The purposes of this study were to improve knowledge of regional vegetation patterns of C-3 and C-4 plants in the North American Great Plains and to use delta(13)C 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 delta(13)C 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 C-3 plant residue C returned to the soil, Results from delta(13)C analyses of soils from near Waldheim, Saskatchewan, to Big Springs, TX, showed a strong north to south decrease in SOC derived from C-3 plants and a corresponding increase from C-4 plants. The delta(13)C analyses gave evidence that C-3 plant residue C (possibly from shrubs) is increasing at the Big Springs, TX, and Lawton, OK, sites, Also, delta(13)C analyses of subsoil and topsoil layers shows evidence of a regional shift to more C-3 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 Jr at Akron, CO, and about 10.5% : during 20 yr at Sidney, NE, The C-14 age of the SOC at 0- to IO-tm depth was 193 yr and at 30 to 45 cm was 4000 yr; C-14 age of nonhydrolyzable C was 2000 and 7000 yr for these same two respective depths, Natural partitioning of the C-13 isotope by the photosynthetic pathways of C-3 and C-4 plants provides a potentially powerful tool to study SOC dynamics at both regional and local scales.
