Wetland soil carbon storage exceeds uplands in an urban natural area (Florida, USA)

Context. Urban greenspaces and natural areas are often recognised for their cultural services, but may also provide ecological services, including carbon (C) sequestration and storage. Aims. This study investigated the strength of the relationship between easily discernable ecosystem characteristics (e.g. topographic position, vegetation, and soil type) and soil C storage, and evaluated common conversion factors and methodologies used in soil C inventories. Methods. Sixty-seven full-depth (up to 5 m) soil cores were collected across nine community types in University of Central Florida's Arboretum (Orlando, Florida, USA) and were analysed for bulk density, organic matter (OM) content, total C, and total nitrogen (N). Key results. Wetlands stored an average of 16 times more C than uplands and C density increased with soil depth. A 70% underestimation of soil C stocks would have occurred if sampling stopped at 50 cm. A strong linear relationship between soil C and OM supports the use of a 0.56 (C:OM) conversion factor for estimating soil organic C. Conclusions. The presence of wetlands is the key predictor of soil C and N storage, but the magnitude of storage varies widely among wetlands. Overall, the 225-ha study area stored 85 482 +/- 3365 Mg of soil C. Implications. Urban natural areas should be evaluated for their ecosystem services separately from their surrounding developed land use/land cover with consideration for C storage potential. Leveraging topographic position, a site-specific soil OM conversion factor, and depth to refusal testing can increase the accuracy and cost-effectiveness of soil C inventories.