Nitrogen Fractionation in Primary Producers Flashcards
A plant with a N isotopic signature of ~ 0ppt is more likely to be:
N-fixing
nitrate user
carnivorous
What would be the expected fractionation (big delta) of a nitrate-limited plant that cannot use atmospheric N?
How does N fixation change the isotopic signature of particulate OM? (heavier, lighter, no effect?)
Animal A has d15N = 6.5 ppt
Animal B has d15N = 10 ppt
What trophic level (herbivore, carnivore) is animal A? B?
If Animal A has d13C = -25 ppt and animal B has d13C = -10 ppt
what can be concluded about these animals diets?
What would you expect the isotopic signature to be of an animal’s DIET if the animal has a d15N = 8 ppt?
Animal A (d15N = 10 ppt) which eats a plant diet (d15N = 5 ppt) would excrete waste with an isotopic signature that is heavier/same/lighter than its body signature?
If animal A has d13C = -25 ppt and animal B has d13C = -10 ppt, then animal A is heavier/lighter than animal B?
Why does d15N vary among terrestrial plants?
plants have evolved different mechanisms for uptaking nitrogen from the environment depending on what is available - N2, NH4, NO3
these types of N have their own isotopic signatures and the mechanism by which a plant takes them up are enzymatic processes which have fractionation (changes to the d15N)
Plants can only use NH4 (ammonium) in their tissues, so nitrate or N2 needs to be converted = fractionation
T or F: the enzymatic processes which facilitate plant uptake of nitrogen have a huge range of E(epsilon = a single enzymatic process)15N
true!
N2 fixation has E = 0-6 ppt v. nitrification (NH4 oxidation into N2O and NO) E = 40-60 ppt
What are the 2 major categories for studying nitrogen in terrestrial plants?
- N2 fixers
- nitrate users (non-fixers) - typically limited by NO3
What is the equation for d15N of a plant?
D(source-plant) = d15Nsource - d15Nplant
rearrange to solve for
d15Nplant = d15Nsource - D(source-plant)
Compare the d15N of a nitrate-using plant that is NOT limited by nitrate to that of a nitrate-using plant this IS limited by nitrate given that:
D(NO3-plant) = 19 ppt
d15NNO3 = 5 ppt
D(source-plant) = d15Nsource - d15Nplant
d15Nplant = d15Nsource - D(source-plant)
plant NOT limited for NO3:
- not limited means that the substrate has not run out and is not all used up (product does not equal substrate)
d15Nplant = 5 - 19
= -14 ppt
plant limited for NO3:
- limited means that the substrate has been used up and product = substrate so D = 0
d15Nplant = 5 - 0
= 5 ppt
*** A nitrate-using plant that is limited by NO3 has a much HEAVIER d15N than one that is not because all of the light isotopes have been used up and what is left is only heavy
Compare the d15N of a N2-fixing plant that is NOT limited by N2 to that of a N2-fixing plant this IS limited by N2 given that:
D(N2-plant) = 2 ppt
d15NN2 = 0 ppt
D(source-plant) = d15Nsource - d15Nplant
d15Nplant = d15Nsource - D(source-plant)
plant NOT limited for N2:
- substrate not all used up
d15Nplant = 0 - 2
= -2 ppt
plant that IS limited for N2:
- substrate all used up so the product = substrate
d15Nplant = 0 - 0
= 0 ppt
*** A N2-fixing plant that is limited by N2 has a slightly heavier d15N than one that is not limited, but very close
Give examples of N2-fixing plants and nitrate-using/non-N2-fixing plants
N2-fixing:
- red alder
- scotch broom
- bracken fern
- lupine
- legumes
non-N2-fixing:
- canola
- coastal Douglas-fir
What are the rules for N fractionation in plants?
same as for kinetic fractionation and photosynthesis:
- lighter isotopes react faster and first
- F is sensitive to supply v. demand (mass balance when reactions are limited, efficient, or using up all substrate)
What are the 2 general rules of thumb for N fractionation in plants?
- N2-fixers have d15N = 0 ppt
- non-N2-fixers have d15N»_space;» 0 ppt
Discuss the example of lemur hairs and howler monkeys and how its related to d15N application contributions of plants to animal diets
the d15N of plant material was studied from foliar tissues (leaves)
the d15N of animal was sampled from lemur and howler monkey hairs
howler monkeys and lemurs are in the same trophic position (both mainly herbivores)
the results showed that as the proportion of the animal diets increased in legume (N2-fixers) consumption, the d15N in the animals was lower (ppt) = it’s the variation in diet between individuals that is causing the change in d15N, not trophic levels
What causes variation in d15N of phytoplankton?
- the d15N of the source (is it NO3, N2, or NH4?)
- the degree of N limitation (supply v. demand)
- the demand for N is created by photosynthesis
Which form of nitrogen is most important for most phytoplankton?
nitrate NO3
How is nitrogen distribution varied in the ocean? Explain why these phenomena occur
nitrate is varied in marine ecosystems with:
limited supply in mid-ocean gyres
- lack of upwelling, most nitrate sinks to bottom of the ocean
seasonal upwelling in coastal upwelling zones causes fluctuations in [NO3] as well as terrestrial inputs of nitrate
high [NO3] in polar seas:
- phytoplankton are limited by other factors such as light, temperature, and trace metals, so there are fewer that exist to use up the NO3
What is an application for studying marine nitrate and phytoplankton ?
paleooceanography - look at sediment cores (nitrate sinks to bottom of ocean and layers) to conduct isotope studies to create historical maps of isotopic variation
What causes nitrate variation in marine systems?
the balance between:
- supply:
- coastal zones: upwelling and terrestrial run off
- mid-ocean gyres: no upwelling, most nitrate sinks - demand:
- how many phytoplankton are present
- south polar seas - phyto limited by light and temp
- N polar seas - phyto limited by trace metals and tempW
which coasts typically have the most upwelling?
eastern coasts of some continents - ex. Peru
How come phytoplankton can still exist in mid-ocean gyres if they have no steady source of NO3 input?
the phyto out there fix N2 (ex. cyanobacteria)
Briefly describe upwelling
upwelling is the process by which cold, nitrate rich, O2-depleted ocean water from the bottom is brought up to the surface for phytoplankton to use up and increase primary productivity
caused by winds and generally occurs on the east coast of some continents
What causes variation in N2-fixer distribution and their rate of N2-fixation?
temperature - typically need warmth
availability of trace metals like iron
light for photosynthesis
Where do most N2-fixing phytoplankton exist?
in mid-ocean gyres or inland seas
where iron is not limiting, but nitrate is
where temps are warm enough and they can get enough light for photosynthesis
iron is limiting in N. seas, light and temp in S
How does isotope distribution vary?
the distribution of d15N in NO3 (substrate) is a mirror image of the distribution of d15N in POM (product)
Particulate Organic Matter (ex. phytoplankton, detritus, bacteria)
the variation in POM is caused by the variation in phytoplankton
Why would you expect coastal areas to be heavier in d15NNO3 and d15NPOM?
this is where NO3 is not limited, where terrestrial run off and upwelling occurs - supply is high and so demand can be high
lots of nitrogen can be taken up and respired = lighter isotopes leave behind heavier
phyto require N for making amino acids - high PP is associated with increased NO3 uptake until [NO3] are depleted at ocean surface
Describe the relationship between d15N of NO3 or POM and rate of primary production in the ocean
As primary productivity increases, more NO3 needs to taken up by phytoplankton (to make amino acids) until NO3 is depleted in the ocean
As primary productivity increases, phyto increase their uptake of NO3 –> making the d15NNO3 heavier until it is all used up;
–> as nitrate is assimilated into the phyto, the d15NPOM gets heavier
why is the water at the bottom of the ocean lower in oxygen than at the surface? how do bacteria deal with this?
no photosynthesis is occurring, so no input of O2 but organisms are continuing to use up O2 for respiration
bacteria have evolved mechanisms to extract oxygen from other sources - anaerobic ex. nitrate via denitrification
Explain denitrification and how it affects the nitrate availability even in upwelling zones and how this relates to the heavy d15N of NO3 and POM
bacteria use NO3 as an electron acceptor instead of O2 - basically they are extracting oxygen from NO3 instead of O2 - anaerobic conditions
so much of the nitrate at the ocean’s floor is used up by denitrifying anaerobic bacteria for respiration that only a small amount gets upwelled to the surface and these bacteria use up the light isotopes first so what does, is heavy
Where is dentrification in the ocean most common?
where deep ocean waters with low [O2] are upwelled to the surface - ex. Peru
where circulation is limited causing high temps and low [O2] - ex. Indian ocean
Describe the relationship between denitrification and d15NPOM
as [O2] in the water is increased, denitrification decreases because bacteria can use O2 to respire
In area where nitrate is primary N source and O2 is high, what factor most influences d15N in the ocean? where does this occur? how does it affect d15NPOM?
primary production most influences d15N
ex. coastal
creates moderate d15N POM
In area where nitrate is primary N source and O2 is low due to upwelling, what factor most influences d15N in the ocean? where does this occur? how does it affect d15NPOM?
denitrification is the most important factor because it removes the lighter isotopes from the water before they can be upwelled to the surface for p.p.
ex. coastal ecosystems on eastern sides of continents with upwelling and tropical systems with limited circulation
high d15NPOM (heavy)
In area where N2 gas is primary N source, what factor most influences d15N in the ocean? where does this occur? how does it affect d15NPOM?
N2-fixation is most important
ex. mid-ocean gyres and inland seas
very light d15NPOM (close to 0)
How does terrestrial run off affect the d15N in coastal ecosystems?
nitrogen can come from rain, rivers, stormwater drains, sewage, etc.
it can cause eutrophication and pollution in which d15Nrun off»_space;> d15Nmarine nitrogen (not universally true)
where normally, d15Nrun off «< d15Nmarine N
What is consumer mediated nutrient recycling (CMNR)? why is it important to coastal systems?
When nitrogen is recycled within a system - dissolved nitrogenous products in consumer-waste
animal (zooplankton or fish) excretion contains NH4+ which is taken up by bacteria for respiration
NH4+ is preferable form of nitrogen (because NO3 has to be converted into NH4 in the organism) so phyto will preferentially uptake ammonium > nitrate
important resource of N in coastal areas cause often nitrate runs out
Which forms of nitrogen can only enter marine system as ‘new nitrogen’ and which can be recycled/regenerated?
new: nitrate and N2
recycled: ammonium NH4
How can NH4 uptake by phytoplankton affect the d15NPOM?
NH4 is lighter than upwelled NO3 so it can cause d15N to be lighter in some areas - depending on season and animal community
