8.2 Biogeochemical cycling and nutrient dynamics

Biogeochemical cycling and nutrient dynamics are crucial to understanding ecosystem-level effects of toxicants. These processes involve the movement of essential elements like carbon, nitrogen, and phosphorus through living and non-living components of ecosystems.

Pollutants can disrupt these cycles, leading to issues like eutrophication and bioaccumulation. Understanding these dynamics helps us grasp how toxicants impact entire ecosystems, not just individual organisms.

Nutrient Cycling

Carbon Cycle

Top images from around the web for Carbon Cycle

• 

  Biogeochemical Cycles · Concepts of Biology View original

  Is this image relevant?

• 

  Soil carbon | Environment, land and water | Queensland Government View original

  Is this image relevant?

• 

  Biogeochemical Cycles and the Flow of Energy in the Earth System | Sustainability: A ... View original

  Is this image relevant?

• 

  Biogeochemical Cycles · Concepts of Biology View original

  Is this image relevant?

• 

  Soil carbon | Environment, land and water | Queensland Government View original

  Is this image relevant?

1 of 3

Top images from around the web for Carbon Cycle

• 

  Biogeochemical Cycles · Concepts of Biology View original

  Is this image relevant?

• 

  Soil carbon | Environment, land and water | Queensland Government View original

  Is this image relevant?

• 

  Biogeochemical Cycles and the Flow of Energy in the Earth System | Sustainability: A ... View original

  Is this image relevant?

• 

  Biogeochemical Cycles · Concepts of Biology View original

  Is this image relevant?

• 

  Soil carbon | Environment, land and water | Queensland Government View original

  Is this image relevant?

1 of 3

• Carbon moves through the environment in various forms including atmospheric carbon dioxide (CO2), organic carbon in living organisms, and inorganic carbon in rocks and minerals
• Photosynthesis by plants and other autotrophs converts atmospheric CO2 into organic compounds, incorporating carbon into the biosphere
• Cellular respiration by organisms releases CO2 back into the atmosphere, completing the cycle
• Decomposition of dead organic matter by microorganisms also returns carbon to the atmosphere as CO2 or methane (CH4)
• Human activities such as burning fossil fuels and deforestation have increased atmospheric CO2 levels, contributing to climate change

Nitrogen Cycle

• Nitrogen is essential for the synthesis of amino acids, proteins, and nucleic acids in living organisms
• Atmospheric nitrogen (N2) is converted into biologically available forms through nitrogen fixation by bacteria and cyanobacteria
  • Symbiotic nitrogen-fixing bacteria (Rhizobium) form nodules on the roots of legumes
  • Free-living nitrogen-fixing bacteria (Azotobacter) and cyanobacteria fix nitrogen in soil and aquatic environments
• Nitrification is the oxidation of ammonia (NH3) to nitrite (NO2-) and then to nitrate (NO3-) by nitrifying bacteria
• Denitrification is the reduction of nitrate to nitrogen gas by denitrifying bacteria, returning nitrogen to the atmosphere
• Assimilation of nitrate and ammonia by plants and microorganisms incorporates nitrogen into the biosphere
• Decomposition of organic matter releases nitrogen back into the soil as ammonia

Phosphorus Cycle and Decomposition Rates

• Phosphorus is a key nutrient for the growth and development of living organisms, found in DNA, RNA, and cell membranes
• The phosphorus cycle is sedimentary, with the main reservoir being rocks and minerals
• Weathering and erosion of rocks release phosphate ions (PO4³⁻) into the soil and water
• Plants absorb phosphate from the soil and incorporate it into their biomass
• Animals obtain phosphorus by consuming plants or other animals
• Decomposition of dead organic matter by microorganisms releases phosphorus back into the soil
• Phosphorus can be lost from ecosystems through runoff and leaching, ending up in aquatic environments and sediments
• Decomposition rates vary depending on factors such as temperature, moisture, and the chemical composition of the organic matter
  • Warm, moist conditions and a low carbon-to-nitrogen ratio (C:N) favor rapid decomposition
  • Cold, dry conditions and a high C:N ratio slow down decomposition rates

Nutrient Dynamics and Pollution

Nutrient Pollution and Eutrophication

• Nutrient pollution occurs when excess nutrients, particularly nitrogen and phosphorus, enter ecosystems from anthropogenic sources
  • Agricultural runoff containing fertilizers and animal waste
  • Sewage and wastewater discharge
  • Fossil fuel combustion releasing nitrogen oxides (NOx)
• Eutrophication is the enrichment of aquatic ecosystems with nutrients, leading to excessive growth of algae and other aquatic plants
• Algal blooms can deplete dissolved oxygen in the water, causing hypoxia and fish kills
• Some algal blooms produce toxins that can harm aquatic life and human health (harmful algal blooms or HABs)
• Eutrophication can lead to changes in species composition, decreased water clarity, and the formation of dead zones in aquatic environments

Bioaccumulation

• Bioaccumulation is the accumulation of toxicants in the tissues of living organisms over time
• Toxicants can be persistent organic pollutants (POPs) such as DDT, PCBs, or heavy metals like mercury and lead
• Bioaccumulation occurs when the rate of uptake of a toxicant exceeds the organism's ability to metabolize or excrete it
• Biomagnification is the increase in toxicant concentration as it moves up the food chain
  • Predators at higher trophic levels accumulate higher concentrations of toxicants than their prey
  • Example: Mercury accumulation in fish, with higher concentrations in larger, long-lived predatory fish like tuna and swordfish
• Bioaccumulation and biomagnification can have adverse effects on the health of organisms and pose risks to human health through consumption of contaminated food

Soil Ecology

Soil Fertility and Microbial Communities

• Soil fertility refers to the ability of soil to support plant growth by providing essential nutrients, water, and a suitable physical environment
• Soil organic matter (SOM) is a key component of soil fertility, consisting of decomposed plant and animal residues
  • SOM improves soil structure, water retention, and nutrient availability
  • Humus is the stable, long-lasting fraction of SOM that contributes to soil fertility
• Soil pH affects nutrient availability and the activity of soil microorganisms
  • Most plants and soil microbes thrive in slightly acidic to neutral pH ranges (6.0-7.5)
  • Extreme pH levels (highly acidic or alkaline) can limit plant growth and microbial activity
• Soil microbial communities play crucial roles in nutrient cycling, decomposition, and plant health
• Bacteria, fungi, and archaea are the most abundant soil microorganisms
  • Bacteria are involved in nitrogen fixation, nitrification, and denitrification
  • Fungi are the primary decomposers of complex organic compounds like lignin and cellulose
  • Mycorrhizal fungi form symbiotic relationships with plant roots, enhancing nutrient and water uptake
• Soil fauna such as earthworms, nematodes, and arthropods contribute to soil mixing, aeration, and the breakdown of organic matter
• Disturbances to soil ecology, such as tillage, pesticide use, and monoculture cropping, can negatively impact soil fertility and microbial communities