Coupling of carbon and nitrogen cycles through humic

Coupling of carbon and nitrogen cycles through humic redox reactions in an alpine stream Diane Mc. Knight, 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

Redox Couples Oxidizing Conditions O 2 H 2 O NO 3 - N 2, NH 4+ Mn(IV) Mn(II) Fe(II) Oxidized Humics SO 42 - Reduced Humics H 2 S Reducing Conditions

CO 2 Photoreduction of Ferric to Ferrous Iron Acetate e- DOM reducing microorganism Reduced DOM Humics act as electron shuttle Oxidized DOM e. Fe 3+ Fe 2+ Ferrous Wheel Hypothesis NO 2 - + DOM-N NO 3 -

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

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

Excitation (nm) Approach: Fluorescence index (Em 450/Em 500 @ 370 nm Ex, and EEM’s (Excitation and emission over a range of wavelengths) Emission (nm) Protein Peak Humic Peaks: (quinone moieties)

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

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

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 Reach 2 0. 1 mg/L Reach 3 0. 1 mg/L

Connectivity of wells Br, Ca, del 18 O & D on July 10 l Ca, del 18 O & D on July 17, 24 l

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

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

Stream Site EEMs S 1 July 10 th (tracer) July 17 th July 24 th S 2 S 3

Well Site EEMs Characteristic Humic Peaks Protein Peaks July 10 th, V 13 July 17 th, V 15 July 17 th, V 25

PARAFAC Components Red-shifted: C 2 (HQ 1), C 3 (HQ 2) 2 Blue-shifted: C 5 (Q) Protein: C 9 5

Ex, Em spectra for HQ 1 and HQ 2 Note: similar excitation spectra

Comparison of HQ 1 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 A Well 1 = No and Low Br Well 2 = High Br F = (Σ HQ 1, HQ 2) / (Q) A C C

Two Components Explain Fluorescence Index

CO 2 Photoreduction of Ferric to Ferrous Iron Acetate e- DOM reducing microorganism Reduced DOM Humics act as electron shuttle Oxidized DOM e. Fe 3+ Fe 2+ Ferrous Wheel Hypothesis NO 2 - + DOM-N NO 3 -

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!!, hotspot of C & N interactions, influencing N transport in alpine systems. Fluorescence index = HQ 1/HQ 2 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.