Transcript N deposition

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 –
the “nitrogen cascade.”
• 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
production
inorganic
organic
decomposition
ecosystem boundary
Organic
matter
(microbes)
Primary
producers
Simple,
soluble
(inorganic)
forms
Hydrologic
losses
Gaseous
losses
Soil organic
matter
(microbes)
Fixation
Fertilizer
Deposition
Plants
Simple,
soluble
(inorganic)
forms
Hydrologic
losses
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 are “boogered” because they 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).
NO, N2O, N2
Fixation
B
Organic N in
organic matter
and microbes
Primary
producers
A
C
F
Fertilizer
Deposition
D
NH4+ → NO2- → NO3E1
E2
Simple, soluble
(inorganic) forms
A = uptake by primary producers.
B = production of detritus
G
C = mineralization.
D = immobilization
E = nitrification (1 = NH4+ oxidation, 2 = NO2- oxidation)
F = denitrification, DNRA, annamox
G = hydrologic loss
NH4+, NO3- ,
dissolved organic N
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, rotting logs?
• 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:
– Anaerobic activity,. E.g. nitrifier denitrification
– PH sensitive
– Limited number of genera
– Limited substrate range (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.
• Staggeringly 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 NO3conditions.
• 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?
Ecosystem (site) controls on terrestrial
nitrogen cycling:
Nutrient → Nutrient →
poor
poor
site
vegetation
Poor →
litter
quality
Nutrient → Nutrient → High →
rich
rich
litter
site
vegetation
quality
Low →
Low productivity
nutrient→ Low loss following disturbance
availability
High
→ High productivity
nutrient
→ High loss following disturbance
availability → Sensitive to saturation
•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.
Open water bodies (lakes, estuaries,
rivers):
Sediment/water interface
Depth (mm)
Water column
N uptake by primary
producers.
Production of detritus
Detritus settles
towards the
sediment
Sediment
Mineralization,
immobilization, nitrification
and denitrification in
layered sediments, as
described to the right.
0
Aerobic –
mineralizaiton,
immboilization
nitrification
4
Anaerobic
denitrification
layer
6
Anaerobic,
sulfate
reduction layer
8
Anaerobic,
methanogensis,
fermentation
12
Streams:
• Nutrient spiraling:
– Uptake lengths
• Patchiness
• Carbon/nitrogen interactions
Nutrient spiraling:
Source: Emily Stanley
Riparian:
Water table
Groundwater flow path
Aquiclude
Riparian ecosystem
Stream
• Urban stream
syndrome:
Natural Channel
Water Table
Stream
Channel with Incision
Due to Increased
Runoff
– High storm flows.
– Incised channels.
– Drier riparian zones
with lower water
tables.
•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?
Gaseous
losses
Soil organic
matter
(microbes)
Fixation
Fertilizer
Deposition
Plants
Simple,
soluble
(inorganic)
forms
Hydrologic
losses
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
Fertilizer
TOTAL
8.7
13.9
22.6
8.7
0
8.7
8.7
100
108.7
Outputs
Streamflow
6.5
0.52
16.4
Retention
Mass
Percent
16.1
71
8.2
94
92.3
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?