Dissolved Organic Carbon (DOC) Isotopic Tracer Reveals the Shift of Landscape Connectivity During Storms in Dry and Wet Seasons

Abstract:

Soil erosion on hillslopes under tillage is a major factor controlling soil organic carbon (OC) redistribution and loss in the Upper Mississippi River Basin. A large portion of OC exported from such intensively managed landscapes is during low-frequency storm events, with the process accelerated by artificial drainage networks connecting the terrestrial with aquatic OC pools. Our study site, Clear Creek Watershed (CCW) - located within the NSF-sponsored IML-CZO, is dominated by row crops cultivated on rolling, loess mantled hills. High-resolution DOC concentration and δ¹³C_DOC in-stream monitoring of storm events along with potential OC sources were used to investigate the export mechanisms, landscape connectivity, and biogeochemical signature of riverine DOC as storm pulses passed from the upper to lower reach of the watershed. In the fall dry season after harvesting, an early pulse of ¹³C-enriched DOC prior to peak discharge suggests a bypassing of the continuum by direct entry through preferential pathways. The export is dominated by upland surface soil and crop residue inputs; this is evidenced by more positive δ¹³C values (-20.5‰ at peak storm pulse) compared to base flow (-26.1‰). During the wet spring/early summer of the growing season, stream DOC concentrations exhibit consistent positive correlation (0.788**) with discharge. The upland surface source is limited by reduced overland flows because of greater interception of precipitation, infiltration and evapotranspiration rates. There is a negative correlation (-0.850*) between rise of DOC concentration and input of isotopically light DOC into stream (~ -23.0‰ at peak DOC compared to -20.6‰ of pre-event). The isotopic record provides good support of this upland source limitation hypothesis as it’s consistent with local C3 dominated riparian sources at peak and greater upland contribution to baseflow through tile drains. Taken together, these distinct δ¹³C patterns of DOC exports exhibited a shift from physical connectivity in dry season to hydrological process dominated connectivity in wet season as results of varied hydrological and physiographic conditions and land management. Our findings provide valuable insights on the intra-storm source shifts of terrestrial C and enhance the ability to predict and model C export under weather extremes.