Impact of Urbanization on CO2 Fluxes and Sources from Houston-Area Watersheds

Rivers are generally supersaturated with CO₂, implying a role as a conduit of soil CO₂ to the atmosphere. Knowledge of the fluxes and sources of CO₂ evaded from rivers is essential to regional carbon budgets. In this study, we directly measured partial pressure of CO₂ (pCO₂) for two nearby subtropical rivers, Buffalo Bayou and Spring Creek in southeast Texas, from June 2007 to May 2008. Buffalo Bayou's watershed is almost entirely contained within the city of Houston, while Spring Creek is not yet urbanized. There is no significant difference in pCO₂ between the two rivers. The overall average pCO₂ is 3589 ± 1464 μatm, and mean CO₂ evasion rate is 6.84 Mg C ha^-1 y^-1, comparable to the Amazon.

Carbon isotopic signatures of riverine dissolved inorganic carbon (DIC) point to different CO₂ sources for the two rivers. DIC in Spring Creek is generally young and more d¹³C-depleted, indicating that a young carbon pool is cycling rapidly and sustaining the high CO₂ concentration in Spring Creek. For Buffalo Bayou, DIC is always depleted in Δ¹⁴C and more enriched in d¹³C, suggesting an input of DIC from carbonate dissolution. This is also supported by a local soil survey, which shows the presence of soil carbonates in the Buffalo Bayou watershed.

There are two possible sources of soil carbonates in the Buffalo Bayou watershed: naturally precipitated pedogenic CaCO₃, and CaCO₃ from shells used to make roads in East Texas in the early 20^th century. In the first scenario, soil carbonates are likely in steady state and thus act only to shift the isotopic signature of respired riverine CO₂. In the second scenario, shells from road building are both a net source of CO₂ to the atmosphere and are altering the C isotopic signature of Houston-area respiration.