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.
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.