Agroforestry: A Carbon Sequestration Solution to Climate Change
Speeding through the famous 5-interstate from Los Angeles to Medford, Oregon, I flew through the agricultural deserts of California's Central Valley. A historic valley that was once lush with vegetation and water, now dusty, dry, and polluted.
Every day you hear about how we need to be planting more trees to sequester carbon to restabilize our increasingly volatile climates around the world. Why then, is the Central Valley of California full of trees growing olives, citrus, almonds, walnuts, pistachios, and stone fruits and yet barely scraping its carbon sequestration potential?
In a word: humus. There is none.
As we just saw, humus comprises of the uppermost layers of soil, also known as the 'O' and 'A' soil horizons. Think "organic," as that topmost layer should be primarily comprised of organic matter— that is, decaying lifeforms (leaves, twigs, dead bugs/animals, etc.).
Now, look at these pictures:
There is no 'O', or 'A' horizon, and there's hardly any 'E' horizon left. What we are looking at is the 4th soil horizon, the 'B' horizon, also known as "subsoil."
Where did the top three horizon layers go? They have eroded away. Why is that?
There are no lifeforms (plants, bugs, animals, soil microbiology) being allowed to regenerate topsoil, and there is frequent tilling (digging up, churning of the soil by machinery). Tilling dries out the soil and breaks up soil aggregates. It effectively kills plant roots and soil microbiology by over-exposing them to air and soil moisture loss.
The top three soil horizons' durability is heavily dependent on two things:
diverse root profiles
diverse soil microbiology
Diverse root profiles exist when various plant species onsite have different root growth features. Some roots grow in a fashion that is either fibrous, tubercular, or tap. It is also ideal to have plant species with root profiles that grow to different soil layer depths. Different soil depth penetrations and growth characteristics of roots create an intricate web structure that holds together the soil layers.
Soil microbiology is responsible for decomposition, the cycling of nutrients, making nutrients biologically available (bioavailable) to other lifeforms (like plants). Their biological activity releases sugars and proteins, which help bind soil particles together, creating aggregated soil chunks. These soil aggregates are more resilient to erosion due to their binding.
Growing trees is not enough. It is imperative to include other small and diverse plants whose root systems will weave together the topsoil layers while leaving the soil untilled(undisturbed) so that the soil microbiology can bind soil particles.
Life on Earth is predominantly made up of carbon-based lifeforms. Soil microbiology accounts for more than 15% of Earth's life forms and has a 32% impact on the variation of the ecosystem's functionality as a whole.
When we create conditions for topsoil to regenerate, more carbon is able to be sequestered. Compare a carbon-rich forest to a carbon-poor desert.
Optimal agroforestry systems encourage topsoil regeneration by creating conditions where biodiversity is supported. This means planting diverse plants, using organic amendments (not using biocides and petrochemical fertilizers), and not disturbing the soil. It's a more natural way of industrialized farming.
Agroforestry systems present many nature-based solutions for climate change. Carbon sequestration is a big one.
Resnick, B., & Zarracina, J. (2018, May 29). All life on Earth, in one staggering chart. Retrieved May 15, 2020, from https://www.vox.com/science-and-health/2018/5/29/17386112/all-life-on-earth-chart-weight-plants-animals-pnas
Milliken, G. (2019, March 18). This Is Exactly How Important Soil Is To Life On Earth. Retrieved May 15, 2020, from https://www.popsci.com/biodiversity-below-ground-is-critical-to-biodiversity-above-ground/