Cultivation of native grasslands depletes soil carbon (C) stocks.   Reestablishment of native tallgrass prairie vegetation after the cessation of agriculture often leads to recovery of whole soil C, which may help mitigate rising atmospheric CO₂.  However, the mechanisms governing the dynamics of accruing soil C stocks are poorly understood.  Most soils have a finite capacity to protect soil organic matter from mineralization, thus they will be a limited sink for anthropogenic CO₂.  The ability of soils to regain or exceed the amounts of C lost from initial cultivation is constrained by plant inputs, soil properties, and environmental conditions.

The goals of this research are to explore how physical protection by hierarchical soil aggregate structure affects the rate of soil C accumulation in restored tallgrass prairie and to determine if different protection mechanisms become saturated despite continuing C inputs. We used a chronosequence approach combined with an exponential model to assess the relative contribution of twelve distinct soil organic matter pools located within four aggregation classes to the total soil C accumulation in restored tallgrass prairie (particulate organic matter (POM), silt, and clay from the non-aggregated pool, macroaggregates, microaggregates, and microaggregates-within-macroaggregates).  The chronosequence, located at the Fermi National Accelerator Laboratory in northeastern Illinois, was comprised of an agricultural field, six restored prairies ranging from 3 to 23 years since planting, and a never-cultivated remnant prairie. 

Results/Conclusions

Initial results indicate that aggregate mass returned to pre-cultivation levels quickly (<10 years), while aggregate-protected C took longer to recover (>50 years).  There was a wide range of C accumulation rates among the fractions, suggesting that some pools reach steady state faster than others, or the protective capacity of some pools has saturated, while other pools continue to accrue C.  Most of the C accumulation occurred in the silt from microaggregates protected within macroaggregates, followed by the unprotected POM.   The clay fractions from each aggregate class contributed the least to total soil C accumulation.  Unprotected POM took the longest to achieve 95% C accumulation, suggesting that C builds in unprotected pools only after the soil protective mechanisms are exhausted.  These results shed light on internal soil C dynamics that should help improve models of total soil C accumulation after establishment of native grassland, lending greater accuracy to predictions of the capabilities and limitations of restored prairie ecosystems as a C sink.