Our paper, Riverine export of aged carbon driven by flow path depth and residence time connecting aged dissolved organic matter export with deeper flow paths, is now available online! (Barnes et al. 2018)
Inland waters represent an important component of the global carbon cycle, releasing between 2-4 Gt of carbon dioxide into the atmosphere annually (Aufdenkampe et al. 2011, Raymond et al. 2013). Lack of inclusion in global carbon balances has led to an overestimate of terrestrial net ecosystem productivity and thus carbon sequestration potential. By characterizing the dissolved organic matter (DOM) in rivers we can better understand the connectivity between the terrestrial and aquatic systems.
Using compiled data from nineteen large rivers – the six largest Arctic watersheds that comprise the Arctic Great Rivers Observatory and thirteen major U.S. watersheds sampled as part of the USGS National Stream Quality Accounting Network program – we were able to show that older carbon is exported with proportionally greater mineral weathering products, suggesting deeper flow paths. This pattern is stoichiometrically consistent across this wide range of ecosystems (boreal to subtropical):
Furthermore this pattern is consistent in both space (more arid systems export older carbon) and time (late in the season, Arctic rivers export older, less aromatic dissolved organic matter). With more aromatic DOM (as indicated by SUVA) corresponding to younger pools of organic matter in both the Arctic and coterminous U.S. watersheds.
Applying these findings more broadly, we can think about how landscape disturbance – drought, land use change – could alter the connectivity of the terrestrial and aquatic ecosystems and thus the nature of DOM (carbon) exports. Thus, providing further insight as to the sources of millennial aged organic matter exports (Butman et al. 2015). Our 2015 analysis illustrates the connection between intensive land use change (urbanization and row crops) and the export of aged DOM. This results links land use change to increased riverine export of aged organic matter. Once in the river the old DOM can be metabolized or photo-oxidized, producing CO2 which is degassed to the atmosphere. As such, the mobilization of aged DOM into streams via land use change is similar to when we burn fossil fuels: our actions are injecting carbon that was trapped in long-term storage into the atmosphere (leading to the rise of atmospheric greenhouse gas concentrations). By connecting age of carbon to the nature of the flow path, we can start to understand how disturbance will further alter the mobilization and export of aged carbon into the biosphere.
This paper will be published as part of a special issue honoring Dr. George Aiken in Environmental Science & Technology. The analysis at the center of this paper was done in response to our learning of George’s illness and the special session in his honor at ACS in April of 2017. In particular, we wanted to re-examine our global radiocarbon DOC dataset using spectral properties of DOM to test various hypotheses put forth in our 2015 paper (Butman et al.) – thus applying the techniques George taught us. Personally, the paper was the best way for me to focus all the emotions wrapped up in losing him and I am grateful that I got to write it with members of my science family. I miss you, George.