have to think about adding something that qualifies it, like either ‘low-temperature biochar’ or ‘wood-based biochar’ or ‘manure-based biochar’.

“We know that, for example, if you take a biochar – that’s produced at let’s say 400 degrees from poultry litter – it may not have a residence time that classifies it as permanent under the Carbon Farming Initiative or the carbon protocol. Because it has to be stable over 100 years. These sorts of biochars have so many nutrients and minerals and don’t have that much of a connected structure that they actually may degrade quicker, or much of it degrades quicker.

“Compare it to biochar produced from oil mallee at 500 degrees, [where] you’re talking about residence times of most of this biochar of thousands of years,” says Krull.

So it’s misleading to say that biochar is stable, biochar can increase crop productivity, or biochar can decrease nitrous oxide emissions because biochar properties vary.

“We know some biochars don’t increase crop productivity, some biochars can actually decrease crop productivity, and some biochars only work in certain soil types,” says Krull. “Under certain conditions biochars do not decrease nitrous oxide emissions. We have to be very careful how we communicate our findings to the general public.”

The best biochars for sequestering carbon derive from woody material which is pyrolysed at relatively high (around 500-degree) temperatures, but crop residues are also good for this use, says Krull. She says that feedstocks that have reasonable levels of nitrogen and phosphorus in them, like biosolids or livestock manures, “may be ideal if you want to increase crop productivity”.

But, again, it depends on which soil types you’re applying the biochar to.

The CSIRO and its partners have been conducting trials on biochar as part of projects funded by the Grains Research Development Corporation (GRDC) – this project focuses on the effect of biochar on increasing nutrient use efficiency particularly in nutrient-poor soils – and the Department of Agriculture, Forests and Fisheries (DAFF) – this project focuses on carbon sequestration and greenhouse-gas mitigation as well as the effect of biochar on different soil types and life cycle analysis.

Krull thinks that the biochar database the CSIRO has assembled in carrying out these two projects is, to date, the largest in the world.

“We figured, before we got started, first of all we need to get an understanding of how much biochar produced from different feedstocks at different temperatures or different production conditions actually differ in their chemistry, biology and physical characteristics,” she says. “Because both funding agencies agreed that this was necessary, we were able to pool resources and that allowed us to increase the database from about 10 biochars to over 80 biochars.”

The data catalogues a wide range of biochar characteristics.

“We’re talking about data that looks at the carbon content, pH, major and minor nutrients, major and minor metals, water-holding capacity, cation-exchange capacity, a wide variety of spectrum of biological, physical and chemical properties,” says Krull.

Using biochar is not just a matter of chucking tonnes of charcoal made from chicken manure that has been pyrolysed at low temperatures onto your paddocks.

“We looked at typical Western Australian acidic sand, a soil from NSW which is a ferrosol – so iron- and aluminium-rich, but also a nutrient-poor soil – and a calcareous soil from South Australia, and a clay vertisol , typical for Queensland,” says Krull. “If you’re particularly looking at which soil responded best to manure biochar addition, it was the NSW ferrosol which showed significant increases in crop productivity.

“The question of all this is ‘why’? We did a multivariate statistical anaylsis on it and, again, it depends on which qualities of the biochar match with the soil. In the ferrosol instance it’s actually the pH, because the ferrosol is quite acidic, and it’s the ameliorating effect of the alkaline biochar that is probably most responsible for driving the increases in productivity plus maybe a little bit of the added nitrogen or phosphorus.”

The same biochar did not work so well on the Western Australian soil.

“[There], it also shows a little bit of [a yield] increase but for the Western Australian soil it seems to be that it was more the wheat-straw biochar where the soil responded better,” says Krull.

The result of all these tests is that farmers will most likely have to refer to a matrix when deciding which biochar to use on their soil, in order to achieve one or both of the benefits that are touted to be derived from application in the paddock.

Tailor made biochar

“You can actually tailor-make the ideal biochar for your conditions,” says Dr Krull.

A matrix of this kind will be available for public consultation at the end of the DAFF project, in the latter part of 2012.

Certification of biochar as a method of carbon sequestration under the Carbon Farming Initiative (CFI) is still to be achieved. The large variability in types of biochars means that certification would take time but Krull says “it wouldn’t be rocket science” to finalise. She anticipates certification will be in place by the latter part of 2012.

“It is intended that increases in soil carbon from the application of biochar will be an eligible activity under the CFI, and thus able to earn CFI carbon credits,” says a spokesperson at the Department of Climate Change and Energy Efficiency. “This, however, will only occur once methods for estimation and reporting are developed.”

DCCEE would not put a date on this, but said that any method for abating carbon must be approved by the federal government. DCCEE has set up the Domestic Offsets Integrity Committee to assess suggested methodologies and provide recommendations to the Minister about their approval.

“The Committee will ensure that methodologies are rigorous and lead to real abatement,” says the spokesperson.

“Methodologies contain the detailed rules for implementing and monitoring specific abatement activities and generating carbon credits under the scheme. Before a biochar project under the CFI can be established, a methodology must be submitted and approved by the Minister for Climate Change and Energy Efficiency.”

DCCEE has set up programs aimed at accelerating this process. The Carbon Farming Futures program will spend over $220 million over 6 years for research and development and to develop estimation methodologies for use in the CFI, including research into soil carbon. And the Climate Change Research Program is providing $20 million to identify land management practices that build soil carbon. It is also providing $3.1 million to assess the benefits and risks of biochar technology in the Australian context.

Krull says making different biochars for different uses would not be difficult. In Melbourne, it was recently announced that a pyrolysis plant would be built using money from a state government grant. Once the rest of the money is found, the plant would be built by Pacific Pyrolysis, a NSW company. This type of plant can produce a range of biochars.

Pacific Pyrolysis, headed by Adriana Downie, has been producing biochars for the CSIRO’s trials.

“Depending on the availability of the feedstock maybe she has a month where she just runs wheat straw, and then that can be stored. Then the next month is biosolids or whatever.

“Ideally, we need more producers simply because of the economics. More producers means more competition, that hopefully brings the price down a little bit. The nice thing about pyrolysis is that it’s so versatile. Today you can do poultry litter and the next day you can do oil mallee. You have to make a few adjustments but you don’t have to change the system, unlike bioenergy systems that can only take certain materials.”

But Krull thinks that very large pyrolysis plants, like the one being planned for Melbourne, are ideal for some but not for others.

“They’re good for the urban approach, for example where you can produce biochars for your local gardens or maybe for the horticulture industry,” she says. “It’s not really feasible for broad-scale agriculture. In those sorts of situations we need to think about mobile biochar production units that either can be leased or that can be transported where the feedstock is generated. Then you have it there where you generate the biochar but also where you apply it, so you don’t have the necessity of either carting feedstocks back and forth or biochar back and forth. You don’t necessarily have the issue of longer-term storage. You can just apply it to the fields.”

Biochar can be applied at different rates and, again, the ideal rate of application for a specific farmer will vary depending on the soil type.

“The other problem that we’re having with giving proper advice at this point in time is that all of our studies have been based on one [single] application only,” says Krull. “If you want to particularly use biochar for carbon sequestration you would think you’d want to apply it several times to maximise your benefit. The question here is should you apply 30 tonnes per hectare of biochar once every five years, or once every two years, or should you apply 10 tonnes per hectare. Again, that sort of research hasn’t been done yet.”

She says that in the NSW soil, researchers saw continuous increases in crop productivity from adding 10, 20 to 30 tonnes of biochar per hectare, even 5 years after the first application.

“But in the Western Australian soil it wasn’t as clear-cut. There we actually saw some decreases at higher rates. Again, it’s the same sort of soil-biochar interaction that needs to be sorted out first.

“In general, application rates of around 10 tonnes per hectare are not going to do you any damage even if you put 10 tonnes per hectare on every  year, for three years let’s say.”

But overloading the soil with biochar can cause problems.

“Once you put it in the soil you have to wait a considerable time until half of it is gone. We want to avoid creating a black desert by applying too much biochar to soil. It has to be the right balance and, again, it depends on the particular soil type.

“In essence, this is really similar to fertiliser or compost application where different requirements and recommendations exist for different soil conditions.”

Krull says that biochar also can increase water-holding capacity and can be used potentially outside agriculture, in the countryside, to absorb paddock run-off in order to protect pristine waterways. As part of the DAFF project, the CSIRO is also undertaking a full lifecycle analysis of biochar.

“This analysis takes into account the energy that goes into making the feedstock and transporting it to the pyrolysis factory, transporting it back to the field etcetera, and so different scenarios are being generated. This will give us a better assessment that then lets us answer this question: what is the net effect of biochar on carbon dioxide in the atmosphere?”

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