Grassland soils may act as biological sinks via the integration of decomposed organic carbon residues into stable soil aggregates. Carbon-rich plant residues, exudates from bacteria and fungi, and fungal hyphae-- both during life and after death-- help cement particles of soil together to form and stabilize aggregates. Although the literature is replete with studies on soil C storage in former agricultural soils and native habitats that show increasing aggregation and soil C storage over time, there are few comparable studies in urban areas. We ask: could restored urban prairie systems exhibit similar belowground ecosystem functioning as their rural and native counterparts? To address this question, we undertook a comparative analysis of two restored Chicago tallgrass prairies- one a former cornfield and the other comprising engineered, urban soils- to document soil C storage over a timescale of decades. We measured the abundance of arbuscular mycorrhizal fungal (AMF) because mycorrhizal fungi immobilize significant amounts of C in living tissue and in recalcitrant compounds that remain in the soil. We quantified root-associated AMF structures and aggregate-associated AMF hyphae to test the hypothesis that prairie restoration in urban environments might constitute a practical strategy to enhance soil C accrual.
We detected an increase in aggregate abundance, AMF hyphal length, and AMF root colonization with time in restored cornfields. Conversely, even after two decades of active restoration management in urban soils, total AMF C pools decreased with time since restoration, and this reduction was driven mostly by a decline in aggregate and hyphal abundance. Similarly, root-associated AMF structures were also more abundant in plants from the restored agricultural field than urban soils. Consequently, C storage in AMF structures in urban soils may be negligible compared with that in a restored agricultural field. The restored urban systems clearly do not exhibit the same belowground ecosystem functioning as their rural counterparts, even though both sites demonstrate a diverse and productive aboveground (plant) community. Possibly other factors, such as soil nutrient availability, may have altered AMF growth. As a result, the restoration of prairies in urban soils may need to incorporate management that enhances AMF abundance if mycorrhizal fungi are to make an increasing contribution to soil C storage.