Title: Conversion of dinitrogen gas N2 to ammonia NH3
1Biological Nitrogen Fixation
Conversion of dinitrogen gas (N2) to ammonia
(NH3) Availability of fixed N often factor most
limiting to plant growth N-fixation ability
limited to few bacteria, either as free-living
organisms or in symbiosis with higher
plants First attempt to increase forest growth
through N-fixation in Lithuania, 1894 (lupines in
Scots pine)
2Basal area accumulation of radiata pine in New
Zealand in relation to addition of lupines,
nitrogen fertilizer, or both.
3Biological N fixation Process
Reduction of atmospheric N2 to ammonia (NH3),
catalyzed by nitrogenase enzyme
N2 8e- 8H 2NH3 H2
Important process because N2 is relatively
inert -biologically unavailable- N N
4N-fixation requires energy input
- Reduction reaction, e- must be added (sensitive
to O2) - Requires 35 kJ of energy per mol of N fixed
(theoretically) - Actual cost 15-30g CH per g of NH3 produced
- Assimilation of NH3 into organic form takes
3.1-3.6 g CH
5Three Types of N-fixers Important in Forest Soils
Cyanobacteria Autotrophic N-fixers, protect
nitrogenase with specialized heterocyst
cells. Hetertrophic bacteria Free-living or
associative with rhizosphere. Use energy from
decomposing organic matter to fix N, protect
nitrogenase by rapidly converting O2 to CO2
through respiration. Symbiotic bacteria Plants
form nodules to house bacteria and provide C as
energy source (Rhizobium/Bradyrhizobium for
legumes, Frankia for non-legumes). Nodules
contain a form of hemoglobin which binds O2,
protecting nitrogenase enzyme.
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7Four Commonly Used Methods to Assess N-fixation
in Forests
- 15N methods (labeling, natural abundance)
- N accretion (total N measured at two points in
time) - Chronosequences (several sites of different age)
- Acetylene reduction assays
CH CH CH2 CH2
815N Methods
Atmospheric N is 99.6336 14N, 0.3664
15N Labeling Add 15N label to soil and measure
ratio of 14N/15N in N-fixing vs. non N-fixing
trees. N-fixers will be less enriched with 15N
and dependence on N-fixation can be accounted
for. Natural Abundance Soils are typically
naturally enriched in 15N compared to atmosphere,
but often very similar. Compare 15N/14N ratio
of N-fixing foliage to non N-fixing reference
plant foliage to get amount of N fixed from
atmosphere.
9N Accretion and Chronosequence Methods
Accretion Measure total N content of an
ecosystem at two points in time. Difference is
amount supplied by N-fixation
precipitation. Accretion method assumes
negligible outputs - underestimates N-fixation at
sites with high leaching or denitrification
rates. Chronosequence Several sites of
different ages used to represent trends within
one site over time. Chronosequence method
assumes all site factors are the same except age.
Similar to accretion method, but more
assumptions.
10N-fixation estimates for snowbrush from a
chronosequence (ovals), and at a nearby site
where accretion was followed within a single site
over time (squares)
11Acetylene Reduction Assays
If average N fixation per gram of nodule were
known, could estimate annual N fixation by
multiplying rate x nodule biomass/ha. Nitrogenase
enzyme will reduce acetylene to ethylene, both
of which are more easily measured than N2 and NH3
CH CH CH2 CH2
Conversion factor of acetylene to N is not
constant, averages between 3 and 10 mol C2H2 per
mol of N2
12N fixation rate by nodules of Himalayan alder
begins to rise after canopy photosynthesis begins
in the mornings, and tapers off as evening
approaches
13N fixation by nodules on red alder trees occurred
for a longer period at a coastal site in Oregon
with a mild, wet climate compared to a higher
elevation, inland site in Washington
14N Fixation is Favored By
- High light intensities
- Adequate moisture supplies
- Warm temperatures
- Adequate nutrient supplies (especially P)
In general, N fixation accelerates N cycling
rates and soil C increases under N-fixing species
15N-Fixing Plants Affect Cycles of Other Nutrients
Acceleration of production caused by N-fixation
increases demands for other nutrients Benefits
of N-fixing species may be limited by the
availability of other nutrients on poor soils P
availability under N-fixing species increases in
some cases and is depressed in others
16Relative changes in availability of N and P for a
range of Albizia proportions in mixture with
Eucalyptus. Supply of N increased with amount of
Albizia in the stand, but supply of P decreased
17Many Silvicultural Strategies Can Employ
N-Fixing Species
- N-fixing species can be used as the crop species
- N-fixers can be interplanted with crop species
to provide N - Can alternate rotations between N-fixers and
crop trees
18Height growth of Eucalyptus and Albizia was
similar for the first 6 yr in pure plots, then
diverged (left graph). In mixtures, Albizia
height growth was suppressed by Eucalyptus,
whereas Albizia increased height growth of
Eucalyptus
19Value of N-Fixing Trees in Forestry
- Depends on
- Market for N-fixer biomass
- Degree to which N-fixers compete with crop trees
- N-fixers increase crop-tree growth on N-poor
sites - N-fixers reduce growth of neighbor trees on
N-rich sites