N2

Cosmic abundance of the elements

Mass number

-2

0

2

4

6

8

10

12

0 10 20 30 40 50 60 70 80 90 100

log

ab

on

da

nce

(ato

me

s / 1

06 Si)

H

He

Li

Be

B

Fe

Ca

Sc

CO

UTh

PbPt

Ni

Tc Pm

N

NeSiS

s2 d10 p6

n 1

18

1 1.0 1

H 1

2 13 14 15 16 17 4.00 2

He 0

2 6.94 3

Li 1

9.01 4

Be 2

10.8 5

B 6

12.0 6

C –4 –2 0

4

14.0 7

N –3 0 3 5

16.0 8

O –2 0

19.0 9

F –1

20.2 10

Ne 0

3 23.0 11

Na 1

24.3 12

Mg 2

3 4 5 6 7 8 9 10 11 12 27.0 13

Al 3

28.1 14

Si 4

30.1 15

P –3 5

32.1 16

S –2 0 4 6

35.5 17

Cl –1

40.0 18

Ar 0

4 39.1 19

K 1

40.1 20

Ca 2

45.0 21

Sc 3

47.9 22

Ti 4

50.9 23

V 5

52.0 24

Cr 3

54.9 25

Mn 4 3 2

55.8 26

Fe 2 3

58.9 27

Co 2 3

58.7 28

Ni 2

63.5 29

Cu 1 2

65.4 30

Zn 2

69.7 31

Ga 3

72.6 32

Ge 4

74.9 33

As 3,5

79.0 34

Se –2 0 4 6

79.9 35

Br –1

83.8 36

Kr 0

5 85.5 37

Rb 1

87.6 38

Sr 2

88.9 39

Y 3

91.2 40

Zr 4

92.9 41

Nb 3 5

95.9 42

Mo 4 6

98 43

Tc 7

101 44

Ru 3 4

103 45

Rh 2 3 4

106 46

Pd 2 4

108 47

Ag 1

112 48

Cd 2

115 49

In 3

119 50

Sn 4,2

122 51

Sb 3,5

128 52

Te –2 0 4 6

127 53

I –1

131 54

Xe 0

6 133 55

Cs 1

137 56

Ba 2

139 57

La 3

178 72

Hf 4

181 73

Ta 5

184 74

W 4 6

186 75

Re 7

190 76

Os 3 4

192 77

Ir 2 4 6

195 78

Pt 2 4

197 79

Au 1 3

200 80

Hg 2

204 81

Ti 1 3

207 82

Pb 2

209 83

Bi 3 5

209 84

Po 2 4

210 85

At –1

222 86

Rn 0

7 223 87

Fr 1

226 88

Ra 2

227 89

Ac 3

f14 d 140 58

Ce 3

141 59

Pr 3

141 59

Nd 3

145 61

Pm 3

150 62

Sm 3

152 63

Eu 3

157 64

Gd 3

159 65

Tb 3

163 66

Dy 3

165 67

Ho 3

167 68

Er 3

169 69

Tm 3

173 70

Yb 3

175 71

Lu 3

232 90

Th 4

231 91

Pa 5

238 92

U 4 6

237 93

Np 4 5

244 94

Pu 4

243 95

Am 3

247 96

Cm 3

247 97

Bk

3 4

251 98

Cf 3

252 99

Es

257 100

Fm

258 101

Md

259 102

No

260 103

Lr

14.0

N-3,0,3,5

Nitrogen species

nitrate NO3– N+V stable oxide of N, highly soluble as an anion

nitrite NO2– N+III intermediate between NO3

– and NH4+

nitrous oxide N2O N+I from lightning and internal combustion engines

nitrogen N2 N0 elemental nitrogen gas

hydroxylamine NH2OH N–I intermediate species during oxidation of NH4+

ammonia NH3 N–III un-ionized ammonia gas

ammonium NH4+ N–III ionized ammonia (dominates below pH 9.23)

urea CO(NH2)2 N–III common fertilizer

amino N R–NH2 N–III organic nitrogen as amine, measured as TKN

-10

-5

0

5

10

15

5 6 7 8 9pH

pe

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

O2/H2O

H2O/H2

SO42–/HS–

NO3–/NO2

–

NO3–/NH4

Eh

(V)

NO3–/N2

NO2–/N2

Fe(OH)3/Fe2+

CO2/CH4

Nitrogen redox

C106H263O110N16P(S)

Elemental composition of algae

Protein

Essential nutrient

Fixing nitrogen for photosynthesisC106H263O110N16P(S)

:NN:

Natural N2 fixation by Rhizobia and

the nitrogenase enzyme(Fe and Fe-Mo proteins)

N2 + 8H+ + 8e− + 16ATP → 2NH3 + H2 + 16ADP + 16 P

adenosine triphosphate (ATP) adenosine diphosphate (ATP)

And organic N scavenged frombiodegradation

R–NH2 + 2H + NH3

NH3 + O2 NO3–

It’s all about getting NH3 and NO3–.

NH3

Human intervention into the nitrogen cycle

Urea from animal waste, urine

CO(NH2)2 + H2O 2NH3 + CO2

Guano from Chile and the Pacific islands

NO3–

Nitroglycerine

4 C3H5(ONO2)3 12 CO2(g) + 10 H2O(g) + 6 N2(g) + O2(g)

Saltpetre KNO3 from India

Haber-Bosch, 1908-10

½N2(g) + 3/2H2(g) ⇌ 2NH3(g)

CH4(g) + 2H2O(g) → CO2 (g) + 4H2(g)

N2 and H2 are reacted over a ferric iron catalyst with Al2O3 & K2O at 250 atm and 450-500°C.

ΔG = –16.5 kJ/mol

Essential nitrogen reactionsDegradation of organic N: –NH2 + H+ NH3

Organically-bound nitrogen is a component of all proteins and plant biomass. Aerobic and anaerobic degradation of such carbon compounds releases this reduced nitrogen in the form of ammonia. Where this occurs in unsaturated materials such as soils or manure, the ammonia can volatilize, or dissolve into water.

Essential nitrogen reactionsDegradation of organic N: –NH2 + H+ NH3

Organically-bound nitrogen is a component of all proteins and plant biomass. Aerobic and anaerobic degradation of such carbon compounds releases this reduced nitrogen in the form of ammonia. Where this occurs in unsaturated materials such as soils or manure, the ammonia can volatilize, or dissolve into water.

Decomposition of urea: CO(NH2)2 + H2O 2NH3 + CO2

Urea is a common form of organic nitrogen that is produced naturally in animals and industrially. It is often applied as fertilizer in granulated form, and breaks down by a bacterially mediated reaction (urease enzyme) to release ammonia for plants.

Ionization of ammonia: NH3 + H+ NH4+ KT = 10–9.23

• high solubility of ammonia in water at neutral pH

• at high pH NH3 represents a large fraction of the total ammonia

• NH3 = NH4+ at pH 9.23.

Ammonia transformations

Ionization of ammonia: NH3 + H+ NH4+ KT = 10–9.23

• high solubility of ammonia in water at neutral pH

• at high pH NH3 represents a large fraction of the total ammonia

• NH3 = NH4+ at pH 9.23.

Volatilization of ammonia: NH3(aq) NH3(g) KH = 101.76

• high Henry’s Law constant for ammonia

• un-ionized NH3 = 246 mg/L at 25˚C for a NH3 partial pressure of one atmosphere

• volatilization from manure, soils and surface waters

• loss from groundwater below the water table is minimal

Ammonia transformations

Ionization of ammonia: NH3 + H+ NH4+ KT = 10–9.23

• high solubility of ammonia in water at neutral pH

• at high pH NH3 represents a large fraction of the total ammonia

• NH3 = NH4+ at pH 9.23.

Volatilization of ammonia: NH3(aq) NH3(g) KH = 101.76

• high Henry’s Law constant for ammonia

• un-ionized NH3 = 246 mg/L at 25˚C for a NH3 partial pressure of one atmosphere

• volatilization from manure, soils and surface waters

• loss from groundwater below the water table is minimal

Sorption of ammonium: Na–clay + NH4+ NH4–clay + Na+

• cation exchange of ammonium onto clay minerals in soils and aquifers

• erosion of NH4-bearing soils is a major sources of contamination in surface waters

• selectivity coefficient for ammonium varies with the clays and competing cations

• transport of NH4+ in groundwater is retarded.

Ammonia transformations

Aerobic nitrification of ammonium: NH4+ + 2O2 NO3

– + H2O + 2H+

G°r = –266.5 kJ/mol

• NH4+ can be oxidized to NO3

–by reaction with elemental oxygen (O2)

• significant energy yield is favorable for bacteria

• two step reaction of oxidation to nitrite by a Nitrosomonas, Nitrobacter and Nitrosospira, and oxidation of nitrite to nitrate by Nitrobacter Pseudomonas.

• reaction is restricted to aerobic environments – manure piles, soils and surface waters.

Ammonia oxidation

Aerobic nitrification of ammonium: NH4+ + 2O2 NO3

– + H2O + 2H+

G°r = –266.5 kJ/mol

• NH4+ can be oxidized to NO3

–by reaction with elemental oxygen (O2)

• significant energy yield is favorable for bacteria

• two step reaction of oxidation to nitrite by a Nitrosomonas, Nitrobacter and Nitrosospira, and oxidation of nitrite to nitrate by Nitrobacter Pseudomonas.

• reaction is restricted to aerobic environments – manure piles, soils and surface waters.

Anaerobic nitrification of ammonium – anammox: 3 NO3– + 5 NH4+ ® 4 N2 + 9 H2O + 2H+ G°r = –282.30 kJ/mol-NH4

+

= –470.50 kJ mol-NO3–

• Recently discovered (1995) less well-known reaction

• thermodynamically very favorable for bacteria

• anaerobic environments with both ammonium and nitrate species are present, such as in waste-water streams, anoxic marine waters and soils.

• NH4+ as an electron donor, with NO3

–, and NO2–, as an electron acceptors, producing N2.

• only known biologically-mediated reaction for conversion of NH4+ to N2.

Ammonia oxidation

Denitrification: 5CH2O + 4NO3– + 4H+ 2N2 + 5CO2 + 7H2O

G°r = –252.47 kJ/mol

• anaerobic reaction - O2 – free conditions required

• low-pe electron donor such as carbon or sulphide

• nitrate is an electron acceptor with nearly the same energy yield as O2

• Pseudomonas denitrificans reduces NO3– to N2 using fixed carbon (biomass)

• Denitrification can also be mediated by chemotrophs such as Thiobacillus denitrificans, which uses sulfide (H2S or pyrite) as a substrate.

• N2 from denitrification becomes overpressured in water as dissolved nitrogen gas

• anaerobic waters with low nitrate concentrations (NO3– limited), denitrification to

N2 gas may not be complete, resulting in the production of N2O gas.

Nitrate reduction back to N2

Nitrate Cycle

UREA HYDROLISIS AND VOLATILIZATION OF AMMONIA

NH3 (gas) CO (NH2) NH3 NH4

+ NO3-

15NH3 + 14NH4+ (aq) 14NH3 (gas) + 15NH4

+ (aq)

Isotope fractionation factor = 1.034

DENITRIFICATION

4NO3- + 5CH2O + 4H+ 2N2 + 5CO2 + H2O

14NO3- + 5FeS2 +14H+ 7N2 + 10SO4

- + 5Fe+2 + H2O

15NNO3-N2 ~ 15 to 20 permil

18ONO3-H2O ~ 8 permil

Isotope Data in Nitrate of Different Origins

Groundwater flow systemRef: Wassenaar, L. 1995. Applied Geochem. 10:391-405

Nitrate distribution in mg/L as NO3-

Septic system plume based on Na concentration

From Aravena, R., Evans, M.L., and Cherry, J.A. 1993. Ground Water, 31: 180-186

Nitrate Concentration in mg/L as NO3-

15N data (‰) in Nitrate

Nitrate distribution in mg/L as N

Ref:Aravena, R and Robertson, W. 1998. Ground Water, 36: 975-982

Oxygen (o) and DOC () concentration profiles

15N (o) and nitrate () concentration profiles

Isotope enrichment trend showing denitrification

Chemical and Isotope Depth Profiles

WHY RIPARIAN ZONES ARE IMPORTANT

• Nitrate is a major groundwater pollutant in agricultural landscapes

• Riparian zones act as a buffer zones to attenuate nitrate associated to contaminated groundwater discharging in rivers and lakes

Conceptual Groundwater Flow Regimes in Riparian Zones

Geological Cross Section of Study Area

Ref: Cey E.E., Rudolph, D.L., Aravena, R., and Parkin, G et al., 1999. Journal of Contaminant Hydrology, 37: 45-67.

Buffer strip

Stream

Manure Spreading

Instrumentation transect perpendicular to the stream

Nitrate vs 15N Data

15N vs 18O data

Nitrate vs DOC