Old and new sinks in the global nitrogen cycle

Bernhard Wehrli (wehrli@eawag.ch)

thanks to Andreas Brand, Martin Maerki, Miriam Reinhardt, Cristian Teodoru, Christian Dinkel, Beat Müller, Carsten Schubert, Alfred Wüest

Eawag: Swiss Federal Institute of Aquatic Science and Technology

03/04/2008 2

Gobal nitrogen cycle and aquatic systems

- how fertilizing the planet affects surface waters - cycling of N and changing surface hydrology

– damming the rivers – constructing new wetlands

- processing N via biogeochemical pathways– denitrification in eutrophic, seiche-driven lakes– elimination of reactive N at aquatic redox boundaries

- conclusions

03/04/2008 3

Agriculture doubles N fixation

Gruber & GallowayNature, 451, 293 (2008)

03/04/2008 4

Global trends in reactive nitrogen sources

0

50

100

150

200

250

300

350

1 11 21 31 41 51 61 71 81 91

fossil fuellegume cropsHaber-Boschlightningnatural fixation

modified afterVitousek et al. (1993) and Gruber & GallowayNature, 451, 293 (2008)

1900 10 20 30 40 50 60 70 80 90 2000

Tg N

year

03/04/2008 5

Global distribution of nitrate emissions

Foley et al. Science, 309, 570 (2005)

03/04/2008 6

NO3- concentrations increased by a

factor of 2.5 from 1960 to 2000

McIsaac et al.Nature, 414, 166 (2001)

03/04/2008 7

Agricultural runoff triggers algal blooms

Beman et al.Nature, 434, 211 (2005)

03/04/2008 8

Efficiency of denitrification in river reaches decreases exponentially with [NO3

-]

Mulholland et al.Nature, 452, 202 (2008)

03/04/2008 9

elimination of Nreactiv in freshwater systems

- only 20-25% of the anthropogenic nitrogen is exported to the sea

- 50% of the global terrestrial denitrification occurs in freshwater

- denitrification capacity in rivers is limited by primary production

- factors controlling N elimination processes – residence time,

– productivity,

– supply of O2 and Corg,

– microbial pathways

03/04/2008 10

Old and new N sinks along the aquatic continuum from land to ocean:

45‘000 large dams managed wetlands

anammoxeutrophic lakes

03/04/2008 11

- 800’000 dams and 45’000 large dams > 15 m

- 0.5 Mio. km2 land inundated (> Caspian Sea)- irrigation > 15% of global food production- hydroelectricity ~20% of total

- rate of dam construction 360 to 170 dams yr-1

- investments 30-45 109 US $ yr-1

2 Understanding the effect ofdams, artificial wetlands

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Global distribution of large dams

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Iron Gates –largest reservoir in Danbue catchment

03/04/2008 14

Is the Iron Gate I Reservoir on the Lower Danube a nutrient sink ?

Iron Gates in Romania / Serbia

length 200 kmsurface area 156 km2

volume 2.7 km3

residence time 5.5 d

03/04/2008 15

Danube runoff at Irong Gates Reservoir

03/04/2008 16

Danube runoff at Irong Gates Reservoir

net sink

net source

Teodoru & Wehrli (2005) Biogeochemistry 76, 539-565

03/04/2008 17

Iron Gates – a small nutrient source?

?

?

03/04/2008 18

Does damming reduce the global nitrate export to the ocean?

- Iron Gates ≠ strong nutrient sink

- extrapolated denitrification = 3 10-5 cm s-1 translates to about 1% denitrification along 200 km

- methods for large low-land rivers needed!

Fig from: Mulholland et al.Nature, 452, 202 (2008)

03/04/2008 19

Reducing the nitrate load with new wetlands

Volume 700 m3

Surface 720 m2

max. depth 3 mCatchment 110 000 m2

Volume 700 m3

Surface 720 m2

max. depth 3 mCatchment 110 000 m2

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Identifying N transformation processes

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Nitrogen dynamics in a pond

constant 15NO3- in spring

shift towards lighter valuesin summer

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Isotopic Mass Balance for 15N

Reinhardt, Müller, Gächter, Wehrli (2006) Environ. Sci. Technol. 40, 3313-3319

03/04/2008 24

- Spring: high nitrate load, oxic pond- Denitrification at the sediment → no fractionation−δ15NO3

- → manure

- High flux > 10 mmol m-2 d-1

- Summer: low nitrate, anoxic water column- Nitrification >> denitrification- low δ15NO3

- → biomass-N

- overall efficiency of 27% limited by anoxia

Tracing N turnover in artificial wetland

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3

EawagKastanienbaum

L. AlpnachL. LucerneL. Zug

Understanding processesdenitrification & anammox

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16 LISA deployments in L. Zug 01/02

N

50 m

80 m

120 m

Maerki, Mueller, Dinkel, Wehrli (2008) L&O (submitted)

03/04/2008 28

High-res. pore water profiles L. Zug

March 2002, 50 m water depth

oxygen top 0.5 mmnitrate top 15 mmammonium top + 30 mm

dept

h (m

m)

100 200 3000

O2

100 200 3000 10 20 300

-3-2-101234

-5

5101520

0

2530

-5

5101520

0

2530

NO3-NH4

+

pore water concentration µM

03/04/2008 29

no oxic respiration below 100 m

O2

0

10

20

30

40

50

0 50 100 150 200

DecMarMay/Jun

Depth (m)

O2 -

flux

(- m

mol

m-2

d-1)

flux

[mm

ol m

-2d-1

]

water depth [m]

27.5 mmol m-2 d-1

03/04/2008 30

0

0.2

0.4

0.6

0.8

1

1.2

0 50 100 150 200

DecMarMay/Jun

Depth (m)

NO3-

NO

3-flu

x [m

mol

m-2

d-1]

depth [m]

constant nitrate flux to 120 m depth

0.65 mmol m-2 d-1

flux chamber &mass balance1.1 mmol/m2 d

Mengis et al.L&O, 1997

03/04/2008 31

NH4+

0.5

1

1.5

2

2.5

3

3.5

4

0 50 100 150 200

DecMarchMay/June

Depths (m)

NH

4+flu

x [m

mol

N m

-2d-1

]

depth [m]

increasing anaerobic mineralizationwith depth

factor 3

03/04/2008 32

switch between aerobic and anerobicbenthic mineralization processes in L. Zug

03/04/2008 33

benthic denitrification in L. Zug

- O2, NO3- fluxes → no trend with water depth

- O2 and NO3- fluxes quite variable between

deployments sediment → heterogeneity or benthic boundary layer dynamics?

- analyze diffuse boundary layer dynamics!

03/04/2008 34

Boundary layer physics and the daily life of microbes

How can we work withsuch a thick diffusiveboundary layer?

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

0 2 4 6 8 10

DOO

ODJδ

222

∆−=

Oxygen concentration [ mg/l ]

Hei

ght a

bove

sed

imen

t [ m

m ]

δD∆O2

Sediment

Water

Oxygen flux:

03/04/2008 35

10-11

10-10

10-9

10-8

12:00 16:00 20:00 24:00 04:00 08:00 12:000

5

10

15

20

25

30

35

August 13/14 2002

Cur

rent

spe

ed u

[ m

m/s

] u1m

ε

1 m above the sediment

Dissipation rate ε [ W

/kg ]Energy dissipation and current velocities

Lake Alpnach velo

city

[mm

/s]

1m a

bove

sed

imen

t

Dis

sipa

tion

[Wkg

]

03/04/2008 36

10-11

10-10

10-9

10-8

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

12:00 16:00 20:00 24:00 04:00 08:00 12:000

5

10

15

20

25

30

35

August 13/14 2002

Cur

rent

spe

ed u

[ m

m/s

] u1m

ε δDBL

Dissipation rate ε [ W

/kg ]

DBL thickness δ

DBL [ m

m ]

DB

L Th

ickn

ess

[mm

]

Lorke et al. (2003) L&O 48, 2077

Currents control boundary layer dynamics

Lake Alpnach

03/04/2008 37

DBL dynamics campaign L. Alpnach

Brand, Dinkel, Reichert, Wehrli, in prep.

03/04/2008 38

Oxygen data and reaction-transport model

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Nitrate microprofiles, DBL model

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Effect of DBL on O2 and NO3- flux

DBL effect > 30% DBL effect > 10%

NitrateOxygen

03/04/2008 41

A „new“ elimination process of Nreactive

Marcel Kuypers

03/04/2008 42

Anaerobic ammonium oxidation byanammox bacteria in the Black Sea

Kuypers et al. (2003) Nature, 422, 608-611

03/04/2008 43

Significance for Black Sea biogeochemistry

- 0.17 mmol NO3- m-2 d-1 mixed downwards

- 0.12 mmol NH4+ m-2 d-1 transported upwards

- up to 0.4 Tg reactive N could be lost via anammox.

- where else?

Teodoru, Friedl, Friedrich, Roehl, Sturm, Wehrli (2007) Marine Chem. 105, 52-59

03/04/2008 44

Anammox in Lake Tanganyika

Schubert, Durisch-Kaiser, Wehrli, Thamdrup, Lam, Kuypers (2006) Env. Microbiol.doi:10.1111/j.1462-2920.2006.001074.x

03/04/2008 45

Conclusions

large lowland rivers are inefficient N sinks

enormous denitrification potential of oxic wetlands

anammox is a key process at redox boundaries in marine and freshwater systems

eutrophication activates denitrification regardless of DBL dimension

mass balance15N

sensors

molecularmicrobiology

03/04/2008 46

thank you

03/04/2008 47

Carbon burial in L. Zug

Meckler et al. L&O, 2004: Higher TOC in anoxic zone butno difference in indicators for decomposition, DI, CI

03/04/2008 48

03/04/2008 49

ADCP (RDI-WH, 600 kHz)

Depth cell

Acoustic beam

Acoustic doppler current profilerADCP

03/04/2008 50

03/04/2008 51 0.1 1 10 100 10001E-14

1E-13

1E-12

1E-11

1E-10

1E-9

1E-8

1E-7

1E-6

1E-5

1E-41/LB

1/LK

Batchelor spectrum

inertial subrange: k-5/3

pow

er s

pect

ral d

ensi

ty [

°C

2 / (r

ad/m

) ]

wave number [ rad/m ]

temperature microstructure and dissipation of turbulent energy