Lecture 15: Nitrogen Cycling

Why would we think N limitation is more likely?

Organisms need P for ATP, and N for amino acids, DNA and RNA

• N and P exist in relatively low [ ] in lakes

What is the redfield ratio?

106:16:1 // C:N:P

• A rule of thumb for organismal nutrient limitation, based on nutrient ratios in oceanic water and cellular ratios from open-ocean phytoplankton

What occurs when N is low in the refield ratio?

When N:P = 20 (>16)

Expect P limitation

What occurs when N is high in the refield ratio?

When N:P = 5 (<16)

Expect N limitation

What does N [ ] mean for lakes?

• Species composition

• Nuisance algae

• Food web structure 

• Nutrient limitation in aquatic systems

What is the main N source?

Nitrogen fixation

What are the main NO₃ sources?

• Fertilizer application

• Atmospheric deposition

• Human sewage in rivers

What is nitrogen fixation?

Cleavage of the N2 bond by specialist bacteria to form NH4+ which becomes available for uptake

What are the main forms of N in lakes?

Particulate organic N (PON)

Total Dissolved N (TDN)

• Dissolved organic N

• Dissolved inorganic N

Exists in multiple redox states; NH4+, NO2-, NO3-

Several gaseous forms of N; N2, NO, N2O

What are the 5 main pathways for N?

1. N-fixation

2. Ammoniafication

3. Assimilation

4. Nitrification

5. Denitrification

What is N-fixation?

Assimilation of N2 after conversion to ammonium

• Undertaken by certain bacteria; cyanobacteria

Energetically costly (>6 ATP / ammonium molecule); inhibited when NO3- and NH4+ are not available

What is ammoniafication?

NH4+ produced as an animal / microbial waste product

When an organism consume organically bound N (DOM, PON) they will either immobilize N or release NH4+

Based on their internal nutrient status (tight vs loose)

What is assimilation?

Bacteria and photosynthetic organisms can take up both NO3- and NH4+ across the cell membrane

NH4+ is required for assimilation; enzymes are used in the cell to convert NO3- ⇒ NO2- ⇒ NH4+

Affinity for NH4+ is higher than NO3-

What is nitrification?

A chemosynthetic process undertaken by nitrifying bacteria in the presence of NH4+ and substantial O2

Energy produced by the conversion of NO3- ⇒ NO2- ⇒ NH4+ is used to reduce CO2 to organic carbon

Requires NH4+ and substantial O2 to come together

Where does most NH4 come from?

From the metalimnion or the water-sediment interface, following turn over, organismal excretion, and downstream of sewage outfall

What is denitrification?

A reduction reaction that couples the oxidation of organic matter to CO2

Requires low O2 and high NO3- ; often occurs where O2 conditions are patchy

Also requires substantial C, so does not always flow to completion

T/F Denitrification is a major N pathway?

True

Expect a full movement through the N cycle when N concentrations are high

What is the N profile for oligotrophic lakes?

TN pools consist of mainly organic N and oxidized inorganic forms; NO3-

What is the N profile for eutrophic lakes?

Epilimnia =TN pools consist of organic N and NO3-

Hypolimnia = TN pools consist of organic N and reduced ammonia