Volume 29 Issue 1, Winter 2024

by Gerco Hoogeweg, JK Black Oak Wildlife Sanctuary Committee

Soil samples were taken from 26 locations at JK Black Oak, including this section of the oak-hickory forest, to help measure carbon sequestration levels over time. Photo by Gerco Hoogeweg

With all the news about climate change and what to do about it, you may have heard the term “carbon sequestration.” But what is it? Carbon sequestration is the capture and storage of carbon that would otherwise be emitted to or remain in the atmosphere as carbon dioxide. Carbon dioxide is a well-known greenhouse gas and a contributor to global warming.

Carbon sequestration can be divided into two types: geologic and biologic.¹

1. Geologic carbon sequestration is the process of storing carbon dioxide in underground geologic formations. The carbon dioxide is usually pressurized until it becomes a liquid, and then it is injected into porous rock formations in geologic basins. This is an artificial way of storing carbon.

2. Biologic carbon sequestration refers to storage of atmospheric carbon in oceans, soils, and vegetation. This is naturally occurring storage of carbon.

Gerco Hoogeweg collecting soil samples. Photo by Sheila Ferguson

Practically speaking, biologic carbon sequestration usually focuses on increasing the number of trees in our landscape and increasing the carbon in the soil. Planting trees is a popular way to sequester carbon. Trees, shrubs, and plants need carbon dioxide to grow. Thus, the more trees we plant, the more carbon dioxide is stored. It is calculated that we need to plant 2.3 billion acres of new trees to help limit the increase in global average temperature to 2.7 degrees Fahrenheit above pre-industrial levels by 2050. This is a vast area that is almost 6,900 times the size of Loudoun County, or 1.2 times the size of the lower 48 states. Globally it translates into 25% more forests compared to what we now have.²

At JK Black Oak we are working on carbon sequestration, albeit on a small scale. Under the terms of the 2022 grant received from Microsoft in collaboration with the Society for Ecological Restoration, we need to estimate the amount of carbon that will be sequestered for our habitat restoration project. You can use online carbon calculators³ to estimate how much carbon is stored in a tree if you know the number and size of the trees.

Instead of aiming for a traditional tree-planting approach to store carbon, we have elected to take a different path. Why not protect what is already there? So rather than digging holes, planting trees, and then watering and protecting them, we went straight to the “protecting” part. As part of the grant, we committed to protect at least 100 tree seedlings with cages and an unknown number of trees in three deer-exclusion areas. For the upcoming three years we will count how many trees are growing in the deer-exclusion areas and how many seedlings will survive in the tree cages.

One of JK Black Oak’s 17 tree seedling count plots. Photo by Sheila Ferguson

Counting each individual tree seedling within the deer-exclusion areas was an impossible task because more than 200 seedlings were found in a single transect during the first count. Therefore, we switched over to a proven and robust scientific approach of using count plots. Seventeen 5-foot by 5-foot plots were established. Within each of the plots, the seedlings were counted and marked. In this first year we estimated that within the three deer-exclusion fences between 7,000 and 7,200 tree seedlings were present — a staggering number. Obviously only a small fraction of those seedlings will survive. Because the tree seedlings are too small, we did not estimate the amount of carbon stored. Toward the end of 2024 we may run our first estimates of carbon storage.

Although soil carbon sequestration is mostly associated with agricultural activities, at JK Black Oak we will monitor soil carbon for the next three years. In the fall of 2023, 26 locations in and around the deer-exclusion areas and tree cages were sampled for soil carbon. The soil samples were mailed in specialized small boxes to the Soil Testing Lab at Virginia Tech⁴ for analysis. In this first year we aimed to establish a baseline of the amount of carbon stored in the topsoil.

The soil samples were taken from the top 8 inches, using a soil auger and probe. The samples were then cleaned using a 2mm soil sieve to remove all large particles, rocks, and root fragments. An 8-inch depth was used because the local soil survey maps showed that the top soil layer was anywhere between 7 and 9 inches thick, and 8 inches happens to be the size of the soil auger. During the sample collection, we noticed that the soil showed an amazing variety of colors. Really dark soils were found in the vernal pools, with red-colored soil along one of the trails, and more yellowish soils on the slopes and lower elevations.

Soils collected at JK Black Oak have many different colors. Photo by Gerco Hoogeweg

Laboratory analysis showed that the soil organic matter (a way of expressing soil carbon) content ranged from 2.3% to 15.4%. The highest organic matter levels were found in the vernal pool soils. When the vernal pools are excluded, the soil organic matter ranged from 2.3% to 6.5%, with an average of 5.0% for the 0-8-inch depth. This information provides our initial baseline of soil carbon at JK Black Oak. It’ll be interesting if we see increases in the soil carbon in the upcoming years. We expect to see little change until 10 to 20 years have passed.

Resources for Additional Information:

“How to Turn Your Backyard Into a Carbon Sink”
“What Plants Are Best for Carbon Sequestration”
A Homeowner’s Guide to Carbon Sequestration

Footnotes:
¹ https://www.usgs.gov/faqs/whats-difference-between-geologic-and-biologic-carbon-sequestration#:~:text=Geologic%20carbon%20sequestration%20is%20the,rock%20
² https://climate.nasa.gov/news/2927/examining-the-viability-of-planting-trees-to-help-mitigate-climate-change/
³ https://8billiontrees.com/carbon-offsets-credits/reduce-co2-emissions/how-many-trees-offset-carbon-emissions/
⁴ https://www.soiltest.vt.edu

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