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Biogeochemical Cycles
Chapter 2
Ecosystems provide vital services
• All life on Earth (including humans) depends on
healthy, functioning ecosystems
• Ecosystem services: essential services provided
by healthy, normally functioning ecosystems
– When human activities damage ecosystems, we must
devote resources to supply these services ourselves
– Example: if we kill off insect predators, farmers must
use synthetic pesticides that harm people and wildlife
• One of the most important ecosystem services:
– Nutrients cycle through the environment in intricate
ways
Ecological processes provide
services
Nutrients circulate through ecosystems
• Nutrients move through the environment in complex ways
– Matter is continually circulated in an ecosystem
• Nutrient (biogeochemical) cycle: the movement of
nutrients through ecosystems
• Pool (reservoir): a location where nutrients remain for
varying amounts of time (residence time)
• Source: a reservoir releases more materials than it accepts
• Sink: a reservoir that are accepts more than it releases
• Flux: the rate at which materials move between reservoirs
– Can change over time
Humans affect nutrient cycling
• Human activities affect nutrient cycling
– Altering fluxes, residence times, and amounts of
nutrients in reservoirs
The water cycle affects all other
cycles
• Water is essential for biochemical reactions and is
involved in nearly every environmental system and cycle
• Hydrologic cycle: the flow of liquid, gaseous, and solid
water through the environment
– Less than 1% is available as fresh water
• Evaporation: conversion of liquid to gaseous water
• Transpiration: release of water vapor by plants
• Precipitation: rain or snow returns water to Earth’s
surface
• Runoff: water flows into streams, lakes, rivers, oceans
Transpiration
Animation: Transpiration
Right-click / Select “Play”
Water is also stored underground
• Infiltration: water soaks down through rock and
soil to recharge aquifers
• Aquifers: underground reservoirs of spongelike
regions of rock and soil that hold …
– Groundwater: water found underground beneath
layers of soil
• Water table: the uppermost level of
groundwater held in an aquifer
• Water in aquifers may be ancient (thousands of
years old)
The hydrologic cycle
Human impacts on the hydrologic cycle
• Humans have affected almost every flux, reservoir, and
residence time in the water cycle
• Damming rivers slows water movement and increases
evaporation
• Removal of vegetation increases runoff and erosion
while decreasing infiltration and transpiration
• Overdrawing surface and groundwater for agriculture,
industry, and domestic uses lowers water tables
• Emitting air pollutants that dissolve in water changes
the nature of precipitation and decreases cleansing
The Carbon Cycle
• Carbon cycle: describes carbon’s route in the
environment
– Carbon forms essential biological molecules
• Through photosynthesis, producers move carbon from
the air and water to organisms
– Respiration returns carbon to the air and water
• Oceans are the second largest reservoir of carbon
– Absorb carbon from the air, land, and organisms
• Decomposition returns carbon to the sediment, the
largest reservoir of carbon
– Ultimately, it may be converted into fossil fuels
The carbon cycle
Humans affect the carbon cycle
• Burning fossil fuels moves carbon from the
ground to the air
– Since mid-1700s, people have added over 275 billion
tons of carbon dioxide to the atmosphere
• Cutting forests and burning fields moves carbon
from organisms to the air
– Less carbon dioxide is removed by photosynthesis
• Today’s atmospheric carbon dioxide reservoir is
the largest in the past 800,000 years
– The driving force behind climate change
The Nitrogen Cycle
• Nitrogen makes up 78% of the atmosphere
• It is contained in proteins, DNA, and RNA
– It is also essential for plant growth
• Nitrogen cycle: describes the routes of nitrogen
through the environment
– Nitrogen gas is inert and cannot be used by
organisms
• It needs lightning, bacteria, or human intervention to
become biologically active and available to organisms
– Then it is a potent fertilizer
Nitrogen must become biologically available
• Nitrogen fixation: nitrogen-fixing soil bacteria or
lightning “fixes” nitrogen gas into ammonium
– Nitrogen-fixing bacteria live in legumes (e.g.,
soybeans)
• Nitrification: bacteria then convert ammonium ions first
into nitrite ions then into nitrate ions
– Plants can take up these ions
• Nitrite and nitrate also come from the air or fertilizers
• Animals obtain nitrogen by eating plants or other
animals
• Denitrifying bacteria: convert nitrates in soil or water to
gaseous nitrogen, releasing it back into the atmosphere
The nitrogen cycle
Humans greatly affect the nitrogen cycle
• Historically, nitrogen fixation was a bottleneck: limited
the flux of nitrogen from air into water-soluble forms
• Industrial fixation fixes nitrogen on a massive scale
– Overwhelming nature’s denitrification abilities
• Excess nitrogen leads to hypoxia in coastal areas
• Nitrogen-based fertilizers strip the soil of other
nutrients
– Reducing soil fertility
• Burning forests and fossil fuels leads to acid
precipitation, adds greenhouse gases, and creates
photochemical smog
The phosphorus cycle
• Phosphorus cycle: describes the routes that
phosphorus takes through the environment
– No significant atmospheric component
– Most phosphorus is in rocks
• With naturally low environmental concentrations,
phosphorus is a limiting factor for plant growth
• Weathering releases phosphorus into water
– Allowing it to be taken up by plants
• Phosphorus is a key component of cell membranes,
DNA, RNA, and other biochemical compounds
The phosphorus cycle
Humans affect the phosphorus
cycle
• Fertilizer from lawns and farmlands
– Increases phosphorus in soil
– Its runoff into water increases phytoplankton blooms
and hypoxia
• Wastewater containing detergents releases
phosphorus to waterways
Controlling nutrient pollution in
waterways
• Reduce fertilizer use in farms and lawns
• Change timing of fertilizer applications to minimize
runoff
• Manage livestock manure applications to farmland
• Plant vegetation “buffers” around streams to trap
runoff
• Restore wetlands and create artificial ones to filter
runoff
• Improve sewage-treatment technologies
• Restore frequently flooded lands
• Reduce fossil fuel combustion
Conclusion
• Life interacts with its abiotic environment in ecosystems
through which energy flows and materials are recycled
• Understanding biogeochemical cycles is crucial
– Humans are changing the ways those cycles function
• Understanding energy, energy flow, and chemistry
increases our understanding of organisms
– How environmental systems function
• Thinking in terms of systems can teach us how to:
– Avoid disrupting Earth’s processes and to mitigate
any disruptions we cause