Nutrient Cycling

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Chapter 19 Nutrient Cycling and Retention
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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.

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Energy Flows, Nutrients Cycle
Nutrient
Solar Radiation
Heat Radiated to Space
Energy Flow Through Ecosystem
Cycles
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Energy Flow Drives Nutrient Cycles
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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

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Fluxes Reservoir Exchangeable Pools
Nutrient Reservoir Pool Flux Flux Exchangeable Pool (s)
Carbon (Gaseous) Atmosphere Photosynthesis Respiration Organic matter
Nitrogen (Gaseous) Atmosphere N-fixation De-nitrification NH4, NO2 in soil water protein in organic matter
Mineral Nutrients Phosphorous Magnesium Calcium Potassium Rock in the Earths crust Weathering Leaching Sedimentation Free ions in soil water organic matter
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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 Carnivores
Organic Matter In Bacteria Fungi
Excretion Decomposition Mineralization
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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.

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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.

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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
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Decomposition Rate Is Directly Related to Actual
Evapotrans-piration Rate
Why ?
Ecosystems with high AE have high rainfall and
high temperature. Good conditions for microbial
activity.
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Decomposition Rate Is Greater In Tropical vs.
Temperate Forests
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Plant Matter w/ High Nutritional Value Decomposes
Faster
Foliage w/ Low CN Ratio and Low Content of
Cellulose and Lignin Decomposes Faster.
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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
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Decomposition Rates Increase with Greater
Nutrient Availability in the Environment
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Decomposition Rate vs. Phosphorus in Stream
Water
At high phosphorous levels, further increases did
not increase decomp- osition rate. WHY NOT ?
At low phosphorous levels, increasing P caused
significant increase in decomposition rate of
leaf matter
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Effect of Grazing on Plant Biomass Turnover
(Nutrient Cycling)
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Prairie Dog Grazing Accelerates Nitrogen
Re-Cycling
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Impacts of Human Activities On Nutrient Cycles
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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.

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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)

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Balancing the Nutrient Budget
Atmospheric Deposition
Rapid Loss
Slowly Replenished
Exchangeable Nutrient Pool In the Soil
Weathering Of Soil Minerals
Nutrients in harvested crop
Soil erosion Nutrient leaching
Manure Fertilization
Decomposition of crop residue
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Harvest Interval and Nutrient Depletion
Long Rotation (Forestry)
---Harvest Interval---
Soil Exchangeable Nutrient Pool
Time
With enough time between harvest removals, the
exchangeable nutrient pool is maintained by
natural fluxes from reservoir pool
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Harvest Interval and Nutrient Depletion
Long rotation
Harvest Interval
Soil Exchangeable Nutrient Pool
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.
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Harvest Intensity and Nutrient Depletion
Corn
Soil Exchangeable Nutrient Pools
Cotton
Time
Crops that remove a larger amount of nutrients
require a longer time period between harvests or
soil nutrient pools will be depleted.
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Harvesting Effects On Different Nutrients
Rapid Input Flux from Reservoir Pool (N)
Soil Exchangeable Nutrient Pool
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).
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Managing Soil FertilityCrop 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
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Managing Soil FertilityCrop Rotation 2 Year
Cycle
N-Fixing Crop Replenish Soil N Pool (Soybean,
Alfalfa)
Nutrient Extractive Crop (Corn, Cotton, Wheat,
Rice)
What about.
Chemical Liming And Fertilization
Other Nutrients (Ca, Mg, K, P) ?
Degraded water retention, aeration, drainage
Soil Organic Matter ?
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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
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A Case StudyAgricultural 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 ???

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A Case StudyAgricultural 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.

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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
The same companies that buy the crops also sell
the seed and fertilizer.
Price per Bushel Decreases
Farmer Income Decreases Grain Sales Receipt
Costs (fuel, seed, fertilizer)
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Agriculture In the Wet Tropics

• A Cautionary Tale of Nutrient Cycling Limits for
  Agriculture

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Total Ecosystem Carbon In Boreal and Tropical
Forest Ecosystems
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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.

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Nutrient Leaching After Slash-and Burn
Calcium
Cut
Burned
Abandoned
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
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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
Rainforest Total NPP 12,742 12,995 12,920
Yucca Crop (edible part) 1,465 1,006 700
Crop Total NPP 5,333 5,294 3150
Weeds Total NPP 300 679 990
Slash-Burn Total NPP 5633 5973 4140
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Phosphorous Dynamics of Slash-and-Burn
P in Atmospheric Dust
P in Soil Minerals
Deposition
Weathering
Insoluble P In the Soil
P in Plant Biomass
Uptake
Plant-Available P In the Soil
Decomposition and Burning
Low pH P precipitates Neutral pH P dissolves
Very low in intact rain forest ecosystems
P-removal in harvested biomass
P-loss to atmosphere in ash from fire
P-loss due to leaching and soil erosion
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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
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Burn Rain Forest (release nutrients from biomass
to soil)
Increase Base Cations Ca, Mg, K in soil
Increased Soil pH
Increase Exchangeable Phosphorous
Decrease Toxic Metals Fe, Mn, Al
Re-Growth of Tropical Rainforest (Recovery Phase)
The Slash-and Burn Cycle
High Crop Yields
Land Abandonment
Removal of Ca, Mg, K In Crops
Loss of Ca Mg, K via Leaching
Decreased Crop Yields Increased Weeds
Decreased Exchangeable P
Decreased Soil pH
Increased Toxic Metals Fe, Mn, Al
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Nutrient Saturation

• The Other Side of Human Impacts on Nutrient Cycles

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Soil Nutrient Capacity vs. Content
Soil Capacity
Soil Content
Plant Uptake - Harvest Loss
Natural Inputs
Decomposition Mineralization N-Fixation Weathering
If losses exceed inputs ? Nutrient
depletion (Content ltlt Capacity)
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Nutrient Saturation
Fertilizer Acid Rain Manure
Soil Capacity
Natural Inputs Human Inputs
Soil Content
Plant Uptake
Leaching to Groundwater
If inputs exceed losses ? Nutrient
saturation (Content Capacity)
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Nitrate Application On U.S. Farms
Indiana
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Algae In Gulf Coast Waters
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Dead Zone Formation
(Hypoxic Bottom Water)
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Gulf of Mexico Dead Zone
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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
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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.

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The End