Water and key nutrients (carbon, nitrogen and phosphorus) are
recycled within ecosystems. Human activities such as agriculture,
industrial activity and energy use disrupt the normal nutrient cycling
patterns. They lead to . . .
pollution
habitat distruction and
climate change.
Understanding normal and altered nutrient cycles gives us a
framework for minimizing the effect of human activities on the
environment and solving some of our environmental problems.
The Water Cycle
Availability of clean, potable water is the major constraint on
population growth, health and productivity.
All of earth’s water has been present since earth first formed.
It is simply recycled from one source to another.
That is why water
pollution is problematic it gets worse over time as
pollutants accumulate.
condensation
precipitation
evaporation
& transpiration
transpiration &
evaporation
melting & runoff
condensation
evaporation
runoff
water reservoir
Pollution Facts
∙
Organic matter (e.g. leftover food, excrement) is bio-degradable.
The environment has an assimilative capacity for these
materials, and they are recycled over time.
∙
Excess organic material causes pollution because the
assimilative capacity of the environment is exceeded. It can also
be a source of disease-causing bacteria and viruses (e.g.
typhoid, typhus, cholera).
∙
Some man-made chemicals (e.g. PCBs) have no assimilative
capacity - they don’t biodegrade and so accumulate.
∙
Other chemicals used in industry (oils, heavy metals) are biodegradable, but cause pollution if released in large amounts into
the environment.
Water Facts:
∙ 98% of the earth’s water is held in oceans, and they are salty
(non-potable). The largest reservoir of fresh water is polar ice
caps and glaciers (1%).
∙
Less than 0.5% of the
earth’s water is fresh
from lakes, streams
and reservoirs. The
remainder is ground
water and in aquifers.
∙
Evaporation from
oceans, rivers and
lakes is the main source of all the fresh water on earth. It is
recycled back in the form of precipitation.
∙
In the U.S., we use ground water and surface fresh water
(lakes, rivers, reservoirs) equally for industrial and residential
purposes.
∙
Industrial use, particularly for
agriculture, is the main
commercial use for water.
∙
If toxins from landfills and
industrial waste seep into
ground water, it poses an
immediate problem.
∙
The ocean has a large assimilative capacity for organic waste
because of its size, and is a tempting dumping site. However,
there are long-term consequences of these practices (toxins build
up in aquatic food chains and damage ecosystems over time).
Keeping it Clean!
Industrialized societies spend a great deal of resources to keep their
supply of potable water clean and safe:
∙
Laws minimize industrial pollution. The Clean Water and
Clean Air Acts in the U.S. are examples. This type of legislation
restricts dumping of chemical waste and mandates proper sewage
treatment.
∙
Sewage treatment systems clean and recycle water used in
human residences. Organic matter and bacteria are removed, and
the water is recycled by evaporation or it is cleaned to the point
where it can safely be released into the ocean. NOTE: Sewage
treatment does not remove chemical pollutants!
∙
Reservoirs are built to trap seasonal snowmelt and rain water
for year-round use. The Hetch-Hetchy system in our area brings
snowmelt/rain to reservoirs in San Mateo. These efforts
effectively increase the available of surface water.
∙
Local ordinances restrict water use in residences and businesses.
There is wide variability in water use across the U.S.
The Nitrogen Cycle
Nitrogen is a critical limiting
nutrient for plants; without it,
productivity falls.
Plants absorb nitrogen mainly
from the soil.
The problem: Most of earth’s
nitrogen is in the atmosphere in
the form of N2 gas (nitrogen
reservoir). Plants can only
use nitrogen in the form of
nitrate (NO3) compounds.
Nitrogen must be moved from the atmospheric reservoir into the
soil and chemically altered into nitrates.
Nitrogen cycling involves
four chemical processes that
turn atmospheric nitrogen or
nitrogen from decomposing
organisms into useful nitrates.
It is performed by different
groups of soil bacteria,
including azobacteria and
rhizobacteria.
The 4 chemical processes are:
ammonification,
nitrification, nitrogen fixation and denitrification.
Ammonification and
nitrification recycle
nitrogen among living
organisms:
∙ Ammonification
decomposes organic
nitrogen into ammonium:
NH2 or NH3 ➔ NH4+.
∙
Nitrification turns ammonia into nitrites, then nitrates:
NH4+ ➔ NO2 ➔ NO3.
Nitrogen fixation and
denitrification recycle nitrogen
between living organisms and
the atmospheric reservoir.
∙
Nitrogen fixation is
performed by bacteria in root
nodule of leguminous plants.
N2 ➔ NH4+ in the first step.
Different bacteria turn NH4+
➔NO3 in subsequent steps.
∙
Denitrification also occurs, returning excess soil nitrates to the
air. NO3 ➔N2. Denitrification is very rare and occurs only in
over-fertilized fields.
The main processes by which soil gains nitrogen are:
ammonification followed by nitrification. Nitrogen fixation
plays a small but important role.
The main way soil loses nitrogen is via uptake by plants, then
ingestion of the plant by animals. Denitrification is rare.
Agriculture and energy use
disrupt nitrogen cycling in the
following ways:
Excess nitrogen fertilizers enter
waterways and cause algal blooms
and premature eutrophication.
Burning fossil fuels increases atmospheric nitrogen oxides,
enhancing the greenhouse effect and leading to acid rain.
Like nitrogen, phosphorus is an
important nutrient without
which primary productivity
falls. Phosphorus is required for
nucleic acids, phospholipids, ATP,
and animal skeletons.
The main phosphorus reservoirs
are rocks and minerals,
dissolved phosphates and in
living organisms. The cycling is relatively simple; phosphates are
moved from one group of organisms to another via excretion,
decomposition and consumption.
Farming disrupts the phosphorus cycle in much the same way as
for nitrogen. phosphate detergents add intermittent phosphate doses
to waterways, causing destructive algae blooms.
ACCELERATED EUTROPHICATION
∙ Eutrophication is a normal process; lakes, ponds, and river banks
gradually fill in due to accumulated silt and nutrients.
∙
Agricultural runoff contains excess fertilizers (nitrates and
phosphates), which accelerate eutrophication.
The ABC’s of eutrophication:
A: Runoff enters lakes and rivers, causing a
producer growth explosion. You get more O2
production during the day, but high O2 use at
night.
B: Producers die when the pulse of nutrients
from the runoff stops. The decomposition of
producers leads to water turbidity, lower
sunlight penetration and oxygen depletion.
C: Rate of detritus production and
accumulated organic matter (from dead
organisms) increases and the water body
begins to fill in prematurely.
The bottom line: In bodies of water that
undergo accelerated eutrophication, higher
order consumers like fish often decrease
because there isn’t enough dissolved oxygen
to support them.
The Carbon Cycle
Most of the available
carbon exists in living
organisms,
sedimentary rocks,
atmospheric CO2 and
fossil fuels.
Carbon is perfectly
recycled via the processes of decomposition, photosynthesis and
respiration.
respiration
C6H12O6 + 6 O2 + 6 H2O
6 CO2 + 12 H2O + energy
photosynthesis
Decomposers liberate
stored carbon from
decaying plants and
animals in the form of
CO2.
Plants can then utilize the
CO2 to form glucose,
liberating oxygen.
Burning fossil fuels and wood increases atmospheric CO2
without increasing the resources (plants) to recycle the carbon.
In the case of wood, plants are removed to use as fuel, creating a
further imbalance between fixation of CO2 into sugars and release of
CO2 into the atmosphere. As a result, atmospheric CO2 rises.
Atmospheric CO2 is a greenhouse gas. It traps heat in earth’s
atmosphere, allowing for a moderately warm climate in most
latitudes. Excess CO2 causes global warming because it enhances
the beneficial greenhouse effect.
Intensive farming also disrupts carbon cycling in many ways:
1. Crops grown in one region are shipped to another for
consumption.
2. Farming requires automation and uses fossil fuels, the burning of
which add atmospheric CO2 .
3. Biomes such as rainforests, tropical grasslands, temperate forests
and temperate grasslands are replaced by farms. Crops are of
similar productivity (recycled CO2 per acre) to temperate
grasslands and forests. However, they are less productive than
tropical biomes they replace. This slows carbon recycling and
increases atmospheric CO2.
4. When forests are burned to clear farms, atmospheric CO2 also
rises due to the wood burning.
Until 200 years ago, carbon was
effectively recycled by
photosynthesis and cellular
respiration.
The normal carbon cycling pattern
resulted in a small amount of CO2 in
the atmosphere.
Humans disrupt the normal carbon cycling pattern by burning
fossil fuels for energy, adding to atmospheric CO2.
Humans also burn down productive forest biomes to make room
for farms and housing for the burgeoning human population.