Terrestrial N cycling

Question 1

how is N transferred

Answer 1

• Mostly below ground - most usable N found in soil
  
  • Transfers among:
    ○ Plants
    ○ Microbes
    ○ Consumers
    ○ environment

Question 2

how is N used by plants? what is limiting the rate?

Answer 2

• Diversity of the cycle composition matters
  
  • most N used by plsnts is from OM decomp (vs other inputs)
  • Conversion of dead OM -> DON by microbes
  • Rate limiting step (dead OM pool> DON pool)

Question 3

why is N important

Answer 3

• Nitrogen is important in the context of a terrestrial system bc the flow of nitrogen does at some point filter through plant systems etc
  ○ Whereas micronutrients are used in a smaller quantity of the plants and not necessarily filtered thru plants - not driven by terrestrial plants

Question 4

why are we looking at nutrients?

Answer 4

○ When we try to understand global patterns, we have to think about a component that can be extrapolated in order to capture the complexity in models or data sampling
Like following energy or water - give us a generalization for ecological systems - overall picture

Question 5

how is DON (dissolved organic N) used

Answer 5

• Ultimately the DON is what funnels the nitrogen in the system
  ○ Used by planys, mycorrhizae, microbes
  ○ DON is a bunch of types of nitrogen in a solution - a “soup”

Question 6

how are microbes limited? how do they use DON to get around it?

Answer 6

• Growth often C limited
  
  • Break down DON and use C to support energy requirements
  • Secrete ammonium into soil when its in excess
  • Not all NH4+ (ammonium) available to plants (microbe immobilization)

Question 7

what do microbes do with DON

Answer 7

mineralize to NH4 - ammonification, not all available to plants bc microbial immobilization

Question 8

gross vs net NH4 immobilization

Answer 8

Gross NH4+ mineralization (total amount mineralized by microbes regardless of fate) - gross NH4 immobilization (total amount taken up by microbes) = Net NH4+ mineralization (net accumulation of N in soil solution available for plant uptake; N in excess of microbial demand

Question 9

Critical Net N mineralization threshhold of DON?

Answer 9

at C:N>25:1, NH+ is immobilized (net immoblization)
at C:N <25:1, NH4 is mineralized (net mineralization)

Question 10

why does the critical net N mineralization exist

Answer 10

microbial C:N approx 10:1 but inefficient at using C (use 40%; respire 60%)
so substrate (DON) needs C:N of 100:4 (25:1) for net N mineralization
this is a generalization; varies with microbial community

Question 11

what factors govern temporal and spatial variation in ammonification

Answer 11

state factors, interactive controls, indirect controls, direct controls

Question 12

Graph of nitrogen avail and flux ?

Answer 12

• The more nitrogen in organic matter the more DON there is
  ○ Gross Nmin and NetNmin follow this pattern
  ○ Hence there’s greater nitrification and less immobilization (bc theres more available to them so they don’t require it from the soil)

Question 13

• Graph of nitrogen pool and nitrogen avail

Answer 13

• More available with more OM
  
  • Green is most usable, yellow is highly usable,

Question 14

fate of NH4 (6)

Answer 14

1. Immobilized by microbes
   
   2. Taken up by a plant
   3. Absorbed on soil - available
   4. Stabilized on clays or SOM - unavailable
   5. Volatilized as ammonia (NH3) - happens when there is too much ammonium
      Nitrification - oxidation of NH4+ by microbes to nitrite, nitrate (NO2-, NO3-)

Question 15

what is nitrification? carried out by?

Answer 15

• Oxidation of: NH4+ -> NO2- - e.g. nitrosolobus bacteria
  
  • Then, NO2- -> NO3- - e.g. nitrobacter bacteria
  • Nitrification- carried out by specific group of autotrophic and heterotrophic microbes (nitrifiers)

Question 16

controls of nitrification?

Answer 16

○ Amount of NH4 - substrate
O2 - most nitrifiers require it

Question 17

trophic ecosystems graph

Answer 17

○ Amount of nitrogen available for use by organisms has a 1:1 relationship
§ Any amount is quickly taken up by microbes and used immediately
○ In temperate its 4:1, not immediately utilized by bacteria bc their community is less active due to adaptations, limitations, and temperate conditions

Question 18

fate of nitrates

Answer 18

1. Immobilized by microbes (used up by them)
   
   2. Taken up by a plant
   3. Leached -> groundwater (more readily leached than NH4)
   4. May absorb in soils
   5. Reduced by denitrifying microbes to Nox, N2O, or N2 (output)
      * Often lost to runoff and ground water bc its soluable
      Gases are particularily important in greenhouse effects