How to Celebrate 25 years in a Soil Carbon Project - Part 2 (of Six) - Soil Carbon Stability

From last week - the breakthrough discovery of a carbon fixing fungi for broad-acre crops becomes a game changer for reliable carbon sequestration and opened the way for productive ACCU generating Soil Carbon Projects in cropping systems.

It's a deceptively monumental idea, a soil carbon fixing inoculum. If the story from the last article fired up your imagination as it did mine, then a quick run through of the basics of how the actual biological mechanism works, may also ignite your inner-soil-nerd and help to give you confidence in your ability to store carbon in your cropping soil, for the long term. This implies the creation of stable carbon...the stuff that stays in the soil and doesn't disappear.

But first, why do I say, “deceptively monumental”? 

Imagine if you will… a large percentage of broadacre crops globally inoculated with a beneficial carbon-fixing-fungi, capable of building significant stable soil carbon stocks and thereby increasing soil fertility and crop productivity each year, (rather than losing soil carbon as we are now as an industry).  There are ~1.5 billion ha’s of annually cropped soils on planet Earth that underpin much of civilization's food security.  ~ 750 million ha's is wheat. What does that impact look like on soil health, food security and atmospheric levels of carbon dioxide?  All from humble beneficial soil fungus that has been selected to do a particularly important job… very effectively. Back of the envelope numbers puts us in the billions of dollars increased ag revenue and into the gigatonnes of carbon dioxide drawdown.

That's the big picture, a tiny fungus, deceptively monumental.

But what about down at the micro level where the fungi do their good work?  A basic understanding of the biology, physics and chemistry at work is at once fascinating and important to understand through the landholder's eyes…to understand the actual working mechanism of amplified fungal mediated soil carbon sequestration.  

 I call it ‘how-the-thing-does-the-thing’ and it actually forms the foundation of why a 25-year soil carbon project now becomes a highly viable and rewarding proposition for a grain farmer.  Plus, it's just very cool soil science stuff that I’ve found most farmers truly get excited about, as well they should… soil health is after all the actual centerpiece of a farm's income and sustainability!

CarbonBuilder carbon-fixing fungal microorganisms do some extraordinarily cool and useful things in the soil. 

 Aggregation is one of my favorite processes in the soil because it has such a profound influence on so many soil health functions such as water infiltration, storage and drainage as well as root health and nutrient availability… and these fungi are kings of soil aggregation.  It has been my experience most farmers intuitively understand that aggregation is structure and good structure means a more productive soil on many levels, non-least ...capturing that valuable rain and getting it down to the subsoil storage tank.

Soil aggregation simply means the fungi wrap up soil particles into tiny balls (microaggregates <250um) with their hyphae (fungi roots), creating aggregated soil structure.   Aggregation allows the soil to breathe so that chemical, physical and biological soil processes can be optimized.  I think of these specialized fungi as the dung beetles of the microscopic world.

Of course, CarbonBuilder fungi’s main game is to ‘trap and convert’ plant carbon (root exudes and dead cells etc.) into two main long term carbon pools, namely; 

1. Aggregate carbon - this is simply carbon trapped inside microaggregates that the fungi have formed. Carbon dating of soil samples shows once inside a microaggregate soil carbon can safely be stored for decades through to centuries.
2.  Mineral Associated Organic Carbon (MAOC) - this is where the long-lived melanin rich carbon exudates the fungi ‘leak’ from their hyphae chemically-cling or ‘sorb’ onto soil mineral surfaces.  It's easier to think of the fungi carbon as glueing itself to soil mineral surfaces like Tarzan's grip.  MAOC is very stable and can safely reside in the soil for decades to even millennia, again as proven by carbon dating.   

Soil carbon produced by these specialized fungi in this way is long lived...stable and safe.

A recent Western Sydney University peer reviewed paper (Involving CarbonBuilder fungi) using 13C radioactive tagging was able to trace C02 out of the air, through the plant, down through the roots, out through the fungi hyphae network attached to those roots and out into the soil.  The reason; to discern the final fate of the tagged carbon atoms from the air into the soil profile.  The soil was then fractionated (split up) into various types of soil carbon to reveal where and how the carbon was stored in the soil.  The paper reveals the fungi are storing atmospheric carbon in the aggregate and MAOC fractions of the soil…the long-term secure carbon pool. Through this exacting carbon mapping method, the paper highlights the actual carbon sequestration mechanism of these fungi very well. It's incredibly cool science with profound implications for agriculture.

  EGUsphere - Non-mycorrhizal root-associated fungi increase soil C stocks and stability via diverse mechanisms (copernicus.org).  

This carbon stability afforded by the fungi is critically important, particularly if you are in a 25-year soil carbon project.  Why? Because you need to be sure the carbon will stay in the soil through droughts and floods and all seasonal conditions.  Producing stable soil carbon enables 25-year soil carbon projects to be a safe option for farmers.

When developing a soil carbon inoculum, the first rule is do no harm to the crop (or anything else).  Indeed, the best way to prove that the fungi will not harm the crop is to show a plant growth stimulatory effect and grain yield.  These specialist fungi are screened to deliver an increased grain yield response in the host crop.

Various plant growth stimulation and yield enhancing mechanisms are at play including enhanced nutrient acquisition, plant growth hormones, and physical rhizosphere impacts such as increased aggregation leading to general soil tilth improvements and root expression in the root zone. 

In a 25-year soil carbon project, the virtuous nature of producing a healthier soil and so a stronger plant year in year out not only increases grain yield potential but also acts as an increased positive carbon feedback mechanism back into the soil to produce more root biomass and carbon rich root exudates.  This extra carbon flow into the soil is simply extra underground carbon resources for the CarbonBuilder fungi to convert into greater quantities of stable aggregate and mineral associated organic carbon.  The crop wins and the soil wins....so the farmer doubly wins.

Very interestingly, a deep dive into the nitrogen dynamics of soil treated with these fungal partners vs untreated has shown that more nitrogen is left behind in the soil when the fungi are inoculated with the plant, when compared to uninoculated crops in replicated rhizobox, small plot and strip trial research.  

This points to a fungal driven mechanism that stimulates the increased production of plant available nitrogen by certain soil organisms.  In turn this also suggests that building carbon in this way does not steal from the applied nitrogen bank, but overall, rather adds to it.  More research is needed however it seems that farmers will not need to add more urea to build soil carbon in this way.  

CarbonBuilder fungi help to stabilize carbon into long term soil reserves. From a gas in the atmosphere to a liquid carbon in the plant to solid carbon in the soil.  In this way I like to think of this biological mechanism as the solidification of carbon dioxide from the air into a long-term carbon into the soil. 

Carbon moves from a gas (carbon dioxide C02) to a liquid (plant glucose C6H1206) to a solid (fungal melanin C18H10N204).... The solidification of carbon!

Note the positive effect on plant growth.

 (Photo courtesy SoilCQuest 2031). 

For an ACCU generating 25-year soil carbon project this is a critical and genuine green light. We can build meaningful levels of soil carbon, and it is stable.

TBC

Keep an eye out next week for Part Three of; ‘How to Celebrate 25 Years in a Soil Carbon Project’ 

New.Possibilities

[ˌpɒsɪˈbɪlɪti]

plural noun

 The state or fact of being possible - likelihood

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For more information contact 

Guy R Webb 

guy@soilcquest.org.au