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Abstract

A pulse of ¹⁴C-labeled dissolved free amino acids (DFAA) was immobilized rapidly from groundwater discharged through cores of hyporheic sediment from an upland stream. Mean turnover distance within the cores was <1.5 cm but, once immobilized, DFAA carbon was retained for longer than would be expected if bacterial cycling was the only retention mechanism. It took 28 weeks for 88% of the original 1.2 µg of DFAA carbon that had been immobilized to be respired and exported from the cores, whereas bacterial respiration alone should have mineralized >99% of this carbon within 7.4-11.4 weeks, depending on the bacterial growth efficiency assumed. Abiotic retention processes associated with organic and mineral surfaces appear to enhance the initial immobilization of DFAA and contribute to a delayed recycling of DFAA carbon within the sediment, increasing the effective availability of this labile organic carbon in the hyporheic zone. Abiotic retention of dissolved organic carbon might help sustain the high levels of bacterial production found in hyporheic sediments while reducing the demand for organic carbon imported into the hyporheic zone.