N2
Cosmic abundance of the elements
12
H
log abondance
6
(atomes / 10 Si)
10
He
8
CO
Ne Si
S
N
6
Fe
Ca
Ni
4
2
Li
0
B
Sc
Pt
Be
Tc
-2
0
10
20
30
40
Pb
Th U
Pm
50
Mass number
60
70
80
90
100
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
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
amino N
R–NH2
elemental nitrogen gas
N–III
common fertilizer
organic nitrogen as amine, measured as TKN
Nitrogen redox
NO2–/N2
1
15
NO3–/N2
O2/H2O
0.8
10
0.6
–
NO3 /NO2
–
Fe(OH)3/Fe2+
5
0.2
NO3–/NH4
0
0
SO42–/HS–
-0.2
CO2/CH4
-5
H2O/H2
-0.4
-10
-0.6
5
6
7
pH
8
9
Eh (V)
pe
0.4
Essential nutrient
Elemental composition of algae
C106H263O110N16P(S)
Protein
:NN:
Fixing nitrogen for
photosynthesis
C106H263O110N16P(S)
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 from
biodegradation
R–NH2 + 2H +  NH3
NH3 + O2  NO3–
It’s all about getting NH3 and NO3–.
Human intervention
into the nitrogen cycle
NH3
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)
ΔG = –16.5 kJ/mol
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.
Essential nitrogen reactions
Degradation 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 reactions
Degradation 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.
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.
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 oxidation
Aerobic nitrification of ammonium:
NH4+ + 2O2  NO3– + H2O + 2H+
DG°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+
DG°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+
DG°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.
Nitrate reduction back to N2
Denitrification: 5CH2O + 4NO3– + 4H+  2N2 + 5CO2 + 7H2O
DG°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 Cycle
UREA HYDROLISIS AND
VOLATILIZATION OF AMMONIA
NH3 (gas)

CO (NH2)  NH3  NH4+  NO315NH
3
+ 14NH4+ (aq)  14NH3 (gas) + 15NH4+ (aq)
Isotope fractionation factor = 1.034
DENITRIFICATION
4NO3 + 5CH2O + 4H  2N2 + 5CO2 + H2O
-
+
14NO3 + 5FeS2 +14H  7N2 + 10SO4 + 5Fe + H2O
-
+
-
+2
e15NNO3-N2 ~ 15 to 20 permil
e18ONO3-H2O ~ 8 permil
Isotope Data in Nitrate of Different Origins
Ref: Wassenaar, L. 1995.
Applied Geochem.
10:391-405
Groundwater flow system
Nitrate distribution in mg/L as NO3-
From Aravena, R., Evans, M.L., and Cherry, J.A. 1993.
Ground Water, 31: 180-186
Septic system plume based on Na concentration
Nitrate Concentration in mg/L as NO3-
15N data (‰) in Nitrate
Ref:Aravena, R and Robertson, W. 1998. Ground
Water, 36: 975-982
Nitrate distribution in mg/L as N
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