The contribution of soil aggregates to carbon sequestration in restored urban grasslands

Background/Question/Methods Carbon stabilization by soil aggregates has been proposed as a principal mechanism for long-term soil organic C sequestration in grassland systems. However, soil disturbances and cultivation in grasslands can reduce soil C content and alter the abundance, distribution, and stability of soil aggregates. A key question is therefore whether soil aggregate C can be reinvigorated when highly disturbed urban soil or former agricultural fields are restored to grasslands. In this study, we examined the impact of different grassland restoration practices and time of restoration on C sequestration in soil microaggregates and macroaggregates over time. Soils were collected from 15 grasslands that differed in management practice and length of restoration. Samples were subsequently separated into four different aggregate sizes by wet sieving, and then dried, ground, and analyzed for C content with an elemental analyzer.

Results/Conclusions Preliminary results suggest that prolonged land management resulted in a C content of macroaggregates (>250µm) over 20% greater than that of microaggregates (<250µm), which indicates an aggregate hierarchy in the later stages of restoration management.  Carbon-depleted aggregates were detected in early restoration sites.  Increasing time of restoration produced an increase in macro- and micro-aggregate-associated C. Our findings support the importance of both aggregate classes as C-stabilization sites in restored urban grasslands, and also provide support for long-term restoration and management practices in these ecosystems.