Diane McKnight, Matt Miller, Rose Cory and Mark Williams Coupling of carbon and nitrogen cycles through humic redox reactions in an alpine stream Diane McKnight, Matt Miller, Rose Cory and Mark Williams Depart. Civil, Environmental & Architectural Engineering, University of Colorado

NWTLTER: C & N transport and reactivity in Green Lakes Valley Response of pristine, cold regions to climate change and N enrichment

Hyporheic Zone: “hotspot” of biogeochemical reactions driven by mixing across redox gradient Stream h2o mixes with groundwater and returns to the stream

Redox Couples Oxidizing Conditions O2 H2O NO3- N2, NH4+ Mn(IV) Mn(II) Fe(III) Fe(II) Oxidized Humics Reduced Humics SO42- H2S Reducing Conditions

DOM reducing microorganism CO2 Acetate Oxidized DOM DOM reducing microorganism Reduced DOM Photoreduction of Ferric to Ferrous Iron e- e- Humics act as electron shuttle Fe3+ Fe2+ NO2- + DOM  DOM-N Ferrous Wheel Hypothesis NO3-

Tracer experiment: Navajo Meadow Stream *formed by snowmelt *elevation~3,750m *formed by snowmelt and glacial runoff *surrounded by alpine wetland *~150m in length

Approach: Tracer injection experiment and modeling with OTIS Main Channel: Lateral inflow Advection Dispersion Transient storage Storage Zone: Transient storage s

Humic Peaks: (quinone moieties) Protein Peak Approach: Fluorescence index (Em450/Em500 @ 370 nm Ex, and EEM’s (Excitation and emission over a range of wavelengths) Excitation (nm) Emission (nm) Humic Peaks: (quinone moieties) Protein Peak

PARAFAC Excitation-emission matrix (EEM) Comp. 1 Comp. 2 Comp. 3

“HQ” “Q” Quinones found in enzymes, e.g ubiquinone, and formed by lignin oxidation. Forms of this complex are found throughout cells Important in electron transfer reactions, such as the oxidation of NADH Also known as coenzyme Q Ubiquinone

Quinone fluorescence AQDS/AHDS useful as models for humic fluorescence

Stream Br- Addition, July 10 Background [Br-] = 0 mg/L Reach 1 Reach 2 Reach 3

Storage Zone Br- Simulation Reach 1 Reach2 Reach 3 0.1 mg/L 0.1 mg/L

Connectivity of wells Br, Ca, del 18O & D on July 10

Stream Chemistry July 10th July 17th July 24th DOC LF FI SR SUVA

Stream-Well Comparisons B A A A A,B B A,B A B B A A A FI Well 1 = No and Low Br, Well 2 = High Br

Stream Site EEMs S1 S2 S3 July 10th (tracer) July 17th July 24th

Well Site EEMs Characteristic Humic Peaks Protein Peaks July 10th, V13

PARAFAC Components Red-shifted: C2 (HQ1), C3 (HQ2) Blue-shifted: C5 (Q) Protein: C9 2 5

Ex, Em spectra for HQ1 and HQ2 Note: similar excitation spectra

Comparison of HQ1 and AHDS Em and Ex spectra: Same ex. max and shape. Emission max are different, probably related to H bonding, solvent, excited state rxns

Comparison of Q and AQDS Very similar ex and em max (ex 260 nm; em max at 418 nm). Similar features of spectra, Q has broader peaks as typical for humics

Stream-Well Comparisons B C A A Well 1 = No and Low Br Well 2 = High Br F = (Σ HQ1, HQ2) / (Q)

Two Components Explain Fluorescence Index Matt’s data appears on click!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

DOM reducing microorganism CO2 Acetate Oxidized DOM DOM reducing microorganism Reduced DOM Photoreduction of Ferric to Ferrous Iron e- e- Humics act as electron shuttle Fe3+ Fe2+ NO2- + DOM  DOM-N Ferrous Wheel Hypothesis NO3-

Ferrous Wheel: Addition of Ferric Nitrate to reduced DOM samples with high ferrous iron concentrations, causes decrease in ferrous due to nitrate reduction. NOTE: Addition of Ferric Citrate causes ferrous iron to INCREASE.

Hyporheic zone interactions, e. g. humic redox Hyporheic zone interactions, e.g. humic redox!!, hotspot of C & N interactions, influencing N transport in alpine systems. Fluorescence index = HQ1/HQ2 FI increases with microbial sources (primary and secondary) Nitrogen and Carbon cycling coupled by biotic and chemical processes

Questions?

Ferrous Wheel Results: Added Ferric Nitrate to Samples with high ferrous iron concentrations.. NOTE: get different results when ferric citrate added, in that case ferrous iron INCREASES.