Soil organic matter accrual and protection by aggregates in a restored tallgrass prairie chronosequence

Continually increasing atmospheric CO₂ from anthropogenic sources is causing global climate change and ecosystem perturbation. Soils offer a means to at least partially offset rising CO₂ by preserving carbon (C) fixed by plant photosynthesis as soil organic matter (SOM). Restoration of cultivated land and degraded soils with grasslands has the potential to rebuild depleted soil C stocks. However, we have a limited mechanistic understanding of the processes leading to the soil C accrual rates seen in restored grasslands and how they can be accelerated. The goals of this research are to explore how physical protection by hierarchical soil aggregates affects the rate of soil C accumulation in restored tallgrass prairie and to determine if different protection mechanisms become saturated despite continuing C inputs. The chronosequence approach, which relies on space-for-time substitution, was used to predict the time to achieve equilibrium C levels in different SOM pools. A chronosequence of reconstructed tallgrass prairies, including an agricultural field (representing time zero) and a remnant native prairie (as a reference for the potential equilibrium condition) at Fermi National Accelerator Laboratory in Batavia, IL, was used to explore the potential for saturation of protective mechanisms. A detailed physical fractionation was performed to determine how fast and how much C can be expected to accumulate in SOM pools with different levels of protection and residence times. Preliminary data suggest that while much of the soil C was protected in microaggregates-within-macroaggregates, inter-microaggregate C within macroaggregates accumulated to levels nearing that in the remnant prairie faster than intra-microaggregate protected C.