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

Global Problem: Increasing Global Problem: Increasing Nitrogen DepositionNitrogen Deposition

NADP: NITRATE PERCENT CHANGE

Lehmann et al., 2005, Environ Poll

NADP: AMMONIUM PERCENT CHANGE

Lehmann et al., 2005, Environ Poll

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

N-CYCLE IS COMPLICATEDN-CYCLE IS COMPLICATED

Aber Spaghetti DiagramAber Spaghetti Diagram

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

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

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

Perturbation: permafrost melting which is increasing N mineralization

PROMISING TOOLPROMISING TOOLPotential ProblemsPotential Problems

• Biome differences

• Year-to-year and site-to-site differences

• Climate change

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

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

Questions?Questions?

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

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

NITRATE LOSSESNITRATE LOSSES

• Increasing N deposition increases net nitrification

• Nitrate mobile• Nitrate export to

surface waters increases as N deposition increases