Soil organic matter (SOM) is the largest terrestrial carbon pool and is both chemically complex and ill-defined. Differences in decomposability of C3- and C4-derived SOM exist, so characterizing the isotopic composition of SOM is critical for estimating its residence time. Tallgrass prairie is the ideal ecosystem to explore the dynamics of the isotopic composition of SOM, since the relative abundance of C3 and C4 plants varies with management. Tallgrass prairie was historically maintained through fire and grazing by megafauna. Frequently burned prairie in the absence of grazers is dominated by C4 grasses. Grazing increases the cover of C3 grasses and forbs and decreases the dominance of C4 grasses, while fire suppression increases C3 woody plant encroachment. To address how isotopic composition of SOM changes in response to contrasting fire and grazing regimes, we determined δ13C and δ15N values of archived soil cores collected between 1982-2015 from four watersheds at the Konza Prairie LTER site. The four watersheds represent a factorial design of different burning and grazing treatments (annual spring burning and long-term (20-yr) fire suppression with and without bison grazing). Additionally, plant community composition of the four watersheds was assessed annually from 1983 – 2015.
Both fire and grazing treatments affected isotopic composition of SOM. Specifically, δ13C values diverged over time among the four watershed treatments and became significantly different in 2002. Isotopic mixing models indicate that across all four watersheds, 10 – 16% of SOM was C3-derived in 1982 while 8 – 26% of SOM was C3-derived in 2015. There was no effect of watershed or time on δ15N values. All four watersheds were initially dominated by C4 grasses before the contrasting fire and grazing treatments went into effect. Annual plant surveys reveal an increase in the ratio of C3/C4 plant cover in all watersheds, but the increase was greatest in watersheds with low fire frequency. The observed ratio of C3/C4 plants was significantly correlated to the modeled ratio of C3/C4 -derived SOM, indicating that changes in plant communities due to land management can lead to detectable changes in SOM isotopic composition on ecologically relevant timescales. These results further suggest that C4- derived SOM may turn over quickly in this ecosystem. Further research should be directed towards the mechanisms behind SOM turnover in the tallgrass prairie.