Nitrogen biogeochemistry. Lecture 1 Universidade do algarve

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1 Nitrogen biogeochemistry Lecture 1 Universidade do algarve

2 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. Life began in an anaerobic world H +, Sulfides, N and water used to give energy ANOXIC LIFE OXIC LIFE

3 In a nutshell.. Bios + geos + chemistry Life + elements + chemical reactions Makes Earth a superorganism

4 Biogeochemistry of Nitrogen: Why Nitrogen is important? Global Nitrogen stocks: Oceanic (organic + inorganic): 771 GT N GT N Terrestrial (Biomass + soil): 18.2 GT N GT N Atmospheric: 43 x 10 5 GT N (99.999% DiNitrogen) Rocks, sediments, Corals :190 x x GT N. Oceanic Nitrogen inventory: Dinitrogen: GT N; Nitrous oxide: 0.2 GT N. Nitrate: 570 GT N; Nitrite: 0.5 GT N; Ammonium: 7 GT N. One GT = grams

5 Ground rules of biogeochemical processes 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. Some terms used to measure these processes: G= Free energy change; G 0 = -n F E 0 where F is Faraday s constant (96485 Coloumb mol -1 ), n is e - transferred & E 0 is redox potential. pe = electron pressure. E h = Half-cell reduction potential. Nitrogen is used for assimilation (incorporation into biomass) and for respiration (e-donor).

6 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 : CO 2 + 2H 2 O + hn (CH 2 O) n + H 2 O + O 2 Chemosynthesis: CO 2 + O 2 + e - (CH 2 O) n + e 0 + H 2 O Oxic Mineralization: NH 2 (CH 2 O) n H 2 O + CO 2 + NH 4 + Suboxic Mineralization: NH 2 (CH 2 O) n + NO 3- H 2 O + CO 2 + Anoxic Mineralization: Occurs using Sulfides. N 2 O + NH 4 +

7 Fate of Nitrogen between land, ocean & atmosphere

8 Major Nitrogen Forms & their oxidation number- Nitrate: NO - 3 +V Nitrogen is an integral Nitrite: NO - 2 +III component of organic Nitrous Oxide: N 2 O +I molecules; its availability in the Nitric Oxide: NO +II environment supports biomass Nitrogen Gas: N 2 0 production and serves as an Ammonia Gas: NH 3 -III important e - donor/oxidizing Ammonium ions: NH + 4 -III agent. Organic amines: RNH 2 -III 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.

9 Li et al 2006 Broadly identified pathways of N-cycle: 1. Remineralization. 2. Ammonification. 3. Nitrification. 4. Denitrification. 5. Nitrogen fixation. 6. Assimilatory N 2 reduction. 7. Assimilation of DON.

10 Sampling methods Seawater Sediments 1. GoFlo sampling bottles 2. In-situ Continuous water sampler Inert eg. Chloride Depletion in upper layers eg. Oxygen Substance consumed at certain depth Substance released into porewater Accumulation in upper layers eg. Silica Substance released into pore water at one depth & removed from pore water at another depth e. SO Release of substance from pore water in deep sediments eg. Sulfide Profiles in general 1. Gravity cores 2. Box cores 3. Piston cores 4. Multicorer 5. Rumohr cores. Pore water extraction: Various methods employed. 1. Centrifugation. 2. Core squeezing method. 3. In-situ pore 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.

11 Some technical aspects : Largely applicable to marine waters Stoichiometry: 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< C) 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.

12 NITROGEN FIXATION Largely biotic (abiotic in atmosphere) N 2 + 6H ATP 2NH ADP Trichodesmium sp. Intensive energy requirement; oxygen sensitive reaction; enzymes require co-factors like Molybdenum & Ferrous, genetically regulated by set of genes called nif-genes. Widely observed both on land & aquatic systems, carried out by bacteria, cyanobacteria and some forms of archaebacteria in symbiotic conditions, freeliving/planktonic & in sediments. N-fixers are called Diazotrophs- 2 Types: autotrophic & heterotrophic. Endosymbiont Rischella sp., host Rhizosolenia sp. Zehr et al 2001

13 Biogeochemical controls Ecological controls Vitousek et al., 2002 Temperature, ph & mixing also affect N-fixation

14 Nitrogen fixation in oceans This 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 (C 2 H 2 )- as it competes for H 2 and gets reduced to C 2 H 4 which is then measured. 3. N 2 fixation using 15 N as tracer results in enrichment of 15 N in PN. 4. Redfield ratio- C:N:P 116:16:1. N < P promotes nitrogen fixation. 5. N*= N- r N:P nitp; 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.

15 Capone et al 2005 Nitrogen fixation in Northern Atlantic

16 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.

17 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(H 2 )O] n ---- nco 2 + H 2 O requires O 2. [O 2 > NO 3 > NO 2 > SO 4 ] Oxygen is inhibitory for denitrification process. 4 step process involving an array of enzymes and production of N 2 O as an important by-product. NO NO NO N 2 O N 2 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.

18 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 NO 3.

19 Naqvi et al 2002 Thomas et al 1996 Idealized Vertical profiles in northern central Arabian Sea & eastern tropical Pacific Ocean. Primary nitrite maxima- Nitrification Secondary nitrite maxima- Denitrification. Primary Ammonium maxima- Ammonification. Secondary Ammonium hump- Remineralization. Primary N 2 O peak- Nitrification. Secondary N 2 O peak- Denitrification. Box in the graph- Anammox.

20 Major OMZ in world oceans Naqvi et al 2003 Helly & Levin, 2004 omparison of NO 2, N 2 O & n w.r.t O 2 levels. Requisite conditions: Strong thermocline/picnocline, High PP, High oxygen demand, Low N:P ratios. Pennington et al 2006 Global map of denitrifying zones based on N*. 3 major sites- Eastern Pacific, Northern Indian Ocean (AS & BoB) and western African coast off Nigeria.

21 Denitrification in sediments Waterlimitation Oxygen-limitation Net gas production NO N 2 O N 2 Denitrification in sediments incubated with a) Nitrate ( ), b) Ammonia ( ) & c) Control ( ). (Seitzinger et al., 1993) % Water-filled pore-space Randers fjord Denitrification rates in Fjord sediments during winter & summer. (Sorensen et al, 1979) Kysing fjord

22 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 NO - 2 N 2 + 2H 2 O G o = -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.

23 Premuzic et al., 1982 Percent organic nitrogen distribution in surface sediments 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.

24 Overview of the Global N-cycle & its link to C & P cycles (Gruber & Galloway, 2008)