Is Biochar a Solution to Climate Change? Maybe, Maybe Not.

A new paper published earlier this week in the scientific journal PLoSOne calls into question whether we know enough about biochar to use it as an important strategy to mitigate climate change. The article, two of whose co-authors formerly worked here at UCS, did a systematic review of the scientific literature on biochar through 2011, and found 311 relevant papers.

But even with all this research, a key question remains unanswered: How long does biochar persist in the soil?

The review was originally done last year as a report, partly funded by UCS, which is available on our website. The authors – Noel Gurwick, Lisa Moore, Charlene Kelly, and Pipa Elias – wanted to see if the published science backed up the oft-discussed possibility of making biochar and adding it to soils as a way to reduce greenhouse gas emissions. (For one recent – indeed, still ongoing – discussion, see the American Carbon Registry’s current comment process on a methodology that would allow compensating such projects in a carbon market. The comment period lasts till October 25).

What is biochar? And how could it reduce CO2 emissions?

Basically, biochar is what you get by using a low-oxygen process called pyrolysis to burn biomass, creating a carbon-rich and supposedly very stable product similar to charcoal. The more volatile parts of the biomass (from the carbon-hydrogen and carbon-oxygen bonds, for you chemists in the crowd) are lost but almost all the carbon remains. Thus, you don’t produce large amounts of the greenhouse gas carbon dioxide, as you do in burning biomass with normal oxygen levels. The product looks like and indeed is chemically very similar to charcoal – that is, mostly just plain carbon in its reduced form.

But, rather than firing up the grill and barbecuing with it, you put it in the soil. This increases the soil’s organic matter content and is thought to have benefits in terms of soil fertility, water-holding capacity, and crop growth. But besides those possible advantages, doing this also keeps the biochar’s carbon from being oxidized, and thus returning to the atmosphere as CO₂, for some period of time

There’s the rub. “Some period of time” – but how long is that? Are we talking a few years or many centuries?

This is critical for whether making biochar and adding it to soil is really a useful way to reduce CO₂ emissions. If it remains stable in the soil for a long period of time, then (other things being equal, and of course they never are), the answer is yes, because you’ve delayed the process by which biomass decomposes and returns to the atmosphere. This means that effectively, you’ve reduced greenhouse gas emissions.

But if the period of time is short, then you haven’t truly reduced emissions, just shifted their timing slightly. So the question of how long biochar is stable in the soil is critical for whether it’s an effective climate mitigation tool.

What does the science say?

What did Gurwick, Moore, Kelly, and Elias find in their review? What is the answer to this key question? Unfortunately, it’s that we don’t know enough to say.

Even though they found 311 peer-reviewed research articles on biochar, the overwhelming majority were studies in the laboratory, not in soils under natural conditions. Of those 311, only seven actually estimated the decomposition rates of biochar in situ. And in those seven, the estimates of the “mean residence time” of biochar in the soil ranged from 8.3 to 3,624 years.

That’s a pretty big range, to say the least. If biochar remains stable in soils for thousands of years, it can be an effective climate mitigation solution. But if we’re talking about less than a decade, it can’t.

This means that scientifically, a critical question remains unanswered. We have hundreds of published studies, but only a handful focus in the issue of stability in the field, and those few don’t agree.

So, the conclusion is, in the words with which practically every published paper seems to end, “we need more research.” Often that seems to be a throw-away line, or (viewed more cynically) a plea for more funding. But with this paper, we have the scientific evidence that it really is the case.

And by the way, if you are going to use some pyrolysed biomass to grill with, I’d recommend turkey burgers and sweet peppers – delicious, and good for both your health and the planet’s.