Soil carbon sampling is done to gather baseline data for long-term evaluation of soil organic carbon sequestration in soils after conversion to seeded grassland and no-till. The overall goal of soil carbon research in Oklahoma is to determine sequestration rates of these practices in specific soils in as much of Oklahoma as possible. Since carbon offsets are a commodity, soil carbon data specific to the state lends credibility to the value of the offsets, increases buyer confidence, and means potentially higher payments to offset providers. The Conservation Commission is committed to expanding soil carbon research in Oklahoma. With funding from Western Farmers Electric Cooperative and the USDA-Natural Resources Conservation Service, we have partnered with Oklahoma State University (OSU) to assess carbon sequestration rates in Oklahoma.
Current sequestration rates
Soil carbon sequestration rates currently used for agriculture offsets in Oklahoma are based mostly on modeling and sampling results from the upper midwestern U.S. with resulting values extrapolated conservatively into regional averages across the U.S. Research currently underway in Oklahoma focuses on direct measurement of organic carbon.
For both sampling methods described below, OSU is evaluating the impact of spatial variations on the accuracy and uncertainty of soil carbon measurements for use in carbon sequestration verification. Assessments of both small scale (meters) and large scale (kilometers) variations are conducted so that future sampling protocols can be developed that minimize sample requirements and insure accurate sequestration measurements for the aggregated acres by carbon offset verifiers
Sample collection (method 1)
Point measurement soil sampling is conducted to determine the average carbon sequestration rate of aggregated acres in an area. At each sample location within a field, 3 soil cores are collected within a 1 m2 area using a hydraulic probe. Each core is 3.5 cm in diameter and collected from a minimum depth of 40 cm. A minimum of 3 locations are sampled within each field. Samples collected are analyzed for bulk density, total SOC, and pH. At each location, surface slope and condition (residue cover, erosion class) are assessed. Sample locations are marked using satellite ground positioning system (GPS) to allow for evaluation of spatial variation in SOC in each field. This also allows for future return to baseline soil sample locations. In addition to selected measured field and soil characteristics, management information is acquired from land managers. This information includes but is not limited to the following: 5 year history of tillage, crop rotation, fertilizer application, total soil potential, and actual yield.
Sample collection (method 2)
The Oklahoma Carbon Program has asked OSU to adapt sample collection method 1 to evaluate the variation among samples to determine the minimum number of samples necessary to validate the average carbon sequestered in a group of aggregated acres within the same ecoregion. The program anticipates using the adapted protocol to determine how many points to sample in fields under carbon contract during verification. To achieve this, OSU will collect some samples with a hand operated probe while still collecting the bulk of the samples with a hydraulic probe in order to adapt the protocol to replace machine power with man power. This will allow for wider scale sampling across the state, particularly by field verifiers during verification. OSU will look at a minimum of 3 locations where both the hydraulic and hand probes are used and assess the impact of the two sample collection methods on carbon stock estimates. Six cores will be collected within a 5m diameter area. This 5 meter area will represent a sampling point. Each core is 3.5 cm in diameter and collected from a minimum depth of 30 cm. OSU will look at the variation in carbon stocks as measured in each core to determine the minimum number of samples required to measure the potential carbon sequestration rate in the area.
Laboratory analysis of samples is the same for both collection methods. Soil cores are sectioned into 0-10, 10-20 and 20-40 cm increments for 40 cm samples, and 0-10, 10-20 and 20-30 cm increments for 30 cm samples. Total carbon and nitrogen are determined by high temperature combustion using a TrueSpec CN analyzer (LECO, Inc., St. Joseph, MI). Soil inorganic carbon is determined using a pressure calcimeter method (Sherrod et al., 2002). Soil organic carbon is determined by the difference between total carbon and inorganic carbon. Soil pH will is determined with an electrode on a 1:1 soil:water paste.
Note: Total carbon is a measure of organic carbon in the surface of most soils in Oklahoma. However, inorganic carbon in the form of calcium carbonate (CaCO3) can be found in the surface of some of our soils in the western part of the state. OSU has found it in experiments as far east as Woodward and Altus. It is important to test for inorganic carbon (CaCO3) because when present it can cause an erroneously high estimate of organic carbon. OSU's soil carbon samples also measure bulk density. Determining bulk density (dry weight/volume) of the soil is important because it varies depending on soil type. In general, bulk density needs to be measured when looking at the amount of carbon stored per acre in order to have a defendable value.
Images courtesy of OSU Cooperative Extension