noc19-bt09 Lecture 12-Nutrient Cycles

Understanding Nutrient Cycles in Ecological Energetics

Definition of Nutrients

• A nutrient is defined as a substance used by an organism to survive, grow, and reproduce.

• Organisms obtain nutrients from food sources, which include proteins, carbohydrates, fats, and minerals.

Types of Nutrients

Macronutrients vs. Micronutrients

• Organisms require certain nutrients in larger quantities (macronutrients) such as carbohydrates, proteins, and fats compared to trace elements like sodium or selenium.

• In plants, primary macronutrients include nitrogen (N), phosphorus (P), and potassium (K), often referenced in fertilizers.

Essential vs. Non-Essential Nutrients

• Essential nutrients are those that cannot be substituted; their absence prevents the completion of an organism's life cycle.

• Three criteria determine if an element is essential:

1. The plant cannot complete its life cycle without it.

2. Its deficiency cannot be compensated by other elements.

3. It must be directly involved in the plant's metabolism.

Key Essential Elements for Plants

Nitrogen

• Nitrogen is crucial for plant growth as it forms part of proteins and nucleic acids (DNA/RNA).

Phosphorus

Phosphorus and Other Essential Nutrients in Plants

The Role of Phosphorus

• Phosphorus is a key component of the cell membrane, which separates the internal environment of the cell from its external surroundings.

• It is essential for plant life; there are no substitutes for phosphorus as it plays a critical role in metabolism, nucleic acids, ATP, and proteins.

Importance of Potassium

• Potassium (K), also known as kalium, is vital for maintaining cation-anion balance necessary for cell turgidity and enzyme activation.

• Turgidity refers to the pressure within a cell; when water is removed, cells become placid. Conversely, inflating a balloon represents turgidity in plants.

• Osmolarity affects turgidity; higher salt concentrations attract more water into cells, increasing their turgidity. Potassium regulates this process and influences stomatal function.

Calcium's Functionality

• Calcium contributes to forming calcium pectate in cell walls, crucial during cell division by separating daughter cells.

• It also activates certain enzymes and plays a role in calcium channels within cell membranes.

Magnesium's Contribution

• Magnesium is an essential part of chlorophyll; without it, plants cannot photosynthesize or respire effectively.

• It also activates various respiration enzymes.

Overview of Nutrient Categories

Macronutrients vs Micronutrients

• Essential elements can be categorized into macronutrients (needed in large amounts) and micronutrients (required in smaller quantities).

Macronutrient Breakdown

Primary Macronutrients

• Carbon (C), Hydrogen (H), and Oxygen (O): sourced from air and water. Carbon comes from CO2 while hydrogen and oxygen come from H2O.

Secondary & Tertiary Macronutrients

• Nitrogen (N), Phosphorus (P), Potassium (K): derived from soil minerals or fertilizers. Sulfur, Calcium, and Magnesium are secondary macronutrients needed less than NPK but still significantly important.

List of Micronutrients

Understanding Biogeochemical Cycles

The Role of Nutrients in Plant Growth

• Plants require various nutrients for growth, but these nutrients are finite on Earth. Their availability is managed through biogeochemical cycles.

Nutrient Cycling Process

• Nutrients absorbed by plants are transferred to animals when they consume the plants, creating a cycle where nutrients return to the soil after decomposition.

• This cyclical movement involves biological (living organisms), geological (earth processes), and chemical processes, collectively termed biogeochemical cycles.

Definition and Components of Biogeochemical Cycles

• A biogeochemical cycle is defined as a pathway through which chemical substances move between biotic (living) and abiotic (non-living) components of the Earth, including lithosphere, atmosphere, and hydrosphere.

• Nutrient pools can exist in various forms: in soil, water, or air. For example, carbon exists in the atmosphere as carbon dioxide before being utilized by plants.

Decomposition and Nutrient Return

• Decomposers like earthworms break down organic matter such as dead leaves into simpler compounds like carbon dioxide, returning essential nutrients back into the nutrient pool.

• Calcium from soil is taken up by plants and eventually returned to the soil through animal waste or decomposition.

Exploring the Nitrogen Cycle

Understanding Nitrogen Pools

• The nitrogen cycle consists of two main pools: atmospheric nitrogen (over 70% of air) and soil nitrates. Plants cannot directly use atmospheric nitrogen; they need it in nitrate form.

Mechanisms for Nitrogen Fixation

• Atmospheric nitrogen reaches the soil primarily through biological fixation by bacteria like rhizobium found in leguminous plant root nodules.

• Lightning also contributes to this process by converting nitrogen gas into nitrates during storms due to high heat and electrical discharge.

Industrial Contributions to Nitrogen Availability

• Industrial fixation methods artificially convert atmospheric nitrogen into nitrates for agricultural purposes, enhancing nutrient availability for crops.

Movement Between Nitrogen Pools

• Denitrification and volcanic activity allow nitrates from the soil pool to revert back into atmospheric nitrogen, completing the cycle.

Understanding the Nitrogen Cycle

Overview of the Nitrogen Cycle

• The nitrogen cycle involves the movement of nitrogen through various forms, including soil nitrates, plants, animals, and decomposers. Decomposers play a crucial role in breaking down dead organic matter.

Key Processes in the Nitrogen Cycle

• Two main components are discussed: pools (storage areas for nitrogen) and fluxes (the rate at which nitrogen moves between these pools).

• Flux refers to how quickly soil nitrates are absorbed by plants, illustrating the dynamic nature of nutrient transfer.

Nitrogen Fixation

• Nitrogen fixation is the conversion of atmospheric nitrogen into ammonia via biological processes, lightning strikes, or industrial methods.

• Biological nitrogen fixation primarily occurs through organisms like rhizobium bacteria found in leguminous plants and certain free-living bacteria such as azotobacter.

Ammonification Process

• Ammonification converts organic nitrogen from decaying plants and animals into ammonia through decomposer activity. For example, bacterial action on proteins can release ammonia back into the environment.

Nitrification Process

• Nitrification transforms toxic ammonia into nitrites and then nitrates through biological oxidation involving specific bacteria like nitrosomonas and nitrobacter.

Industrial Processes Related to Nitrogen

Haber Process

• The Haber process synthesizes ammonia from nitrogen and hydrogen gases under high temperature and pressure with a catalyst.

Oswald Process

• Following ammonia production, the Oswald process further oxidizes it to produce nitric acid (HNO3), which can be combined with bases to form nitrate salts like sodium nitrate (NaNO3).

Introduction to the Carbon Cycle

Main Pools of Carbon

• The carbon cycle focuses on atmospheric carbon as its primary pool but also includes carbon stored in oceans, soils, and forests.

Weathering Process

• Weathering involves reactions between atmospheric carbon dioxide and minerals in rocks. For instance, calcium hydroxide reacts with CO2 to form calcium carbonate (CaCO3), releasing water in the process.

Carbon Cycle and Its Processes

Carbon Movement Between Pools

• Carbon in the atmosphere can become trapped in rocks as calcium carbonate (CaCO₃), which is released through tectonic processes such as plate collisions and volcanic activity.

• Heating CaCO₃ produces calcium oxide and carbon dioxide (CO₂), allowing carbon to transition from the atmosphere to the lithosphere, demonstrating its mobility between different pools.

• CO₂ can dissolve in ocean water, forming carbonic acid (H₂CO₃), illustrating how atmospheric carbon can enter oceanic systems through dissolution.

• Through photosynthesis, producers convert atmospheric carbon into biomass, which then moves through food webs as it is consumed by herbivores and carnivores.

• Organic matter can be transformed into fossil fuels via lithification, where buried plants and animals gradually convert into substances like petroleum or coal.

Release of Carbon Back to Atmosphere

• Fossil fuels release carbon back into the atmosphere when burned, completing a significant part of the carbon cycle through combustion processes.

Water Cycle Overview

Stages of Water Cycle

• The water cycle involves evaporation from bodies of water due to solar heat, leading to condensation and cloud formation.

• Precipitation occurs when clouds cool down enough for water droplets to fall as rain onto land or water bodies.

• Rainwater either percolates into soil or runs off into streams and rivers, eventually reaching larger bodies of water like oceans.

Additional Processes in Water Cycle

• Plants absorb groundwater; during transpiration, they release moisture back into the atmosphere, contributing further to the cycle's continuity.

Phosphorus Cycle Dynamics

Phosphorus Sources and Transformation

• The phosphorus cycle primarily revolves around rock phosphates that weather over time to form soluble soil phosphates.

Movement Through Ecosystems

• Soil phosphates are taken up by plants; this phosphorus then transfers through food chains from plants to animals and decomposers before returning to soil.

Phosphorus and Sulfur Cycles: Understanding Natural Processes

Phosphorus Cycle

• The phosphorus cycle involves the conversion of oceanic sediments into rock phosphate through tectonic processes, which can later be exposed to weathering.

• Weathering allows phosphorus to re-enter the ecosystem, where it is taken up by plants and transferred through the food chain to animals and decomposers.

Sulfur Cycle

• The sulfur cycle begins with sulfur compounds in the atmosphere that react with water during rainfall, forming sulfates absorbed by plants.

• Human activities, particularly fossil fuel combustion, release significant amounts of sulfur dioxide (SO₂), contributing to acid rain and soil acidity.

• Volcanic eruptions also release large quantities of sulfur into the atmosphere, which can lead to further environmental impacts.

Human Impact on Biogeochemical Cycles

Carbon Cycle

• Increased carbon dioxide emissions from burning fossil fuels are leading to climate change and global warming due to rising atmospheric concentrations.

Sulfur Cycle Impacts

• Industrial processes contribute significantly to SO₂ emissions, affecting air quality and leading to health issues related to acid rain.

Nitrogen Cycle Changes

• Industrial nitrogen fixation alters natural nitrogen levels in ecosystems; excess nitrates from fertilizers can disrupt ecological balance.

• This alteration may result in increased plant growth but also leads to negative consequences like eutrophication in aquatic systems.

Water Cycle Dynamics in Forest Ecosystems

Rainfall Interactions

• Rainfall in forests interacts with soil through infiltration or runoff; some water is intercepted by trees, affecting its flow rate back into the ecosystem.

Water Evaporation Processes

Impact of Deforestation on Stream Flows

Changes in Stream Flow Due to Forest Cutting

• The Kuczera curve illustrates the impact of forest cutting on stream flows, showing a drastic reduction in average annual yield from streams over time.

• Initially, it may seem counterintuitive that removing trees decreases stream flow; however, trees absorb water and release it into the atmosphere, limiting water reaching streams.

• After deforestation, stream flow can drop from 1200 mm to 600 mm within approximately 20 years, indicating a significant decline in available water.

• Recovery of stream flow back to normal levels (1200 mm) can take around 150 years as forests gradually regrow.

Nutrient Dynamics and Plant Growth Post-Deforestation

• Following deforestation, nutrients remain in the soil and precipitation continues; thus, some plant growth occurs despite tree removal.

• As plants grow, they convert nutrients into biomass (cellulose), raising questions about their water usage through transpiration.

Transpiration and Water Availability Over Time

• In young forests (around 15 years old), overstorey transpiration is high while runoff is low due to significant water loss through plant processes.

• As forests mature, overstorey transpiration decreases significantly because larger plants require less water for maintenance compared to growth phases.

• Conversely, understorey transpiration increases slightly as smaller plants utilize more water. This shift leads to an increase in runoff as losses decrease.

Effects of Deforestation on Water Chemistry

• Deforestation also impacts the pH levels of stream waters; healthy watersheds typically have a pH around 5.5 to 6.

• In areas where deforestation occurs, such as specific watersheds with cleared trees draining into streams, pH levels can drop significantly.

Consequences of Acidic Water Post-Deforestation

• A lower pH indicates increased acidity; for instance, a pH close to 4 is highly acidic and detrimental for aquatic organisms' survival.

Impact of Deforestation on Water Quality and Nutrient Cycles

Nitrate Concentration Changes

• The most significant change observed is in nitrate concentration, with undisturbed forests showing around 2 mg/L of nitrates.

• In deforested watersheds, nitrate levels can spike to as high as 60 mg/L shortly after deforestation, indicating a drastic increase.

• Water with 2 mg/L nitrates is considered potable for both humans and animals; however, water with 60 mg/L poses health risks such as methaemoglobinaemia.

Algal Blooms and Ecosystem Impact

• High nitrate concentrations can lead to algal blooms, which were notably observed in the Potomac River.

• The increase from 2 mg/L to 60 mg/L represents a thirty-fold rise in nitrate levels due to deforestation.

Other Nutrient Concentrations Affected

• Sulfate concentrations decrease significantly from about 7 mg/L to approximately 3 mg/L following deforestation.

• Calcium concentration rises dramatically from 10 mg/L to up to 110 mg/L due to soil exposure after tree removal.

Soil and Nutrient Dynamics

• The increase in calcium is attributed not only to soil disturbance but also because plant roots that previously held the soil are removed.

• Magnesium levels more than double post-deforestation; potassium increases fourfold, sodium nearly doubles, and aluminum concentration triples.

Human Activities and Nutrient Cycles

• Deforestation leads to substantial changes in water quality affecting drinking sources for wildlife and humans alike.

• Fertilizer runoff contributes further nutrient loading into water bodies, exacerbating algal blooms that deplete oxygen levels necessary for aquatic life.

Summary of Key Concepts Discussed

• The lecture covered nutrient classifications (macro vs. micro), their roles, essential elements criteria, generalized nutrient cycles, and specific human impacts like deforestation on these cycles.