Carbon isotope ratios of Great Plains soils and wheat-fallow systems

The purposes of this study were to improve knowledge of regional vegetation patterns of C₃ and C₄ plants in the North American Great Plains and to use δ¹³C methodology and long-term research sites to determine contributions of small-grain crops to total soil organic carbon (SOC) now present. Archived and recent soil samples were used. Detailed soil sampling was in 1993 at long-term sites near Akron, CO, and Sidney, NE. After soil sieving, drying, and deliming, SOC and δ¹³C were determined using an automated C/N analyzer interfaced to an isotope-ratio mass spectrometer. Yield records from long-term experimental sites were used to estimate the amount of C₃ plant residue C returned to the soil. Results from δ¹³ analyses of soils from near Waldheim, Saskatchewan, to Big Springs, TX, showed a strong north to south decrease in SOC derived from C₃ plants and a corresponding increase from C₄ plants. The δ¹³ analyses gave evidence that C₃ plant residue C (possibly from shrubs) is increasing at the Big Springs, TX, and Lawton, OK, sites. Also, δ¹³C analyses of subsoil and topsoil layers shows evidence of a regional shift to more C₃ species, possibly because of a cooler climate during the past few hundreds to thousands of years. Data from long-term research sites indicate that the efficiency of incorporation of small-grain crop residue C was about 5.4% during 84 yr at Akron, CO, and about 10.5% during 20 yr at Sidney, NE. The ¹⁴C age of the SOC at 0- to 10-cm depth was 193 yr and at 30 to 45 cm was 4000 yr; ¹⁴C age of nonhydrolyzable C was 2000 and 7000 yr for these same two respective depths. Natural partitioning of the ¹³C isotope by the photosynthetic pathways of C₃ and C₄ plants provides a potentially powerful tool to study SOC dynamics at both regional and local scales.