Soils are a critical component of terrestrial ecosystems and the global carbon (C) cycle.  Cultivation can deplete soil C stocks by up to 50%, resulting in diminished soil fertility and increased CO₂ release.  Restoration of native vegetation, such as tallgrass prairie, has been shown to help soils recover part of this lost C.  However, the factors that control the rate of soil C recovery are poorly understood.  A series of tallgrass prairie restorations with a well described soil C accrual rate at the Fermi National Accelerator Laboratory provides an opportunity to observe mechanisms controlling potential soil C accumulation.  Our previous work revealed that high soil moisture and presence of C₄ grass promoted soil C accrual, but the relative importance of these effects was not clear.  The objective of this study was to partition the influence of moisture and vegetation on soil C recovery in restored prairies.  We measured soil C in nine prairies situated over a soil drainage gradient 19 to 29 years after restoration.  Within each prairie, soil was collected from patches dominated by C₃ plants and from nearby patches comprised of a mixture C₄ and C₃ plants characteristic of tallgrass prairies. 

Results/Conclusions

We found that soil moisture exerts stronger control than vegetation type over soil C accumulation in restored tallgrass prairies.  After accounting for variation in bulk density, the most poorly drained plots had 42% more C than better-drained plots after two to three decades of prairie development.  Vegetation controls over soil C accrual were less pronounced and not statistically significant, though patches with exclusively C₃ plants, which typically have higher quality residues than C₄ plants, tended to have higher C stocks than patches with mixed vegetation.  These results suggest that soil moisture is a key factor governing C accumulation.  High moisture may enhance C accrual via two mechanisms: (i) by promoting plant growth and thus organic inputs to soil and (ii) by inhibiting microbial decomposition of soil organic matter.  The effects of soil moisture on soil C appears to be much stronger than variations in prairie vegetation, which showed little control over soil C accumulation.  Thus, edaphic and climatic factors appear to be more important than vegetation in determining the rate of soil C accrual.