nitrogen cycling

Question 1

nitrogen inputs?

Answer 1

chemical weathering, atmospheric deposition, anthropogenic fixation, biological fixation of atmospheric o2

Question 2

outputs?

Answer 2

gaseous and solute
denitrification
roles of N gases

Question 3

what does nutrient cycling involve

Answer 3

1. nutrient inputs
2. internal cycling
3. nutrient loss
   transfers among plants microbes consumers & enivronment

Question 4

describe chemical weather of rocks

Answer 4

sedimentary rocks 75% surface rock - can contain N
minor contribution in most ecosystems

Question 5

describe deposition

Answer 5

gaseous, particulate, and dissolved sources
from other ecosystems - > leaching
atmospheric:
-wet (precip/fog) and dry (dust/aerosols)
-lightning N2 to No3-
approx. 5% total N fixed in systems

Question 6

anthropogenic fixation (increasing importance)

Answer 6

production of fertilizers (N2 to NH3)
fossil fuel combustion - releases NOx
domestic animals (NH3 to atmosphere)
often transported long distances

Question 7

biological fixation

Answer 7

main pathway for N entry into natural systems
reduction of N2 to NH4 by microbes

Question 8

role of nitrogenase in biol fix

Answer 8

catalyst
denatured in presence of O2 - needs anaerobic environment
- energetically expensive; needs souce of C and P (ATP)
- temp constrains C supply and nitrogenase activity - prominant in tropics and contstrained at high lat.

Question 9

organisms involved in biol fix?

Answer 9

• N fixing bacteria in symbiotic association with plants have the highest rates of N fixation
  ○ Rhizobium bacteria symbiotic with legumes
  ○ Frankia spp associated with alders
  ○ Physical association

Question 10

describe bacteria symbiosis with plant for N fixation

Answer 10

heterotrophic N fixers
○ Plants provide high energy © environment in return for N
○ From root nodules around bacteria (protect from O2 - it degrades)
○ Also free living bacterial (low fixation); bacteria in termite guts (important in tropics)
○ Even the absoprtion of nutrients is not a free process- active transport involved

Question 11

phototrophic N fixers

Answer 11

○ Free living cyanobacteria (bluegreen algae, Nostoc)
○ Cyanobacteria symbiotic with fungi (ie lichens)
§ Important in early successional ecosystems
○ Cyanobacteria symbiotic with plants (Azolla fern and Nostoc in rice paddies)
Often have specialized cells (heterocysts) that protect nitrogenase from O2

Question 12

Biotic and Abiotic limits on N fixation

Answer 12

• Biotic and abiotic constraints on N fixation
  
  1. Energetically expensive
     * Approx 2-4x higher cost than N uptake from soil
     * Association with symbiotic N fixers strongest in N poor soils
     * High energy = high light environments (photosynthesis of symbiotic plants) -> most common in early successional species
  2. Nutrient co-limitation (e.g. P (for ATP))
     * May be ultimate control over N supply and NPP in some systems
  3. Herbivory
     * High N content of N-fixing plants makes them preferred forage (tradeoff)

Question 13

N and P in natural ecosystems -

Answer 13

inputs and outputs «< internal tranfers
closed systems
but human activities = open system

Question 14

look at nitrogen inputs graph

Answer 14

• Time series
  
  • Human caused inputs now = or exceed natural inputs
    Trade off btwn human needs and planets needs