8.2 Biogeochemical cycling and nutrient dynamics
4 min read•august 7, 2024
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 through living and non-living components of ecosystems.
Pollutants can disrupt these cycles, leading to issues like and . Understanding these dynamics helps us grasp how toxicants impact entire ecosystems, not just individual organisms.
Nutrient Cycling
Carbon Cycle
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- 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
- of dead 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 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
- is the oxidation of ammonia (NH3) to nitrite (NO2-) and then to nitrate (NO3-) by nitrifying bacteria
- is the reduction of nitrate to nitrogen gas by denitrifying bacteria, returning nitrogen to the atmosphere
- 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 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
- 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
Key Terms to Review (20)
Assimilation: Assimilation refers to the process by which organisms incorporate nutrients and elements from their environment into their own biological structures. This concept is crucial in understanding how energy and matter flow through ecosystems, as it highlights the role of living organisms in biogeochemical cycles and nutrient dynamics, where they transform available resources into forms that can be utilized for growth and reproduction.
Bioaccumulation: Bioaccumulation is the process by which organisms accumulate contaminants in their bodies over time, often from their environment or food sources. This phenomenon can lead to higher concentrations of harmful substances in the tissues of an organism compared to the surrounding environment, significantly impacting health and ecological dynamics.
Biomagnification: Biomagnification is the process by which the concentration of toxic substances increases as they move up the food chain, affecting organisms at higher trophic levels more severely. This phenomenon connects various aspects of ecological interactions and highlights the importance of understanding how pollutants behave in ecosystems and impact wildlife health.
Carbon cycle: The carbon cycle is the natural process through which carbon atoms are exchanged among the atmosphere, land, ocean, and living organisms. This cycle is essential for maintaining the balance of carbon in the environment and plays a crucial role in regulating Earth's climate and supporting life. It encompasses various processes including photosynthesis, respiration, decomposition, and fossil fuel combustion, demonstrating how carbon moves between different spheres of the Earth.
Decomposition: Decomposition is the biological process by which organic matter is broken down into simpler substances, primarily by the action of microorganisms, fungi, and detritivores. This process plays a crucial role in the recycling of nutrients within ecosystems, ensuring that essential elements such as carbon, nitrogen, and phosphorus are made available for uptake by plants and other organisms, which is vital for maintaining ecosystem health and productivity.
Denitrification: Denitrification is a microbial process that converts nitrates (NO₃⁻) and nitrites (NO₂⁻) in the soil back into nitrogen gas (N₂) or, to a lesser extent, nitrous oxide (N₂O), which is released into the atmosphere. This process plays a crucial role in the nitrogen cycle, helping to maintain ecosystem balance and prevent the accumulation of excess nitrates that can lead to environmental issues like water pollution and eutrophication.
Eutrophication: Eutrophication is the process by which a body of water becomes overly enriched with nutrients, leading to excessive growth of algae and other aquatic plants. This nutrient overload, often caused by runoff from agricultural land and urban areas, disrupts the natural balance of ecosystems, ultimately harming aquatic life and water quality.
Fertilization: Fertilization is the process by which male and female gametes unite to form a zygote, initiating the development of a new organism. This process is critical as it not only leads to the formation of new life but also influences genetic diversity and population dynamics within ecosystems, impacting biogeochemical cycling and nutrient dynamics as organisms grow and decompose.
Limiting Nutrients: Limiting nutrients are essential elements that are in short supply and restrict the growth of organisms in an ecosystem. These nutrients are crucial for biological processes, but when they are not available in sufficient quantities, they can limit productivity and influence the overall health of an ecosystem. Understanding which nutrients are limiting helps explain patterns of growth and biomass production in various environments.
Mass balance modeling: Mass balance modeling is a systematic approach used to quantify the input, output, and storage of substances in a defined system over time. This technique helps in understanding how nutrients and pollutants cycle through ecosystems, allowing for the evaluation of nutrient dynamics and the effects of human activities on biogeochemical cycles.
Nitrification: Nitrification is a two-step biological process in which ammonia is converted into nitrite and then into nitrate by specific bacteria in the soil. This process plays a crucial role in the nitrogen cycle, as it transforms nitrogen from organic matter into forms that plants can readily absorb, thereby facilitating nutrient dynamics in ecosystems.
Nitrogen Cycle: The nitrogen cycle is the series of processes by which nitrogen and its compounds are interconverted in the environment and in living organisms. This cycle is crucial for maintaining ecosystem health as it involves the transformation of nitrogen from atmospheric gas into organic compounds and back again, ensuring that all living things have access to this essential nutrient.
Nitrogen fixation: Nitrogen fixation is the process by which atmospheric nitrogen gas (N₂) is converted into ammonia (NH₃) or related compounds in soil, making it accessible to plants. This process is essential for life on Earth because it transforms inert nitrogen from the atmosphere into a usable form that supports plant growth, ultimately impacting nutrient dynamics and biogeochemical cycling within ecosystems.
Nutrient Uptake: Nutrient uptake refers to the process by which living organisms, particularly plants and microorganisms, absorb essential nutrients from their environment, mainly soil or water. This process is crucial for maintaining growth, development, and overall health, as nutrients like nitrogen, phosphorus, and potassium are vital for various biological functions, including metabolism and energy transfer.
Organic matter: Organic matter refers to the carbon-based compounds that are derived from the decay of plant and animal materials. This includes a wide variety of substances such as dead leaves, decomposed organisms, and other biological materials that contribute to soil fertility and nutrient cycling. Organic matter plays a crucial role in maintaining ecosystem health by supporting microbial activity and enhancing soil structure.
Phosphate: Phosphate is a chemical compound that contains the phosphate ion (PO₄³⁻), which plays a crucial role in biological systems and is an essential nutrient for plants and animals. In the context of nutrient dynamics and biogeochemical cycling, phosphates are vital for energy transfer through ATP, DNA and RNA synthesis, and overall cellular function. They cycle through various environmental compartments, including soil, water, and living organisms, influencing ecosystem productivity and health.
Phosphorus: Phosphorus is a vital chemical element, essential for life, that plays a key role in energy transfer, photosynthesis, and the formation of DNA and RNA. In the context of nutrient dynamics, phosphorus exists in various forms and cycles through different environmental compartments, influencing ecosystem productivity and health.
Primary productivity: Primary productivity is the rate at which autotrophs, primarily plants and phytoplankton, convert light energy into chemical energy through photosynthesis. This process forms the foundation of energy flow in an ecosystem, supporting all other trophic levels. High primary productivity indicates a robust ecosystem, while disruptions can lead to diminished ecological health and altered nutrient dynamics.
Stable Isotope Analysis: Stable isotope analysis is a technique used to measure the relative abundance of stable isotopes of elements in various biological and environmental samples. This method provides insights into biogeochemical processes, nutrient dynamics, and ecological interactions by revealing information about sources, pathways, and transformations of nutrients in ecosystems.
Sulfur: Sulfur is a non-metallic element with the atomic number 16, known for its role in various biological processes and as a critical component in biogeochemical cycling. It is essential for the synthesis of amino acids and proteins, which makes it vital for life. In the context of nutrient dynamics, sulfur cycles through different environmental compartments, influencing ecosystem health and productivity.