🪺Environmental Biology Unit 5 – Biogeochemical & Nutrient Cycles

Key Concepts and Definitions

• Biogeochemical cycles describe the movement and exchange of matter and energy between the biosphere, atmosphere, hydrosphere, and geosphere
• Nutrients are essential elements required by organisms for growth, development, and reproduction
• Reservoirs are locations where matter is stored for short or long periods (atmosphere, oceans, soil)
• Fluxes represent the movement of matter between reservoirs
• Residence time is the average time a molecule spends in a reservoir before moving to another
• Limiting nutrients are essential elements that limit biological productivity when scarce (nitrogen, phosphorus)
• Eutrophication is the excessive growth of algae and aquatic plants due to nutrient enrichment, leading to oxygen depletion and ecosystem degradation

Earth's Spheres and Their Interactions

• The biosphere consists of all living organisms and their interactions with the environment
• The atmosphere is the gaseous layer surrounding the Earth, composed primarily of nitrogen and oxygen
• The hydrosphere encompasses all water on Earth, including oceans, lakes, rivers, and groundwater
• The geosphere is the solid Earth, including rocks, minerals, and soil
• Biogeochemical cycles involve the exchange of matter between Earth's spheres
  • Carbon dioxide ($CO_2$) is exchanged between the atmosphere and biosphere through photosynthesis and respiration
  • Nitrogen is transferred from the atmosphere to the biosphere through nitrogen fixation by bacteria and lightning
• Anthropogenic activities, such as fossil fuel combustion and deforestation, alter the natural balance of biogeochemical cycles

The Carbon Cycle

• Carbon is a fundamental element for life, found in all organic compounds
• The carbon cycle involves the exchange of carbon between the atmosphere, biosphere, hydrosphere, and geosphere
• Photosynthesis by plants and algae converts atmospheric $CO_2$ into organic compounds, storing carbon in biomass
• Respiration by organisms releases $CO_2$ back into the atmosphere
• Decomposition of organic matter by microorganisms releases carbon into the soil and atmosphere
• Oceans absorb $CO_2$ from the atmosphere, forming carbonic acid ($H_2CO_3$) and affecting ocean pH
• Fossil fuel combustion and deforestation release stored carbon back into the atmosphere, contributing to climate change

The Nitrogen Cycle

• Nitrogen is an essential nutrient for plant growth and protein synthesis
• The nitrogen cycle involves the transfer of nitrogen between the atmosphere, biosphere, and geosphere
• Nitrogen fixation converts atmospheric nitrogen ($N_2$) into biologically available forms (ammonia, $NH_3$)
  • Biological nitrogen fixation is carried out by bacteria in root nodules of legumes (soybeans, alfalfa)
  • Lightning converts $N_2$ into nitrates ($NO_3^-$), which enter the soil through precipitation
• Nitrification is the conversion of ammonia to nitrites ($NO_2^-$) and then to nitrates by soil bacteria
• Assimilation is the uptake of nitrates by plants, incorporating nitrogen into organic compounds
• Ammonification is the decomposition of organic matter, releasing ammonia back into the soil
• Denitrification is the conversion of nitrates back into atmospheric nitrogen by anaerobic bacteria

The Phosphorus Cycle

• Phosphorus is a crucial nutrient for plant growth, DNA, and energy transfer (ATP)
• The phosphorus cycle is a sedimentary cycle, with the primary reservoir being rocks and minerals
• Weathering and erosion of rocks release phosphates ($PO_4^{3-}$) into the soil and water
• Plants absorb phosphates from the soil, incorporating them into organic compounds
• Decomposition of organic matter by microorganisms releases phosphates back into the soil
• Runoff and leaching transport phosphates to aquatic ecosystems, potentially causing eutrophication
• Phosphorus is a limiting nutrient in many ecosystems due to its slow release from rocks and lack of a gaseous phase

The Water Cycle

• The water cycle, also known as the hydrologic cycle, describes the continuous movement of water on Earth
• Evaporation is the conversion of liquid water into water vapor, primarily from oceans and other water bodies
• Transpiration is the release of water vapor from plants through stomata during photosynthesis
• Condensation is the formation of liquid water droplets from water vapor, leading to cloud formation
• Precipitation is the falling of water from the atmosphere as rain, snow, sleet, or hail
• Infiltration is the movement of water into the soil, recharging groundwater aquifers
• Runoff is the flow of water over land, eventually reaching streams, rivers, and oceans
• The water cycle is driven by solar energy and is essential for maintaining Earth's climate and ecosystems

Human Impacts on Biogeochemical Cycles

• Anthropogenic activities have significantly altered the natural balance of biogeochemical cycles
• Fossil fuel combustion releases stored carbon into the atmosphere, contributing to climate change and ocean acidification
• Deforestation reduces carbon sequestration and alters the water cycle by reducing evapotranspiration
• Agricultural practices, such as fertilizer use and livestock production, increase nitrogen and phosphorus inputs to ecosystems
  • Excess nutrients lead to eutrophication, causing algal blooms and hypoxic zones (Gulf of Mexico)
• Urbanization and land-use changes alter the water cycle by increasing surface runoff and reducing infiltration
• Damming of rivers disrupts the natural flow of water and sediment, affecting aquatic ecosystems and nutrient transport
• Sustainable management practices, such as reforestation, precision agriculture, and wastewater treatment, can help mitigate human impacts on biogeochemical cycles

Ecological Importance and Applications

• Biogeochemical cycles are essential for maintaining the balance and productivity of ecosystems
• Nutrient availability determines the growth and distribution of plants, which form the base of food webs
• Disruptions in biogeochemical cycles can lead to ecosystem degradation, biodiversity loss, and reduced ecosystem services
• Understanding biogeochemical cycles is crucial for developing sustainable agriculture and forestry practices
  • Crop rotation and nitrogen-fixing cover crops can reduce the need for synthetic fertilizers
  • Agroforestry systems combine trees and crops, enhancing nutrient cycling and soil health
• Biogeochemical models help predict the impacts of climate change and land-use changes on ecosystems and inform management decisions
• Bioremediation techniques utilize microorganisms to break down pollutants and restore contaminated sites by harnessing natural biogeochemical processes
• Studying biogeochemical cycles provides insights into the resilience and adaptability of ecosystems in the face of global environmental changes