EVSC 100 Lecture 4

Biogeochemistry

 the (re)cycling of matter (including nutrients) - organic and inorganic - resulting in the movement and transformation of chemical elements and compounds between living organisms (biosphere), the atmosphere, the geosphere, and the hydrosphere.

• Different biogeochemical cycles →O, H₂O, C, N, P, S

Microorganisms are:

1. Microorganisms are perhaps the main drivers of biogeochemical cycles on Earth, and without them, life (everything) as we know it would be completely different from our present day experience (single celled, or colony of cells and primarily bacteria).

   1. Metabolic processes: The life-sustaining chemical reactions which occur in organisms, with 3 main functions:

      1. Conversion of the energy in food to energy available to run cellular processes → think ATP

      2. Conversion of food (matter) to building blocks for proteins, lipids, nucleic acids, and some carbohydrates

      3. Elimination of waste 

6 basic types types of biogeochemical cycles

• Biogeochemical cycles → sedimentary cycle, gaseous cycle

  • Sedimentary cycle: 

    • Sulphur cycle

    • Phosphorous cycle

  • Gaseous cycle:

    • Carbon cycle

    • Oxygen cycle

    • Water cycle

    • Nitrogen cycle 

Why is the nitrogen cycle important?

• 1st: Nitrogen is a limiting resource of ecosystems because it is very important for plant growth and productivity (amino acids → proteins → nucleic acids [DNA, RNA] and chlorophyll), but it is difficult for plants to acquire because it is necessary to transform atmospheric elemental nitrogen into usable forms.

• 2nd: Nitrogen is the most abundant element of our atmosphere (N₂ ) followed by Oxygen (O2 ), which is good because it is relatively inert (not flammable like oxygen) and it is not toxic (like too much oxygen - cell membrane damage).

4 Key Steps of a Nitrogen Cycle

1. Nitrogen fixation

2. Nitrification

3. Ammonification

4. Denitrification

Nitrogen fixation:

• Atmospheric nitrogen must be processed, or "fixed", into a usable form to be taken up by plants - fixation results in Ammonia (NH3 ) and can occur by bacteria and lightning

  • Fixed can also be thought of as combining, in this case nitrogen is combined with hydrogen by breaking the triple N₂ bond with ATP in the presence of water and free electrons.

  • “Fixing” in the soil occurs in root nodules of nitrogen fixing plants, and in the soil where bacteria are found. Fixing in the soil occurs under anaerobic or anoxic (oxygen free) conditions.

Nitrification:

• A different set of bacteria from step 1 converts Ammonia (NH₃) to Nitrite (NO₂), and then another set of bacteria converts Nitrite to Nitrate (NO₃), which is then used by plants 

  • This occurs through oxidation, in this case the Ammonia and Nitrite accepts an electron e^- from a donor chemical compound → O₂. 

  • This requires breaking the double bond, and this occurs by mediating bacteria

  • Nitrification occurs under oxygenated conditions

Ammonification:

• This process comes into play when a plant or animal dies, or when an animal expels waste like urine. Initially, the nitrogren is bound to other molecules in an organic form. The molecules are broken down by bacteria and fungi and the nitrogen is released as ammonia (NH3).

  • The ammonia then becomes available for uptake by plants and other microorganisms for growth, or it can go through nitrification and then denitrification and then released back to the atmosphere.

  • Ammonification occurs under oxygenated conditions

Denitrification:

• In the final step of the nitrogen cycle, anaerobic bacteria can turn nitrates back into nitrogen gas. 

• This process, like the process of turning nitrogen gas into ammonia, occurs under anoxic (oxygen free) conditions so it often occurs deep in the soil, or in wet environments where mud and muck keep oxygen at bay

• In some ecosystems, denitrification is a valuable process to prevent nitrogen compounds in the soil from building up to dangerous levels

Phosphorous Cycle

• Phosphorus is a limiting resource of ecosystems because it is an essential nutrient for sustaining life on Earth (energy transfer via ATP and ADP, nucleic acids [DNA, RNA] and cell membranes, phospholipids [fats and oils], insect exoskeletons [Calcium Phosphate]…),

• But it is a scarce resource that is dependent upon the geologic processes of tectonic uplift and weathering of phosphorus rich rocks. 

• Phosphorus occurs in many chemical forms, but the most common is the Phosphate Ion → PO₄ ^-3

4 Key Steps of Phosphorous Cycle

1. Rock Weathering

2. Environmental Distribution

3. Organic Decomposition

4. Geologic Burial

Rock Weathering:

• Over time, rain and weathering cause rocks, which are phosphate rich minerals, to release phosphate ions, which dissolve in water → very slow process on human timescales

  • The release of phosphate ions from rocks is dependent on the:

    • Rates of tectonic uplife, which brings rocks to the surface of the Earth

    • Rates of landscape erosion which can also expose rocks to the surface of the Earth

    • Rates of rock erosion and weathering on exposed rock surfaces

Environmental Distribution:

• Once phosphate ions are freed from rocks, over yet more time, the dissolved ions are distributed throughout the environment via the movement of water into and through soils, and water into rivers, which then flow to lakes and oceans.

• Once in soils, plants can take up inorganic phosphate ions, supporting their growth. 

• Animals gain phosphate for their biological purposes by eating plants that contain phosphate. 

Organic Decomposition:

• When plants uptake inorganic phosphate, or when animals consume plants with phosphate, the phosphate is incorporated into organic molecules (DNA, RNA, etc.) to support biological functions.

• When the plants and animals die, the phosphate is re-released back into the environment where it is acted upon by bacteria which solubilize the phosphate via a process called mineralization.

• Phosphorus also comes back to the soil through waste excretions from animals

Geologic Burial:

• When inorganic phosphate reaches oceans, it is taken up by marine and aquatic plants and animals. Once these biological organisms die, their tissues decompose, releasing the phosphate through decomposition.

• After decomposition,  the inorganic phosphate is buried where it undergoes the rock cycle, only to be brought back to the surface in the future by tectonic processes, completing the cycle.

Eutrophication

Excessive richness of nutrients in a lake or other body of water, frequently due to runoff from the land, causes a dense growth of plant life and death of animal life from lack of oxygen.

Top sources of Nutrient Pollution:

• Municipal sewage

• Agricultural fertilizers

• Livestock waste

• Stormwater drainage

• Aquaculture 

Human Impacts to the N and P Cycles 

1. Industrial and intentional biological fixation of nitrogen (through planting N-fixing crops), and mining of phosphorous, increases the amount of reactive nitrogen, and speeds up the release of phosphorus in the environment. 

2. N and P in fertilizer can leave farmlands in water] – as well as from industrial and municipal sewage and wastewater discharges – all of these impacts are associated with eutrophication of freshwaters and coastal zones. 

3. Harvesting of crops from farmland enhances the need for P based fertilizers because the removal of vegetation material results in a loss of P stores that could help to replenish P in soils. The cycle goes on… 

4. Combustion of oil, natural gas and coal releases N in the form of NO₂→ greenhouse gas, produces smog, eutrophication & acid rain.

Possible Interventions to Human Impacts

• Alternative farming practice → stop the practice of industrial farms, and focus back on family farms growing food crops that humans need for sustenance, as opposed to dividend creation 

• Better support to farmers in field design → runoff and shallow flow from farmlands can be designed to interface with a range of treatment wetland configurations to remove excess P and N from water. 

• Regenerative farming practice → promote crop rotations and soil regeneration practices to limit fertilizer application and need, and build healthier and more productive soils. 

• Reducing our dependence on fossil fuels in all forms for energy production and transit