Temperature controls production but hydrology regulates export of dissolved organic carbon at the catchment scale

Lateral carbon flux through river networks is an important and poorly understood component of the global carbon budget. This work investigates how temperature and hydrology control the production and export of dissolved organic carbon (DOC) in the Susquehanna Shale Hills Critical Zone Observatory in Pennsylvania, USA. Using field measurements of daily stream discharge, evapotranspiration, and stream DOC concentration, we calibrated the catchment-scale biogeochemical reactive transport model BioRT-Flux-PIHM (Biogeochemical Reactive Transport-Flux-Penn State Integrated Hydrologic Model, BFP), which met the satisfactory standard of a Nash-Sutcliffe efficiency (NSE) value greater than 0.5. We used the calibrated model to estimate and compare the daily DOC production rates (<span classCombining double low line"inline-formula">Rp</span>; the sum of the local DOC production rates in individual grid cells) and export rate (<span classCombining double low line"inline-formula">Re</span>; the product of the concentration and discharge at the stream outlet, or load).

Results showed that daily <span classCombining double low line"inline-formula">Rp</span> varied by less than an order of magnitude, primarily depending on seasonal temperature. In contrast, daily <span classCombining double low line"inline-formula">Re</span> varied by more than 3 orders of magnitude and was strongly associated with variation in discharge and hydrological connectivity. In summer, high temperature and evapotranspiration dried and disconnected hillslopes from the stream, driving <span classCombining double low line"inline-formula">Rp</span> to its maximum but <span classCombining double low line"inline-formula">Re</span> to its minimum. During this period, the stream only exported DOC from the organic-poor groundwater and from organic-rich soil water in the swales bordering the stream. The DOC produced accumulated in hillslopes and was later flushed out during the wet and cold period (winter and spring) when <span classCombining double low line"inline-formula">Re</span> peaked as the stream reconnected with uphill and <span classCombining double low line"inline-formula">Rp</span> reached its minimum.

The model reproduced the observed concentration-discharge (C-Q) relationship characterized by an unusual flushing-dilution pattern with maximum concentrations at intermediate discharge, indicating three end-members of source waters. A sensitivity analysis indicated that this nonlinearity was caused by shifts in the relative contribution of different source waters to the stream under different flow conditions. At low discharge, stream water reflected the chemistry of organic-poor groundwater; at intermediate discharge, stream water was dominated by the organic-rich soil water from swales; at high discharge, the stream reflected uphill soil water with an intermediate DOC concentration. This pattern persisted regardless of the DOC production rate as long as the contribution of deeper groundwater flow remained low (<span classCombining double low line"inline-formula">&lt;18</span>&thinsp;% of the streamflow). When groundwater flow increased above 18&thinsp;%, comparable amounts of groundwater and swale soil water mixed in the stream and masked the high DOC concentration from swales. In that case, the C-Q patterns switched to a flushing-only pattern with increasing<span idCombining double low line"page946"/> DOC concentration at high discharge. These results depict a conceptual model that the catchment serves as a producer and storage reservoir for DOC under hot and dry conditions and transitions into a DOC exporter under wet and cold conditions. This study also illustrates how different controls on DOC production and export - temperature and hydrological flow paths, respectively - can create temporal asynchrony at the catchment scale. Future warming and increasing hydrological extremes could accentuate this asynchrony, with DOC production occurring primarily during dry periods and lateral export of DOC dominating in major storm events.

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