David G. Khachatrian

How can burning be good for soil?

Had yet another moment where I realized how little I know about plants and plenty of other things. Was prompted by a continuation to a metaphorical joke: the metaphor was to a field, and I suggested that it may be good to burn the remainder when you’re down to improve the soil quality for next season. “But why is that good for the soil?” I wondered. And with that, I went forward!

Here’s hoping I consolidated the information well. Welcome to know where my understanding could use more work – an evening of reading does not an expert ecologist make. If only Tony Stark could teach me his ways\~

“It’s alive!”

Let’s remember that soil is teeming with life: microscopic life in the form of microbes, as well as insects and earthworms and other such critters. The living and dead organic matter is one of the main factors that differentiate soil from plain ol’ dirt.

Microbes aren’t limited to the soil either. Different species have different preferred environments/surroundings, so you’ll find different species on tree logs, plants, etc. But for now, let’s consider the soil microbes. To survive, they often metabolize dead organic matter deposited on the surface of the ground, dubbed the litter layer of the soil. Some of the (bio)chemical reactions that let the microbes “eat” also release inorganic nutrients from the unusable forms they existed in the once-living organic matter into forms that a plant’s roots can absorb. For example, the nitrogen atoms that are locked in peptide bonds may as well not exist as far as a plant is concerned. But if a microbe could “extract” the nitrogen and release it as an ammonium ion – yum! The process of forming absorable solutes of important inorganic elements (minerals) by decomposing organic matter (specifically by redox reactions) is called mineralization in soil science.

That some forms of an atom are more readily absorbed than others shouldn’t be too surprising. For an extreme example of humans, what is dietary fiber other than entire chains of carbohydrates that we can’t absorb at all due to the linkage between the repeating unit (and due to our inability to “pull” in large molecules through the membranes of our intestines)? There’s also cooking. The example that comes to mind is how the beta-carotene in tomatoes changes to a much more absorbable form after cooking the tomatoes. (And also the whole “cooking vastly increased net calories gained per meal, which is probably why we could afford to develop brains with ‘higher’ function in the first place” thing.)

Problems on the forest front.

In any case, the microbes at the surface of the soils, logs, etc. provide a slow and steady supply of inorganic nutrients into the soil itself (which can get pulled into a plant’s roots via establishing an electrochemical gradient). This is generally good if things are tending toward a steady state that is good for the ecosystem.

But what if things go sour? For example, what if the rate of nitrogen fixation (the formation of abosrbable nitrogen compounds) is much smaller than the rate of nitrogen consumption by plants? (Remember that plants, like people, need to make proteins to survive/function properly.) Then letting things progress on their own probably isn’t the best idea, as many plants may die and pull out so much nitrogen from the soil that nothing would be able to grow for years (or you’d need to dump a lot of fertilizer – which, incidentally, is “good for soil” because they contain absorbable forms of these desperately needed minerals). Perhaps even more important, what if you have trees surrounded by tall, dry shrubs? When a passing breeze causes enough friction for one of those things to ignite like tinder, it can spread uncontrollably, catch on to the surrounding trees, burn everything down! Obviously no good.

Combustion: Decomposition on steroids.

Enter fire. Since “decomposition” in this context mainly involves oxidizing the compounds in organic matter, combustion amounts to extremely rapid decomposition. In addition, by controlling the rate/temperature of combustion, the dry shrubs can be burned up and the fire put out before the fire catches onto any currently living nearby trees (which, due to being “wetter” and having a much smaller surface-area-to-volume ratio than shrubs, don’t catch fire as quickly). (Of course, this latter, probably more important case could more cleanly be solved by precise, manual removal of the shrubs.)

However, as is often the case, doing the job quickly means doing the job sloppily. In this case, the “sloppiness” comes from the inefficiency of organic matter conversion. You’re technically making all the organic matter and their inorganic nutrients potentially absorable, but you’re losing an incredible amount of those nutrients to vaporization. Wood ignites at ~175 degrees Celsius (°C), all of the gases within it evaporate at ~600°C, and the resulting charcoal burns at ~1100°C. Meanwhile, the nitrogen compounds in organic matter become volatile at around ~300-400°C, and so a ton of the nitrogen ends up forming N2 and floating away – over 90%! The part that does remain does so by getting transferred downward into the soil. This is due in part to the large temperature gradient (while the surface burns at ~1000°C, the soil stays at ~200°C due to poor heat conduction) and probably also in part to a large concentration gradient at/near the combustion site (a ton of nitrogen is escaping upward, so at least some stays near the initial site).

Not only that, remember how we said there are microbes in other environments, like on logs? Well, some of those microbes are nitrogen-fixing microbes, and when you burn the log, you kill the microbes. When the log’s gone, there’s no environment for new microbes to move into. Sad day for microorganisms.

Harnessing, and coming to terms with, prescribed burning.

The huge loss of nutrient reserves in the ecosystem inherent to prescribed burning means that it must be used carefully. It’s a controlled burn meant to only reach incomplete combustion – ecologists aren’t pyromaniacs. We want to keep around the homes of those useful nitrogen-fixing bacteria, and we want to leave enough organic matter so that the microbes that migrate upward to the soil surface after the fire can continue metabolize organic matter and provide that slow and steady supply of inorganic nutrients.

Of course, as we’ve noted before, prescribed burning is the “quick” way of dealing with the underlying issues, not necessarily the best. As much as the people involved intend for the fire to be carefully managed, weather conditions and other unforeseen factors can make it not all that well controlled. As for alternatives, as mentioned before, manual removal of potential fire-starters is a more labor-intensive but less potentially destructive method of mitigating the possibility of unexpected wildfires. And though I joked about dumping a ton of fertilizer, the introduction of the inorganic nutrients themselves or the bacteria that make such nutrients absorbable could in fact balance the rates of consumption and production without all the waste prescribed burning entails.

It’s impressive how the saying that “poison, at the right doses, is medicine” relies on largely “hidden” biochemical truths in the physiology of both humans and forests. I’m just glad there are people out there assessing the health of natural ecosystems, without which we’d be toast! Here’s hoping any interventions we make as a result of such assessments, prescribed burning or otherwise, lead to good outcomes.

Bonus fact: Potash and potassium.

As we’ve discussed above, fire expedites the decomposition process and quickly yields bioavailable forms of different nutrients. The resulting wood ash can be soaked in water, which releases potassium ions (another important inorganic nutrient) into solution. Potassium salts can then be evaporated out of the resulting solution. Since the ash-soaking process often occurred in pots, they referred to the results as potash. Later, in the early nineteenth century, potassium was derived from this potash. And as did the compound itself, so did its (English) name.

References

Main resource:

Extra resources:

Bonus fact:

We glossed over how plants/plant roots take up nutrients from the soil. A more detailed account can be found at Nature: Plant-Soil Interactions: Nutrient Uptake.

- DK