Nitrogen biogeochemistry

Lecture 1

Universidade do algarve

Cycling of elements in the early stages of earth was slow, dependent on extreme conditions – temperature, pressure, high energy radiations.. Purely geochemical cycling.With the beginning of life…scenario changed at a faster rate..beginning of Bio-Geo-Chemical cycles.

OXIC LIFE

ANOXIC LIFE

Life began in an anaerobic worldH+, Sulfides, N and water used to give energy

In a nutshell……..

Bios + geos + chemistry

Life + elements + chemical reactions

Makes Earth a super-organism

Global Nitrogen stocks:

Oceanic (organic + inorganic): 771 GT N + 22578 GT N

Terrestrial (Biomass + soil): 18.2 GT N + 460 GT N

Atmospheric: 43 x 105 GT N (99.999% DiNitrogen)

Rocks, sediments, Corals :190 x 106 + 4 x 105 + 120 GT N.

Oceanic Nitrogen inventory:

Dinitrogen: 22000 GT N; Nitrous oxide: 0.2 GT N.

Nitrate: 570 GT N; Nitrite: 0.5 GT N; Ammonium: 7 GT N.One GT = 1015 grams

Biogeochemistry of Nitrogen: Why Nitrogen is important?

All biogeochemical reactions are redox reactions mediated by biology.

Availability of an element depends on its oxidation number.

All reactions follow the laws of thermodynamics.

Ground rules of biogeochemical processes

Some terms used to measure these processes: ∆G= Free energy change; ∆G0= -n F∆E0’ where F is Faraday’s constant (96485 Coloumb mol-1), n is e- transferred & E0 is redox potential. pe = electron pressure. Eh = Half-cell reduction potential.

Nitrogen is used for assimilation (incorporation into biomass) and for respiration (e-donor).

Organic matter:

Produced biological entities during their life

Contribute either as living/dead/detritus

Produced by the reduction of inorganic carbon/nitrogen.

Major Pathways-

Photosynthesis : CO2 + 2H2O + hn (CH2O)n + H2O + O2

Chemosynthesis: CO2 + O2 + e- (CH2O)n + e0 + H2O Oxic Mineralization: NH2(CH2O)n H2O + CO2 + NH4

+

Suboxic Mineralization: NH2(CH2O)n + NO3- H2O + CO2 +

N2O + NH4+

Anoxic Mineralization: Occurs using Sulfides.

Fate of Nitrogen between land, ocean & atmosphere

Major Nitrogen Forms & their oxidation number-

Nitrate: NO3- +V

Nitrite: NO2- +III

Nitrous Oxide: N2O +INitric Oxide: NO +IINitrogen Gas: N2 0Ammonia Gas: NH3 -IIIAmmonium ions: NH4

+ -IIIOrganic amines: RNH2 -III

Nitrogen is an integral component of organic molecules; its availability in the environment supports biomass production and serves as an important e- donor/oxidizing agent.

In marine environments, Nitrogen is an important limiting nutrient that regulates other elemental cycles.

Nitrogen source- 1. New Nitrogen- mainly through external inputs including upwelling & biological nitrogen fixation.2. Regenerated- Mainly through biological mineralization of OM. Primary production based on the nitrogen source termed as “New production” and “Regenerative production”.

Broadly identified pathways of N-cycle:

1. Remineralization.2. Ammonification.3. Nitrification.4. Denitrification.5. Nitrogen fixation.6. Assimilatory N2 reduction.7. Assimilation of DON.

Li e

t al 2

006

Sampling methods

Seawater Sediments

1. Gravity cores2. Box cores3. Piston cores4. Multicorer5. Rumohr cores.

1. GoFlo sampling bottles2. In-situ Continuous water sampler

Pore water extraction: Various methods employed.

1. Centrifugation. 2. Core squeezing method. 3. In-situpore water sampler.

4. Ex-situ pore-water sampler.

The pore water samples can then be measured for dissolved gases, nutrients, alkalinity, organic matter & trace metals.

Inert eg. ChlorideDepletion in upper layers eg. OxygenSubstance consumed at certain depth Substance released into porewaterAccumulation in upper layers eg. SilicaSubstance released into pore water at one depth & removed from pore water at another depth e. SO4

--.Release of substance from pore water in deep sediments eg. Sulfide

Profiles in general

Some technical aspects : Largely applicable to marine watersStoichiometry: Nitrogen has elemental relationship with O, C & P. Ideal stoichiometry is called Redfield stoichiometry or ratio. -∆O: ∆C: ∆N: ∆P= 138:106:16:1. Variations in the ratio suggests alteration in N or P.

N:P generally are linearly correlated. However, low oxygen environs alter the slope from the expected trend. The deficit in actual nitrate from expected nitrate is called nitrate deficit. In deep oxic waters, this may alsobe due to partial nitrification.

N*= [N-N] (Deutsch et al, 1997), defined N* as the difference between observed N and estimated N for a given P value.

Remineralization ratio is the ratio of P/N/C/-O in deep oxic waters (θ< 10-120C) which is largely influenced by OM, microbial heterotrophy & temperature.This ratio varies in different oceans indicating changes in OM chemistry & microbial community.

F-ratio is defined as the fraction of primary production that is supported by nitrate and is a major cause of flux of OM to ocean sediments.

Largely biotic (abiotic in atmosphere)N2 + 6H+ + 16 ATP 2NH3 + 16ADP

Intensive energy requirement; oxygen sensitive reaction; enzymes require co-factors like Molybdenum & Ferrous, genetically regulated by set of genes called nif-genes.

Endosymbiont Rischella sp., host Rhizosolenia sp.

Trichodesmium

sp.

Widely observed both on land & aquatic systems, carried out by bacteria, cyanobacteria and some forms of archaebacteria in symbiotic conditions, free-living/planktonic & in sediments. N-fixers are called Diazotrophs- 2 Types: autotrophic & heterotrophic.

NITROGEN FIXATION

Zehr

et a

l 200

1

Vito

usek

et a

l., 2

002

Biogeochemical controls Ecological controls

Temperature, pH & mixing also affect N-fixation

Nitrogen fixation in oceansThis is a surface ocean phenomenon- euphotic layer which is in contact with lower atmosphere.

Direct & Indirect ways of estimating nitrogen fixation in open oceans-1. Direct ways- Acetylene inhibition method.2. Indirect ways- Mass balance (N:P); 15N- dilution/uptake; N*.Nitrogen fixation is inhibited by acetylene (C2H2)- as it competes for H2and gets reduced to C2H4 which is then measured.3. N2 fixation using 15N as tracer results in enrichment of 15N in PN. 4. Redfield ratio- C:N:P 116:16:1. N < P promotes nitrogen fixation.

5. N*= N- rN:PnitP; N* is regenerated N & P, N:P

nit is stoichiometric constant N:P during remineralization or nitrification and r is a constant.N* positive means P-limitation and negative means N-limitation.

Cap

one

et a

l 200

5

Nitrogen fixation in Northern Atlantic

Impact of Fe-limitation on C-fixation & N-fixation

High PP in tropical-sub-tropical Atlantic, northern Indian Ocean and Eastern Pacific coincides with elevated Fe concentrations and N-fixation.

Nitrogen in decomposition of OM- Denitrification

Denitrification is a heterotrophic process wherein nitrate is used to meet oxygen demand. Elemental Nitrogen is the end product and a major source of atmospheric nitrogen.

[C(H2)O]n---- nCO2 + H2O requires O2. [O2> NO3> NO2> SO4]

Oxygen is inhibitory for denitrification process.

4 step process involving an array of enzymes and production of N2O as an important by-product.

NO-3 - NO-

2 - NO N2O N2

Regulated at genetic level by nir-genes and enzymes are reductases.

Important co-factors: Molybdenum, Ferrous, Manganese.

Occurs in oxygen minimum waters of the ocean depths and sediments.

Techniques to measure denitrification rates Regulatory Factors

Water column:1. Physical stratification.2. Organic loading.3. Sedimentation rates.4. Residence time.5. Mineralization rates.6. Availability of Fe/Mo.

Sediments:1. Porosity.2. Organic loading.3. Overlying water

conditions.4. Mixing/disturbances.5. Availability of NO3.

Naq

vi e

t al 2

002

Thomas et al 1996

Primary nitrite maxima- NitrificationSecondary nitrite maxima- Denitrification.Primary Ammonium maxima- Ammonification.Secondary Ammonium hump- Remineralization.Primary N2O peak- Nitrification.Secondary N2O peak- Denitrification.Box in the graph- Anammox.

Idealized Vertical profiles in northern central Arabian Sea & eastern tropical Pacific Ocean.

Global map of denitrifying zones based on N*. 3 major sites- Eastern Pacific, Northern Indian Ocean (AS & BoB) and western African coast off Nigeria.

Penn

ingt

on e

t al 2

006

Major OMZ in world oceans

Hel

ly &

Lev

in, 2

004

Comparison of NO2, N2O & Mn w.r.t O2 levels.

Naq

vi e

t al 2

003

Requisite conditions: Strong thermocline/picnocline, High PP, High oxygen demand, Low N:P ratios.

0 20 40 60 80 100

N2

N2O

NO

Denitrification in sediments

% Water-filled pore-space

Net

gas

pro

duct

ion

Water-limitation

Oxygen-limitation

Denitrification in sediments incubated with a) Nitrate (•), b) Ammonia (♦) & c) Control (■). (Seitzinger et al., 1993)

Denitrification rates in Fjord sediments during winter & summer. (Sorensen et al, 1979)

Randers fjord Kysing fjord

Anammox- Anaerobic Ammonium Oxidation

First postulated by Broda (1977), anaerobic ammonium oxidation with nitrite would yield energy for lithogenic C-fixation and alternate source of atmospheric nitrogen.NH+

4 + NO-2 N2 + 2H2O ∆Go = -360 kJ/mol.

First anammox organisms identified in wastewater plants, belong to genus Planctomycetales (Mulder et al 1995).

Currently, 20-50% of oceanic nitrogen efflux is believed to be from these bacteria.

Not much is known about this process in open oceans or in sediments.

Black Sea and Nigerian shelf waters are well-established sites for Anammox activity.

Percent organic nitrogen distribution in surface sediments

Premuzic et al., 1982

Major source of PON in ocean sediments is brought by vertical sedimentation of particles from the surface. In coastal sediments, inputs from rivers, anthropogenic sources, algal beds and coastal vegetation add to the PON load.High N-load in sediment also means slow mineralization in the water-column.

Overview of the Global N-cycle & its link to C & P cycles

(Gruber & Galloway, 2008)