The critical zone comprises the globally important terrestrial carbon (C) pool where organic carbon in cycled, stabilized in the soil or mobilized and transported in stream waters. Whereas the assessment of in-stream dissolved organic and inorganic carbon (DOC and DIC) concentrations is important to assess its contribution to catchments C losses, it is equally important to assess the contribution of in-stream processes impacting C cycling. In this study we compare stream waters at the outlet of two headwater catchments in the Jemez River Basin (JRB), NM as part of the JRB-SCM Critical Zone Observatory (CZO) in order to test the potential to employ combined measures of C quantity and DOM quality to assess in-stream C cycling. One relatively small (3.7 km2), steep forested catchment containing small dispersed wetland areas is drained by LaJara creek that, due to shorter residence time, can be assumed to be less impacted by in-stream C-cycling. In contrast, Jaramillo creek drains a large (11.7 km2) catchment and meanders through large wetland areas at the catchment outlet promoting in stream C-cycling. At both catchment outlets, grab samples and flume discharge data were collected before snowmelt and six times during the falling snowmelt hydrograph from March to July 2010, and analyzed for DOC and DIC concentrations and DOM quality (fluorescence spectroscopy). Stream waters from both catchments contain the highest DOC concentrations at the onset of snowmelt, but peak DOC concentrations in Jaramillo creek were twice as high (10.5 mg/L compared to 5.2 mg/L for LaJara). DIC/DOC ratios of both catchments are negatively correlated with discharge and can be used as an indicator for hydrologic transit times where increasing DIC/DOC ratios point to longer transit times. In the case of LaJara creek the correlation is linear (r2= 0.90) suggesting a simple discharge related flushing of shallow soil C and thus an increase of DOC. For Jaramillo creek in contrast, DIC/DOC ratios show a negative power law relation to discharge (r2= 0.82), indicating additional feedback mechanisms presumably due to prolonged, wetland impacted flow paths. In general, longer hydrologic residence times during baseflow appear to be associated with increased respiration and carbon mineralization (increased DIC). In contrast, increased DOC concentrations and decreased DIC/DOC ratios at high discharge suggest flushing of DOC from surface or riparian soils and minimal processing during transport. Fluorescence index values (FI) as measures for DOM quality also correlate with discharge and constrain DOM sources. FI values for both catchments are consistent with allochtonous DOC (FI< 1.5) with lower values for Jaramillo creek (FI ranging between 1.28 and 1.37), indicating that the larger wetland area increases the amount of plant-derived DOM, thus decreasing the FI. Positive linear correlation between DIC/DOC ratio and FI further confirm the higher amount of in-stream processing at low discharge and therefore a relative increase of autochtonous DOM. These results show the potential of combined measures of C quantity and DOM quality to trace landscape-scale controls over in-stream C cycling processes.
Perdrial, J.N., McIntosh, J.C., Brooks, P.D., Chorover, J. (2010): DOM as a potential tracer for in-stream processes in small mountain catchments (JRB-SCM Critical Zone Observatory). AGU Fall Meeting Abstract B12A-05..