Ever clean your room by shoving everything under the bed?
What if we could do this with all the extra carbon dioxide in our atmosphere — just shove it in the ground?
Well, some people are actually already trying this and have poured millions of dollars, research, and time into this strategy,
known as soil sequestration.
The thinking goes that soil can lock carbon underground and help with the climate crisis.
But much like how messes don’t disappear when shoved into your closet,
soil carbon may not stick around in the ways we had previously thought.
In fact, our entire understanding of how carbon dioxide stays in the soil
may need some MAJOR reworking.
To be clear: We do know that carbon can stick around in soil for centuries or even millennia.
If we wanna store carbon in the soil,
we first need to understand what that thin layer of material covering Earth is actually made of.
At its most basic, soil is made up of minerals, air, water, and organic matter like leaves.
Organisms feast on this organic matter, breaking it down and releasing carbon dioxide, or CO2, into the air.
With one major exception: humus.
First described in 1786, this group of complex organic molecules has long been considered stable enough
to resist decomposition from microbes,
which instead ate the other, shorter molecules present in the soil.
This meant that all of the carbon stored inside these humus particles sort of just stayed put.
Because of this, some scientists have been trying to harness the potential of humus as a carbon sink.
The Salk Institute’s Harnessing Plants Initiative has received millions of dollars in funding
to research methods of creating more humus.
But we still don’t know how carbon cycles in soil
and there’s very little we actually understand about humus.
Like, does it even exist?
In a groundbreaking study from 2015,
the researchers argue that the available data collected on soil doesn’t actually provide any evidence
for the formation of humus in soils...
mostly because traditional soil extraction methods don’t analyze the material as precisely as they should.
Traditionally, scientists study soil organic matter by adding a harsh alkaline treatment to the soil sample.
This treatment causes different components of the soil organic matter to react differently —
some of it becomes more soluble in water, whereas some of it isn’t affected by the treatment at all.
To account for these differences, scientists concluded that there were different types of humic substances in the soil…
instead of trying to figure out how soil organic matter functions as a whole.
Basically, it created a lot of confusion.
But now, scientists have a way more sophisticated way to study soil organic matter:
put a lil’ bit of soil in a magnetic field, beam it with timed radiofrequency pulses, and examine it directly.
Recent studies have found that humus mostly contains components from plants and microbes —
meaning that humus may be a snack for microorganisms too.
And if they’re able to digest humus…that means the carbon that we once thought was locked away…
is actually being released back into the atmosphere.
So you can imagine how this changes our plans for having humus capture CO2 underground.
But remember, carbon CAN stick around in the soil for a very long time.
So if humus isn’t sequestering carbon...what is?
Well, one idea is that minerals and clumps of soil particles, or aggregates, could be responsible.
They could present a challenge to microbes looking for a snack,
essentially “locking away” carbon in the soil and preventing them from digesting it.
But we don’t have enough studies about this interaction or how it’s vulnerable to natural and human processes
like freezing, thawing, or cultivation.
Meanwhile, we’ve gotta apply our new understanding of humus to climate crisis solutions,
starting with our climate models.
Back when scientists started making models of the global climate,
they simplified them by putting soil carbon into ‘short-term’ and ‘long-term’ pools.
Those classifications continued to be used for the next few decades.
So now a bunch of models, like the one used by the Intergovernmental Panel on Climate Change,
overestimate how much carbon is sequestered underground
and underestimate how much carbon comes out.
Which becomes a bigger problem when you factor in that warm soil releases wayyyyy more CO2.
The reality is that we just don’t have precise enough ways to figure out how soil carbon cycles,
so it’s really hard to write a perfect model.
But researchers could add certain elements to our models,
like the fraction of carbon associated with minerals.
And since soil microbes are such huge drivers of carbon flow,
maybe adding microbial biomass or the microbial growth rate into the mix will be the move too.
Even though it’s a blow to learn that long-term carbon storage in humus doesn’t actually happen,
it is great to have this knowledge to better inform our decisions.
Science isn’t perfect, but it’s ever-improving our understanding of the world.