This article was researched, written and published by UnderstandingAg.com.  Go to https://understandingag.com/the-carbon-chronicles-part-1/ to read the article in situ.

Understanding AG: The Carbon Chronicles

In Three Parts

By Brian Dougherty

Part 1: The Crop-Carbon Connection

In soil I lie, a secret unseen,
A cornerstone of life, in every green.
With humble microbes, a dance serene,
In Earth's embrace, my role is keen.

In the ground I dwell, unseen but grand,
A vital link in life’s command.
With microbes and roots, a partnership planned,
In the soil I'm hidden, yet readily at hand.

From forest floors to fields so wide,
I'm in the roots where plants provide.
In life’s cycles, a constant guide,
Guess who am I, in the soil I hide?

Carbon. It’s everywhere, and you hear about it all the time if you are involved in agriculture. But have you ever thought about why carbon is so important to us? I wrote the riddle above with a little (okay a lot of) help from ChatGPT in a whimsical attempt to provoke some thought about this question.

Carbon is the building block of life. Every living and formerly living thing has carbon as part of its makeup. Globally, carbon cycles in timescales reaching tens of millions of years, from the atmosphere to deep in the earth’s crust and back again, with many temporary stops along the way. These shorter layovers where carbon cycles through plants, animals, and soil are fundamentally important for food production.

Carbon is unique, because it has the ability to form bonds with a vast number of different elements, and also with itself. Hydrocarbons like coal, oil, and natural gas that we burn for energy are just carbon and hydrogen atoms arranged in rings or chains of varying length. Carbon performs the same function in soil. It is the energy (food) source that powers microbial activity. Microbial activity in the soil is the engine that drives everything from nutrient cycling to decomposition to water purification. This microbial action creates the glues that build soil aggregates, and soil aggregates are the building blocks for healthy, well-functioning soil.

Carbon has been described as the “currency” of the soil. It is the unit of exchange that enables the biological community to function. Plants take in carbon from CO2 in the atmosphere and convert that carbon to various types of sugars and other compounds. Some of those sugars are traded with microbes in the soil in the form of root exudates. In exchange, the community of bacteria and other soil organisms cycle nutrients and make them available for plant uptake. Mycorrhizal fungi are particularly important partners that tap directly into plant roots and deliver nutrients and water in exchange for the carbon they need to survive.

Other sources and forms of carbon also play a role in this underground economy. Soil organic matter is a catch-all term that describes everything from decomposing residue to humic substances to thin films of carbon compounds stuck to soil particles to sticky proteins that glue soil aggregates together. Organic matter is about 50% carbon on average. It serves as a critical buffer for physical, chemical, and biological stability in the soil. Generally, the more organic matter a soil has, the greater its capacity to store and supply nutrients and water. Low organic matter soils are less productive and usually less profitable to farm.

Think of organic matter as a “savings account” in the soil, whereas root exudates are more like cash flow that funds the checking account for everyday transactions. If there is extra cash in the checking account, it can be put towards savings for a rainy day. Microbes can use all forms of organic matter as an energy source. If the checking account runs dry, they will tap into the savings account to get their food. Without a continual influx of carbon, the biological community in the soil suffers, just like your community would suffer if the grocery store shelves were running empty. Without food to eat, productivity declines and the community can no longer perform essential functions that support plant growth.

Carbon must continually cycle through the soil via root exudates and decomposing residue to maintain the checking and savings accounts. If the cycle slows, organic matter levels will decline as the microbes eat through their cash reserves. Farmers and ranchers then eat through their own cash reserves buying expensive inputs to try to replace the lost soil function and productivity. Think of carbon as the cash cow that drives farm and ranch profit. If you invest in cycling more carbon through your soil, you will be paid many dividends in the long run.

In part 2 of this series, we will dig into the details of where this “cash flow” of carbon comes from, where it goes, and how it all works at the field scale

 

Carbon Chronicles: Part 2, Where Does the Carbon Go?

I'm a traveler unseen, in cycles I flow,
From the air to the earth, far down below.
Plants welcome me in their leafy grace,
As part of a process, essential to embrace.

I dance with the sun, in the atmosphere high,
Yet in oceans and soil, you'll find me nearby.
Roots release me when they exhale,
Guess my name, for I never fail.

Where do I journey, where do I roam?
In this grand cycle, I find my home.
A puzzle of nature, a mystery in prose,
Tell me dear friend, where do I go?

In part 1, we reviewed the important role that carbon plays in the soil. Now let’s focus on how that carbon moves though agroecosystems at the field scale. Below is a basic diagram showing how carbon cycles through a corn field yielding around 200 bu/acre. This is a classic ‘stocks and flows’ diagram that engineers like to create. I confess to being a recovering engineer, but this is a useful way to conceptualize an invisible process that’s happening in crop fields and in pastures. The box represents the carbon stored in your soil. The actual amount will vary depending on your soil organic matter (SOM) percentage and how deep in the soil you measure. Our family farm in Northeast Iowa with 3% SOM in the top six inches has about 67,000 lbs. of carbon per acre in the top two feet of soil. You can measure this in your own soil by running a total nutrient digestion (TND) soil test.

 The box is your “storage tank” of carbon in the soil. The arrows represent flows into and out of your tank. The inflow starts with the plant pulling in carbon (CO₂) from the atmosphere during photosynthesis. An average corn crop in Iowa is pulling in around 30,000 lbs. of carbon per acre annually. That’s a big number! Where does it all go? Some carbon accumulates in the plant as it grows and ends up as crop residue and root biomass that decays after harvest. Plant biomass is around 40% - 45% carbon on a dry basis. Some carbon is pumped into the soil via root exudates. Root exudates are sugars (liquid carbon) that plants pump into the soil to feed microbes in exchange for nutrients. Another portion of the carbon is used for root respiration.

Now let’s look at the outflows. A portion of the carbon is given off by the plant aboveground as CO₂. Soil microbes respire a large portion of the carbon as CO₂ while they go about the business of living and dying. The root system of the plant also respires and releases CO₂ into the soil and eventually back to the atmosphere. You may have heard people say that soil ‘breathes’. The diagram shows that this is true. Plant roots and soil microbes take in oxygen and give off CO₂ just like we do. Another portion of carbon is removed when the grain is harvested. There are other smaller outflows when carbon is lost by soil erosion or dissolved carbon leaches deeper into the soil or out through a drainage line. The actual carbon flows and interactions between plants and soil are much more complex and intertwined but this is a basic model for the carbon cycle of a corn crop. The same process is happening everywhere that plants grow.

Figure 1. Carbon flow estimates for a 200 bu/acre corn crop.

The inflow and outflow numbers are averages based on research. The schematic illustrates that all of this carbon is cycling through the plant and soil. It does not necessarily stay in the soil. This is what engineers call a “steady state” system. Subtract the outflows from the inflows and you get zero. The soil organic matter in our box will neither increase nor decrease in this scenario.

Did you spot the missing arrow? It’s the tiny input from fertilizer and seed. We typically add very little carbon to our fields unless we are adding a lot of manure or compost. That is okay because plants can do the work for us, but we need to remember that to increase soil organic matter, we need the flows into the box to exceed the flows out of the box.

Unfortunately, human activity over the past several millennia, and especially the last 150 years, has made the inflows smaller and the outflows bigger. This is why we have lost a large portion of soil organic matter worldwide. We need to reverse this process, but how? It's simple.  At least in theory. Do less of the things that make the outflows bigger and do more of the things that make the inflows bigger.

Let’s tackle the outflows first. Soil disturbance makes the outflows bigger. Tillage increases erosion and accelerates organic matter decay. The disturbance and rapid addition of oxygen to the soil creates a huge spike in microbial activity. The microbes use carbon (organic matter) as a food source. The result is a rapid spike in CO₂ emissions from the soil. Minimize soil disturbance and leave residue in the field for soil armor to prevent these losses.

Biomass removal also makes the outflows bigger. We have to eat so some removal is inevitable, but we can minimize the negative effects with good management. We need to be very careful with schemes that remove biomass for energy production or other uses. In the long run, we also need to do a much better job of returning carbon and other nutrients to the soil through composting of food waste and humanure to complete the cycle. Composting humanure was a common practice historically, but now it is often contaminated with pharmaceuticals and heavy metals. In nature, all ‘waste’ is recycled. The fact that our humanure is too contaminated to be safely returned to the soil should give us all pause about what we are putting into our bodies and how removed we have become from the cycle of life that sustains us.

Microbial and root respiration are actually good outflows. We measure this as CO₂ respiration using a Haney soil test. A larger flow of carbon through roots and root exudates to microbes and back to the atmosphere is a general indicator of a healthier plant and healthier soil. Too little flow of carbon to feed soil microbes is one of the most common problems we encounter on farms.

In part three of this series, we’ll take a deeper dive to look at carbon inflows and examine one key regenerative principle that can supercharge that inflow on your farm or ranch.

Carbon Chronicles: Part 3, Capturing, Cycling More Carbon

The first two parts of this series examined the critical role of carbon in powering plant and soil life (all terrestrial life) and the cyclic outflow from an average corn field. Now let’s look at inflows.

Unsurprisingly, the lack of photosynthesizing plant cover throughout the growing season makes the carbon inflows smaller. Maintaining living cover throughout the growing season is the best and easiest way to increase carbon flow into the soil. Plant photosynthesis is by far our biggest arrow. The corn crop in the diagram emerges and reaches maturity in about 120 days. The remaining growing days are lost opportunity if there is no living cover. Adding a cover crop adds a new inflow, and it’s more likely that a portion of that carbon will stay in the soil if that cover crop is not harvested. Adding a perennial to the crop rotation can also drive a large increase in photosynthesis. Diversity in the cover crops or perennials increases photosynthetic activity even more. Farmers switching from tillage and fallow to a no-till cover crop system or from set stocked to adaptive grazing often see an increase in soil organic matter. Why? Because they have reduced the outflow and increased the inflow of carbon. You can also add carbon via humic products or compost, but the most efficient route is to let plants do the work for us.

Fertilizers and animal manures are a special case, because whether the ultimate result is net positive or negative depends on how they are managed. Animals in a well-managed adaptive grazing system tend to have an overall net positive impact. This can be quite large and drives the most rapid increases in soil organic matter that can be achieved without bringing in a large amount of biomass from somewhere else. On the other hand, poor fertility management can have a detrimental effect. Overapplying fertilizers or manure (especially high N fertilizers or anaerobic pit manure) can create nutrient imbalances, increase salt loads, and disrupt soil function. Adding too much nitrogen has the same result as tillage. It can spike microbial activity and drive the conversion of organic matter to CO₂. However, manure does add carbon to the system so the net effect is hard to predict. Excesses of other nutrients can also cause nutritional imbalances in the plant that limit productivity.

Figure 1. Carbon flow estimates for a 200 bu/acre corn crop.

There is one more way to increase the inflow of carbon that is almost always overlooked. Can you guess what it is? Many researchers believe that it is not possible to increase soil organic matter much beyond 0.1% per year. They prove this using the exact same math that you see in the diagram. But what if one of the underlying assumptions is wrong? Photosynthetic efficiency is typically assumed to be constant when these calculations are done. This is the amount of light energy that is converted into chemical energy during photosynthesis.

But what if that chemical energy input is not constant? What happens if the plant converts a greater percentage of sunlight, CO₂, and water to biomass and root exudates? We now have a bigger arrow from the same ground cover. What if that same corn crop pulled in 50,000 lb of carbon per acre instead of 30,000? What if your adaptive grazing system leads to a large increase in plant productivity and carrying capacity? Now you have a much larger flow through the plants and extra carbon available for storage in the soil.

There is research that actually proves this, but it’s often disregarded because it is very hard to predict. Root exudation can vary from as little as 5% to as much as 95% of photosynthate production. That is a huge range and will vary depending on plant species, stage of growth, environmental conditions, and soil function. The majority of soil organic matter actually comes from microbes consuming root exudates and creating carbon compounds that stick to soil particles. In the diagram, root exudation is 20% of photosynthate production. What if we had more exudates?

To improve photosynthetic efficiency, the plant must be able to access adequate mineral nutrition in the proper balance and grow in well aggregated soil that provides optimal conditions for the plant. What limits photosynthesis? The two main inputs are CO₂ and water, thus one or both are most likely limiting factors. Following the soil health principles to improve water infiltration and water holding capacity will increase water availability to the plant. What about CO₂? Look at the diagram. Is there a source of CO₂ right where the plant needs it? Yes! You can improve photosynthetic efficiency by increasing microbial activity and CO₂ respiration from the soil. Nurture life in the soil by minimizing disturbance, armoring the surface, keeping living roots in the soil, adding diversity, and incorporating livestock. Remember, it’s a cycle. The plant-soil system is generating and recapturing CO₂ right in the plant canopy and feeding itself in perhaps the most elegant cycle in all of nature.

Learning how to create and maintain conditions that improve photosynthetic efficiency is the next frontier in regenerative land management. Following the 6-3-4^TM is the foundation upon which we can build. It is a time-tested way to reverse degradation, improve soil function, increase organic matter, and cycle more carbon through the soil.

Carbon is the currency of terrestrial life. Understanding how that currency is exchanged, and what we can do to positively affect the soil/plant carbon cycle, will lead to increased soil organic matter, healthier soil plants and animals—and more profitable and resilient farms and ranches. In other words…

In realms unseen, I take my flight,

A quest to capture carbon's might.

From skies so high to earth below,

I'm the secret, that makes it all grow.

 

In gardens, fields, or nature's space,

I work my magic, a silent embrace.

With animals and crops, I weave my lore,

Returning carbon to where it was before.

 

What am I, this subtle force?

In green and brown, I chart my course.

In decay, transformation I enact,

Tell me now, what is this silent act?