Soil Organic Carbon Storage in the Chicagoland Ecosystem.

This research examined soil organic carbon (SOC) storage in soils across the seven county Chicagoland ecosystem. A total of 190 plots were stratified and sampled across five land-use classes. In total, 1,293 samples were characterized to determine SOC by depth distributions across the Chicago urban ecosystem. We tested three hypotheses with this data: 1) the majority of urban SOC will be contained in deeper soil layers; 2) SOC in this urban ecosystem will meet or exceed SOC contents in other terrestrial systems; and, 3) spatial patterns in urban SOC will be most related to vegetation patterns and anthropogenic factors and not related to other soil forming factors of relief, parent material and climate. Our first hypothesis was confirmed and we found approximately 75% of SOC at depths greater than 25 cm. As expected, the concentration of SOC (%) is greatest at the surface and decreases with depth. However, due to increases in soil density with depth the stock of SOC (kg m^-2) increases with depth, peaking in the 50-80 cm depth. This finding is important given the majority of research attempting to detail urban SOC storage has focused on surface soil sampling, which would substantially underestimate the actual SOC stock in urban soils. The second hypothesis that urban SOC stock is large when compared to other non-urban systems is supported. The mean SOC stock to a 1-m depth across the Chicagoland ecosystem was 36 kg m^-2, with a range of 4-132 kg m^-2. These soils contain much greater amounts of SOC compared to local forest and prairie soils (7-15 kg m^-2). Urban SOC storage was not found to be as large as organic soils in wetlands, which often contain approximately 200 kg m^-2. Our last hypothesis was partly supported, in that our models were able to predict urban SOC storage with a variety of anthropogenic factors (e.g, land-use, impervious surface area). However, inclusion of vegetation factors, such as basal area and increment growth, did not improve SOC models. Furthermore, other soil forming factors related to climate, relief and parent material were not useful in predicting urban SOC. Future research will be directed towards refining of our urban SOC models and testing them with remotely sensed data. It is our hope that these models might eventually be developed into spatial data layers, which could then be tested across other urban ecosystems to predict SOC storage.