Biogeochemical cyclesWater Cycle, Carbon Cycle, Nitrogen Cycle, and Phosphorus Cycle

Biogeochemical cycles - the movement of chemical elements and compounds between biological (living organisms) and geological (earth, atmosphere, water) components of the Earth.

These cycles play a crucial role in maintaining ecosystem stability and sustaining life.

The major biogeochemical cycles include the Water Cycle, Carbon Cycle, Nitrogen Cycle, and Phosphorus Cycle.

1. Biogeochemical Cycles: Water, Carbon, Nitrogen, and Phosphorus Dr. Vividha Raunekar

2. Biogeochemical Cycles: Water, Carbon, Nitrogen, and Phosphorus Biogeochemical cycles - the movement of chemical elements and compounds between biological (living organisms) and geological (earth, atmosphere, water) components of the Earth. These cycles play a crucial role in maintaining ecosystem stability and sustaining life. The major biogeochemical cycles include the Water Cycle , Carbon Cycle , Nitrogen Cycle , and Phosphorus Cycle .

3. 1. Water Cycle (Hydrological Cycle) The water cycle describes the continuous movement of water within the Earth’s atmosphere, surface, and underground reservoirs. It is driven by solar energy and involves various physical processes.

4. Major Steps of the Water Cycle: Evaporation: Solar heat causes water from oceans, rivers, lakes, and soil to convert into water vapor. Transpiration (water loss from plant surfaces) and sublimation (direct conversion of ice to vapor) also contribute to water vapor in the atmosphere. Condensation: Water vapor rises, cools, and condenses into tiny droplets, forming clouds. Precipitation: When the droplets in clouds grow large enough, they fall back to Earth as rain, snow, sleet, or hail. Infiltration & Percolation: Some precipitation infiltrates the soil, replenishing groundwater. Water may percolate deeper into aquifers. Runoff & Collection: Excess water flows over land into rivers, lakes, and oceans, completing the cycle.

5. Importance of the Water Cycle: Maintains global climate balance. Provides fresh water to terrestrial ecosystems. Regulates temperature and weather patterns.

6. Carbon Cycle The carbon cycle describes the movement of carbon between the biosphere, atmosphere, hydrosphere, and geosphere. Carbon is an essential element of life, found in organic molecules like carbohydrates, proteins, lipids, and nucleic acids.

7. Major Steps of the Carbon Cycle: Photosynthesis: Plants, algae, and cyanobacteria absorb atmospheric CO₂ and use it to produce organic molecules (glucose) via photosynthesis. Respiration: Organisms (plants, animals, and microbes) consume organic molecules and release CO₂ back into the atmosphere through cellular respiration. Decomposition: Dead organisms and waste products are decomposed by bacteria and fungi, releasing carbon into the soil or atmosphere. Carbon Sequestration (Sedimentation & Fossilization): Some organic carbon is buried over millions of years, forming fossil fuels (coal, oil, natural gas). Carbon can also be stored in ocean sediments, forming carbonate rocks (e.g., limestone). Combustion of Fossil Fuels: Human activities like burning fossil fuels release stored carbon as CO₂ into the atmosphere, contributing to climate change. Oceanic Carbon Exchange: The ocean absorbs CO₂ from the atmosphere, forming carbonic acid, which influences ocean pH and marine ecosystems. Marine organisms use dissolved carbon to build calcium carbonate shells, which later contribute to sedimentary rock formation.

8. Importance of the Carbon Cycle: Regulates global temperature and climate. Provides carbon for biological molecules and ecosystems. Excess atmospheric CO₂ contributes to global warming.

9. Nitrogen Cycle The nitrogen cycle describes the movement of nitrogen through the atmosphere, soil, water, and living organisms. Nitrogen is a crucial component of amino acids, proteins, DNA, and RNA. However, atmospheric nitrogen (N₂) is unusable by most organisms and must be converted into biologically available forms.

10. Major Steps of the Nitrogen Cycle: Nitrogen Fixation: Atmospheric nitrogen (N₂) is converted into ammonia (NH₃) by nitrogen-fixing bacteria (e.g., Rhizobium in legume root nodules, Azotobacter in soil). Lightning and industrial processes (Haber-Bosch process) also fix nitrogen. Nitrification: Ammonia (NH₃) is converted into nitrites (NO₂⁻) by Nitrosomonas bacteria. Nitrites (NO₂⁻) are further converted into nitrates (NO₃⁻) by Nitrobacter bacteria. Assimilation: Plants absorb nitrates (NO₃⁻) from the soil and incorporate nitrogen into organic molecules (proteins, nucleic acids). Animals obtain nitrogen by consuming plants or other animals. Ammonification (Decomposition): Decomposers (bacteria and fungi) break down dead organisms and waste, converting organic nitrogen back into ammonia (NH₃) or ammonium (NH₄⁺). Denitrification: Denitrifying bacteria (e.g., Pseudomonas, Clostridium) convert nitrates (NO₃⁻) back into atmospheric nitrogen (N₂), completing the cycle.

11. Importance of the Nitrogen Cycle: Essential for protein synthesis in all organisms. Maintains soil fertility and ecosystem productivity. Human activities (fertilizer use, fossil fuel burning) disrupt the nitrogen cycle, causing pollution (e.g., eutrophication, acid rain).

12. Phosphorus Cycle The phosphorus cycle differs from other cycles because it does not involve a gaseous phase . Phosphorus is mainly found in rocks, soil, and living organisms. It is essential for DNA, RNA, ATP, and phospholipids in cell membranes.

13. Major Steps of the Phosphorus Cycle: Weathering & Erosion: Phosphorus is released from rocks through weathering, entering the soil and water systems. Absorption by Plants & Animals: Plants absorb phosphate (PO₄³⁻) from soil and incorporate it into biological molecules. Herbivores and carnivores obtain phosphorus through their diet. Decomposition & Mineralization: When organisms die, decomposers (bacteria, fungi) break down their tissues, returning phosphorus to the soil or sediments. Sedimentation & Geological Uplift: Phosphates in water bodies settle into sediments and can eventually form new rock formations. Over geological time, tectonic activity uplifts these rocks, restarting the cycle.

14. Importance of the Phosphorus Cycle: Essential for energy transfer (ATP) and genetic material. Key nutrient for plant growth (limiting nutrient in many ecosystems). Human activities (fertilizer use, sewage discharge) contribute to eutrophication, harming aquatic life.

15. Pools, Fluxes, and Mass Budget Models in Biogeochemical Cycles Understanding biogeochemical cycles requires analyzing how elements and compounds are stored ( pools ) and how they move between these storage locations ( fluxes ). These dynamics can be quantified using mass budget models , which help scientists track changes, predict future trends, and assess human impacts on ecosystems.

16. Pools (Reservoirs) in Biogeochemical Cycles A pool (reservoir) refers to a storage location where a chemical element or compound is accumulated for varying periods. Pools can be categorized based on their stability and exchange rates: Major types of pools: Atmospheric Pools – Gases like CO₂ (carbon cycle), N₂ (nitrogen cycle), and H₂O vapor (water cycle) are stored in the atmosphere. Biospheric Pools – Living organisms store elements in organic molecules (e.g., plants store carbon in glucose via photosynthesis). Soil and Sediment Pools – Organic matter, humus, minerals, and fossilized material in soils or ocean sediments. Hydrospheric Pools – Elements dissolved in water bodies (e.g., dissolved CO₂ in oceans, phosphates in lakes). Geospheric (Lithospheric) Pools – Long-term storage in rocks, minerals, and fossil fuels (e.g., calcium carbonate in limestone stores carbon).

17. Cycle Major Pools Water Cycle Atmosphere (water vapor), oceans, lakes, glaciers, groundwater Carbon Cycle Atmosphere (CO₂), terrestrial and marine biomass, fossil fuels, sedimentary rocks (limestone) Nitrogen Cycle Atmosphere (N₂), soil (nitrates, ammonium), living organisms, water bodies Phosphorus Cycle Rocks and minerals, soil (phosphate ions), living organisms, water sediments Examples of Pools in Biogeochemical Cycles

18. Fluxes (Transfers) in Biogeochemical Cycles A flux refers to the movement of an element or compound between pools. Fluxes are dynamic processes that regulate the flow of matter through ecosystems. Fluxes can be natural (e.g., photosynthesis, evaporation, sedimentation) or anthropogenic (e.g., fossil fuel combustion, deforestation, industrial emissions).. Flux Rates and Residence Time Flux rate refers to how fast an element moves between pools. Residence time is the average time an element stays in a pool before moving to another. Example: Atmospheric CO₂ has a short residence time (~5 years), whereas carbon stored in fossil fuels may remain for millions of years.

19. Cycle Major Fluxes Water Cycle Evaporation, transpiration, precipitation, runoff, infiltration Carbon Cycle Photosynthesis, respiration, decomposition, combustion, ocean-atmosphere exchange Nitrogen Cycle Nitrogen fixation, nitrification, assimilation, ammonification, denitrification Phosphorus Cycle Weathering, leaching, assimilation, decomposition, sedimentation Examples of Fluxes in Biogeochemical Cycles

20. Mass Budget Models in Biogeochemical Cycles A mass budget model quantifies the inputs, outputs, and storage of an element in a system. These models help estimate whether a system is in equilibrium, gaining, or losing mass over time. Components of a Mass Budget Model Inputs (Influxes) – Addition of an element to a system. Outputs (Outfluxes) – Removal of an element from a system. Storage Change (ΔS) – Net accumulation or loss within a pool. The general mass balance equation is: Input−Output=ΔS where ΔS is the net storage change in a reservoir.

21. Mass Budget Models in Biogeochemical Cycles (a) Water Cycle Mass Budget For a watershed or ecosystem: Precipitation−( Evaporation+Runoff+Infiltration )= Δ S If precipitation > outputs, water accumulates (e.g., lake expansion). If precipitation < outputs, water loss occurs (e.g., drought conditions).

22. (b) Carbon Cycle Mass Budget For an ecosystem: Photosynthesis−( Respiration+Decomposition+Fossil  Fuel Combustion)= Δ S Positive ΔS : More carbon is stored (e.g., in forests). Negative ΔS : More carbon is lost (e.g., deforestation, burning fossil fuels). This model helps assess carbon sequestration in forests and oceans.

23. (c) Nitrogen Cycle Mass Budget For agricultural systems: Fertilizer  Input+Nitrogen  Fixation−(Crop  Harvest+Leaching+Denitrification )= Δ S Excess nitrogen runoff can lead to eutrophication in aquatic systems. High nitrogen losses can deplete soil fertility, requiring more fertilizer. (d) Phosphorus Cycle Mass Budget For a lake ecosystem: Weathering of  Rocks+Fertilizer  Runoff−( Sedimentation+Plant  Uptake)=ΔS If input > output, phosphorus accumulates, leading to algal blooms . If output > input, phosphorus depletion affects aquatic productivity.

24. Human Activity Effect on Biogeochemical Cycles Deforestation Reduces carbon storage, increases atmospheric CO₂ Fossil Fuel Burning Increases CO₂ emissions, contributing to climate change Fertilizer Use Alters nitrogen and phosphorus cycles, causing eutrophication Urbanization Increases surface runoff, reduces groundwater recharge Industrial Emissions Increases nitrogen oxides, causing acid rain Human Impact and Alteration of Biogeochemical Cycles Human activities disrupt natural mass budgets , leading to environmental problems:

25. Applications of Mass Budget Models Climate Change Studies – Predicting future CO₂ levels and global warming effects. Agriculture & Soil Management – Assessing nutrient cycling for sustainable farming. Water Resource Management – Understanding precipitation, infiltration, and water scarcity. Ecosystem Conservation – Tracking nutrient depletion and pollution levels in forests, oceans, and wetlands.

26. Rates of Biogeochemical Processes: Productivity, Decomposition, Trophic Transfer, Turnover, and Mean Residence Time In biogeochemical cycles, rates of processes determine how quickly elements and energy move through ecosystems. These rates control the balance of nutrients, energy flow, and ecosystem stability. Key processes include productivity, decomposition, trophic transfer, turnover, and mean residence time (MRT) . Understanding these rates helps ecologists assess ecosystem function, sustainability, and human impact.

27. Productivity (Primary Production) Definition: Productivity refers to the rate at which energy is converted into organic material by autotrophs (producers) through photosynthesis or chemosynthesis. It determines the energy available for higher trophic levels. Types of Productivity: Gross Primary Productivity (GPP) The total rate at which plants convert solar energy into chemical energy (organic matter) via photosynthesis. Measured in grams of carbon fixed per square meter per year (g C/m²/ yr ). Net Primary Productivity (NPP) The energy remaining after plants use some for respiration ( R ). NPP = GPP - R Represents energy available to herbivores and decomposers . Secondary Productivity The rate at which consumers (herbivores, carnivores) convert assimilated food energy into their own biomass.

28. Ecosystem NPP (g C/m²/yr) Tropical Rainforest 2,000–3,000 Grasslands 600–1,500 Open Ocean 50–150 Coral Reefs 1,500–2,500 Factors Affecting Productivity: Light Availability – Higher in tropical forests, lower in deep oceans. Temperature – Higher temperatures increase enzyme activity, boosting productivity. Nutrient Availability – Nitrogen (N), Phosphorus (P), and Iron (Fe) limit productivity in terrestrial and aquatic ecosystems. Water Availability – Essential for plant photosynthesis and transpiration. Ecosystem Productivity Examples:

29. Decomposition: Definition: Decomposition is the process of breaking down organic matter into simpler substances (CO₂, water, nutrients). It is crucial for nutrient recycling in ecosystems. Stages of Decomposition: Leaching – Water dissolves and carries away soluble compounds. Fragmentation – Detritivores (earthworms, insects) break down organic matter into smaller pieces. Chemical Breakdown – Microbes (bacteria, fungi) break complex molecules (proteins, carbohydrates, lipids) into simpler compounds. Decomposition Rate Influences: Temperature – Higher temperatures increase microbial activity (faster decomposition in tropical forests than in arctic tundras ). Moisture – Wet conditions promote microbial activity, but extreme waterlogging slows oxygen-dependent decomposition. Nutrient Availability – High nitrogen speeds up microbial processes. Litter Quality – High lignin (woody plants) slows decomposition, while nitrogen-rich material decomposes faster.

30. Decomposition Rate Measurement: Litter Decay Rate Constant (k) : Ecosystem Decomposition Rate Tropical Rainforest Fast (weeks to months) Temperate Forest Moderate (months to years) Boreal Forest Slow (years to decades) Peat Bogs Extremely Slow (centuries) Ecosystem Differences in Decomposition Rates:

31. Trophic Transfer Efficiency Definition: Trophic transfer efficiency (TTE) is the proportion of energy or biomass transferred from one trophic level to the next. It typically ranges from 5% to 20% , meaning most energy is lost as heat (respiration) or waste. Key Efficiency Measures: Consumption Efficiency (CE) Fraction of available energy consumed by the next trophic level. Example: Herbivores may consume 40% of plant biomass. Assimilation Efficiency (AE) Fraction of consumed energy that is absorbed (not lost as feces). Carnivores (~80-90%) > Herbivores (~30-50%) because plant material is harder to digest. Production Efficiency (PE) Fraction of assimilated energy converted into new biomass. Ectotherms (fish, reptiles) ~50% > Endotherms (mammals, birds) ~10% due to energy lost as heat. Ecological Efficiency (EE) Overall efficiency of energy transfer between trophic levels: EE=CE×AE×PE

32. Turnover Rate Definition: Turnover rate is the rate at which a particular pool of an element or compound is replaced over time. Turnover Rate= Flux In or Out/ Pool size Element/Component Turnover Time Atmospheric CO₂ ~5 years Soil Organic Carbon ~50–100 years Phosphorus in Rocks ~10,000 years Forest Biomass ~20–200 years High Turnover – Rapid replacement (e.g., atmospheric CO₂, plankton populations). Low Turnover – Slow replacement (e.g., carbon in deep oceans, phosphorus in rocks). Examples of Turnover Times:

33. Mean Residence Time (MRT) Definition: MRT (also called residence time ) is the average time a molecule stays in a reservoir before being transferred to another. MRT= Pool Size​ /Flux Rate Short MRT → Rapid cycling (e.g., water in rivers, atmospheric CO₂). Long MRT → Slow cycling (e.g., phosphorus in sediments, carbon in fossil fuels). Pool MRT Water in Atmosphere ~9 days Carbon in Atmosphere ~5 years Soil Nitrogen ~1–5 years Oceanic Carbon ~500 years Phosphorus in Rocks ~10,000 years Examples of MRT in Different Reservoirs:

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