Biogeochemical Cycles
Biogeochemical Cycles: Comprehensive Study Notes
Building Blocks of Life: Macronutrients (CHOPNS)
Macronutrients are essential elements that form the base for organic life forms:
Carbon (C):
Base for organic life forms; found in carbohydrates, proteins, nucleic acids, and lipids.
Hydrogen (H):
Essential for hydrogen bonding, which occurs only with Nitrogen (N), Oxygen (O), and Fluorine (F).
Oxygen (O):
Crucial for aerobic respiration.
Phosphorus (P):
A limiting factor in aquatic systems.
A component of teeth and bones.
Essential for ATP (adenosine triphosphate).
Nitrogen (N):
A key component of DNA and proteins.
An important plant nutrient.
A limiting factor in marine systems.
Sulfur (S):
Found in DNA and proteins.
Fundamental Laws Governing Cycles
First Law of Thermodynamics: Energy cannot be created or destroyed in a closed system.
Law of Conservation of Mass (or Matter): Matter cannot be created or destroyed.
These laws highlight that as energy flows through ecosystems, matter also cycles.
Key Terminology for Biogeochemical Cycles
To understand biogeochemical cycles, familiarity with specific terms is necessary:
Process: Actions or activities within a cycle, often ending in "-tion" (e.g., evaporation, respiration, nitrification).
Reservoir / Sink / Storage: A location where a significant amount of a nutrient or element is held for a period of time.
Source: A point or area from which a substance originates or is released.
Anthropogenic: Caused or influenced by humans.
Hydrologic Cycle (Water Cycle)
Learning Objective and Essential Knowledge
Learning Objective: Explain the steps and reservoir interactions in the hydrologic cycle.
Essential Knowledge:
The hydrologic cycle, powered by the sun, involves the movement of water in its solid, liquid, and gaseous phases between sources and sinks.
Oceans are the primary reservoir of water on Earth's surface, with ice caps and groundwater serving as much smaller reservoirs.
Characteristics and Distribution of Earth's Water
Driven by the sun and gravity.
71\% of Earth's surface is covered by water.
Approximately 97\% of Earth's water is saltwater, with an average salinity of 35 parts per thousand (ppt) or 3.5\%.
Approximately 3\% is freshwater.
Only about 0.024\% of Earth's water is available for direct human consumption.
Key Processes in the Hydrologic Cycle
Evaporation: The conversion of liquid water into water vapor, primarily from surface water bodies and land.
Transpiration: The evaporation of water from the leaves (stoma) of plants, where water extracted from the soil by roots is transported throughout the plant and then released as vapor.
Condensation: The conversion of water vapor into droplets of liquid water, forming clouds or dew.
Precipitation: Water released from clouds in the form of rain, sleet, hail, or snow.
Infiltration: The movement of water from the surface into the soil.
Percolation: The downward flow of water through soil and permeable rock formations to groundwater storage areas called aquifers.
Runoff: The downslope surface movement of water, eventually returning to the sea or other major water bodies to continue the cycle.
Sublimation: The process where ice or snow changes directly into water vapor without first melting.
Deposition: The process where water vapor changes directly into ice (e.g., frost).
Groundwater Recharge and Flow: Water infiltrating the ground and moving through underground aquifers.
Reservoirs in the Hydrologic Cycle
Oceans: Primary reservoir, accounting for roughly 97\% of Earth's water.
Ice Caps and Glaciers: Significant reservoirs of freshwater, though smaller than oceans.
Groundwater Storage (Aquifers): Underground layers of permeable rock that hold water.
Lakes and Rivers: Surface freshwater bodies.
Atmosphere: Stores water vapor.
Soil Moisture: Water held within the soil layer.
Plants and Animals: Contain water as part of their biological composition.
Permafrost: Permanently frozen ground containing ice.
Human Impacts on the Hydrologic Cycle
Human activities significantly alter the natural water cycle:
Groundwater Depletion: Excessive pumping for agriculture, industry, and drinking water leads to declining water tables and aquifer exhaustion.
Clearing Vegetation (Deforestation): Reduces transpiration and infiltration, leading to increased surface runoff, soil erosion, and potential flooding.
Dams and Water Diversion Projects: Alter natural river flows, create reservoirs, change local evaporation rates, and impact ecosystems downstream.
Covering Land with Crops and Buildings: Reduces infiltration and percolation, leading to increased surface runoff and decreased groundwater recharge. This can also increase flooding.
Point Source Pollution: Contaminates surface water and groundwater.
Global Warming: Leads to changes in precipitation patterns, increased melting of glaciers and ice caps, and altered evaporation rates, intensifying the cycle in some areas and causing droughts in others.
Carbon Cycle
Fundamental Processes
Photosynthesis:
Performed by plants, algae, and some bacteria.
Converts atmospheric carbon dioxide into usable carbon (carbohydrates).
Equation: 6CO2 + 6H2O + \text{Solar energy} \rightarrow C6H{12}O6 + 6O2
Energy is consumed, carbohydrates (glucose, e.g., C6H{12}O6) are produced, and oxygen (O2) is released as a waste product.
Chemosynthesis:
Found in some bacteria, particularly in deep-sea vents.
Equation: CO2 + S^{2-} + H2O \rightarrow (CH2O)n + SO_4^{2-}
Converts carbon dioxide using chemical energy (e.g., from sulfide compounds) into organic matter.
Aerobic Respiration:
Performed by all organisms.
Converts carbohydrates back into carbon dioxide, releasing energy for use.
Equation: C6H{12}O6 + 6O2 \rightarrow 6CO2 + 6H2O + \text{Energy}
Oxygen is consumed, carbon dioxide (CO2) and water (H2O) are released as waste products.
Air-Sea Gas Exchange:
Oceans absorb CO_2 from the atmosphere (carbon sink).
Oceans release CO_2 to the atmosphere (carbon source).
Storage in Producers and Consumers: Carbon is stored in living organisms. Upon death, this organic matter enters sediments, leading to longer-term storage.
Thawing Permafrost: Releases stored carbon (as CO2 or methane, CH4) as frozen organic matter decomposes.
Burning Carbon-Containing Compounds: Examples include forest fires and the combustion of fossil fuels, which release large amounts of CO_2 into the atmosphere.
Carbon Reservoirs and Average Residence Times
Atmosphere: Short term (mainly as CO_2 gas), approximately 750 GtC (Gigatons of Carbon).
Vegetation: Approximately 610 GtC.
Soils: Middle term, around 25-30 years (carbonate sediments, rocks), approximately 1,580 GtC.
Surface Ocean: Short to middle term, approximately 1,020 GtC.
Deep Ocean: Long term, around 1,500 years (deep marine sediments), approximately 38,100 GtC.
Marine Biota: Approximately 3 GtC.
Dissolved Organic Carbon: Less than 700 GtC.
Sediments: Approximately 150 GtC.
Rocks (Limestone, Marble): Very long term.
Fossil Carbon (Coal, Oil, Natural Gas): Very long term, approximately 4,000 GtC.
Carbon Fluxes (in GtC/yr)
Plant uptake (Photosynthesis): 121.3 from atmosphere to vegetation.
Respiration from vegetation: 60 from vegetation to atmosphere.
Respiration/Decomposition from soils: 60 from soils to atmosphere.
Ocean absorption: 92 from atmosphere to surface ocean.
Ocean release: 90 from surface ocean to atmosphere.
Surface ocean to deep ocean: 50.
Deep ocean to surface ocean: 91.6.
Fossil fuels and cement production: 5.5 from fossil fuels to atmosphere.
Rivers: carbon transport, approximately 1.6.
Storage in Ice and Snow (Sediments): Approximately 0.2.
Net atmospheric increase: 0.5 GtC/yr (balance of fluxes).
Human Impacts on the Carbon Cycle
Human activities have profoundly impacted the carbon cycle, primarily leading to an increase in atmospheric CO_2:
Combustion of Fossil Fuels: Burning hydrocarbons (e.g., Hx Cy + O2 \rightarrow CO2 + H2O for complete combustion) releases vast amounts of stored carbon into the atmosphere as CO2.
Deforestation:
Loss of Carbon Sink: Forests act as major carbon sinks through photosynthesis. Deforestation reduces this capacity.
Slash-and-Burn Techniques: Directly release large quantities of CO_2 and other greenhouse gases into the atmosphere.
Climate Change Feedback Loops:
Warmer Oceans: May release more dissolved CO_2 as water warms (reduced solubility of gases in warmer liquids).
Ocean Acidification: Increased atmospheric CO2 leads to more CO2 being absorbed by oceans, forming carbonic acid, which increases ocean acidity and negatively impacts marine life.
Warming Permafrost: Leads to decomposition of organic matter in previously frozen soils, releasing methane (CH4), a potent greenhouse gas, and CO2.
Atmospheric Carbon Dioxide Concentrations
Measured at Mauna Loa, Hawaii, showing a continuous increase over decades from approximately 310 ppmv in 1960 to over 390 ppmv in 2010, with an annual seasonal cycle superimposed.
Nitrogen Cycle
Essential Knowledge
The nitrogen cycle describes the movement of atoms and molecules containing nitrogen between sources and sinks.
Most nitrogen compounds in the nitrogen cycle have relatively short residence times in their reservoirs.
Nitrogen fixation is the process where atmospheric nitrogen (N2) is converted into forms (primarily ammonia, NH3) usable by plants and for plant tissue synthesis.
The atmosphere is the major reservoir of nitrogen (N_2).
Characteristics and Major Stores
Major Store: Atmosphere, primarily as molecular nitrogen (N_2), which constitutes about 78\% of the air. It is biologically inert in this form.
Limiting Nutrient: Nitrogen is often a limiting nutrient in marine ecosystems.
Key Processes in the Nitrogen Cycle
Nitrogen Fixation: Converts atmospheric nitrogen (N2) into usable forms (ammonia, NH3).
Abiotic Fixation: High-energy events like lightning and cosmic radiation convert N_2 to nitrogen oxides.
Biotic Fixation: Soil microorganisms (e.g., Rhizobium bacteria) or free-living bacteria convert N2 into ammonia (NH3). Many symbiotic bacteria are found in the root nodules of leguminous plants (clover, alfalfa, soybeans, chickpeas).
Ammonification:
Decomposers break down nitrogenous wastes and organic matter from dead plants and animals, producing ammonia (NH3) or ammonium ions (NH4^+).
For example, NH4^+ is produced from NH3.
Nitrification: Converts ammonia/ammonium into forms usable by most plants.
Step A: Nitrosomonas bacteria convert ammonia (NH3) or ammonium (NH4^+) ions to nitrite (NO_2^-) ions.
Step B: Nitrobacter bacteria convert nitrite (NO2^-) ions to nitrate (NO3^-) ions.
Assimilation (Uptake):
Plants absorb inorganic nitrogen compounds (predominantly nitrate, NO_3^-, but some can use ammonia and ammonium) through their roots and incorporate them into organic molecules like proteins and DNA.
Heterotrophs (animals) assimilate nitrogen by consuming plants or other animals.
Denitrification:
Anaerobic bacteria (e.g., Pseudomonas) convert nitrates (NO3^-) in the soil back into gaseous molecular nitrogen (N2) and nitrous oxide (N_2O), returning them to the atmosphere. This process occurs in oxygen-poor conditions.
Nitrogen Reservoirs and Fluxes (in Teragrams N/year for fluxes)
Atmospheric Nitrogen (N_2): Major reservoir.
Soil Microbes: Store approximately 1,200 Teragrams (Tg) of nitrogen.
Fluxes:
Atmospheric fixation: 100 Tg N/year.
Biological fixation (free-living and symbiotic): 228 Tg N/year.
Industrial fixation (Haber-Bosch process): 100 Tg N/year.
Denitrification: 193 Tg N/year.
Ammonia volatilization: 5 Tg N/year.
Plant & animal wastes / decaying organic matter: contribute to ammonification.
Leaching of nitrates to groundwater: 36 Tg N/year.
Human Impacts on the Nitrogen Cycle
Anthropogenic activities have significantly altered the nitrogen cycle, often leading to environmental problems:
Combustion of Fossil Fuels: Releases nitrogen oxides (NO_x) into the atmosphere.
NOx contributes to photochemical smog and acid rain (HNO3 formation).
Fertilizers (Haber-Bosch Process): Industrial production of nitrogen fertilizers (100 Tg N/year input).
Nitrogen Runoff: Excess nutrients from agricultural runoff enter aquatic ecosystems.
Eutrophication: This runoff leads to excess nutrients in water bodies, causing rapid algae blooms.
Oxygen Depletion: When algae die, their decomposition by bacteria consumes dissolved oxygen (DO), leading to declines in DO and potential death of fish and other aquatic life (hypoxia/anoxia).
Nitrous Oxide (N2O) Release: Fertilizers can release N2O, which is both a potent greenhouse gas and an ozone-depleting compound.
Planting Excessive Nitrogen-Fixing Crops: Increases the rate of nitrogen fixation, adding more usable nitrogen to the soil than natural processes.
Runoff from Feedlots (Manure): Concentrated animal waste from large-scale livestock operations contributes significant amounts of nitrogen (and phosphorus) to surrounding environments through runoff, intensifying eutrophication. The total human input of nitrogen has risen sharply from near 0 Tg N/year in 1900 to over 250 Tg N/year by 2000, with projections to continue increasing.
Phosphorous Cycle
Characteristics
Sedimentary Cycle: Primarily involves the movement of phosphorus through rock, soil, and water, with very little atmospheric component. It is a slow cycle.
Limiting Nutrient: Phosphorus is often a limiting nutrient in aquatic systems due to its relatively low availability.
Major Stores
Phosphate Rock: The largest reservoir of phosphorus, primarily in mineral forms like apatite (Ca3(PO4)_2). It reaches the surface through geological uplift and weathering.
Marine Sediments: Significant long-term storage of phosphorus.
Dissolved in Soils and Water: Available for uptake by producers.
Guano: Accumulations of bird or bat excrement, rich in phosphorus and nitrogen, serving as a natural fertilizer.
Key Processes
Weathering: Releases phosphate from rocks into soil and water.
Uptake by Producers: Plants absorb dissolved phosphate from soil or water.
Consumption: Animals obtain phosphorus by eating plants or other animals.
Decomposition: Decomposers break down organic matter, returning phosphate to soil and water.
Sedimentation: Phosphate in aquatic systems can settle and become incorporated into marine sediments, eventually forming sedimentary rock.
Mining: Phosphate rock is mined, primarily as apatite (Ca3(PO4)_2). The largest mine is reportedly near Tampa, FL.
Biological Fixation: Mycorrhizal fungi form symbiotic relationships with plant roots, enhancing phosphorus uptake by plants (keystone species for phosphorus absorption).
Phosphorus Reservoirs and Fluxes
Sedimentary Rocks: Largest pool, approximately 840,000,000 units (e.g., Gt).
Soil: Contains approximately 125,000 units.
Oceans (dissolved): Approximately 1,000 units, about 1,000 times the amount in marine organisms.
Oceans (organisms): Approximately 85 units.
Detritus: Approximately 650 units.
Fresh Water: Approximately 90 units.
Land Organisms: Approximately 2,600 units.
Mineable Rock: Approximately 19,000 units.
Fluxes:
Application of fertilizer to soil: 200 units.
Runoff from land to oceans: 21 units.
Atmospheric deposition of P onto land: 1.0 units.
Atmospheric deposition into oceans: 0.03 units.
Movement from land to atmosphere: 3.2 units.
Movement from oceans to atmosphere: 4.2 units.
Cycling between organisms and soil: Not explicitly numbered in diagram but implied.
Uplift of marine sedimentary rocks returns phosphorus to terrestrial ecosystems.
Human Impacts on the Phosphorous Cycle
Human activities accelerate the phosphorus cycle and can lead to depletion and pollution:
Over-Mining: Phosphorus is being removed from phosphate rock reservoirs faster than it can be replenished by natural geological processes, making it a non-renewable resource in human timescales.
Fertilizers:
Lead to Excess Nutrients: Runoff from excessive use of phosphate fertilizers.
Eutrophication: Similar to nitrogen, this causes algae blooms in aquatic systems.
Oxygen Depletion: Dead algae decompose, consuming dissolved oxygen, leading to the death of fish and other aquatic life.
Phosphate-Containing Detergents: Historically, detergents were a major source of phosphate pollution, contributing to eutrophication in waterways (though many regions now ban or limit their use).
Sulfur Cycle
Characteristics
Gaseous Cycle: Sulfur can exist in gaseous forms (e.g., SO2, H2S, DMS), facilitating its movement through the atmosphere.
Major Stores
Rocks: Primary long-term reservoir, particularly in sulfide minerals (e.g., iron sulfide, Fe2S) and sulfate minerals (e.g., calcium sulfate, CaSO4).
Sediments: Sulfur is stored in ocean sediments.
Oceans: Dissolved sulfates (SO_4^{2-}) are present in oceans.
Natural Sources of Sulfur Release
Volcanoes: Release sulfur dioxide (SO2) and hydrogen sulfide (H2S) into the atmosphere, which can then react to form sulfuric acid (H2SO4).
DMS (Dimethyl Sulfide): Produced by marine phytoplankton in the oceans, contributing to atmospheric sulfur.
Hydrogen Sulfide (H_2S): Released from the decay of organic matter in anaerobic conditions (e.g., wetlands, swamps).
Sea Spray: Releases sulfates (SO_4^{2-}) from the ocean into the atmosphere from wind and wave action (approximately 35 units).
Biogenic H2S, (CH3)_2S: Approximately 33 units from biological activity.
Key Processes and Fluxes
Breakdown of organic matter by decomposers releases sulfur.
Volcanic emissions: Approximately 10 units of H2S, SO2, and sulfates.
Rainfall and dry deposition over land: Approximately 100 units of SO_2 and sulfates.
Rainfall, dry deposition, and absorption over oceans: Approximately 35 units of SO_2 and sulfates.
Sulfur applied to soil and transported by rivers to oceans: Approximately 15 units.
SO_2 and sulfates taken up by plants.
Human Impacts on the Sulfur Cycle
Human activities have significantly increased the atmospheric concentration of sulfur compounds:
Combustion of Coal and Petroleum: Releases large quantities of sulfur oxides (SOx), primarily sulfur dioxide (SO2), into the atmosphere.
Acid Rain: SO2 reacts with water vapor and other atmospheric chemicals to form sulfuric acid (H2SO_4), which is a major component of acid rain. Acid rain harms ecosystems, corrodes infrastructure, and acidifies soils and water bodies.
Smelting Operations: Extracting metals from sulfide ores also releases SO_2.
Historical Evidence: Ice core samples show a significant increase in atmospheric sulfur concentrations since the Industrial Revolution, highlighting the anthropogenic impact.