The Global Nitrogen Cycle

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1 OCN 401 The Global Nitrogen Cycle ( ) Fig Units are g N/yr (Tg)

2 Role of N in Biogeochemistry Bioavailability of N (and/or P) can limit NPP on land/oceans; controls size of biomass N has multiple oxidation states: -3 in NH 3 ammonia 0 in N 2 nitrogen +1 in N 2 O nitrous oxide +2 in NO nitric oxide +3 in NO - 2 nitrite ion +4 in NO 2 nitrogen dioxide +5 in NO - 3 nitrate ion Microbial processes exploit the redox gradients (for energy purposes) and thereby mediate fluxes Most abundant form of N is atmospheric N 2 is fixed (oxidized) by bacteria in N-poor regions. Denitrifying bacteria reverse process.

3 Processes within the Global Nitrogen Cycle

4 N-bioavailability: fixed N Balance of fixation and denitrification affects global N availability Pool of available inorganic N is always small. Like P bioavailable N is rapidly taken up All biologically available N originally came from atmospheric N, via lightning, meteor impacts etc., or by biological fixation Current rates are dominated by biological fixation: Lightning <3-5 x N yr -1 Biological fixation 140 x yr -1

5 Nitrogen reservoirs Atmosphere Soil organic Terrestrial biomass Total oceans (NO 3 ) Ocean dissolved N 2 gas Oceanic biota 3.5 x g x g 3.5 x g 570 x g 1.1 x g 4.7 x g Annual terrestrial N requirement 1200 x g N yr -1 12% from N fixation Most N from recycling

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7 Nitrogen Fixaton Total terrestrial biological N fixation 140 x g N yr x g N yr -1 from leguminous crops (enzymemediated) and > 80 x g N yr -1 from fertilizer addition Haber process: burn natural gas (CH 4 ) to produce H 2 react H 2 with N 2 under high T and P to produce NH 3 20 x g N yr -1 from fossil fuel combustion N N, triple bond takes massive energy to break In total, 240 x g N yr -1 of newly fixed N is delivered from the atmosphere to the Earth s land surface each year: 40% via natural processes 60 % via human-derived sources

8 Nitrogen cycle Terrestrial N fixation 240 x g N yr -1 Rivers carry ~ 36 x g N yr -1 to the oceans Denitrification in sediments balances N inputs Global estimates of terrestrial denitrification x g N yr -1 Wetlands account for ~50% Estimate pre-industrial denitrification at 70 x g yr -1 Denitrification leads to N 2 and N 2 O production 22:1 From N 2 O increase in atmosphere can calculate increase in denitrification of 90 x g N yr -1 Biomass burning also leads to denitrification of ~ 50 x g N yr -1 (as N 2 )

9 Oceans Rivers to oceans ~ 36 x g dissolved N yr -1 Biological fixation ~ 15 x g N yr -1 Precipitation ~30 x10 12 g N yr -1 (recycled NH 3 ; but ignores DON) Deep water N pool 570 x g N yr -1 Most NPP supported by recycling Little burial of N in marine sediments Oceanic N cycle closed by denitrification in low O 2 environments ~ 110 x g N yr -1

10 Temporal variations in global N cycle Early atmosphere dominated by N 2 and CO 2 N fixation by lightning and meteor shock waves (6 % of current rate) May have been important for origin of life Lack of N may have lead to early evolution of N fixers MRT of N in atmosphere wrt fixation by lightning ~ 1.3 x 10 9 yrs Add in biological fixation MRT decreases to ~ 20 x 10 6 yrs Denitrification important for maintaining atm N level Not sure if denitrification evolved before or after O 2 in atmosphere; since denitrifiers can tolerate low O 2, perhaps after Nitrification evolved after O 2 in atmosphere--process requires it Both processes at least 1 x 10 9 yrs old

11 N isotopes in Paleoceanography Can use 15 N/ 14 N in sedimentary organic N and forams to assess paleo N chemistry Denitrification removes NO 3 - from ocean, increases 15 N/ 14 N in DOM, residual NO 3 - is heavy (enriched in 15 N) MRT of N in ocean water is ~ 8000 yrs N may not be in steady state High N inputs during LGM support high denitrification rates today High NPP during LGM and lower 15 N/ 14 N in sediments support this notion Changes in N cycling may accompany climatic changes

12 Anthropogenic activity Planting of N fixers, fertilizer production >80 x g yr -1 (from N+H at high T) Fossil fuel combustion ~ 20 x g yr -1 produces NO x Some NO x transported long distances -- seen in Greenland ice Total fluxes fixed N = 240 x g yr -1 40% natural, 60% anthropogenic Groundwater N increasing from fertilizer leaching: 11 x g yr -1 From Holland et al., 2005 EOS 86, 254

13 Nitrous oxide Nitrogen fixation and denitrification both produce N 2 O as a byproduct N 2 O is an important greenhouse gas; its impact is 300x that of a molecule of CO 2, also destroys O 3 Sink for N 2 O is stratospheric destruction via photolysis 80% N 2, 20% NO which destroys O 3 MRT N 2 O in atmos ~ 120 yrs -- uniformly distributed Seawater source of N 2 O to atmos, but some consumption occurs via denitrification within water column

14 Global Nitrous Oxide Cycle

15 Soils are main source of N 2 O to atmosphere, Cultivation increases N 2 O production rate as increases nitrification and denitrification rates From Holland et al., 2005 EOS 86, 254 Manure production tracks N 2 O increases But NOAA says Sinks (including atmos increase) slightly more than identified sources, yet we observe an increase in atm N 2 O -- more research needed

16 Global Budget of Atmospheric Ammonia

17 The Global Ammonia Cycle

18 Global Emissions of NOx

19 Global Nitrogen Oxides (NO 2 and NO) Cycle

20 Relationship between NO 3 - in Ice Cores and NO x Emissions

21 N vs. P Biogeochemistry Whereas N is found at valance states ranging from -3 to +5 (e.g., NO 3- ), and microbes capitalize on transformations for energy, P is almost always at a valance of +5, as PO 4 3-.

22 Linked N and P Cycling P as ATP important in all biochemical processes, including C fixation, protein production Primary production requires both N & P, both tend to be present at low - limiting levels Redfield C:N:P used to predict biomass production N-fixation rates inversely related to N:P in soil High demand for P by N-fixing organisms links global C- N-P cycles, with P being the ultimate limiting nutrient Despite this, NPP often shows an immediate response to N additions It continues to be a puzzle, and source of debate

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26 Global Land-Ocean Nitrogen Cycle

27 Organic Nitrogen Fluxes Land to Ocean

28 Sources and Sinks of Nitrous Oxide (10 12 g N/yr)

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