Relation of Microbial Biomass Carbon and Aggregate Size Distribution to Soil Carbon Pools Four Years After Urban Soil Rehabilitation.

Urbanization typically degrades soils, making soil rehabilitation to improve structure and function over time desirable. We evaluated the potential of soil rehabilitation to enhance the regenerative capacity of degraded urban soils by altering soil carbon (C) storage, microbial biomass, and aggregate size distribution. In 2007, six replicates of four treatments [minimal effort (10 cm topsoil replaced), enhanced topsoil (10 cm topsoil replaced plus rototilling), profile rebuilding (compost amendment incorporated via subsoiling to 60-cm depth, 10 cm topsoil replaced plus rototilling), and undisturbed (no grading, scraping or improvement methods)] were installed in a completely randomized design in Montgomery County, VA. Both Shottower loam (fine, kaolinitic, mesic Typic Paleudults) and Slabtown loam (fine-loamy, mixed, mesic Aquic Paleudalfs) are present at the site. All plots except undisturbed were subjected to typical land development practices to replicate urbanized land as pretreatment: topsoil removed and subsoil compacted to a bulk density of 1.98 g cm^-3. All plots were planted with five tree species. In June 2011, we measured soil C pools (available, aggregate-protected and mineral-bound C pool), microbial biomass C (MBC), and aggregate size distribution 1 m from Acer rubrum trees at 0-5, 5-10 and 15-30 cm depths. Undisturbed soils had greater total C in the surface 10 cm (12.10 and 7.73 Mg ha^-1, respectively) than other soils due to a larger mineral-bound C pool. However, at the 15-30cm depth, urban soil rehabilitation via profile rebuilding increased the total C pool (6.12 Mg ha^-1) and MBC (149.47 mg C kg^-1 soil) compared to the other treatments. Undisturbed soils had greatest macro-aggregate (250-2000µm) proportion at all three depths (0.34, 0.44 and 0.26 g sand-free aggregate g^-1 soil, respectively). Typical land development practices broke down macro-aggregates and decreased C storage, especially within the mineral-bound C pool. Urban soil rehabilitation increased microbial biomass and C storage in subsoil.