Chapter 19Chapter 19Nutrient Cycling Nutrient Cycling

and Retentionand Retention

ObjectivesObjectives

Students will be able to describe the major Students will be able to describe the major reservoirs of important nutrients and the reservoirs of important nutrients and the processes that move nutrients between these processes that move nutrients between these pools and plant-usable exchangeable pools.pools and plant-usable exchangeable pools.

Students will be able to describe factors that Students will be able to describe factors that control biological nutrient cycling.control biological nutrient cycling.

Students will be able to describe experiments Students will be able to describe experiments to test the influence of factors on biological to test the influence of factors on biological nutrient cycling.nutrient cycling.

Energy Flows, Nutrients CycleEnergy Flows, Nutrients Cycle

Energy Flow Through Ecosystem

So

lar

Rad

iati

on

Hea

t R

adia

ted

to

Sp

ace

Hea

t R

adia

ted

to

Sp

ace

Energy Flow Drives Nutrient CyclesEnergy Flow Drives Nutrient Cycles

Nutrient PoolsNutrient Pools

Reservoir Pools:Reservoir Pools: The largest pool where The largest pool where most of the nutrient is found most of the nutrient is found

AtmosphereAtmosphere HydrosphereHydrosphere LithosphereLithosphere

Exchangeable Pools:Exchangeable Pools: The pool / chemical The pool / chemical form(s) of nutrients that are available for use form(s) of nutrients that are available for use by living organisms by living organisms – Dissolved in waterDissolved in water– Free ions on soil particlesFree ions on soil particles– Organic matterOrganic matter

Fluxes: ReservoirFluxes: Reservoir Exchangeable Pools Exchangeable Pools

NutrientNutrientReservoirReservoir

PoolPoolFluxFlux FluxFlux

ExchangeableExchangeablePool (s)Pool (s)

Carbon Carbon (Gaseous)(Gaseous)

AtmosphereAtmosphere PhotosynthesisPhotosynthesis

RespirationRespiration

Organic matterOrganic matter

NitrogenNitrogen(Gaseous)(Gaseous)

AtmosphereAtmosphere N-fixationN-fixation

De-nitrificationDe-nitrification

NHNH44, NO, NO22 in soil in soil

& water; protein & water; protein in organic matterin organic matter

MineralMineralNutrients:Nutrients:PhosphorousPhosphorousMagnesiumMagnesiumCalciumCalciumPotassiumPotassium

Rock in the Rock in the Earth’s crustEarth’s crust

WeatheringWeathering

Leaching &Leaching &SedimentationSedimentation

Free ions in soil Free ions in soil & water; organic & water; organic mattermatter

Generalized Nutrient CycleGeneralized Nutrient CycleReservoir Pool

Atmosphere Lithosphere

Abiotic Exchangeable PoolSoil Water

Organic Matter in Plants

Dead Organic Matter Organic Matterin Herbivores

Organic Matter in Carnivores

Organic MatterIn Detritivores

Organic MatterIn Bacteria & Fungi Excretion Decomposition Mineralization

Nutrient CyclingNutrient Cycling

Fluxes from reservoir to exchangeable pools are Fluxes from reservoir to exchangeable pools are often slow (weathering, N-fixation).often slow (weathering, N-fixation).

Most nutrients in exchangeable pools are present Most nutrients in exchangeable pools are present due to nutrient cycling.due to nutrient cycling.

– DecompositionDecomposition

– MineralizationMineralization

Losses from the exchangeable pool due to erosion, Losses from the exchangeable pool due to erosion, harvesting, sedimentation must be replaced by harvesting, sedimentation must be replaced by fluxes from reservoir pool.fluxes from reservoir pool.

Gaseous nutrients are replaced more rapidly than Gaseous nutrients are replaced more rapidly than mineral (sedimentary) nutrients.mineral (sedimentary) nutrients.

Factors That Influence Rate of Factors That Influence Rate of Decomposition and Nutrient CyclingDecomposition and Nutrient Cycling

Climate:Climate: Metabolic rate of organisms in Metabolic rate of organisms in detrital food web controlled by temperature detrital food web controlled by temperature and water availability.and water availability.

Nutrient Availability (in environment and in Nutrient Availability (in environment and in dead organic matter):dead organic matter): Low nutrient content Low nutrient content in DOM and in the environment slows in DOM and in the environment slows population growth of decomposer species.population growth of decomposer species.

GrazersGrazers accelerate the breakdown of plant accelerate the breakdown of plant organic matter and nutrient re-cycling.organic matter and nutrient re-cycling.

Decomposition of Decomposition of Tree LeavesTree LeavesDry vs. Wet EnvironmentsDry 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 Decomposition Rate Is Directly Related to Is Directly Related to Actual Evapotrans-Actual Evapotrans-

piration Ratepiration Rate

Why ?

Ecosystems with high AE have high rainfall and high temperature.

Good conditions for microbial activity.

Decomposition Decomposition Rate Is Greater In Rate Is Greater In Tropical vs. Temperate ForestsTropical vs. Temperate Forests

Plant Matter w/ High Plant Matter w/ High Nutritional Value Nutritional Value

Decomposes FasterDecomposes Faster

Foliage w/ Low C:N Ratio and Low Content of Cellulose and Lignin Decomposes Faster.

Decomposition Rate vs. Decomposition Rate vs. Lignin and Lignin and NNitrogen itrogen

CContentontent of Leaf Matter of Leaf Matter

Warmer

Cooler

BadFood

Why is the ground in a pine forest covered with dead pine needles ?

Is this a problem ?

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 decomp-osition rate. WHY NOT ?

At low phosphorous levels, increasing P caused significant increase in decomposition rate of leaf matter

Effect of Effect of GGrazing on razing on PPlant lant BBiomass iomass TTurnoverurnover (Nutrient Cycling) (Nutrient Cycling)

Prairie Dog Grazing Accelerates Prairie Dog Grazing Accelerates Nitrogen Re-CyclingNitrogen Re-Cycling

Impacts of Human Activities On Impacts of Human Activities On Nutrient CyclesNutrient Cycles

ObjectivesObjectives

Students will be able to describe how Students will be able to describe how agriculture and forestry impact soil nutrient agriculture and forestry impact soil nutrient budgets.budgets.

– How factors of rotation length, harvest intensity, How factors of rotation length, harvest intensity, and nature of the nutrient influence impact.and nature of the nutrient influence impact.

– Consequences / Mitigation of Consequences / Mitigation of nutrient depletionnutrient depletion

Students will be able to describe how human Students will be able to describe how human activities can saturate natural ecosystem activities can saturate natural ecosystem nutrient pools and the consequences of nutrient pools and the consequences of nutrient saturationnutrient saturation..

Agriculture and ForestryAgriculture and Forestry

HarvestingHarvesting of biomass and of biomass and soil erosionsoil erosion from from human crop systems remove nutrients from human crop systems remove nutrients from the ecosystem.the ecosystem.

Natural fluxes from reservoir pool replenish Natural fluxes from reservoir pool replenish exchangeable nutrient pools, depending on exchangeable nutrient pools, depending on rates of input vs. output in harvests.rates of input vs. output in harvests.

Additions of manure and chemical fertilizer Additions of manure and chemical fertilizer often necessary to maintain exchangeable often necessary to maintain exchangeable nutrient pools in soil (and productivity)nutrient pools in soil (and productivity)

Balancing the Nutrient BudgetBalancing the Nutrient Budget

Exchangeable Nutrient Pool

In the Soil

WeatheringOf Soil Minerals

AtmosphericDeposition

Manure

Fertiliz

ation

Nutrients inharvested crop

Soil erosion

Nutrient leachingDecompositionof crop residue

RapidLoss

SlowlyReplenished

Harvest Interval and Nutrient DepletionHarvest Interval and Nutrient DepletionS

oil

Exc

han

gea

ble

Nu

trie

nt

Po

ol

Time

With enough time between harvest removals, the exchangeable nutrient pool is maintained by natural fluxes from reservoir pool

Long Rotation (Forestry)|---Harvest Interval---|

Harvest Interval and Nutrient DepletionHarvest Interval and Nutrient DepletionS

oil

Exc

han

gea

ble

Nu

trie

nt

Po

ol

Time

With insufficient time between harvests to allow for natural replenishment, soil nutrient pools are depleted.Crop production will decrease over time.

Long rotation

Short Rotation (Agriculture)

HarvestInterval

Harvest Intensity and Nutrient DepletionHarvest Intensity and Nutrient Depletion

Corn

Cotton

So

il E

xch

ang

eab

le N

utr

ien

t P

oo

ls

Time

Crops that remove a larger amount of nutrients require a longer time period between harvests or soil nutrient pools will be depleted.

Harvesting Effects On Different NutrientsHarvesting Effects On Different NutrientsS

oil

Exc

han

gea

ble

Nu

trie

nt

Po

ol

Time

Rapid Input Flux from Reservoir Pool (N)

Slow Input Flux from Reservoir Pool (P)

Slowly cycled mineral nutrients (Ca, Mg, K, P) are more readily depleted than more rapidly cycled gaseous nutrients (N, C, S).

Managing Soil FertilityManaging Soil FertilityCrop Rotation: 4 Year CycleCrop Rotation: 4 Year Cycle

Nutrient Extractive Crop(Corn, Cotton, Wheat, Rice)

Fallow Year (No Crop)Replenish Soil Nutrients

Hay, Grass Cover

“Green Manure” CropReplenish Soil Organic Matter

Hay, Alfalfa

N-Fixing CropReplenish Soil N Pool

(Soybean, Alfalfa)

Managing Soil FertilityManaging Soil FertilityCrop Rotation: 2 Year CycleCrop Rotation: 2 Year Cycle

Nutrient Extractive Crop(Corn, Cotton, Wheat, Rice)

N-Fixing CropReplenish Soil N Pool

(Soybean, Alfalfa)

What about….

Other Nutrients (Ca, Mg, K, P) ?Chemical LimingAnd Fertilization

Soil Organic Matter ? Degraded water retention, aeration, drainage

Corn YieldU.S.A

WheatYield

Major gains in crop production from the Green Revolution required massive increases in the use of chemical fertilizer

Fertilizer Use and the Green Revolution

A Case StudyA Case StudyAgricultural Trends In Georgia (USA): 1940 – 1990Agricultural Trends In Georgia (USA): 1940 – 1990

Acreage of agricultural land decreased by Acreage of agricultural land decreased by 50% (farm abandonment)50% (farm abandonment)

State-wide total agricultural crop production State-wide total agricultural crop production increased by 100%increased by 100%

Crop yield per acre increased 4-fold.Crop yield per acre increased 4-fold.

How did this happen ???How did this happen ???

A Case StudyA Case StudyAgricultural Trends In Georgia (USA): 1940 – 1990Agricultural Trends In Georgia (USA): 1940 – 1990

Total use of fertilizer (per acre) increasedTotal use of fertilizer (per acre) increased 7-fold7-fold

Use of Nitrogen fertilizer increased Use of Nitrogen fertilizer increased 11-fold11-fold..

Is this a problem ?Is this a problem ?

– Excess nutrients from fertilizer washes into Excess nutrients from fertilizer washes into streams, lakes, and groundwater (more later).streams, lakes, and groundwater (more later).

– Dependence on expensive fertilizers puts farmers Dependence on expensive fertilizers puts farmers at economic risk.at economic risk.

Agricultural EconomicsAgricultural Economics

Fertilization IncreasesCrop Yields (and also Costs)

Increased Grain Supply to Consumer Market

Price per Bushel Decreases

Farmer Income Decreases:Grain Sales Receipt – Costs (fuel, seed, fertilizer)

N-fertilizer made using fossil fuel. Sensitive to price fluctuations

The same companies that buy the crops also sell the seed and fertilizer.

Agriculture In the Wet TropicsAgriculture In the Wet Tropics

A Cautionary Tale of Nutrient A Cautionary Tale of Nutrient Cycling Limits for AgricultureCycling Limits for Agriculture

Total Ecosystem Carbon In Boreal Total Ecosystem Carbon In Boreal and Tropical Forest Ecosystemsand Tropical Forest Ecosystems

0

20

40

60

80

100

Boreal Tropical

Forest Type

% T

ota

l E

cosy

stem

Car

bo

n

Leaf

Wood

Litter

Soil

Slash-and-Burn AgricultureSlash-and-Burn Agriculture

Cut-down and burn forest vegetation to Cut-down and burn forest vegetation to release nutrients to the soil.release nutrients to the soil.

Initially, crop yields are high.Initially, crop yields are high.

Crop yields progressively decline.Crop yields progressively decline.

Field abandoned after 3 to 5 years.Field abandoned after 3 to 5 years.

Sustainable w/ SMALL human populations, Sustainable w/ SMALL human populations, but NOT w/ large human populations.but NOT w/ large human populations.

Nutrient Leaching After Slash-and BurnNutrient Leaching After Slash-and Burn

0

1

2

3

4

Kg

/ h

a /

mo Control

Cut-Burned

1976 1977 1978 1979 1980

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

Primary Productivity (kg / ha/ yr) of Primary Productivity (kg / ha/ yr) of Rain Forest vs. Slash-and-Burn CropRain Forest vs. Slash-and-Burn Crop

Year 1Year 1After BurnAfter Burn

Year 2Year 2After BurnAfter Burn

Year 3Year 3After BurnAfter Burn

RainforestRainforestTotal NPPTotal NPP

12,74212,742 12,99512,995 12,92012,920

Yucca CropYucca Crop(edible part)(edible part)

1,4651,465 1,0061,006 700700

CropCropTotal NPPTotal NPP

5,3335,333 5,2945,294 31503150

““Weeds”Weeds”Total NPPTotal NPP

300300 679679 990990

Slash-BurnSlash-BurnTotal NPPTotal NPP

56335633 59735973 41404140

Phosphorous Dynamics of Slash-and-BurnPhosphorous Dynamics of Slash-and-Burn

P in Atmospheric Dust P in Soil Minerals

Plant-Available PIn the Soil

Insoluble PIn the Soil

P in PlantBiomass

Deposition Weathering

Uptake

Low pH – P precipitatesNeutral pH – P dissolves

Decompositionand Burning

P-removalin harvested

biomass

P-loss to atmosphere

in ash from fire

P-loss due to leaching andsoil erosion

Very lowVery lowin intactin intactrain forestrain forestecosystemsecosystems

Phosphorous Dynamics in Tropical Phosphorous Dynamics in Tropical Soil After Slash-and-BurnSoil After Slash-and-Burn

ControlControlOne month One month since burnsince burn

20 months 20 months since burnsince burn

4 years after 4 years after abandonmentabandonment

Available P Available P (ppm)(ppm)

4.84.8 7.47.4 13.013.0 4.74.7

Soil pHSoil pH 4.254.25 4.964.96 5.155.15 4.524.52

Total P Total P (ppm)(ppm)

200200 130130 250250 300300

% Available% Available 2.42.4 5.75.7 5.25.2 1.61.6

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)

Increase Base Cations Ca, Mg,

K in soil

IncreasedSoil pH

IncreaseExchangeablePhosphorous

Decrease ToxicMetals Fe, Mn, Al

High Crop Yields

Removal ofCa, Mg, KIn Crops

Loss of CaMg, K viaLeaching

DecreasedSoil pH

Decreased Exchangeable P

Increased ToxicMetals Fe, Mn, Al

Decreased Crop Yields

&Increased“Weeds”

LandAbandonment

Re-Growth ofTropical Rainforest(Recovery Phase)

The Slash-andBurn Cycle

Nutrient SaturationNutrient SaturationThe Other Side of Human The Other Side of Human

Impacts on Nutrient CyclesImpacts on Nutrient Cycles

Soil Nutrient Capacity vs. ContentSoil Nutrient Capacity vs. Content

SoilContent

NaturalInputs

Plant Uptake -Harvest Loss

DecompositionMineralizationN-FixationWeathering

If losses exceed inputs If losses exceed inputs ► Nutrient depletion► Nutrient depletion(Content << Capacity)(Content << Capacity)

Nutrient SaturationNutrient Saturation

SoilContent

Natural Inputs+

Human Inputs

PlantUptake

FertilizerAcid RainManure

Leaching to

GroundwaterIf inputs exceed losses If inputs exceed losses ► Nutrient saturation► Nutrient saturation

(Content = Capacity)(Content = Capacity)

Nitrate Application On U.S. FarmsNitrate Application On U.S. Farms

IndianaIndiana

Algae In Gulf Coast WatersAlgae In Gulf Coast Waters

““Dead Zone” FormationDead Zone” Formation

(Hypoxic Bottom Water)(Hypoxic Bottom Water)

Gulf of Mexico Dead ZoneGulf of Mexico Dead Zone

““Acid Rain” Adds Excess NutrientsAcid Rain” Adds Excess NutrientsExcess Inputs of N and S From AtmosphereExcess 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 AlIncreased Toxic Soluble Al

Decreased Plant Growth“Forest Decline”

Excess Al Leaches Into StreamsAl Toxicity Kills Aquatic Organisms

“Dead Lakes”

SummarySummary

““Sustainability” of agricultural production Sustainability” of agricultural production systems and “Health” of natural ecosystems systems and “Health” of natural ecosystems require balancing of nutrient budgets.require balancing of nutrient budgets.

Nutrient depletion of agricultural systems Nutrient depletion of agricultural systems requires expensive chemical fertilization that requires expensive chemical fertilization that may not be sustainable long-term.may not be sustainable long-term.

Nutrient saturation of natural systems is a Nutrient saturation of natural systems is a major risk to ecosystem health.major risk to ecosystem health.

The EndThe End