CARBON-ISOTOPE RATIOS OF GREAT-PLAINS SOILS AND IN WHEAT-FALLOW SYSTEMS

The purposes of this study were to improve knowledge of regional veget ation patterns of C-3 and C-4 plants in the North American Great Plain s and to use delta(13)C methodology and long-term research sites to de termine contributions of small-grain crops to total soil organic carbo n (SOC) now present, Archived and recent soil samples were used, Detai led soil sampling was in 1993 at long-term sites near Akron, CO, and S idney, NE, After soil sieving, drying, and deliming, SOC and delta(13) C were determined using an automated C/N analyzer interfaced to an iso tope-ratio mass spectrometer, Yield records from long-term experimenta l sites were used to estimate the amount of C-3 plant residue C return ed to the soil, Results from delta(13)C analyses of soils from near Wa ldheim, Saskatchewan, to Big Springs, TX, showed a strong north to sou th decrease in SOC derived from C-3 plants and a corresponding increas e from C-4 plants. The delta(13)C analyses gave evidence that C-3 plan t residue C (possibly from shrubs) is increasing at the Big Springs, T X, and Lawton, OK, sites, Also, delta(13)C analyses of subsoil and top soil layers shows evidence of a regional shift to more C-3 species, po ssibly because of a cooler climate during the past few hundreds to tho usands 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 3 0 to 45 cm was 4000 yr; C-14 age of nonhydrolyzable C was 2000 and 700 0 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 pro vides a potentially powerful tool to study SOC dynamics at both region al and local scales.