Temporal changes in soil organic carbon and other near-surface soil properties under native tallgrass prairie in the Arkansas River Valley

One climate-change mitigation pathway is soil carbon (C) sequestration. Native prairies have long been known for their soil C sequestration abilities, but the full extent of their sequestration capacity is not completely understood, particularly across diverse regions. This study evaluated the effects of soil depth and soil map unit-prairie combination on changes in various soil properties from 2018 to 2022 in the top 20 cm of two native tallgrass prairies in west-central Arkansas. Soil bulk density, pH, electrical conductivity, total nitrogen, total C (TC), soil organic matter (SOM), and Mehlich-3 extractable soil nutrients were evaluated from the 0- to 10-cm and 10- to 20-cm depths in six soil map units across the two prairie ecosystems. Results indicated that soil pH was greater (p < 0.05) in 2022 than in 2018 in all six soil map unit-prairie combinations, but remained acidic, averaging 5.1 in the top 10 cm. Soil organic matter was lower (p < 0.05) in 2022 than in 2018 in all soil depth–prairie combinations, ranging from −11.3 to 11.3 Mg ha⁻¹ in the 0–10 cm of one combination to 3.3 Mg ha⁻¹ in the 10- to 20-cm depth of a different combination. Averaged across soil map unit-prairie combination, soil TC was lower (p < 0.05) in 2022 than in 2018 in both soil depths. Results showed that soil C sequestration did not occur during the 4-year study period, potentially meaning that the prairies have already reached an equilibrium soil C content and may not continue to sequester soil C indefinitely.