HOW DO STABLE CARBON ISOTOPE VALUES OF VEGETATION AND BULK SOIL ORGANIC MATTER RECORD WET VERSUS DRY CONDITIONS IN THE SHORTGRASS PRAIRIE ECOSYSTEM OF SOUTHWESTERN KANSAS, USA?

Carbon isotope (δ¹³C) values of bulk soil organic matter (SOM) from paleosols are used as proxies for reconstructing paleovegetation and paleoclimate. These proxies have been widely used in the Meade Basin of southwestern Kansas, USA for investigating the evolution and expansion of C₄ grasslands. During wet parts of the year, it is expected that more SOM will be preserved in the soil profile; while in the drier parts of the year more pedogenic carbonates will precipitate. Drought-like conditions can be expressed isotopically through plant tissues and SOM as more positive δ¹³C values due to water stress or a shift in vegetation coverage. This potentially biases the paleosol SOM record towards wet growing season conditions. Our study investigates this proxy bias by comparing δ¹³C values of vegetation collected in the Meade Basin during severe drought conditions of 2012 and 2013 to this year’s moderately wet growing season. We will also compare the δ¹³C values of vegetative biomass to the δ¹³C values of SOM at the same locations, to investigate how carbon fixed during wet versus dry growing seasons is incorporated into the soil profile. Aboveground vegetative biomass samples were taken at 16 sites throughout the Meade Basin, each of the selected sites has isotopic data of soil and vegetation from previous drought years. Our preliminary data shows an increase in aboveground biomass in this year’s relatively wetter growing season. We predict to see more positive δ¹³C plant values during the drought years and more negative δ¹³C values in this year’s wetter growing season, as consistent with prior studies. We also expect the δ¹³C values of SOM to reflect similar δ¹³C values as vegetation during the wetter growing season conditions, as a factor of more biomass produced and preserved as SOM in the soil. Understanding the isotope dynamics of carbon in plant tissue and soil organic matter during these wet and dry growing seasons will improve our interpretation of carbon isotopes as proxies in paleosols.