Investigating stoichiometric regulation of priming effects in the hyporheic zone

The significance of hyporheic zone (HZ) in carbon cycling has emerged in recent decades as an important component of global change. Groundwater-river water mixing in the HZ stimulates aerobic respiration, possibly by enhancing the cycling of N-bearing organic matter (OM). Organic N may also prime the degradation of complex OM in the HZ. However, the extent to which organic N supports microbial metabolism, and specifically the turnover of complex OM, in the hyporheic zone remains unknown. We hypothesize that N-free and N-bearing organic compounds impart different magnitudes of priming via their influences on underlying microbial metabolisms.

We conducted a batch reactor experiment to evaluate the impact of organic N on the priming of complex organic material [Salix fragilis: (W)] using stable isotope tracing of 13C. To measure aerobic respiration rates, 20 g of HZ sediment from the Columbia River in eastern Washington State along with 28 mL of synthetic groundwater-river water mixture was added in an optical O₂ sensor implanted reactor. Glucose (G) and glucosamine (Gn) were amended in 34 treatments with and without of 13C-labeled leaf water extract of S. fragilis in a variety of ratios to a final concentration of 9 mgC/l. Each reactor was incubated in the dark on a shaker and O₂ was measured hourly until reaching a threshold concentration of 3.3 mg/L. At the conclusion of the incubation, we sub-sampled sediment and water from each batch reactor for isotopic assisted ultrahigh resolution metabolomics, enzyme assays and environmental genomics. Aerobic respiration was approximately 12.5% higher in batch reactors with both simple (G, Gn) and complex (W) OM as compared to individual OM sources, providing evidence for priming in hyporheic zones independent of N content. Overall, our results will unravel the molecular mechanisms of priming derived elevated respiration in the HZ and highlight our understanding of the links between aquatic and terrestrial carbon cycling.