Outline:• Nitrogen – the global picture.

• Nutrient cycles in context.

• Nitrogen cycle processes:– N fixation– Mineralization/immobilization– Nitrification– Dissimilatory processes (denitrification, DNRA, annamox)– Leaching– Plant uptake/litterfall

• Regulation at the ecosystem scale

• Human influences

• Nitrogen balances

Why are we obsessed with N:• The most commonly limiting nutrient in terrestrial

systems, especially temperate.

• Also limiting in marine, estuarine systems.

• N cycle is more complex than most.

• Human manipulation of the N cycle is intense.

• N can be become a drinking water pollutant and agent of eutrophication.

• N gases contribute to the greenhouse effect, ozone production/destruction.

Pools and fluxes of N – global:

• Pools (g N)– Atmosphere – 3.8 x 1021

– Terrestrial biomass – 3.5 x 1015

– Soil organic matter – 95 x 1015

• Fluxes (g x 1012 per year)– Fixation – 190– Cycling by land plants - 1200– Cycling in ocean - 6000

Pools and fluxes of N – terrestrial:

production

decomposition

ecosystem boundary

organicinorganic

Primary producers

Simple, soluble

(inorganic) forms

Detritus

Hydrologic losses

Plants Simple, soluble

(inorganic) forms

Detritus

Gaseous losses

Hydrologic losses

Fixation

Fertilizer

Deposition

Key things to remember about nutrient cycles:

• They are a by-product of energy flow in the ecosystem. Energy flow (terrestrial) is 20% trophic, 80% detrital. The biggest function of the detrital flow is nutrient regeneration.

• Primary producers require inorganic (or at least simple), soluble nutrient forms.

• Inorganic forms are subject to loss, especially hydrologic.

• Inorganic pool responds to disturbance, e.g. clearcut, deposition, fertilization

• Microbes mediate the organic to inorganic transformation (mineralization).

Primary producers

NH4+ → NO2

- → NO3-

Simple, soluble (inorganic) forms

Organic N in organic matter and microbes

NO, N2O, N2

NH4+, NO3

- , dissolved organic N

Fixation

Fertilizer

Deposition

A

B

C

D

E1

F

G

A = uptake by primary producers.

B = production of detritus

C = mineralization.

D = immobilization

E = nitrification (1 = NH4+ oxidation, 2 = NO2

- oxidation)

F = denitrification, DNRA, annamox

G = hydrologic loss

E2

N fixation:

• Pathway: N2 → NH3

• Energetically expensive due to triple bond, requires 15 ATP/mole to break. Aerobic oxidation of glucose yields 26 ATP/mole, anaerobic yields less than 5. The industrial process uses high temperature and pressure to make NH3.

• Where do we find N fixation – whenever you have abundant energy sources, e.g. Legumes-Rhizobia, Cyanobacteria, Frankia-alders

• Non-symbiotic fixation is rare, but there is still uncertainty about this.

N Mineralization:• Pathway: Organic N → NH4

+

• Organic to inorganic (mineral, or simple) transformation. Release of NH4

+ from amino acids, nucleotide bases.

• Should we redefine to include simple organics?

• An energy-driven process. Think like a microbe.

• Occurs under both aerobic and anaerobic conditions.

Immobilization:• Pathway: NH4

+ → Organic N

• Uptake of inorganic N to support growth.

• Again, energy driven. Microbes reluctantly need N to acquire carbon and energy.

• Aerobic and anaerobic.

• Balance between mineralization and immobilization controlled by the C:N ratio of the substrate:– 25:1 is considered to be breakpoint– Sawdust = 225:1, oat straw = 80:1, Compost = 10:1.

• Microbes and plants will produce enzymes to acquire specific nutrients that they need.

Nitrification:• Pathway: NH4

+ → NO2- → NO3

-

• Unique process carried out by strange aerobic chemoautotrophic

bacteria. They acquire energy from the oxidation of ammonia.

• Strong regulation by ammonia, and especially by the competition with roots and heterotrophs (immobilizers). They are lousy competitors because of slow growth rates.

• Key to losses. Without NO3-, the N cycle would be very

conservative.

• Source of N2O

• The physiology literature is a pack of lies, albeit generally true:– An aerobic process? Not entirely . . . .– pH sensitive? Not really . . . – Limited number of genera? For sure not!– Limited substrate range? Unh-uh, e.g., TCE, methane)

Dissimilatory processes:• Anaerobic microbial processes that convert

nitrate into more reduced forms (ammonia or N gases).– Denitrification - anaerobic respiration of nitrate

to produce nitrogen gases.– Dissimilatory nitrate reduction to ammonia

(DNRA)– Anaerobic ammonium oxidation (Annamox)

Denitrification:• Pathway: NO3

- → NO2- → NO → N2O → N2

• Anaerobic (mostly), heterotrophs (sort of mostly), nitrate as electron acceptor.

• Thought to be low in most terrestrial ecosystems, but should balance fixation on a global basis, e.g. very high rates in oceans.

• Very high rates (25 g N/m-2/y) in wetlands with high nitrate.

DNRA:• Pathway: NO3

- → NH3

• Anaerobic

• Carried out by fermenters and/or S-oxidizers.

• Dumps more electrons than denitrification, may be favored under high C, low NO3

- conditions.

• May contribute to N retention because NH3 is more stable than NO3

-

Annamox:• Pathway: NH4

+ +NO2- → N2

• Anaerobic

• Carried out poorly characterized group of bacteria, driven by hydrazine (rocket fuel).

• Discovered in waste treatment, shown to be important in ocean.

• May be most important in anaerobic ecosystems with limited labile C, e.g., deep ocean, deep lakes.

Leaching:

• NO3- is more mobile than NH4

+.

• Some plants may be adapted to this mobility.

• Are ecosystems “adapted” to minimize hydrologic loss?

• DON – may be an unregulatable loss, the source of persistent N limitation.

Uptake/detritus dynamics by primary producers:• Uptake:

– Plants have many strategies for taking up N.– Uptake of organic N is hot topic.– Ability to exploit soil N reserves critical for

“down regulation” of stimulation of production by elevated CO2

– Will N deposition lead to P limitation?

• Detritus:– In many cases, production of detritus is the

main (e.g, 80%) fate of primary production.– Root turnover is of great current interest. How

fast? Is it much more functionally important than leaf litter?

Nutrient → Nutrient → Poor → Low → Low productivitypoor poor litter nutrient→ Low loss following disturbancesite vegetation quality availability

Nutrient → Nutrient → High → High → High productivityrich rich litter nutrient → High loss following disturbancesite vegetation quality availability → Sensitive to saturation

Ecosystem (site) controls on terrestrial nitrogen cycling:

• This conceptual framework has been incorporated into models and has been applied to many studies and applications, e.g. clearcutting, trace gas fluxes, water quality, N saturation, climate change, etc.

• Can these site controls be overcome by exotic species invasion, e.g Ailanthus invasion?

• Can these site controls be overcome by input, e.g. N deposition?

Terrestrial: N cycling, plant succession andecosystem development

• Young systems with no biotic control over the abiotic environment (e.g. plants) have high loss.

• Aggrading system – plant and organic matter pools are increasing.

• Mature system – plants and organic matter no longer increasing so losses should go up. Doesn’t always happen, e.g. dead wood, denitrification.

Open water bodies (lakes, estuaries, rivers):

• Water column versus sediment.

• Redox layering in sediment.

• Coupled response to nutrient additions:– Productivity and organic loading to sediment.– Feedbacks with anaerobic conditions.

N uptake by primary producers.

Production of detritus

Detritus settles towards the

sediment

Mineralization, immobilization, nitrification and denitrification in layered sediments, as described to the right.

Water column

Sediment

Aerobic – mineralizaiton, immboilization nitrification

Anaerobic denitrification layer

Anaerobic, sulfate reduction layer

Anaerobic, methano-gensis, fermentation

Sediment/water interfaceDepth (mm)

0

4

6

12

8

Open water bodies (lakes, estuaries, rivers):

Epilimnion

Hypolimnion

Streams:

• Nutrient spiraling:– Uptake lengths

• Patchiness

• Carbon/nitrogen interactions

Nutrient spiraling:

Source: Emily Stanley

Riparian ecosystemStream

Aquiclude

Water table

Groundwater flow path

Riparian:

Natural Channel • Urban stream syndrome:– High storm flows.– Incised channels.– Drier riparian zones

with lower water tables.

Channel with Incision Due to Increased Runoff

Water Table Stream

• Channel Erosion• Nonfunctional Floodplain• Dry Riparian Soils

Agriculture:• Remove plants.

• Add fertilizer.

• Reduce SOM (increase decomposition by disturbance, litter quality, harvest).

• Given these constraints, how much can we increase efficiency and decrease loss without sacrificing productivity?

• How did we get here?

Plants Simple, soluble

(inorganic) forms

Soil organic matter

(microbes)

Gaseous losses

Hydrologic losses

Fixation

Fertilizer

Deposition

N deposition:• Will N saturation ever occur given:

– Disturbance frequency.– Abiotic uptake.– DON leaching– Two kinds of results:

• Fertilizer studies show very, very high retention.• Gradient studies show sensitive response to inputs.

• Will the plants change, e.g., overcoming site controls as discussed above?

• Will we lose biodiversity, e.g., Trillium?

N balances: The enigma of missing N• Balance = Inputs – outputs.

• Lots of N “missing” N in balances computed at all scales.

• Where does all the N go:– Soil?– Plants?– Denitrification:

• Soil• Stream• Estuary

• Great environmental relevance:– Estuarine loading– Atmpospheric chemistry– Critical loads

22 year N balance, continuous corn in Iowa”:

Source: Steinheimer et al. (1998)

Source: Howarth et al. (1996)

N BUDGETS 1999 - 2001

Suburban Forested Agriculture

------------------- kg N ha-1 y-1 ------------------

Inputs

Atmosphere 8.7 8.7 8.7

Fertilizer 13.9 0 100

TOTAL 22.6 8.7 108.7

Outputs

Streamflow 6.5 0.52 16.4

Retention

Mass 16.1 8.2 92.3

Percent 71 94 85

Landscape thinking:• Is this ecosystem potentially a sink or source

of N?– N rich (natural, fertilizer)– Disturbance– Sink: Wet, high organic matter, high pH

• How is this ecosystem “connected?”– Internal controls:

• Soil texture and leaching• Soil structure, drainage and cover affect infiltration

and runoff.– Where does the ecosystem “sit” in the

landscape?