Differential soil organic carbon storage at forb- and grass-dominated plant communities, 33 years after tallgrass prairie restoration

Background and aims: Dominance of C₄ grasses has been proposed as a means of increasing soil organic carbon (SOC) sequestration in restored tallgrass prairies. However, this hypothesis has not been tested on long time scales and under realistic (e.g. N-limited) environmental conditions. We sampled a restoration in southern Illinois 33 years after establishment to determine the effects of varying plant communities on SOC sequestration in the top 50 cm of soil. Methods: SOC, total nitrogen (TN), and the stable isotopic composition of SOC (δ¹³C) were used to calculate SOC sequestration rates, N storage, and the relative contributions of C₃ vs. C₄ plant communities as a function of soil depth. Results: While both a forb-dominated and a mixed forb-grass plant community showed positive sequestration rates (0.56 ± 0.13 and 0.27 ± 0.10 Mg C ha^-1 yr^-1, respectively), a C₄ grass-dominated community showed SOC losses after 33 years of restoration (-0.31 ± 0.08 Mg C ha^-1 yr^-1). Soil δ¹³C values were significantly more negative for forb-dominated plant communities, increasing the confidence that plant communities were stable over time and an important contributor to differences in SOC stocks among transects. Conclusion: These results suggest that functional diversity may be necessary to sustain sequestration rates on the scale of decades, and that dominance of C₄ grasses, favored by frequent burning, may lead to SOC losses over time.