THE INFLUENCE OF GRAZING ON VEGETATION AND SOIL ORGANIC MATTER IN GRASSLANDS OF SOUTHWESTERN KANSAS AND IMPLICATIONS FOR USING MODERN DATASETS TO INTERPRET PALEOSOL RECORDS

Paleovegetation proxies derived from paleosols, such as carbon isotope (δ¹³C) records of bulk organic matter (SOM), and pedogenic carbonates, have provided key insights into the Cenozoic evolution of the Great Plains C₄ grasslands. In order to interpret paleosol δ¹³C records we rely upon our understanding of modern plant and soil carbon dynamics in comparable biomes. We have focused on the Meade Basin, Kansas, in an area where modern grasslands overlay a suite of Miocene to Pleistocene paleosols. Within this small region there is a range of microhabitats that comprise the grassland biome. Previous research has characterized the variability in plant community composition, and SOM geochemistry among these modern microhabitats. But another potentially important source of variability in the vegetation composition and SOM among open grassland sites is the influence of grazing by large herbivores (cattle). To address this issue, we sampled abundant grasses and herbaceous plant species, and collected samples from 30cm soil profiles for isotopic and elemental analyses in adjacent areas that have been minimally grazed for the past several decades, and heavily grazed over the same time period.

The δ¹³C values of major plant types (C₄ and C₃ grasses, and C₃ herbs) were similar in grazed and ungrazed areas. Aboveground vegetative biomass was higher at ungrazed sites, and plant community compositions were different between areas. The SOM δ¹³C profiles from the grazed soil pits (N=3) were indistinguishable from each other, whereas the ungrazed profiles were substantially different among pits (N=3). Specifically, ungrazed pits had non-overlapping values at all depths above 20cm, and a 3.5‰ range in the δ¹³C values of the shallowest samples (compared to 0.3‰ for shallow grazed samples). This translates to an estimated range of 64-87% C₄ grass carbon in the ungrazed shallow samples, versus 77-79% C₄ in the grazed shallow samples. The weight percent carbon (%C) values were similar across pits, although ungrazed %C was slightly higher in the top 10cm. Our results suggest that grazing in open Meade grasslands influences both vegetation and SOM carbon, and this may be a factor to consider when using modern datasets to interpret paleosol records.