The Cycling of Nitrogen

N is an important nutrient that frequently limits primary productivity in aquatic ecosystems

It is rare in the earth’s crust, but makes up 79% of the atmosphere (N2)(oxidation state =0)

Most algae and plants require NO3¯(+5) (NO2 ¯) (+3) or NH3 (NH4

+) (-3)to synthesize amino acids to make proteins

N-fixing microorganisms can take up N2 and convert it to NH3

N2 + 3H2 → 2NH3

Many plants have N-fixing mutualists (eg Azolla)

Denitrifying bacteria can convert NO3¯ back to N2

Azolla, an aquatic fern used in rice culture

•The leaves of this aquatic fern have cavities that harbour filamentous cyanobacteriaAnabaena azollae

•The large cells (heterocysts) are specialized for N-fixation

•Traditional rice farming in many countries involve planting Azolla to build up N concentrations in rice paddy.

•Nutrients like N and P tend to accumulate in the hypolimnion during summer stratification—sedimentation.

•In eutrophic lakes the deep layers become very depleted in O2

•NO3-—the most oxidized form of

N occurs highest in the water column where there is O2 present

•NH4+ or NH3, the most reduced

form is prevalent deep where O2 is absent or nearly so

•N2O and NO2-—are intermediate

oxidation states

The Nitrogen cycle involves many different oxidation states, and the redox processes are facilitated by plants and wide variety of bacteria

-3

+3

+5

0

+1

Nitrite

Photoautotrophs (PA) Chemoautotrophs(CA)

CH

Chemoheterotrophs (CH)

PA

CH

This graph shows Nitrate concentrationsIn large rivers as a function of human population density

This graph shows Nitrate export from large river watersheds as a function of human population density

Question?? ExplainPrairie rivers and watersheds are ‘high’ in Nitrates and Nitrate export, even though population density is low.