Biogeochemical Cycling Notes

Biogeochemical Cycling

• "Matter cycles, energy flows"

• Two main cycles:

  • The Carbon Cycle

  • The Nitrogen Cycle

Learning Outcomes

• How carbon and nitrogen cycle through ecosystems and around the globe.

• How traits involved with cycling evolve.

What is Biogeochemistry?

• Study of elemental cycles on molecular → global scales.

The Carbon Cycle

• Components:

  • CO_2 in the atmosphere

  • Photosynthesis by producers

  • Plant respiration

  • Carbon fixation

  • Animal respiration by consumers

  • Fossils and fossil fuels

• Pools and Fluxes

  • Pool: Quantity of an element contained within a part of the earth system (aka stock or reservoir).

  • Flux: Quantity of an element moved from one pool to another in a given time.

  • Transformation: Change of an element from one chemical form to another within or between pools (e.g., Organic C to CO_2).

• Units:

  • Pools measured in Pg C (petagrams of carbon).

  • Fluxes measured in Pg C yr-1 (petagrams of carbon per year).

  • 1 petagram = 1 gigaton (billion metric tons).

• Simplified Model:

  • Land

  • Atmosphere

  • Ocean

• Flux Processes:

  • Photosynthesis

  • Respiration

  • Combustion

  • Weathering

• Human Impact: Humans are altering many fluxes.

• Photosynthesis and Plant Respiration

  • Global GPP (Gross Primary Production) = 123 PgC/yr

  • ~50% of GPP goes to plant respiration, resulting in NPP (Net Primary Production) of 60-70 PgC/yr.

  • NPP is split ~evenly above and belowground.

  • Most NPP becomes litter.

    • 25-30 PgC/yr aboveground

    • 25-30 PgC/yr belowground

• Heterotrophic Respiration

  • Microbes: 44 PgC/yr

  • Herbivores: 3 PgC/yr

• Fossil Fuel Emissions and Cement Production

  • 10 PgC/yr

• Biomass Burning

  • 2.5 PgC/yr

• Ocean

  • Ocean is at approximate equilibrium with the atmosphere.

  • NPP: 51 PgC/yr

  • CO_2 exchange occurs.

  • The ocean currently takes up slightly more CO_2 than it emits.

  • Organic matter and biogenic carbonates sink to the seafloor, storing carbon in sediments.

• (Geological) Carbon Cycle

  • Rock weathering consumes CO2 and releases bicarbonate (HCO3) to oceans.

  • HCO_3 is taken up by marine life, settles to the bottom, and becomes rock.

  • Rock is eventually subducted and erupted, releasing the carbon.

Evolution of C Cycling Traits

• Carbon fluxes mediated by organisms’ traits.

• Traits are subject to natural selection.

• Example: Schaum et al. 2017

  • Chlamydomonas reinhardtii (green algae) raised in freshwater mesocosms at ambient or warmed (+4 ºC) temperatures for 10 years.

  • Warmed isolates evolved higher optimal temperature range.

  • Warmed isolates had higher net photosynthesis (P-R) rates.

The Nitrogen Cycle

• Components:

  • Atmospheric N_2

  • Reactive N (Atmospheric N2O, NOx, NH_3)

  • Biomass burning N

  • Industrial N fixation

  • Fossil fuel N

  • N fixation by crops

  • Natural biological N fixation

  • Animal N

  • Atmospheric deposition

  • Vegetation

  • Soils

  • Marine biota

  • Detritus

• Simplified Model:

  • Atmosphere N_2

  • N fixation

  • Plants

  • Organic N (Litter/ SOM)

  • NH_4 +

  • NO_3 -

  • NO_2 -

  • Nitrification

  • Immobilization

  • Leaching

  • Denitrification ( NO2 - → NO → N2O → N_2 )

  • Acid rain

Biological Nitrogen Fixation

• Equation: N2 + 16 ATP + 16 H2O + 8 H^+ → 2 NH3 + H2 + 16 ADP + 16 PO_4^{3-}

• Extremely energy-intensive process!

• Nitrogenase enzyme = responsible for most nitrogen in living biomass today!

• Performed by free-living and root-symbiotic microbes.

• Most prevalent in the tropics.

• Example: Molybdenum limitation of asymbiotic nitrogen fixation in tropical forest soils (Barron et al. 2009)

  • Demonstrated through experiments involving phosphorus and molybdenum.

• Example: Dr. Michelle Wong's research in Southeastern Amazon (Wong et al. 2021)

  • Southeastern Amazon gets less airborne Mo deposition.

  • Question: Does that mean BNF there is Mo-limited?

• Wong et al. 2020

  • Mo additions did not consistently increase BNF.

  • Possibly because soil N availability higher in SE Amazon.

Denitrification (To Complete or Not to Complete?)

• Each step of denitrification produces (less and less) energy.

• Each step has its own associated enzymes.

• Denitrification can be complete (producing N2) or incomplete (producing N2O).

• Process: NO3 - → NO2 - → NO → N2O → N2

Complete vs. Incomplete Denitrification

• Bergsma et al. 2002

• Compared denitrification N2O vs. N2 production between…

  • Cropped farm field (corn-soy-wheat) - fertilized

  • Early-successional “old field”

• Incomplete denitrification favored in cropped soil watered + fertilized simultaneously.

• Cropped soils contain microbes adapted to higher nitrate availability.