Organic carbon thermodynamics elucidate spatiotemporal mechanisms governing hyporheic zone biogeochemical cycling
This element of the PNNL SFA has contributed to a predictive understanding of river corridor hydrobiogeochemical function by revealing processes governing the oxidation of organic carbon (OC) in the hyporheic zone. Groundwater-surface water mixing zones (i.e., hyporheic zones) exhibit enhanced biogeochemical cycling, and strongly influence river corridor function. Processes governing biogeochemical cycling in these zones remain a crucial uncertainty in watershed-scale process models. To reveal governing processes we combined ultra-high resolution OC characterization, geophysical monitoring, microbial ecology, and biogeochemical assays to determine novel processes associated with the concept of ‘priming’ and how those processes are influenced by (i) riparian vegetation and (ii) hydrologic mixing in the Columbia River hyporheic zone. Here, priming occurs when microbial oxidation of thermodynamically unfavorable OC is fueled by the addition of more bioavailable OC. Field surveys of sediment-associated OC (and related biogeochemical data) conflicted with priming expectations. We found that inputs of thermodynamically favorable OC protected thermodynamically unfavorable mineral-bound OC. We also found that the presence of riparian vegetation shifted biochemical pathways that drive the oxidation of OC. Further, despite the respective oxidation of water-soluble vs. mineral-bound OC pools in dense and sparsely vegetated areas, thermodynamically favorable OC was preferentially depleted in both areas. This suggests universal thermodynamic principles underlie biogeochemical cycling in the hyporheic zone. In contrast, field surveys of pore water were consistent with priming. These data revealed that groundwater contains thermodynamically favorable OC at low concentrations while surface water contains thermodynamically unfavorable OC at higher concentrations. As such, OC oxidation is concentration-limited in groundwater and thermodynamically-limited in surface water. When groundwater mixes with surface water thermodynamically favorable OC in groundwater primes microbial communities to oxidize less favorable OC compounds in surface water. We also show concomitant deterministic shifts in hyporheic zone microbial communities and shifts in OC biochemical transformations. These results provide a mechanistic foundation for modeling hyporheic zone biogeochemistry under temporally dynamic mixing conditions.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 1559110
- Report Number(s):
- PNNL-SA-125130
- Resource Relation:
- Conference: Proceedings of the Subsurface Biogeochemical Research and Terrestrial Ecosystem Science Joint Meeting April 25-26, 2017, Potomac, Maryland
- Country of Publication:
- United States
- Language:
- English
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