Large stocks of deep soil carbon are vulnerable to decomposition

Authors: Phillips, Claire; PENG, QUIPING - Washington State University; Huggins, David

Submitted to: WSU Dryland Field Day Abstracts
Publication Type: Experiment Station
Publication Acceptance Date: 4/8/2024
Publication Date: N/A
Citation: N/A

Interpretive Summary: To establish carbon credits for agricultural soil carbon sequestration it is necessary to monitor carbon change following the adoption of practices such as no-till management. A long-term no-till study was initiated at the Cook Agronomy Farm in 1998 and has been used to evaluate changes in soil organic carbon. We previously reported that the site lost an average of 13.2 Mg C ha-1 over the 17 years following no-till adoption. Although carbon markets currently require monitoring carbon change to only 30 cm depth, we found that 57% of initial soil carbon stocks, and 74% of subsequent soil carbonlosses occurred between 30-153 cm depth. Building on those observations, here we present results from a laboratory study to that measured the potential decomposition of soil carbon under optimal laboratory conditions. In 2015, we collected 25 soil cores from 5 different soil series at Cook Agronomy Farm, sectioned them into 10 cm increments in the top 30 cm and by soil horizon below 30 cm, and incubated them for 1 year in the laboratory. We found that the fraction of soil carbon that decomposed in the incubations increased with depth. The total amount of readily-decomposable soil carbon was 51% greater for 30-153 cm depth compared to 0-30 cm depth. These results help explain the continued soil carbon losses that have occurred at Cook Agronomy Farm. These losses may be related to the silt loam texture of soils in the Palouse, which have little soil aggregation. These findings provide further evidence that monitoring below 30 cm depth is important for accurate monitoring of soil carbon change following no-till management. These results are relevant to a wide cross-section of agricultural professionals who work on measuring and monitoring soil carbon change following adoption of conservation practices.

Technical Abstract: A long-term no-till study was initiated at the Cook Agronomy Farm in 1998 and has been used to evaluate changes in soil organic carbon. We previously reported that the site lost an average of 13.2 Mg C ha-1 from 0-153 cm depth over the 17 years following no-till adoption. Although carbon markets currently require monitoring carbon change to only 30 cm depth, we found that 57% of initial SOC stocks, and 74% of SOC losses occurred between 30-153 cm depth. Building on those observations, here we present results from a laboratory study to evaluate the vulnerability of SOC stocks to decomposition under optimal laboratory conditions. In 2015, we collected 25 soil cores from 5 different soil series at Cook Agronomy Farm, sectioned them by depth into 10 cm increments in the top 30 cm and by soil horizon below 30 cm, and incubated them for 1 year in the laboratory. Although soils had decreasing SOC concentrations with soil depth, we found that the fraction vulnerable to decomposition increased with depth. We modeled the production of CO2 over the course of the incubation to compute kinetic pools of SOC contributing to decomposition. The amount of SOC in the pools with transit times of = 2 years was 51% greater below 30 cm than near the soil surface. Stocks of SOC pools with transit times of = 2 years averaged 12.1 Mg C ha-1 for 0-30 cm and 18.3 Mg C ha-1 for 30-153 cm. These results provide further evidence that accurate measurements of SOC changes require monitoring below 30 cm. The high vulnerability of deep SOC may be related to low levels of water-stable aggregates in silt loam soils of the Palouse, which are important for physically protecting SOC from microbial decomposers. These stocks are vulnerable to loss under the wetter soil conditions that have developed following no-till adoption.