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Nitrogen Cycle. Most of Nitrogen is in the atmosphere. 14 N = 99.6% 15 N = 0.4% Air is standard for d 15 N Range is –20 to +20 ‰. Assimilation and Dissimilation Nitrogen Cycling Pathways. Assimilation NO 3 and NH 4 uptake (NO 3 or NH 4 org N) N 2 Fixation (N 2 org N) - PowerPoint PPT Presentation
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Nitrogen Cycle
• Most of Nitrogen is in the atmosphere.
• 14N = 99.6% 15N = 0.4%• Air is standard for 15N • Range is –20 to +20 ‰
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Assimilation and Dissimilation Nitrogen Cycling Pathways
Assimilation
NO3 and NH4 uptake (NO3 or NH4 org N)
N2 Fixation (N2 org N)
Dissimilation
-Mineralization (org N NH4)
-Nitrification (NH4 NO3)
-Denitrification (NO3 N2)
-Annamox (NO2 + NH4 N2)
(in marine systems)
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15N Range in Terrestrial Plants
Non N-fixers = -6 to +6 ‰ N-Fixers = 0±3 ‰
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KIE during Nitrate Assimilation by Terrestrial Plants
• Large range of KIE values measured in culture experiments
• Range: - 3 to –24 ‰ (Mariotti, 1980)
-3 to –7 ‰ (Kohl and Shearer, 1980)
-0.3±0.6 ‰ (38 species) (Mariotti, 1982)
• Measured KIE depends significantly on culture conditions (growth rate, NO3 concentration)
• Typically, KIE increased with increased nitrate concentration.
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N2 Fixation in Plants
• Plants have 15N = 0±3 ‰ similar to 15N of N2 in air.
-molecular diffusion of N2 is air (soil pockets) yields KIE = -9 ‰. N2 diffusion in water has KIE = -1.3 ‰
-also you would expect a significant KIE during the breaking of a N-N bond to convert to NH4
-this seems to imply that aqueous N2 diffusion limits N2 fixation. What other explanation?
• Haber Process (N2 + 3H2 2NH3) yields commercial fertilizers with 15N of ~ 0±3 ‰
(high temp (>300ºC) and complete conversionlittle KIE)
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KIE during NO3 and NH4 Uptake
in Plankton Cultures
KIE ranged from 0 to –15 ‰ with a mean of ~ –7 ‰
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Effect of growth rate of KIE in cultures
As growth rate increased, KIE decreased. High concentrations (mmolar) of NO3 and NH4.
NO3 SubstrateNO3 Substrate
+
NH4 Substrate
A
A
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KIE during N2, NO3 and NH4 uptake
(same species Anabaena)
KIEs
N2 = -2 ‰
NO3 = -11 to –13 ‰
NH4 = -14 ‰
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KIE during NO3 assimilation under closer to ambient concentrations (uM vs mM)
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15N variations in marine plankton
Three Ocean Regimes
NO3 replete (I)
NO3 limited (II)
N2 fixation (III)
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15N of Plankton (PON) in Mixed Layer depends on NO3 Concentration
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Effect of KIE on 15N of NO3
KIE during nitrate assimilation enriches 15N of remaining NO3 pool
Both regions yields a KIE = -9.1 ‰. What is advantage of this approach?
Southern Ocean Sta Papa
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15N of NH4 in Delaware Estuary
KIE for NH4 uptake = -20±? ‰ (Rayleigh)
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Summary of KIE during N Assimilation
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Steps in nitrate assimilation
~15 ‰ offset between internal and external NO3 pools
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15N based N Budget at ALOHA and BATS
At ALOHA, N2 fixation contributes 50% of new nitrogen.
At BATS, N2 fixation is a negligible N source.
ALOHA near Hawaii BATS near Bermuda
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