Soil Carbon: To Hoard of Cycle?
The hype around sequestering carbon in our agricultural soils has become ubiquitous. As evidenced by the latest blockchain-backed startups, a race is underway to capture a US market valued at $4.6 trillion and simultaneously reverse climate change. But with all this zeal for locking up carbon in the soil, we are missing the fundamental principle of soil carbon: its value is in decomposition, not sequestration.
Soil carbon is a component of the broader category of organic compounds referred to as soil organic matter.
Approximately 58% of soil organic matter is carbon. The total soil organic matter pool includes plant detritus, roots, root exudates, microorganisms, and stable humic substances. Organic matter is what animates our soils; without its dynamic presence, our soils would be devoid of life.
The production of organic matter begins with photosynthesis, a natural process by which green leaves convert carbon dioxide and water into biochemical energy. This energy is converted into carbon-based material, from roots and shoots to sugar exudates. The exudates, roots, and other plant material components are broken down and joined together to form more complex and stable molecules. This process, initiated by bacteria and finished by fungi, is called humification. Humic substances, ranging from fulvic and humic acid to humin are the final products of humification, forming the soil’s stable carbon pool. Known as recalcitrant carbon, the stable pool is produced as most of the starting pool of soil carbon is lost to the atmosphere due to microbial respiration.
This dynamic transformation of soil organic matter is at the heart of optimal ecosystem health. The decay of soil organic matter results in more nutrient cycling, increased plant productivity, higher rates of photosynthesis, and, therefore more carbon injected into the soil.
We go wrong with the myopic focus on sequestering carbon in soils. To maximize carbon sequestration requires constraining the dynamic process of microbial decay and the cascade of life-giving processes that spring forth.
Industrial grain farms are reaping the benefits of monetizing the carbon they can lock up in the soil with half-hearted practices like no-till, cover cropping, and ever more reliant on exorbitant rates of glyphosate and synthetic fertilizers.
While these soils are sequestering some carbon, mainly due to the absence of tillage, the health of the soil, its fertility, microbial communities, and ability to capture and hold precious water are also absent.
Take the example of a farm where the carbon cycle is in full motion; organic matter is constantly added to the soil as root exudates and plant detritus. Meanwhile, microorganisms ebb and flow as they break down the organic matter, preserving a small portion of the total pool as stable soil carbon and releasing a majority of the carbon they feed on as CO2 due to microbial respiration. The benefits of fully cycling organic matter and soil carbon are immeasurable. Plants receive the mineral nutrients they require in the optimal form at the precise time, returning a richer composition of root exudates to the soil to encourage microorganisms to continue. Arresting this cycle at the halfway point by simply maximizing the sequestration of carbon is to the soil what hoarding all the money and not spending it is to the economy. Carbon is the currency of soil and must be spent to foster more life.
We need to see farms as whole systems, not isolated parts. The health of a farm and the resulting vibrancy of the food they produce depends on a fluid connection between each dynamic part. The myopic emphasis on a single thread of the complex tapestry of the soil/plant system is a fast track to mismanagement. Life begets life, and from decay springs forth more life. When we can increase soil life by encouraging the complete cycling of soil carbon and its inevitable decay, we make way for more life across the system. Crops thrive, water is captured and stored, and people benefit from a more nutritious harvest.
