Nitrogen Cycle

Nitrogen

  • Nitrogen is a cycle that describes the continuous movement of nitrogen through the atmosphere, soil, living organisms, and water 

  • Nitrogen is mediated by biology and bacteria (particularly cyanobacteria)

  • Nutrient: any element that an organism must take in to live, grow and reproduce

  • Macronutrients: C, H, N, O, P, S

    • Hydrologic = C and H (water)

    • Gaseous = C and N (main nutrients in atmosphere)

    • Sedimentary = P and S (mostly in rocks and soil)

  • N2 is not particularly soluble in water 

    • like other gases, solubility is temperature dependent ; as temperatures increase, the solubility of nitrogen in water decreases

Liebig’s Law of Minimum

  • The element present in the lowest concentration relative to its demand limits growth

    • “You’re only as strong as your weakest link”

    • In the nitrogen cycle, this means that deficiencies in nitrogen can hamper plant growth and affect overall ecosystem productivity

  • Composition of Nitrogen

    • Biosphere = nitrogen is moderate (0.27)

    • Lithosphere = nitrogen is limiting (~0)

    • Hydrosphere = nitrogen is limiting (~0)

      • this can cause lake’s algae production to shut down or create massive algae blooms which use up all the nitrogen in the lake because nitrogen is in such demand (30000) 

    • Atmosphere = nitrogen is abundant (78.3)

    • Demand/Supply (in water) = 30000

Principal Forms of Nitrogen

  1. Gas - N2

  2. Inorganic N: Ammonia (NH4+), Hydroxylamine (NH2OH), Nitrate (NO32-) most oxidized from which is needed for organisms for protein development and nucleic acids, Nitrite (NO2-)

  1. Organic N (in algae): DON = dissolved organic N, PON = particulate organic N

Particulate vs. Dissolved

  • Filter water → Particulate (on filter) or dissolved (in filtrate)

Sources of Nitrogen to a Lake

  • 1) Direct deposition onto lake surface

    • generally considered minor, but now significant

    • comes from dust and acid rain (which is partly nitric acid), and when combustion occurs, nitrogen oxides (NOx) are produced, which can then be deposited back to the earth through precipitation

  • 2) Allochthonous (from outside) inputs from surface and groundwater

    • not a lot of nitrogen in most natural soils

    • depends on: bedrock, soil, vegetation, and microbial activities (e.g. limestone vs igneous)

    • human activities have accelerated nitrogen inputs (e.g. agriculture and fertilizers)

      • famers need to use more fertilizer every year because the fertilizer will drive down into lakes

      • nitrogen comes from farm animals trough their waste

      • sewage from cities can drive down into lakes with high levels of nitrogen

    • Fritz Harber changed the nitrogen cycle because he found a way to industrially create nitrogen fertilizer, which allowed for greater agricultural productivity but also contributed to environmental issues such as eutrophication in aquatic ecosystems

  • 3) Autochthonous (from inside) N fixation

    • N2 + 6 e- + 8H+ → 2 NH4+ = uses up a lot of energy

      • Process is key for cyanobacteria, but they only fix nitrogen when it is extremely limiting (usually occurs in late summer)

    • a) Blue-green algae (cyanobacteria) 

      • Their heterocysts are the technology they use to fix nitrogen

    • Anabaena (blue-green algae)

      • use akinete that are their resting phases and they are formed when there is lack of nitrogen available 

    • Microcystis (blue-green algae)

      • cannot fix nitrogen

      • *Not all cyanobacteria can fix nitrogen

    • b) Heterotrophic Bacteria can fix nitrogen but at much lower rates

    • Photosynthetic bacteria (green and purple sulfur bacteria) can fix nitrogen but at much lower rates 

    • c) Wetland sources (e.g. alder)

      • Alder has bacterial symbionts associated with their roots that can fix nitrogen

  • 4) Sediments

    • there is a lot of nitrogen in sediments (e.g. 200 kg N/ha)

      • however the nitrogen can be trapped in the sediments if sediment surface is oxidized 

Losses of Nitrogen From a Lake

  • 1) Outflow from basin

  • 2) Denitrification (NO3- → N2)

  • Sedimentation of inorganic and organic cpds to the sediments

Summary

  • N is a major nutrient

  • N-cycle is complex and microbially driven

  • The distribution of N in a lake changes vertically and seasonally

  • There are important differences in N-cycling between oligotrophic and eutrophic lakes