Soil organic carbon on North Carolina farms

How much carbon is being stored under pastures on your farm? Last month’s article outlined the approach of where to sample, how to sample, how to process samples, and what calculations are needed to make this determination of soil carbon on your farm. This article now gives you some idea as to how much carbon is present under a diversity of forage and grazing lands throughout North Carolina.

A few years ago, Dr. Matt Poore and I were driving from one farm to another. Along the way, we got to talking about what the soil health condition of various pastures might be as we zoomed around the curves in the road. Some pastures had an abundance of tall dogfennel with short bermudagrass being voraciously consumed by hungry cattle. Other pastures had a good foot of mostly tall fescue canopy intermixed with various other forages that were waiting for the next stocking event. Dr. Poore commented that one of the pastures surely didn’t have good soil health, and the other looked like it probably had a lot of soil carbon and overall good health. I pondered the issue and wondered if that seemingly obvious comment might really be true or not. I then replied, “How would we know if we didn’t measure it?” This conversation stuck with me for some months afterwards. Eventually an opportunity appeared that allowed me to start making measurements of soil carbon on private farms throughout the southeastern U.S. One of my goals was quickly formulated to know how much carbon might actually be stored under the diversity of pastures in North Carolina!

In the fall of 2021, I started taking notes about what the most important management variables might be that could influence how well pastures performed as far as forage growth and soil organic matter formation, as well as what might be the diversity of management choices made by producers in the state. By the fall of 2022, I was really interested in getting answers, and since I had just completed sampling soil from a series of farms in Virginia earlier in the year, I was ready to embark on one of the largest soil sampling efforts of my career. I had honed my sampling techniques in this earlier effort and just needed to find some farms.

I wanted the decision of which farms to select to be both systematic and somewhat random to avoid my own biases. I sent out a query to almost all livestock and agricultural agents in the 100 counties of North Carolina to help me find a few farmers that would be willing to participate in this research project. The target number of farmers to be selected in any one county was scaled to the acreage of pasture reported in the U.S. Census of Agriculture for each county. Which farm was selected in a county was at the discretion of each county agent. With farms sampled in 70 counties across the state, my biases were left at the office, and I felt good that I could get a fair representation of soil carbon under forage and grazing lands in North Carolina. Collecting soil from the state with the number and location of farms scaled to the acreage of pasture ensured the approach was systematic. 

I collected five cores at depths of 0 to 4 inches, 4 to 12 inches, and 12 to 24 inches from two sampling zones in each field for one or two fields per farm and repeated this approach on 206 farms from December 12 to March 31. While I was working like a machine, I also enjoyed the scenery and conversations along the way. In my opinion, I achieved my goal of being unbiased and systematic.

Soil organic carbon was measured on 1944 samples in this study. The three depth increments sampled per soil profile were necessary to quantify the soil organic carbon stock (mass of carbon stored in the top foot of soil) using a non-linear regression approach. Carbon concentrations of the 648 profiles were also used to separate the influence of relatively recent management from that of long-term pedogenesis. I’ll explore this separation more in an upcoming issue. For now, I’ll share with you the summarized results of soil organic carbon stock in the top foot of soil.

Soil organic carbon stock was expected to be greater in the Blue Ridge region than in the Coastal Plain region because of differences in soil type and climatic conditions. Soils in the Coastal Plain are typically sandier than in the Piedmont and Blue Ridge. Sand particles are not as reactive with organic matter, and therefore don’t hold onto and protect carbon like clay and silt particles do. Generally cooler temperature in the higher elevation of the Blue Ridge also tends to preserve carbon in soil by slowing soil microbial activity.

The stock of soil organic carbon in the top foot of soil was computed for each of the soil profiles separately, and average values for a county are shown in Table 1. Counties were grouped by the three physiographic regions of the Coastal Plain, Piedmont, and Blue Ridge.

References

Bliss, N.B., Waltman, S.B., West, L.T., Neale, A., & Mehaffey, M. (2014) Distribution of soil organic carbon in the conterminous United States. Progress in soil science. Springer. https://doi.org/10.1007/978-3-319-04084-4_9