Transcript BIOGEOCHEMISTRY OF NITROGEN

BIOGEOCHEMISTRY OF NITROGEN
I.
II.
III.
IV.
V.
Introduction
N-cycle, and the Biochemistry of N
Global N Patterns/Budget (Galloway et al. 1995)
Patterns of N at the HBEF
Inputs, Effects and Management of Anthropogenic
N in the Northeast
I. INTRODUCTION
Nitrogen is a difficult element to study. Nitrogen has many different species, phases
and oxidation states.
Reduced
Oxidized
-III
0
+I
+II
+III
+IV
+V
NH4+
N2
N2O
NO
NO2-
NO2
NO3-
NH3
(molecular N)
(nitrous
(nitrite)
(nitrogen
dioxide)
(nitrate)
oxide)
(nitric
oxide)
(g)
(g)
(aq)
(g)
(aq)
Org N
(g, aq, s)
(g)
Nitrogen is an interesting element because some pools (N2, Org N) are large and
generally unavailable.
N is an important element because:
1.
It is a macronutrient (protein);
2.
At elevated concentrations, it may cause adverse environmental effects (NH3,
NH4+, NO2-, NO3-).
II. N-CYCLE, AND THE BIOCHEMISTRY OF N
N Utilization
1.
Assimilatory - used for biosynthetic reactions (amino acid production), not
directly used in energy metabolism - All living organisms require N.
2.
Dissimilatory processes - Nitrogen is taken up in a particular form (oxidized or
reduced), for specialized reactions involving ATP production and excretion of a
N product. Dissimilatory N is not incorporated into the physical or biochemical
structure of an organism - Only a few specialized organisms can utilize
dissimilatory processes.
N Assimilation
Nitrogen in biomass largely occurs as the reduced oxidation state (-III), so this is the
energetically favored form of N. However, NO3- is generally preferred by plants.
This may be due to greater mobility of NO3-. Energy must be expended by plants or
microbes to extract NH4+ from soil sediments. Also competition of NH4+ with other
cations on enzymes.
Plants/microorganisms can commonly assimilate NH4+, NO3- in water or soil.
Organic N is rarely used as an N source. Some coniferous trees have been shown to
assimilate dissolved organic N.
N-cycle, and the Biochemistry of N (cont.)
If N is taken in as NH4+, it is directly used by organisms in biosynthesis.
If N is assimilated as NO3-, it must be reduced within the cell.
Two enzymes are involved:
1.
Nitrate reductase - contains molybdenum
NO3- + NADH + H+ = NO2- + H2O + NAD+
2.
Nitrite reductase
NO2- + 3NADH + 5H+ = NH4+ + 2H2O + 3NAD+
Some organisms have the unique characteristic to assimilate molecular N - nitrogen
fixation.
This process requires the enzyme, nitrogenase, which is a complex protein containing
iron, molybdenum and inorganic S as part of its structure.
The process is extremely energy-intensive, as you might expect, to break a triple
bond.
N  N
N-cycle, and the Biochemistry of N (cont.)
Only a few species of microorganisms can fix nitrogen. These include free-living
organisms (asymbotic, e.g. Clostribium, Azobacter, Azospirillum, and Anabena) and
organisms in symbiotic relationships with roots (e.g. Rhizobium, Frankia).
N2 + 10H+ + 8e- + nATP + nH2O = 2NH4+ + H2 + nADP + nH2PO4where:
n = 12 - 20 (exact number uncertain)
Ammonium assimilation occurs by two enzymatic routes:
1.
Glutamine synthetase
COO-
CONH2
CH2
CH2
CH2 + NH4++ATP =
CH2 + ADP + H2O
CHNH3+
CHNH3+
COO-
COO-
glutamate
glutamine
N-cycle, and the Biochemistry of N (cont.)
2.
Glutamate dehydrogenase
COO-
COO-
CH2
CH2
CH2 + NADH + H+ + NH4+ =
CH2 + NAD + H2O
C=O
CHNH3+
COO-
COO-
-ketoglutarate
glutamate
N-cycle, and the Biochemistry of N (cont.)
In addition, Glutamate synthase is used in plants and microorganisms to convert
amido-nitrogen of glutamine back to glutamate for amino acid systems.
Glutamate synthase
COO-
CONH2
COO-
CH2
CH2
CH2
CH2 + NADH + H+ +
CH2 =
C=O
CHNH3+
CHNH3+
COO-
COO-
COO-
-ketoglutarate
glutamine
glutamate
2 CH2 + NAD+
N-cycle, and the Biochemistry of N (cont.)
Mineralization
Mineralization is the decomposition of organic matter to inorganic matter. This
is accomplished by heterotrophic microbes. The release of N is generally
thought to be a by-product of the use of soil organic C as an energy source.
R - NH2 = NH3 + H2O = NH4+ + OHMineralization of organic matter is critical to the supply of nutrients to vegetation
in terrestrial environments (see Table).
Mineralization is directly related to the nitrogen content of soil and the
availability of organic carbon. Vegetation with high C/N in litter generally
shows low rates of mineralization in soil.
Urea
NH2
urease
C = O + 2H2O + 2H+
=
NH2
2NH4+ + H2CO3
Percentage of the annual requirement of nutrients for growth in the Northern
Hardwoods Forest at Hubbard Brook, New Hampshire, that could be supplied by
various sources of available nutrients*
Process
Growth requirement (kg ha-1 yr-1)
Percentage of the requirement that could by
supplied by:
Intersystem inputs
Atmospheric
Rock weathering
Intrasystem transfers
Reabsorption
Detritus turnover (includes return in throughfall
and stemflow)
N
P
K
Ca
Mg
115.4
12.3
66.9
62.2
9.5
18
0
0
1
1
11
4
34
6
37
31
69
28
67
4
87
0
85
2
87
*Calculated using Eqs. 6.2 and 6.3. Reabsorption data are from Ryan and Bormann
(1982). Data for N, K, Ca, and Mg are from Likens and Bormann (1995) and for P
from Yanai (1992).
N-cycle, and the Biochemistry of N (cont.)
Nitrogen Dissimilation
Nitrification - the oxidation of NH4+
NH4+ + 2O2 = NO3- + H2O + 2H+
Two different species of lithotrophic organisms are responsible for this reaction.
Nitrosomonas
NH4+ + 3/2 O2
ammonia oxidase
=
NO2- + 2H+ + H2O
This oxidation/reduction sequence is not direct but includes an electron transport
chain in which 1 mol of ATP is produced per mol of NH4+ oxidized.
This sequence is continued by the organism.
Nitrobacter
NO2- + ½O2
nitrite oxidase
=
NO3-
The electron produced from the oxidation of NO2- is also coupled with an electron
transport cycle producing 1 mol of ATP.
N-cycle, and the Biochemistry of N (cont.)
Nitrification can also be accomplished by heterotrophic bacteria.
Nitrification is an important process because many factors influence it and because it
converts nitrogen from an immobile form (NH4+) to a mobile form (NO3-).
Because the organisms which mediate nitrification reactions are specific
populations, they are easily disrupted.
1.
Lithotrophic organisms use inorganic C (CO2) to produce organic C
through the Calvin cycle. This process is very energy intensive so these
organisms have slow growth rates.
2.
Nitrifiers, require well-oxygenated conditions.
3.
Very sensitive to toxicants, trace metals.
4.
Sensitive to pH (< 6?).
N2O and NO are released via nitrification.
N-cycle, and the Biochemistry of N (cont.)
Denitrification
Denitrification is the process by which N is used as the terminal electron acceptor in
a reduction reaction.
This may be conducted by species: Pseudomonas, Bacillus, Vibrio and
Thiobacillus.
Because organisms favor O2 reduction due to energetics, denitrification only
proceeds under anaerobic conditions.
Organisms produce 2 mol ATP per mol NO3- reduced.
The process proceeds through an electron transport chain. The reductant is
generally organic matter, generally sugars or simple compounds (methanol used in
waste water treatment). Reduced sulfur compounds can also be used (sulfur,
sulfide). These electrons are transferred to the electron transport chain where the
reduction occurs.
In this process, NO3- is first reduced to NO2-.
NO3- + NADH + H+ = NO2- + NAD+ + H2O
Through this process, ATP is produced.
N-cycle, and the Biochemistry of N (cont.)
Subsequent reactions may occur:
NO2- + 2H+ + e- = NO + H2O
NO2- + 3H+ + 2e- = ½N2O + 3/2 H2O
NO2- + 4H+ + 3e- = ½N2 + 2H2O
NO + 2H+ + 2e- = ½N2 + H2O
½N2O + H+ + e- = ½N2 + ½H2O
The overall reaction to N2 is
C6H12O6 + 24/5 NO3- + 24/5 H+ = 6CO2 + 12/5 N2 + 42/5 H2O
The "leaky pipe" hypothesis suggests that trace gases, N2O and NO, are by-products
of nitrification and denitrification.
N-cycle, and the Biochemistry of N (cont.)
Mechanisms of N Immobilization
1.
Plant assimilation
2.
Microbial (thought to predominate)
bacteria
fungal
C5H7O2N
higher C:N
Critical C:N  20-25
Above, microbial growth is N limited
Little N leaching
Below, microbial growth is C limited
N leaching occurs
3.
Nitrification, distribution of NH4+, NO3Abiotic immobilization
Cation exchange X- - Na+ + NH4+ = X- - NH4+ + Na+
No significant mechanism for abiotic immobilization of NO3- (anion exchange
weak).
N-cycle, and the Biochemistry of N (cont.)
N-Volatilization
NH4+ participates in an acid-base reaction.
NH4+ = NH3 + H+
;
pKa = 9.1
NH3 also has the ability to volatilize.
NH3 aq = NH3
g
As a result, NH3 can volatilize, but the reaction is only quantitatively important
under high pH conditions.
Forest soils are generally acidic, so NH3 volatilization is an insignificant process.
In agricultural lands, application of fertilizer (manure) can result in high pH
conditions and significant NH3volatilization.
N-cycle, and the Biochemistry of N (cont.)
Stable Isotopes of N
Stable isotopes of N can provide insight into biogeochemistry.
1.
Addition tracer experiments
2.
Natural abundance observations
There are two stable isotopes of N:
air composition
15N
=
0.0037
14N
=
0.9963
15N/14N
= 1/272
Nitrogen isotopes are reported in values of per mil relative to atmospheric air.
Delta notation
15
15

N
N 

 14

N sample 14 N std 
15

 N =
x 1000
15


N


14
N std


N-cycle, and the Biochemistry of N (cont.)
Let's consider an example:
sample
15N
= 0.00371
std
15N
= 0.00370
 0.00371 0.00370 
 0.99629  0.99630 
15
 N sample = 
 x 1000
0.00370


0.99630


= 2.7 % 0
Note that this example suggests that the sample is slightly enriched in 15N relative to
the standard (+ sign).
A negative value would indicate that the sample is depleted relative to the standard.
In most terrestrial ecosystems, 15N values range from -10 to +15 %o. In absolute
abundance, this represents a range of 0.3626 to 0.3718 atom % 15N.
Rule of thumb  Organisms prefer the light isotope (14N) over the heavy isotope
(15N) in transformations (see figures).
t2
t1
SOM
SOM
δ 15N
NO3
Isotope Enrichment Effect on Ammonium
SOM
N mineralization
NH
+
4
 15N
NH
+
nitrification
4
-
NO 3
N-cycle, and the Biochemistry of N (cont.)
The 15N of a cumulative product is always lighter than the residual reactant.
Consider denitrification. Say that this process fractionates by 5, 10, 20 %o from an
initial NO3- of 0 %o. The first bit of product (N2) is lighter than the reactant by the
fractionation factor. As the reaction proceeds to completion, the product becomes
progressively heavier until, at the end, it reaches its initial composition. The reactant
also becomes progressively heavier until it is used up.
Several factors influence the degree of fractionation:
1.
Specific process.
2.
Size of the pool. Large pools exhibit large fractionation, small pools exhibit little
fractionation.
3.
Temperature.
N-cycle, and the Biochemistry of N (cont.)
N Fractionation
Process
N fixation
Qualitative Charge
Literature
small
-3 to +1 %o
microbial
small
-1.6 to +1 (-0.52)%o
plant
small
-2.2 to +0.5 (-0.25)%o
Mineralization
small
-1 to +1 %o
Nitrification
large
-12 to -29 %o
Volatilization
large
> 20 %o
Sorption/desorption
small
1 to 8 %o
Denitrification
large
-40 to 5 %o
Assimilation
N-cycle, and the Biochemistry of N (cont.)
Observations in the Literature
Terrestrial Ecosystem Compartments
1.
2.
3.
Plants are slightly depleted.
Organic soils are enriched.
Mineral soils are more enriched.
The 15N of plants is similar to what they assimilate (little fractionation). Variations in
plant 15N are due to:
1.
rooting depth;
2.
NO3- vs. NH4+ preference;
deeper roots  more enriched
NH4+  more enriched
Rates of N Cycling
In general, 15N increases in ecosystems with increased rates of N cycling due to
fractionation associated with nitrification and NO3- loss.
This is sometimes quantified as an enrichment factor (15N leaf See observations from Walker Branch, TN and Hubbard Brook.

15N
soil).
3
15N (per mil)
2
A
Reference
Clear-cut
1
0
-1
Streamwater [NO3-] mol L-1
-2
-3
1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000
400
300
200
100
B
0
1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000
Pardo et al., 2002 (Can. J For Res)
N-cycle, and the Biochemistry of N (cont.)
Food Web Studies
Food web studies show an enrichment in 15N.
N isotope scientists like to say you are what you eat, plus 3 %o.
See figure.
Use of 18O and 15N as a Tracer of Ecosystem N Retention
There are some drawbacks to using 15N as an ecosystem tracer due to its relatively
narrow range. 18O associated with NO3- offers additional information as a tracer.
Durke et al. (1994) used 15N and 18O together to evaluate the retention of atmospheric
NO3- to forests in Germany.
See tables.
16
12

 N
Carbon and Nitrogen
Stable Isotopes in
Oneida
Lake Food Web from
Mitchell
et al. (1996)
14
10
8
6
-35
-30

 C
SEDIMENT
SESTON
DAPHNIA
ZEBRA MUSSEL FLESH
YELLOW PERCH
SHAD
WALLEYE
-25
Adult Walleye
Adult Yellow Perch
Young-of-the-Year Fish
(e.g., Yellow Perch and Gizzard Shad)
Benthic Macroinvertebrates
Pseudofeces
Zooplankton
(e.g., Daphnia)
Zebra Mussels
Benthic Algae
Phytoplankton
N-cycle, and the Biochemistry of N (cont.)
Table 1. Characteristics of sites studied.
Atmospheric input1
Site
No.
Site Condition*
Springwater output
NO3(mmol m-2 yr-1)
NH4+
(mmol m-2 yr-1)
NO3concentration2
(mol 1-1)
Total NO3output3
(mmol m-2 yr-1)
1
Healthy
46
92
53
29
2
Declining, peaty
soil
57
107
43
30
3
Slightly declining
74
108
98
69
4
Healthy
74
81
225
90
5
Slightly declining,
limed
45
91
361
90
6
Slightly declining,
limed
60
81
191
96
7
Strongly declining
88
96
167
117
8
Strongly declining
36
59
274
137
SEE NEXT PAGE FOR FOOTNOTES
N-cycle, and the Biochemistry of N (cont.)
Site conditions, atmospheric inputs of nitrogen to the watersheds, and NO3- output
characteristics of eight forest springs in the Fichtelgebirge (northeast Bavaria,
Germany).
*Definitions: slightly declining, single trees affected by needle yellowing and crown
thinning; strongly declining, all trees affected.
1Extrapolated
from measurements of throughfall sampled between 15 April and 15
December 1992 with ten funnels per site.
2Volume-weighted
3Modelled
mean of monthly measurements in 1991 and 1992.
from volume-weighted mean NO3- concentration and seepage.
N-cycle, and the Biochemistry of N (cont.)
Table 2. Nitrate in spring water
Output on NO3-atm in spring water
Site No.
Fraction of total NO3- output
Absolute flux
Fraction of NO3- input
(%)
(mmol m-2 yr-1)
(%)
1
25
8
16
2
46
15
24
3
28
21
26
4
16
18
20
5
15
16
30
6
14
16
23
7
44
54
59
8
30
45
114*
*This value (>100% recovery) could have been caused by errors in the input-output balance,
or by temporal NO3-atm storage in the aquifer.
III.
GLOBAL N PATTERNS/BUDGET
Across the Earth, N largely occurs as N2 in the atmosphere (78%) and in the ocean
and in soil.
Nitrogen is divided into two broad classes:
1.
2.
Reactive
-
NOy = NOx (NO + NO2) +
any oxidized N with a single atom of N
-
NHx = NH4+ + NH3
-
organic N
Unreactive -
See tables.
N2
-
N2O
-
organic N (soil)
Global N Patterns/budget (cont.)
Table 1. Estimates of the active pools in the global nitrogen cycle.
million tonnes N
Air
N2
N2O
3 900 000 000
1 400
Land
Plants
Animals
of which people
Soil organic matter
of which microbial biomass
15 000
200
10
150 000
6 000
Sea
Plants
Animals
In solution or suspension
of which NO3--N
of which NH4+-N
Dissolved N2
300
200
1 200 000
570 000
7 000
22 000 000
Global N Patterns/budget (cont.)
Table 2. Production of combined nitrogen gases by land, sea and air.
Gas
Atmospheric-stock,
million tonnes N
Residence time
in atmosphere
Annual production, million
tonnes N per year
NH3
<1
6 days
54 ± 8
N2O
1400
170 years
14 ± 7
NOx
<1
5 days
48 ± 15
Global N Patterns/budget (cont.)
Table 3. Distribution of nitrogen (g m-2) between plant biomass and aboveground litter and plant uptake in difference bioclimate zones. Calculated from
Baztlevich and Soderlund and Svenson.
Biomass
(g N m-2)
Litter
(g N m-2)
Turnover time
in litter (yr)
Plant uptake
(g m-2 yr-1)
Polar areas
12
106
66
1.6
Boreal areas
15
76
12
6.4
Sub-boreal areas
57
10
1.0
10.0
humid
137
22
14.5
semi-arid
22
5
11.0
arid
13
3
4.3
Subtropical areas
73
9
0.4
21.2
humid
161
18
37.8
semi-arid
68
10
20.7
arid
22
3
11.0
Tropical areas
165
6
0.2
29.3
humid
287
9
46.4
semi-arid
88
6
21.6
arid
8
1
3.4
Total terrestrial
94
23
1.3
17.2
Global N Patterns/budget (cont.)
Preindustrial N budget
The transfer of reactive to unreactive N was balanced.
N2, N2O produced by denitrification in oceans and soil.
NH3 is released by volatilization.
NH4+ = NH3(aq) + H+
;
pKa = 9.3
NH3(aq) = NH3(g) 
This process occurs only under high pH conditions.
NH3 is released by burning of plants.
NH3 is very reactive and has a short residence time in the atmosphere.
NH3 + H2O = NH4+ + OH-
Global N Patterns/budget (cont.)
NO can be formed by
1.
Oxidation of N2 by lightning;
2.
Soil microbes;
3.
Burning of biomass.
NO, NOx are very reactive and have a short residence time in the atmosphere.
In the preindustrial world, N inputs were largely retained where they were deposited.
Nitrogen is a tightly conserved element in terrestrial environments because it is the
growth limiting nutrient.
Global N Patterns/budget (cont.)
NH4+ - relatively immobile form of N
a.
b.
Strongly assimilated by biota due to energetics;
Abiotically retained on soil cation exchange sites.
NO3- - relatively mobile from of N
a.
No significant mechanism of abiotic retention.
Nitrification is a key process regulating the mobility of N.
Riverine fluxes of N are thought to be 75 - 120 kg N/km2-yr and this is thought to
largely occur as particulate organic N.
Galloway et al.: N Fixation: Anthropogenic Influence
(Tg N/yr)
Galloway et al.: N Fixation: Anthropogenic Influence
(Tg N/yr)
Global N Patterns/budget (cont.)
Anthropogenic Sources
Human activity has had a profound effect on the N cycle.
Three processes largely contribute to this disturbance, through anthropogenic
nitrogen fixation.
1.
Energy production - under high temperature combustion processes, unreactive
nitrogen is converted to reactive nitrogen by two processes.
a.
Thermal NOx
1000oK
N2 + O2
b.

2NO
Fuel NOx - the oxidation of organic N in fuels
Natural gas - very low 0%
Coal - up to 3%
Both thermal and fuel NOx can be significant, but fuel NOx is often the dominant
source.
Global N Patterns/budget (cont.)
2.
Fertilizer-Most anthropogenic fertilizers are either NH3 or urea produced from
NH3.
This material is produced by the Haber process.
4N2 + 12H2 = 8NH3
This is a very energy intensive process if natural gas is the energy source for H2,
as it usually is.
3CH4 + 6H2O + 4N2 = 3CO2 + 8NH3
3.
Production of legumes and other crops allows for the conversion of N2 to
reactive N by increasing biological nitrogen fixation.
Legumes include:
Soybeans
Ground nuts (peanuts)
Pulses (lentils)
Forage (alfalfa, clover)
(Tg N/yr)
Galloway et al.: N Fixation: Anthropogenic Influence
(Tg N/yr)
Global Population and Reactive Nitrogen Trends
200
150
4
Natural N Fixation
100
2
50
0
1850
1870
Population
1890
1910
Haber Bosch
From Galloway et al. 2002. In review.
1930
C-BNF
1950
1970
Fossil Fuel
1990
Total Nr
0
2010
Tg N yr-1
Human Population (billions)
6
Global N Patterns/budget (cont.)
All three categories of anthropogenic nitrogen fixation have increased, but most
significant is fertilizer consumption. The rate of fertilizer consumption is increasing
particularly in Asia.
There are two critical questions in response to this change.
1.
What is the fate of this fixed N; and
2.
What are the effects of this increase in fixed N?
IV.
PATTERNS OF N AT THE HBEF (HBEF W6, CPW, BNA)
Bormann et al.
(1977)
1965-76
1990-95
464
524
549
Dry Deposition
52
54
N-fixation
36
36
464
560
639
286
310
64
38
38
0
0
286
348
102
Biomass
643
1328
167
Forest Floor
550
0
0
314
314
1430
56
Inputs
Bulk Precipitation
Total Inputs
Outputs
Streamwater
DIN
DON
Denitrification
Total Outputs
Changes in Pools
Mineral Soil
Net Retention
1014