Why does biochar disappear?

Many on this list and I have had the same experience.  Once you dig your biochar into the soil, you can see it for a few months.  But in a couple of years, you can't see it at all, even if you dig it up in the right place and try to find it.  

I have to rely on remembering where I put it in the first place.

Why does biochar disappear?

John S
PDX OR

Does it disappear, or does it just break up into such tiny pieces that you can't see it with the naked eye?

Nancy Reading wrote:Does it disappear, or does it just break up into such tiny pieces that you can't see it with the naked eye?

Freeze, thaw, wet, dry and interaction with the soil microbiome would certainly accelerate that.

Earthworms will ingest small pieces and their little gizzards grind them up. If you're lucky you might find their burrows and see the black lining. It can't really go "away" unless you have a really hot fire that penetrates into the ground along with enough oxygen to combust it, and if that happens you've got bigger problems than accounting for your biochar.

You could do some basic testing on some samples to see what your soil carbon fraction is. By weighing samples before and after pyrolysis in a retort, and again after doing complete combustion, you'll get total carbon versus what was already in pyrogenic form. The steps go like this:

1. Collect some samples.
2. Oven dry the material to drive all the water off.
3. Weigh each dry sample.
4. Pyrolyse the samples in a closed (not tightly sealed) retort at minimum 400 degrees C (baking tins in a wood fire will do the trick).
5. Weigh again. The difference is the volatile matter that was driven off by gasification. 58% of this was carbon. In soil that has had biochar added, the change will be smaller than in a similar sample without biochar, so you can get a reasonable estimate of the biochar percentage if it's not too tiny.
6. Burn the samples, this time with good airflow, all the way to ash and minerals.
7. Weigh them one last time. The difference between this measurement and the initial one in step 3 is total soil organic matter. If there is no biochar in the soil, 58% of this figure was carbon.

It's more just getting mixed in than disappearing. Biochar lasts literally for centuries.

I have mostly thought along the lines that you are all saying.  I think they get distributed more widely after the weather and small animals eat them.  In the original terra preta, it is still visible today which areas are literally "dark earth" and which are the more commonly seen light brown soil from the area.  

The areas in my yard that dramatically improved after application of biochar years ago are still dramatically improved.  The produce from those trees is still remarkably better and hasn't tailed off.  

John S
PDX OR

Geez dude, relax. If you can't find it, you have succeeded en masse. The soil biota own it now.  

This thread has me looking over some scholarly papers to try and better understand the mechanism at hand. A lot of the papers I found point towards mechanical breakdown of biochar in the soil as others have pointed out in the replies.

The thing that I'm now interested in is that different biochar break down at different rates. This seems to be from both the source material and how hot of an environment the biochar is created in. Perhaps there is a way to create biochar from a certain material that is less inclined to break down rapidly?

Do you use different base materials for your biochar?

I use wood as the source material, but I will sometimes throw in some bones.  Wood is dense and it has a lot of mass.  It will produce more biochar than an equal volume of almost any other organic material. I live in an area where trees grow exceptionally well, and people are often trying to get rid of wood.   I have read that in alkaline environments, conifer wood might be better than hardwood, because it is acidic enough to have less of an alkalizing effect on the soil ph.

John S
PDX OR

Therer are fractions of biochar that do breakdown. Other fractions are stable for centuries.
I guess your soil is sandy: Clay and biochar builds, similarly to clay and humus, clay-biochar complexes, which are more stable.

Some of the studies about Tera preta that I've read suggest that it's usually quite deep and was built up over very long periods.

John, that makes me wonder if you soil hasn't reached "critical mass" of enough biochar yet?

Most of my boichar goes into the 1/2 barrels or other raised beds. When I do have to dig them, I certainly still find the larger pieces that I added in the past. Similarly, I find it in my compost bins.

However, I have been trying to biochar my neighbour's sawmill sawdust, and I suspect it will be much harder to find!  I don't think my soil is reliably a single colour, so I won't be able to go by colour change either.

Jay Angler wrote: Some of the studies about Tera preta that I've read suggest that it's usually quite deep and was built up over very long periods.

John, that makes me wonder if you soil hasn't reached "critical mass" of enough biochar yet?

.

Excellent question, Jay.  I don't know the answer to it.  I know that the texture of the soil that I am adding biochar to is greatly different than what has been biocharred.  The unbiocharred soil is still dense clay below the first few inches. I have mostly completed my goal of getting biochar within 2-3 feet of every part of my garden.   Soil that has been biocharred or even soil that is near biochar seems to be of much higher quality: good draining and retaining of moisture, with much better airflow and crumbliness (not a word!).  :)
John S
PDX OR