Carbon Sequestration in Urban Forest Soils.

Urban development in the conterminous United States increasingly converts forests to urban land uses, and potentially reduces the terrestrial carbon (C) pool. Otherwise, urban forests mitigate abrupt climate change (ACC) and the urban heat island effect by altering the surface energy balance, providing renewable energy from wood, and sequestering atmospheric CO2 in trees and in the soil organic carbon (SOC) pool. However, data are scanty as C cycle science has focused on natural ecosystems and not on settlements. We study soil properties and their C storage and stabilization to 1-m depth in urban forests in relation to soils disturbed by agricultural activities and to relatively undisturbed forest soils in Columbus, Ohio, USA. The urban forest soils store up to 210 Mg C ha-1 which is about twice the amount stored in agricultural soils but less than the pool stored in peatlands. In contrast to naturally developed soil profiles, burial of C-rich surface horizons and of anthropogenic organic matter contribute to increasing C pools (14.3 and 29.3 Mg C ha-1 in 30-40cm and 90-100cm, respectively), and increasing C/N ratios (10.4 and 49.3 in 30-40cm and 90-100cm, respectively) with increasing depth in urban forest soil profiles. These preliminary results indicate that maximizing net benefits of urban forest land uses on mitigating increases in atmospheric CO2 concentrations must also include strategies to manage the SOC pool aside the tree C pool as about 8.5 % of the land area of Ohio is classified as urban. Yet, terrestrial C pools in Ohio are underestimated by the exclusion of urban land cover.