What Is Carbon?

Carbon farming, carbon markets, and carbon sequestration are just a few of the many terms that farmers are constantly hearing at conferences, webinars, and reading in articles. However, agronomists, or crop scientists, rarely cover the basics of carbon and why it is essential to crops and the environment. Let's start at the beginning- "What is carbon?" Carbon is a chemical element in the atmosphere, plants and soil, fossil fuels, and the ocean. All these groups serve as carbon reservoirs or sinks, where it is stored in various forms (solids, liquids, or gases). Carbon is constantly changing forms and moving from one reservoir to another through different mechanisms that occur naturally or due to human activity.

Each of these reservoirs has a specific role in carbon transfer and agriculture.

Atmospheric Carbon:

Atmospheric carbon is present in the form of gases like carbon dioxide (CO₂) and methane (CH₄). Carbon dioxide is the raw material plants require for growth through a light-powered process called photosynthesis. The plant will use carbon-based building blocks (or carbohydrates) from photosynthesis to construct cell walls, fibers, stems, leaves, flowers, roots, tubers, wood, branches, grain, and fruits.

Soil Carbon:

Soil carbon can be present in inorganic or organic forms. Inorganic forms of carbon include calcium carbonates, like dolomite and limestone. Organic forms of carbon are present in living and decomposing plants, animals, and microbes. In addition, plants, microbes, and the soil fauna interact by releasing organic carbon into the soil. For example, actively growing roots release carbon-containing substances into the soil that will serve as food for microbial communities and as signals to recruit beneficial nitrogen-fixing bacteria and mycorrhizae. So, this interaction between plants, soil fauna, and microbes contributes to the soil organic carbon pool. When microbes decompose plant debris, carbon is released into the atmosphere as CO₂.

Fossil Fuels:

Crude oil and coal are carbon-rich products derived from the decomposed remains of ancestral plants and animals. Fossil fuels are mainly used to generate electricity or manufacture products that power gasoline and diesel engines. Combustion of fossil fuels releases CO₂ back into the atmosphere and other gases that are considered pollutants. 

Oceanic Carbon:

In the ocean, atmospheric carbon dioxide dissolves in water. Marine organisms use carbon as a building material for shells, and aquatic plants and microorganisms for photosynthesis. Many fossil fuels come from the long process of aquatic decomposition and carbon cycling within the ocean. The oceans also contribute to some of our soil amendments like fish meal and seaweed or kelp-based products.

Why do we have to care about carbon in agriculture?

Carbon compounds provide the fuel that all living organisms need to survive. We already mentioned the importance of carbon dioxide for photosynthesis in plants, but what else does it do? Soil organic carbon, which makes up more than half of soil organic matter and therefore is often referred to as such, is composed mainly of residues of plants and animals. As this material forms in the soil, it increases the soil structure, and its ability to retain water and hold nutrients that could otherwise be lost through leaching or erosion. Carbon also supplies food for living organisms in the soil, which increases residue breakdown, thus promoting the release of nutrients into the soil solution. For a more in-depth look at soil organic carbon and how it improves soils, read Agronomy Fact Sheet 41: Soil Organic Carbon by Cornell University.

No-till and cover crop systems often have the highest amount of soil organic carbon because the microbes and organic carbon are allowed to form long acid chains rather than being exposed to air and released as carbon dioxide. Adding cover crops or forages to a system increases the volume of roots in the soil throughout the year, meaning there is more food for microbes and more opportunity for building up soil organic carbon. No-till and cover crops are sometimes referred to as carbon sequestration practices because they help to keep the carbon in the soil, compared to traditional methods.

Too much of a good thing?

But if carbon is so good, why should we be concerned about rising CO₂ levels in the atmosphere? Human activities have led to large amounts of carbon dioxide being released into the atmosphere. Consequently, larger carbon sinks, for example, the ocean, cannot keep up with removing all the additional CO₂ from the atmosphere, breaking the natural balance of carbon exchange among the reservoirs. Carbon dioxide levels continue to set record highs, reaching levels beyond the concentrations that have caused the Earth's natural warming over the last million years (1). Gaseous CO₂ absorbs heat, making this gas a critical contributor to the increased temperatures we have experienced in the previous decades. Increased temperatures have led to changes in precipitation patterns, increased frequency of droughts and heatwaves, and heavy downpours (1). Climate predictions for Pennsylvania indicate that at the end of the 21^st century, corn production will face a greater risk of drought and high-temperature stress during critical growth stages (2). Increased climate variability and exposure to more extreme weather events counteract the positive effects that CO₂ levels in the atmosphere may have on agriculture and plant productivity in some regions of the country. In a nutshell, in many cases - like corn production in Pennsylvania, more carbon in the soil is better than more carbon in the atmosphere.

Reference

1. Lindsey, R. 2021. Climate Change: Atmospheric Carbon Dioxide. NOAA Climate.gov. (Accessed May 24^th, 2022)
2. Prasad R, Gunn SK, Rotz CA, Karsten H, Roth G, et al. (2018). Projected climate and agronomic implications for corn production in the Northeastern United States. PLOS ONE 13(6): e0198623.