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Ecosystem Nitrogen Status Indicator: DIN to DON Ratio in Rivers

This study by Mark Williams, CU Boulder, Dave Clow, USGS, and Tamara Blett, NPS, focuses on the ratio of dissolved inorganic nitrogen (DIN) to dissolved organic nitrogen (DON) in annual riverine flux as a metric for assessing ecosystem nitrogen status. DIN is the form of nitrogen readily used by plants and microbes, while DON is generally recalcitrant and not easily utilized. The study explores how the DIN:DON ratio can serve as a vital sign for resource managers to evaluate nitrogen deposition impacts on ecosystem health. Climate change, biome variations, and nitrate losses are considered challenges to this novel indicator.

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Ecosystem Nitrogen Status Indicator: DIN to DON Ratio in Rivers

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  1. A NOVEL INDICATOR OF ECOSYSTEM N STATUS: RATIO OF DIN TO DON IN ANNUAL RIVERINE FLUX Mark Williams, CU Boulder Dave Clow, USGS Tamara Blett, NPS

  2. Global Problem: Increasing Nitrogen Deposition

  3. NADP: NITRATE PERCENT CHANGE Lehmann et al., 2005, Environ Poll

  4. NADP: AMMONIUM PERCENT CHANGE Lehmann et al., 2005, Environ Poll

  5. NPS RESOURCE MANAGERS • In the hot seat • Need metrics • to evaluate • ecosystem N status • before we have dead • fish and dead trees • Simpler the better • Identify thresholds

  6. N-CYCLE IS COMPLICATED

  7. Aber Spaghetti Diagram

  8. DIN:DON RATIO IN ANNUAL DISCHARGE PROVIDES A METRIC FOR ECOSYSTEM NITROGEN STATUS

  9. DISSOLVED ORGANIC 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

  10. DISSOLVED INORGANIC NITROGEN (DIN=NH4+ + NO3-) • 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

  11. Perturbation: permafrost melting which is increasing N mineralization

  12. PROMISING TOOLPotential Problems • Biome differences • Year-to-year and site-to-site differences • Climate change

  13. Very wet year flushes out more nitrate. Non-linear response

  14. THE 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

  15. Questions?

  16. 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

  17. DON DOES NOT RESPOND TO N ADDITIONS • 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

  18. NITRATE LOSSES • Increasing N deposition increases net nitrification • Nitrate mobile • Nitrate export to surface waters increases as N deposition increases

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