Nitrogen Fractionation in Primary Producers

Question 1

A plant with a N isotopic signature of ~ 0ppt is more likely to be:

N-fixing
nitrate user
carnivorous

Question 2

What would be the expected fractionation (big delta) of a nitrate-limited plant that cannot use atmospheric N?

Question 3

How does N fixation change the isotopic signature of particulate OM? (heavier, lighter, no effect?)

Question 4

Animal A has d15N = 6.5 ppt

Animal B has d15N = 10 ppt

What trophic level (herbivore, carnivore) is animal A? B?

Question 5

If Animal A has d13C = -25 ppt and animal B has d13C = -10 ppt

what can be concluded about these animals diets?

Question 6

What would you expect the isotopic signature to be of an animal’s DIET if the animal has a d15N = 8 ppt?

Question 7

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?

Question 8

If animal A has d13C = -25 ppt and animal B has d13C = -10 ppt, then animal A is heavier/lighter than animal B?

Question 9

Why does d15N vary among terrestrial plants?

Answer 9

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

Question 10

T or F: the enzymatic processes which facilitate plant uptake of nitrogen have a huge range of E(epsilon = a single enzymatic process)15N

Answer 10

true!

N2 fixation has E = 0-6 ppt v. nitrification (NH4 oxidation into N2O and NO) E = 40-60 ppt

Question 11

What are the 2 major categories for studying nitrogen in terrestrial plants?

Answer 11

1. N2 fixers
2. nitrate users (non-fixers) - typically limited by NO3

Question 12

What is the equation for d15N of a plant?

Answer 12

D(source-plant) = d15Nsource - d15Nplant

rearrange to solve for

d15Nplant = d15Nsource - D(source-plant)

Question 13

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

Answer 13

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

Question 14

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

Answer 14

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

Question 15

Give examples of N2-fixing plants and nitrate-using/non-N2-fixing plants

Answer 15

N2-fixing:
- red alder
- scotch broom
- bracken fern
- lupine
- legumes

non-N2-fixing:
- canola
- coastal Douglas-fir

Question 16

What are the rules for N fractionation in plants?

Answer 16

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)

Question 17

What are the 2 general rules of thumb for N fractionation in plants?

Answer 17

1. N2-fixers have d15N = 0 ppt
2. non-N2-fixers have d15N&raquo_space;» 0 ppt

Question 18

Discuss the example of lemur hairs and howler monkeys and how its related to d15N application contributions of plants to animal diets

Answer 18

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

Question 19

What causes variation in d15N of phytoplankton?

Answer 19

1. the d15N of the source (is it NO3, N2, or NH4?)
2. the degree of N limitation (supply v. demand)
   - the demand for N is created by photosynthesis

Question 20

Which form of nitrogen is most important for most phytoplankton?

Answer 20

nitrate NO3

Question 21

How is nitrogen distribution varied in the ocean? Explain why these phenomena occur

Answer 21

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

Question 22

What is an application for studying marine nitrate and phytoplankton ?

Answer 22

paleooceanography - look at sediment cores (nitrate sinks to bottom of ocean and layers) to conduct isotope studies to create historical maps of isotopic variation

Question 23

What causes nitrate variation in marine systems?

Answer 23

the balance between:

1. supply:
   - coastal zones: upwelling and terrestrial run off
   - mid-ocean gyres: no upwelling, most nitrate sinks
2. 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

Question 24

which coasts typically have the most upwelling?

Answer 24

eastern coasts of some continents - ex. Peru

Question 25

How come phytoplankton can still exist in mid-ocean gyres if they have no steady source of NO3 input?

Answer 25

the phyto out there fix N2 (ex. cyanobacteria)

Question 26

Briefly describe upwelling

Answer 26

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

Question 27

What causes variation in N2-fixer distribution and their rate of N2-fixation?

Answer 27

temperature - typically need warmth
availability of trace metals like iron
light for photosynthesis

Question 28

Where do most N2-fixing phytoplankton exist?

Answer 28

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

Question 29

How does isotope distribution vary?

Answer 29

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

Question 30

Why would you expect coastal areas to be heavier in d15NNO3 and d15NPOM?

Answer 30

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

Question 31

Describe the relationship between d15N of NO3 or POM and rate of primary production in the ocean

Answer 31

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

Question 32

why is the water at the bottom of the ocean lower in oxygen than at the surface? how do bacteria deal with this?

Answer 32

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

Question 33

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

Answer 33

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

Question 34

Where is dentrification in the ocean most common?

Answer 34

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

Question 35

Describe the relationship between denitrification and d15NPOM

Answer 35

as [O2] in the water is increased, denitrification decreases because bacteria can use O2 to respire

Question 36

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?

Answer 36

primary production most influences d15N

ex. coastal

creates moderate d15N POM

Question 37

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?

Answer 37

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)

Question 38

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?

Answer 38

N2-fixation is most important

ex. mid-ocean gyres and inland seas

very light d15NPOM (close to 0)

Question 39

How does terrestrial run off affect the d15N in coastal ecosystems?

Answer 39

nitrogen can come from rain, rivers, stormwater drains, sewage, etc.

it can cause eutrophication and pollution in which d15Nrun off&raquo_space;> d15Nmarine nitrogen (not universally true)

where normally, d15Nrun off «< d15Nmarine N

Question 40

What is consumer mediated nutrient recycling (CMNR)? why is it important to coastal systems?

Answer 40

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

Question 41

Which forms of nitrogen can only enter marine system as ‘new nitrogen’ and which can be recycled/regenerated?

Answer 41

new: nitrate and N2
recycled: ammonium NH4

Question 42

How can NH4 uptake by phytoplankton affect the d15NPOM?

Answer 42

NH4 is lighter than upwelled NO3 so it can cause d15N to be lighter in some areas - depending on season and animal community