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A NOVEL INDICATOR OF A NOVEL INDICATOR OF ECOSYSTEM N STATUS: ECOSYSTEM N STATUS: RATIO OF DIN TO DON IN RATIO OF DIN TO DON IN ANNUAL RIVERINE FLUXANNUAL RIVERINE FLUX
Mark Williams, CU BoulderMark Williams, CU Boulder
Dave Clow, USGSDave Clow, USGS
Tamara Blett, NPSTamara Blett, NPS
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Global Problem: Increasing Global Problem: Increasing Nitrogen DepositionNitrogen Deposition
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NADP: NITRATE PERCENT CHANGE
Lehmann et al., 2005, Environ Poll
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NADP: AMMONIUM PERCENT CHANGE
Lehmann et al., 2005, Environ Poll
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NPS RESOURCE MANAGERS
•Need metricsto evaluate ecosystem N status before we have deadfish and dead trees
•Simpler the better
•Identify thresholds
•In the hot seat
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N-CYCLE IS COMPLICATEDN-CYCLE IS COMPLICATED
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Aber Spaghetti DiagramAber Spaghetti Diagram
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DIN:DON RATIO IN ANNUAL DIN:DON RATIO IN ANNUAL DISCHARGE PROVIDES A DISCHARGE PROVIDES A METRIC FOR ECOSYSTEM METRIC FOR ECOSYSTEM
NITROGEN STATUSNITROGEN STATUS
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DISSOLVED ORGANIC DISSOLVED ORGANIC NITROGEN (DON)NITROGEN (DON)
• Developed from soil organic nitrogen• Generally recalcitrant organic nitrogen• Not tasty to microbes• Companion to dissolved organic carbon (DOC) • Not generally measured• Difference of TDN minus DIN• Dominant form of N loss in pristine catchments
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DISSOLVED INORGANIC DISSOLVED INORGANIC NITROGEN (DIN=NHNITROGEN (DIN=NH44
++ + NO + NO33--))
• DIN is the form of nitrogen used by plants and microbes
• Microbes respond immediately to increased available DIN (fertilizer, atm)
• DIN tightly recycled in N-limited ecosystems
• DIN rarely in surface waters
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Perturbation: permafrost melting which is increasing N mineralization
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PROMISING TOOLPROMISING TOOLPotential ProblemsPotential Problems
• Biome differences
• Year-to-year and site-to-site differences
• Climate change
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Very wet year flushes out more nitrate. Non-linear response
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THE DIN and DON STORYTHE DIN and DON STORY
• Shows promise as an indicator of ecosystem N status
• Interannual and other variations need to be addressed
• May provide a simple vital sign to resource managers
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Questions?Questions?
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HYPOTHESIS HYPOTHESIS
• DON export not related to N input
• N deposition acclerates N mineralization
• DIN increases much faster than DON
• DIN:DON ratio metric for ecosystem N status
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DON DOES NOT RESPOND TO N DON DOES NOT RESPOND TO N ADDITIONSADDITIONS
• LEAKY FAUCET HYPOTHESIS
• Persistent “leak” of DON from catchments
• DON is decoupled from microbial demand for N.
• DON export coupled to soil standing stock of C, N
• Lag between N inputs and DON export
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NITRATE LOSSESNITRATE LOSSES
• Increasing N deposition increases net nitrification
• Nitrate mobile• Nitrate export to
surface waters increases as N deposition increases