Chapter 19
Nutrient Cycling
and Retention
Objectives
Students will be able to describe the major
reservoirs of important nutrients and the
processes that move nutrients between these
pools and plant-usable exchangeable pools.
Students will be able to describe factors that
control biological nutrient cycling.
Students will be able to describe experiments
to test the influence of factors on biological
nutrient cycling.
Energy Flow Through Ecosystem
Heat Radiated to Space
Solar Radiation
Energy Flows, Nutrients Cycle
Energy Flow Drives Nutrient Cycles
Nutrient Pools
Reservoir Pools: The largest pool where
most of the nutrient is found
Atmosphere Hydrosphere
Lithosphere
Exchangeable Pools: The pool / chemical
form(s) of nutrients that are available for use
by living organisms
– Dissolved in water
– Free ions on soil particles
– Organic matter
Fluxes: Reservoir
Nutrient
Carbon
(Gaseous)
Reservoir
Pool
Atmosphere
Exchangeable Pools
Flux
Flux
Photosynthesis
Exchangeable
Pool (s)
Organic matter
Respiration
Nitrogen
(Gaseous)
Atmosphere
N-fixation
De-nitrification
Mineral
Nutrients:
Phosphorous
Magnesium
Calcium
Potassium
Rock in the
Earth’s crust
Weathering
Leaching &
Sedimentation
NH4, NO2 in soil
& water; protein
in organic matter
Free ions in soil
& water; organic
matter
Generalized Nutrient Cycle
Reservoir Pool
Atmosphere Lithosphere
Abiotic Exchangeable Pool
Soil Water
Organic Matter in Plants
Organic Matter
in Herbivores
Dead Organic Matter
Organic Matter
In Detritivores
Organic Matter
In Bacteria & Fungi
Organic Matter
in Carnivores
Excretion
Decomposition
Mineralization
Nutrient Cycling
Fluxes from reservoir to exchangeable pools are
often slow (weathering, N-fixation).
Most nutrients in exchangeable pools are present
due to nutrient cycling.
– Decomposition
– Mineralization
Losses from the exchangeable pool due to erosion,
harvesting, sedimentation must be replaced by
fluxes from reservoir pool.
Gaseous nutrients are replaced more rapidly than
mineral (sedimentary) nutrients.
Factors That Influence Rate of
Decomposition and Nutrient Cycling
Climate: Metabolic rate of organisms in
detrital food web controlled by temperature
and water availability.
Nutrient Availability (in environment and in
dead organic matter): Low nutrient content
in DOM and in the environment slows
population growth of decomposer species.
Grazers accelerate the breakdown of plant
organic matter and nutrient re-cycling.
Decomposition of Tree Leaves
Dry vs. Wet Environments
Decrease in mass of
dead organic matter
over time is the measure
of decomposition rate
Leaves
decomposed
faster in the wet
environment
Decomposition Rate
Is Directly Related to
Actual Evapotranspiration Rate
Why ?
Ecosystems with
high AE have high
rainfall and high
temperature.
Good conditions for
microbial activity.
Decomposition Rate Is Greater In
Tropical vs. Temperate Forests
Plant Matter w/ High
Nutritional Value
Decomposes Faster
Foliage w/ Low C:N
Ratio and Low Content
of Cellulose and Lignin
Decomposes Faster.
Decomposition Rate vs.
Lignin and Nitrogen
Content of Leaf Matter
Warmer
Why is the ground in a
pine forest covered with
dead pine needles ?
Is this a problem ?
Cooler
Bad
Food
Decomposition Rates Increase with Greater Nutrient
Availability in the Environment
Decomposition Rate vs. [Phosphorus] in Stream Water
At high phosphorous levels, further
increases did not increase decomposition rate. WHY NOT ?
At low phosphorous levels,
increasing P caused significant
increase in decomposition rate
of leaf matter
Effect of Grazing on Plant Biomass
Turnover (Nutrient Cycling)
Prairie Dog Grazing Accelerates Nitrogen
Re-Cycling
Impacts of Human Activities
On Nutrient Cycles
Objectives
Students will be able to describe how
agriculture and forestry impact soil nutrient
budgets.
– How factors of rotation length, harvest intensity,
and nature of the nutrient influence impact.
– Consequences / Mitigation of nutrient depletion
Students will be able to describe how human
activities can saturate natural ecosystem
nutrient pools and the consequences of
nutrient saturation.
Agriculture and Forestry
Harvesting of biomass and soil erosion from
human crop systems remove nutrients from
the ecosystem.
Natural fluxes from reservoir pool replenish
exchangeable nutrient pools, depending on
rates of input vs. output in harvests.
Additions of manure and chemical fertilizer
often necessary to maintain exchangeable
nutrient pools in soil (and productivity)
Balancing the Nutrient Budget
Slowly
Replenished
Weathering
Of Soil Minerals
Rapid
Loss
Exchangeable
Nutrient Pool
In the Soil
Decomposition
of crop residue
Nutrients in
harvested crop
Soil Exchangeable Nutrient Pool
Harvest Interval and Nutrient Depletion
Long Rotation (Forestry)
|---Harvest Interval---|
Time
With enough time between harvest removals, the exchangeable
nutrient pool is maintained by natural fluxes from reservoir pool
Soil Exchangeable Nutrient Pool
Harvest Interval and Nutrient Depletion
Long rotation
Harvest
Interval
Short Rotation (Agriculture)
Time
With insufficient time between harvests to allow for
natural replenishment, soil nutrient pools are depleted.
Crop production will decrease over time.
Soil Exchangeable Nutrient Pools
Harvest Intensity and Nutrient Depletion
Corn
Cotton
Time
Crops that remove a larger amount of nutrients require
a longer time period between harvests or soil nutrient
pools will be depleted.
Soil Exchangeable Nutrient Pool
Harvesting Effects On Different Nutrients
Rapid Input Flux from Reservoir Pool (N)
Slow Input Flux from Reservoir Pool (P)
Time
Slowly cycled mineral nutrients (Ca, Mg, K, P) are
more readily depleted than more rapidly cycled
gaseous nutrients (N, C, S).
Managing Soil Fertility
Crop Rotation: 4 Year Cycle
Nutrient Extractive Crop
(Corn, Cotton, Wheat, Rice)
N-Fixing Crop
Replenish Soil N Pool
(Soybean, Alfalfa)
Fallow Year (No Crop)
Replenish Soil Nutrients
Hay, Grass Cover
“Green Manure” Crop
Replenish Soil Organic Matter
Hay, Alfalfa
Managing Soil Fertility
Crop Rotation: 2 Year Cycle
N-Fixing Crop
Replenish Soil N Pool
(Soybean, Alfalfa)
Nutrient Extractive Crop
(Corn, Cotton, Wheat, Rice)
What about….
Other Nutrients (Ca, Mg, K, P) ?
Soil Organic Matter ?
Chemical Liming
And Fertilization
Degraded water retention,
aeration, drainage
Fertilizer Use and
the Green Revolution
Corn Yield
U.S.A
Wheat
Yield
Major gains in crop production
from the Green Revolution
required massive increases in the
use of chemical fertilizer
A Case Study
Agricultural Trends In Georgia (USA): 1940 – 1990
Acreage of agricultural land decreased by
50% (farm abandonment)
State-wide total agricultural crop production
increased by 100%
Crop yield per acre increased 4-fold.
How did this happen ???
A Case Study
Agricultural Trends In Georgia (USA): 1940 – 1990
Total use of fertilizer (per acre) increased 7-fold
Use of Nitrogen fertilizer increased 11-fold.
Is this a problem ?
– Excess nutrients from fertilizer washes into
streams, lakes, and groundwater (more later).
– Dependence on expensive fertilizers puts farmers
at economic risk.
Agricultural Economics
Fertilization Increases
Crop Yields (and also Costs)
Increased Grain Supply
to Consumer Market
N-fertilizer made
using fossil fuel.
Sensitive to price
fluctuations
Price per Bushel
Decreases
The same companies
that buy the crops
also sell the seed and
fertilizer.
Farmer Income Decreases:
Grain Sales Receipt – Costs (fuel, seed, fertilizer)
Agriculture In the Wet Tropics
A Cautionary Tale of Nutrient
Cycling Limits for Agriculture
Total Ecosystem Carbon In Boreal
and Tropical Forest Ecosystems
% Total Ecosystem Carbon
100
80
Leaf
60
Wood
Litter
40
Soil
20
0
Boreal
Tropical
Forest Type
Slash-and-Burn Agriculture
Cut-down and burn forest vegetation to
release nutrients to the soil.
Initially, crop yields are high.
Crop yields progressively decline.
Field abandoned after 3 to 5 years.
Sustainable w/ SMALL human populations,
but NOT w/ large human populations.
Kg / ha / mo
Nutrient Leaching After Slash-and Burn
4
3
Calcium
Control
Cut-Burned
Cut
2
Burned
Abandoned
1
0
1976
1977
1978
1979
1980
High crop yields immediately after burn are associated with a large
pulse of basic cations into the soil from the burned vegetation
Decreasing crop yields over 3-5 years associated with decreased
pools of basic cations in the soil
Primary Productivity (kg / ha/ yr) of
Rain Forest vs. Slash-and-Burn Crop
Year 1
After Burn
Year 2
After Burn
Year 3
After Burn
12,742
12,995
12,920
1,465
1,006
700
Crop
Total NPP
“Weeds”
Total NPP
5,333
5,294
3150
300
679
990
Slash-Burn
Total NPP
5633
5973
4140
Rainforest
Total NPP
Yucca Crop
(edible part)
Phosphorous Dynamics of Slash-and-Burn
P in Soil Minerals
P in Atmospheric Dust
Deposition
P in Plant
Biomass
P-removal
in harvested
biomass
Weathering
Insoluble P
In the Soil
Plant-Available P
In the Soil
P-loss to
atmosphere
in ash from fire
P-loss due to
leaching and
soil erosion
Low pH – P precipitates
Neutral pH – P dissolves
Very low
in intact
rain forest
ecosystems
Phosphorous Dynamics in Tropical
Soil After Slash-and-Burn
Control
One month
since burn
20 months
since burn
4 years after
abandonment
Available P
(ppm)
4.8
7.4
13.0
4.7
Soil pH
4.25
4.96
5.15
4.52
Total P
(ppm)
200
130
250
300
% Available
2.4
5.7
5.2
1.6
There is a large pool of soil phosphorous, but only a small percentage
is available for plant uptake. Ash from burning increases soil pH,
increasing the amount of plant-available P
Burn Rain Forest
(release nutrients
from biomass to soil)
Re-Growth of
Tropical Rainforest
(Recovery Phase)
Land
Abandonment
Decreased Crop
Yields
&
Increased
“Weeds”
Increase Base
Cations Ca, Mg,
K in soil
Increased
Soil pH
Increase
Exchangeable
Phosphorous
The Slash-and
Burn Cycle
Decrease Toxic
Metals Fe, Mn, Al
High Crop Yields
Removal of
Ca, Mg, K
In Crops
Decreased
Exchangeable P
Increased Toxic
Metals Fe, Mn, Al
Loss of Ca
Mg, K via
Leaching
Decreased
Soil pH
Nutrient Saturation
The Other Side of Human
Impacts on Nutrient Cycles
Soil Nutrient Capacity vs. Content
Natural
Inputs
Soil
Content
Plant Uptake Harvest Loss
Decomposition
Mineralization
N-Fixation
Weathering
If losses exceed inputs ► Nutrient depletion
(Content << Capacity)
Nutrient Saturation
Fertilizer
Acid Rain
Manure
Natural Inputs
+
Human Inputs
Soil
Content
If inputs exceed losses ► Nutrient saturation
(Content = Capacity)
Plant
Uptake
Nitrate Application On U.S. Farms
Indiana
Algae In Gulf Coast Waters
“Dead Zone” Formation
(Hypoxic Bottom Water)
Gulf of Mexico Dead Zone
“Acid Rain” Adds Excess Nutrients
Excess Inputs of N and S From Atmosphere
Soil Saturated w/ N and S
Excess NO3- and SO4- Leach From Soil
Base Cations Ca, Mg, K, Na Leach From Soil
Decreased Soil pH
Increased Toxic Soluble Al
Decreased Plant Growth
“Forest Decline”
Excess Al Leaches Into Streams
Al Toxicity Kills Aquatic Organisms
“Dead Lakes”
Summary
“Sustainability” of agricultural production
systems and “Health” of natural ecosystems
require balancing of nutrient budgets.
Nutrient depletion of agricultural systems
requires expensive chemical fertilization that
may not be sustainable long-term.
Nutrient saturation of natural systems is a
major risk to ecosystem health.
The End