Chapter 12: The Global Cycles of Nitrogen and Phosphorus

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Chapter 12 The Global Cycles of Nitrogen and
Phosphorus
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Nitrogen Unique among nutrients in many ways

• The most commonly limiting nutrient in
  terrestrial ecosystems
• Most abundant form on earth N2 gas (78 of
  atmosphere by volume) - is unavailable to plants
• Valence states from -3 (NH3) to 5 (NO3-)
• No mineral source
• SOM stores nearly all soil N
• Atmosphere is main reserve, but unavailable must
  be fixed

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Nitrogen Unique among nutrients in many ways

• Very low soil available pools relative to uptake
  - that is why it is most commonly limiting
• Volatile phases (NH3, N2O, N2)
• Can be taken up as cation (NH4) or anion (NO3-)
  for
• Assimilating NH4 costs 2-5 of plant energy,
  NO3- costs 15
• Deposition can be major input in polluted areas
• Form in plants proteins, amino acids

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Nitrogen Cycling in soils Biologically controlled
Atmospheric Deposition
N2 fixation
N2, N2O
Plant N
Litterfall Root turnover
Uptake
Denitrification
Uptake
Mineralization
Organic N
NH4
NO3-
Nitrification
Immobilization
Leaching
Immobilization
Clay-fixed NH4
Includes mostly microbial (biotic) but also
soime abiotic immobilization
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The global N cycle (Fig 12.2)

• Atmosphere contains largest pool (3.9 x 1021 g)
• Relatively small amount in terrestrial biomass
  3.5 x 1015 g)
• Relatively small amount in soil organic matter 95
  to 140 x 1015 g)
• Mean CN ratios
• Biomass 160
• Soil organic matter 15
• N produces much more C per unit input in biomass
  than in soil

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The global N cycle (Fig 12.2)

• All N in terrestrial system was derived from the
  atmosphere
• N fixation
• Estimated at 140 x 1012 g yr-1
• 10 kg ha-1 yr-1 average
• Not distributed uniformly - high rates under N
  fixers (clover, lupine, alder, Ceanothus)
• About 40 x 1012 g yr-1 in ag systems (e.g.,
  soybeans)
• Asmymbiotic (free-living) N fixation ranges from
  1 to 5 x 1012 g yr-1

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The global N cycle (Fig 12.2)

• All N in terrestrial system was derived from the
  atmosphere
• Lightning (lt3 x 1012 g yr-1)
• Atmospheric deposition
• Currently 100 x 1012 g yr-1
• Pre-industrial lt20 x 1012 g yr-1
• Mean residence time for N in terrestrial system
  is 700 yr
• Mean residence time for soil N is gt100 yr
• Thus, N cycles through both the atmosphere and
  soil at a slower rate than C

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The global N cycle (Fig 12.2)

• N needed for terrestrial production
• Assuming net primary production of 60 x 1015 g
  yr-1
• Assuming mean CN of primary production is 50
• Need about 1200 x 1012 g N yr-1
• N fixation supplies only about 12 of this (140)
• The remainder must come from recycling, as
  discussed earlier

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The global N cycle (Fig 12.2)

• In total, approximately 240 x 1012 g N are
  delivered to the earth per year
• Lightning lt3 x 1012 g yr-1
• Atmospheric deposition 100 x 1012 g yr-1
• N2 fixation 140 x 1012 g N yr-1
• Of the total, 60 is caused by humans
  (fertilization, N fixation, pollution), 40 is
  natural
• VG, global N cycle, Fig 12.2, p. 386

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The global N cycle (Fig 12.2)

• Atmospheric deposition
• Currently 100 x 1012 g yr-1
• Pre-industrial lt20 x 1012 g yr-1
• Fossil fuel combustion 20 x 1012 g yr-1

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The global N cycle (Fig 12.2)

• Agricultural activities gt 80 x 1012 g yr-1
• Mainly fertilization
• The Haber process
• 3CH4 6H2O -gt 3CO2 12H2
• 4N2 12H2--gt 8NH

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The global N cycle (Fig 12.2)

• In the absence of N removal, earth would contain
  a large amount of N in a very short time
• River export
• Rivers export 36 x 1012 g yr-1 - - but this may
  be too low, does not include particulate, DON
• About half of current river export is
  human-induced

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• N fertilization
• An absolute necessity for crop production
• A problem needing a solution

• Groundwater nitrate buildup a problem in ag
  areas, mostly because of fertilization
• N fertilization is a special problem
• Nitrogen will not remain in ionic form on soil
  exchanger for prolonged periods
• NH4 is consumed by plants, heterotrophs when N
  is limiting

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• N fertilization
• Available N does not remain elevated for long
• Frequent refertilization is necessary

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N fertilization

• Fertilizer Efficiency
• Defined as percentage of added fertilizer that is
  actually used by the target plants
• Generally 5-40 for N

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N fertilization

• Why is N fertilizer efficiency so low?
• Things we cannot help
• Microbial immobilization (most important for N)
• Abiotic N fixation in organic matter
• Things we can help
• Bad timing get it in when roots grow
• Incorrect amounts
• Too low feeds microbes, which are most efficient
  competitors
• Too high in the case of N causes nitrate leaching
  losses

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N fertilization

• Excessive N fertilization leads to acidification
  and groundwater NO3- pollution
• Balancing N fertilization rates and timing with
  crop needs is a major problem
• We must fertilize with N to eat!

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Denitrification

• The microbially-mediated reduction of nitrate
  (NO3-) to N2, N2O, NO
• NO3- carbohydrates --------gt CO2 N2, NO, N20
  (mostly N2)
• Required No O2, presence of NO3- carbohydrates
• Denitrification is the major mechanism of N loss
  from desert/range soils.
• Global estimates 13 to 233 x 1012 g yr-1
• Major uncertainty!!

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Fire effects on N

• N gasified (not volatilized) to NH3, NOx, and N2
  during fires
• gt90 N in whatever burns is lost as gas
• Globally, biomass burning may have contributed 50
  x 1012 g N yr-1 according to the book - probably
  higher now

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Fire effects on N

• N gasified (not volatilized) to NH3, NOx, and N2
  during fires
• gt90 N in whatever burns is lost as gas
• Globally, biomass burning may have contributed 50
  x 1012 g N yr-1 according to the book - probably
  higher now

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Global N cycling

• Balancing the global N cycle, they ignore
  recycling - e.g.
• NH3 from fires rapidly re-absorbs in terrestrial
  environment
• Natural emissions of NOx from soils (how is this
  determined?)
• Tables 12.1 and 12.2

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Global N cycling Oceans

• Oceans receive
• From rivers 36 x 1012 g N yr-1
• From N2 fixation 15 x 1012 g N yr-1
• From atmospheric deposition 30 x 1012 g N yr-1
• Note that while rivers are a rather small
  proportion of terrestrial N cycle, they are 40
  of input to oceans

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Global N cycling Oceans

• Surface ocean contains very little inorganic N
• Deep oceans contain large amounts of inorganic N
  
• (570 x 1012 g N yr-1) from organic matter
  decomposition
• Permanent burial of N in sediments is small
• Most N sent deep eventually denitrifies and is
  lost
• Denitrification fluxes may be large 110 x 1012
  g N yr-1

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Nitrous oxide N2O

• Greenhouse gas - 300 x as effective as CO2
• Increasing at an annual rate of 0.3
• The only significant sink is stratospheric
  destruction
• Soils may also constitute a small sink
• Mean residence time in atmosphere is about 120 yr
• Sources are poorly known
• Natural
• Oceans nitrification from deep sources
• Soils - mostly tropical wet soils
• Table 12.3

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Nitrous oxide N2O

• Greenhouse gas - 300 x as effective as CO2
• Increasing at an annual rate of 0.3
• Greenland ice cores show it has been much lower
  (Fig 12.5)
• The only significant sink is stratospheric
  destruction
• Soils may also constitute a small sink
• Mean residence time in atmosphere is about 120 yr
• Table 12.3

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Nitrous oxide N2O

• Sources are poorly known
• Mostly from denitrification and nitrification in
  soils
• Natural (10 x 1012 g yr-1)
• Oceans nitrification from deep sources (4 x 1012
  g yr-1)
• Soils - mostly tropical wet soils (6 x 1012 g
  yr-1)
• Anthropogenic (5.7 x 1012 g yr-1)
• Cultivated soils (3.5 x 1012 g yr-1)
• Biomass burning (0.5 x 1012 g yr-1)
• Industrial sources (1.3 x 1012 g yr-1)
• Cattle and feedlots (0.4 x 1012 g yr-1)
• Table 12.3

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Nitrogen Saturation

• N-saturation
• Many definitions. We will use
• N leaching is approximately equal to N
  deposition
• N leaching is nearly all NO3-, with some organic
  N
• N deposition includes NH4, NO3-, organic N
• Very rare in forest ecosystems because most are
  N-limited
• Has been found in polluted sites (near farms,
  much of Holland, Smoky Mountains), especially
  with low N uptake (old forests, coniferous
  vegetation)
• Also found in
• Red alder (too much fixation) and
• Some naturally N-rich sites (e.g., Turkey Lakes,
  ONT)

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Nitrogen Saturation

• The level of N deposition that will produce
  N-saturation varies enormously with tree
  species and age (i.e., N uptake)
• The next figure shows the range of N uptake in a
  variety of forests in the Integrated Forest Study
• Smokies Tower, Becking and Beech sites have both
  high deposition and low uptake therefore they
  are N-saturated
• The same rates of deposition in the Duke Loblolly
  site would probably produce faster growth but no
  N-saturation

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Nitrogen Saturation

• NO3- leaching in saturated sites is highly
  dependent on deposition rates, plant and
  microbial uptake
• Unlike the case with S, N will NOT be retained
  with higher N inputs once the site is
  N-saturated
• There is no mechanism for N retention at higher
  inputs
• Further N can be retained after N-saturation
  only by
• Adding high CN ratio material to soil (more
  microbial uptake)
• Planting younger or higher N-demanding trees
  (more
• plant uptake)

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Nitrogen Saturation

• If N deposition is reduced, NO3- leaching usually
  decreases quickly and sharply (no bleeding effect
  like for SO42-)
• N retention is controlled by completely different
  factors and behaves completely differently with
  changes in N input than S retention is with
  changes in S input!!

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The Global Phosphorus Cycle
No significant gaseous component Atmospheric
transport as dust and particles is small (1 x
1012 g yr-1) Major source of available P is not
created by microbial reactions, but by chemical
reactions Apatite in parent material is he major
primary source (in contrast to N, where there is
no major mineral source)
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Characteristics of the Phosphorus Cycle

• Second most commonly limiting (second on fert
  bag)
• Taken up in anion form (H2PO4-, HPO42- at higher
  pH)
• Both soil mineral (apatite) and organic sources
  are important
• Deposition is unimportant
• Strongly retained by adsorption to soils
• Forms in plants ADP, ATP (plant energy
  currency)

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Characteristics of the Phosphorus Cycle

• Leaching unimportant
• Particulate loss (sediment in streams, erosion)
  is major pathway for loss from terrestrial
  ecosystems
• Leaching lt deposition (net gain) nearly always
• Due to biological uptake when N is limiting
• Due to adsorption and precipitation alway
• Large, inorganic adsorbed or apatite pools
• Small or large "available" soil pools

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Characteristics of the Phosphorus Cycle

• Fertilization
• Large effect of P fixation
• Adsorption to Fe, Al hydrous oxides so strongly
  that P is unavailable
• Common in Andisols (volcanics) and Oxisols
  (tropical highly oxidized soils)
• Duration of increases in soil available P depend
  largely on P fixation
• Long-term in non-P-fixing soils
• Short-term in P fixing soils

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Phosphorus Fertilization
P-fixing soils
Non-P-fixing soils
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Characteristics of the Phosphorus Cycle

• CP ratios
• Mineralization when lt2001
• Immobilization when gt3001

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Phosphorus in the Oceans

• Main input is via rivers 21 x 1012 g P yr-1
• Only 10 of this is ortho-P
• Remainder is adsorbed to particles, unavailable
  to marine biota
• What ortho-P enters oceans is precipitated mostly
  as apatite
• Ca5(PO4)3OH lt--gt 5Ca2 3HPO42- 4HCO3- H2O
• Ksp 10-58
• So ortho-P concentrations are approx. 3 x 10-6 M

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Phosphorus in the Oceans

• Turnover of P in surface oceans is days
• 90 of P taken up is recycled
• Some goes deep and is entrained in sediments (2 x
  1012 g P yr-1)
• The latter is the long-term P sink
• Recycled to the terrestrial system only over very
  long-terms, with uplift
• Therefore, we have what agronomists call the
  phosphorus problem
• A finite supply, in essence, which is being
  slowly sequestered in the ocean
• Accelerated erosion increases this problem