Chap 10: Water cycle, nutrient cycles and Food Production

What does the water cycle describe

The natural, continuous movement of water between atmospheric, aquatic and terrestrial compartments driven by solar energy and gravity

• closed system

• ensures water is distributed and renewed across the planet, supporting ecosystems and agriculture

How does deforestation distrupt the water cycle?

Fewer trees to absorb water and release water into the atmosphere through transpiration

• reduces local humidity and rainfall

• increase surface runoff leading to flooding

Key components of the global water cycle

1. Evaporation - water evaporates from land, water and organisms

2. Precipitation - water condenses and falls as rain or snow

3. Run off - water moves over the ground surface into rivers and eventually the ocean

4. Infiltration - flow of water from ground surface into ground due to gravity

5. Storage - water can be stored in one place e,g ice caps, oceans

How do plants play an important role in the water cycle?

Regulates how water moves through the ecosystem through

• Interception
  - some rainwater evaporates from tree leaves before it reaches the ground (control soil erosion)
  
  

• Transpiration
  - roots take up water and return in to the atmosphere through the stomata
  
  

• Infiltration
  - tree roots and leaf litter on the ground create spaces in the soil that help water flow into the soil which decrease surface runoff
  
  

• Evapotranspiration
  - the combined process of water surface evaporation, soil moisture evaporation and plant transpiration
  - plants with deep roots can bring water from deep underground to the surface during transpiration —> more water vapour enters the atmosphere

Define abstraction for irrigation

Abstraction: Process of collecting water from natural sources (lakes, river, groundwater) for human purposes (drinking water, agriculture, industrial purposes)

Irrigation: the practise of applying controlled amounts of water to land to grow crops, landscape plants and lawns

• around 70% of global freshwater used by human —> irrigation

How does irrigation modify the water cycle?

• diverting runoff and/or surface water in lakes and rivers —> for crops or livestock

• diverting groundwater —> for crops or livestock

• increasing evaporation (water at the surface) and transpiration (plant leaves) —> increase water vapour in the atmosphere, decrease water in lakes, rives and aquifers

Case study of over abstraction for irrigation

Aral sea in Central Asia

• at the boundary of Kazakhstan to the north and Uzbekistan to the south

• Once a large saltwater lake of Central Asia and worlds fourth largest body of inland water

• During 1960s 2 rivers (Amu Darya and Sry Darya) draining into the aral sea were diverted to irrigate dry farmland for crops e.g Cotton

• Aral sea began to shrink significantly —> loss of fishery

Why irrigation problems were face in Uzbekistan/dry regions

Tends to be inefficient

• Lots of water sitting at the surface of hot climates —> evaporation
  - so instead of efficiently watering the crops, water was loss through evaporation
  
  

Salinization

• After evaporation, dissolved salts are left behind

• Remaining water becomes increasingly salty

• Salinization reduces soil fertility, making it harder for crops to grow

Irrigation must be ____ and _____ properly to reduce consequences

managed and planned

Changes in land cover (deforestation)

Modifications of the earths surface, primarily by human activities (urbanization, agricultural expansion)

• Land development creates impervious surfaces through urbanization
  -increase concrete and hard surfaces, decrease soil and vegetation to absorb water

• more rain water, increased runoff and flooding in and downstream of urban area

What is impervious surfaces?

Surfaces that water cannot penetrate

How does urbanization cause the increase in temperature?

Concrete and solid materials like asphalt have heat absorbing capabilities

Impact of deforestation on the water cycle depends on?

1.What type of vegetation replaces the forest?

• e.g if secondary forest may be able to replace interception and evapotranspiration of water that the primary forest used to provide

• if replaced plants have dense forest canopy (evapotranspiration) and deep roots (absorbtion of water)

2.What happens to the soil during deforestation

• direct sunlight can lead to soil compaction, erosion and loss of organic matter

• reduce the ability to absorb and hold water

• less water to infiltrate the ground and instead cause surface runoff increasing flooding

What does interception mean?

Precipitation that is caught by plant canopies such as leaves, branches and trunks preventing it from reaching the ground

Example of the type of vegetation that replaces the forest

Young oil plantations (< 5 yrs old)

• evotranspire 20% less water than rainforest trees

• increased runoff and downstream flooding

• contribute to flooding

Mature oil plantations (>8 yrs old)

• evapotranspire 10% more than rainforest trees

• less soil infiltration and runoff

• decreased water available downstream

• reduce water availability

Both are not better than each other the rainforest is the best

Logging

causes soil to become compacted

• spaces between the soil particles are squeezed togther reducing water infiltration

• increased flooding

bare soil easily washed away than soil covered by vegetation

• without tree roots to hold the soil in place

• increased risk of soil erosion (topsoil) into rivers —> process called siltation and sedimentation

• accumulation of sediments caused rivers to become shallow, river cannot carry as much water

• cause flooding

Construction of dams

Dams alter the runoff component of the water cycle

Positive

Control of river flows - benefits people living in flood-prone regions (more control over release of water)

Negative

Flora and fauna downstream are affected

• many fish use seasonal flooding as a signal to swim upstream for reproduction

• physical barrier to upstream movements of migratory animals e.g salmon

• moving water (river) —> deep, still water (reservoir): changes aquatic communities

Potential disasters if dams break or overflow —> habitat loss due to reservoir flooding

Displacement of people —> affecting livelihood

Emission of GHG

• CO2 released from soil and dying vegetation when the reservoir is first flooded with water

• Methane formed when organic matter decays under anaerobic conditions at the bottom of the reservoir
  - release is increased by turbines in dams constructed for hydroelectricity generation: turbines circulate water and sediments at the bottom of the reservoir

Case study for the construction of dams

Bakun Dam in Malaysia

• Malaysia largest hydroelectric power station

• flooded 700km² of rainforest and farmland —> habitat loss

• displaced 9000 people

Nutrient cycle

Natural processes that recycle essential elements to the atmosphere, lithosphere, hydrosphere and biosphere

• Nitrogen cycle

• Phosphorous cycle

Nitrogen general information

• Abundant in the earth 78% of the air we breath

• key co mponent of molecules which are essential to living organisms (amino acid, ATP, DNA and RNA)

• N2 is chemically unreactive —> not in a usable form for most living organisms

• N2 needs to be converted / fixed into NH4+ or other reactive nitrogen compounds which can be used by living organisms

• Reactive nitrogen - NH4+ and nitrate ions are essential for plants and animals but tend to relatively scarce in soils, despite abundant N2 in atmosphere

Nitrogen cycle

The movement of nitrogen within and between the atmosphere, biosphere, hydrosphere and geosphere which allow it to be used by organisms

Nitrogen fixation

converts atmospheric nitrogen into useable forms such as nitrate which plants can absorb

How can nitrogen fixation happen?

• Nitrogen fixing bacteria that live in the soil or root nodules of plants like legumes

• Lightning converts nitrogen gas into nitrates during storms

• Artificial process industrially that produces synthetic fertilizers with nitrogen

Excess nitrogen from ferilizers in soil cause?

Runoff into river causing algae bloom in river

5 main processes of the nitrogen cycle

1. Nitrogen fixation

2. Nitrification

3. Assimilation

4. Ammonification

5. Denitrification

Nitrogen fixation

Atmospheric nitrogen is converted to ammonia (ammonium when dissolved in water)

• making nitrogen available to plants

Some bacteria can convert N2 into ammonium NH4+ by nitrogen fixation

• bacteria free living in soils or water

• form symbiotic association with plants or other organisms (termites - have nitrogen fixing bacteria in their guts) recycle nitrogen

• Lightning strikes, volcanic eruption, combustion - high energy to break triple bond in atmospheric nitrogen and combine with oxygen or hydrogen to form compounds like ammonia

Nitrogen fixation example

Legumes have root nodules that contain nitrogen fixing bacteria called Rhizobium (mutualistic symbiosis)

• plants receive nitrates or ammonium ions from the bacteria

• bacteria receive carbohydrates from photosynthesis by the plant

Nitrification

Some ammonia is converted to nitrite(NO2-) and then nitrate(NO3-) by nitrifying soil bacteria

• nitrate taken up by plants and used to build proteins, nucleic acids

• animals cannot take up nitrate or ammonium ions directly but obtain nitrogen in their diet by eating plants or other animals

Assimilation

Uptake of ammonium NH4+ or nitrate NO3- ions by plants and animals

• some ammonium ions dissolved in water are taken up directly from the soil by plant roots

Ammonification

is a form of decomposition

• many bacteria and fungi breakdown organic matter (dead plants/animals) into ammonium NH4+

• releases N2 for uptake by other organisms

Denitrification

Process that returns nitrogen back to the atmosphere as nitrogen (N2) or nitrous oxide (N2O)

• Some bacteria in waterlogged soils and lake or ocean sediments convert: NH3 and NH4+ —> nitrogen gas, N2 and nitrous oxide, N2O

• N2 and N2O gases are released back into the atmosphere ready to start the N cycle again

Human alteration of Nitrogen cycle

Fertilizers: supply essential nitrogen

Uses the Haber-Bosch process —> synthesize ammonia from nitrogen and hydrogen under high heat and pressure. Common types

• urea - 46% nitrogen

• ammonium nitrate (34-35%)

Ends up on agricultural land and in sewage

• nitrogen runoff causes eutrophication - algae blooms

• N2O is a GHG and contributes to the destruction of the ozone layer

• nitrogen oxide released by burning fossil fuels and sunlight contribute to photochemical smog in cities

Phosphorous cycle

Most of the Earth’s phosphorus is stored in rocks and sediments

• is slowly released by natural processes like weathering, leaching, and mining

Phosphorous in land and freshwater ecosystems

Phosphorous cycles through plants and animals, returning to the soil through decay

• some phosphorous reaches the ocean, where it circulates in shallow waters or sinks to deep areas

• part of ATP, DNA, RNA

Is there phosphorus in the atmosphere?

No it is not other than as rock dust

• phosphorous cycle only involves water, earth crusts and living organisms

• most stored in earths crust as phosphate salts orthophosphate PO43- in rocks and ocean sediments (sedimentary cycle)

Phosphorous cycle

Movement of phosphorous within and between the biosphere, hydrosphere and geosphere

Weathering phosphorous

Releases from rocks —> soil and water

• some taken up by plants and algae eaten by herbivores and carnivores —> back to soil and water through extraction and decomposition

• water transports dissolved phosphorous and eroded soil particles containing phosphorous into sea and ocean sediments

  • new rocks form as ocean sediments are compressed or become rocks on land due to plate tectonics or changes in sea lvl

Types of weathering (breaks rocks into smaller particles)

Weathering can be:

Physical
- mechanical breakdown of rock without changing chemical composition
- e.g frost wedging water enter through cracks and freezes, reptitive actions cause cracks to widen

Chemical
- chemical reaction that alters chemicals within the rocks
- errosion

Biological
- Living organisms help break down rocks
- e.g roots grow between rocks

Phosphorous cycle

1.Weathering

2.Absorbtion by plants
3.Consumption by animals
4.Decomposition and Mineralization
5.Leaching and runoff

Weathering

• Phosphorus is abundant in rocks

• Rocks breakdown —> phosphate salts are released into the soil

Absorbtion by plants

Plants absorb dissolved phosphate salts from soil or water

• Phosphate salts have low solubility

• Aquatic plants absorb inorganic phosphorus from deep water layers

• growth in aquatic ecosystems can be limited

Consumption by animals

Get phosphorous by eating plants or other herbivore animals

• rate of cycle is faster in plants and animals when compared to rocks

Decomposition and Mineralization

when plants and animals die

• microorganisms break them and convert organic phosphorous into inorganic forms in soil

• overtime settles into sediments and rocks which release it again through weathering

Leaching and runoff

Phosphorus is carried by surface runoff into rivers, lakes, and eventually the ocean

• waste and bodies of marine organisms sink to the ocean floor forming sedimentary rocks

• overtime phosphorous rich rocks lifted onto land by plate movements or seal lvl changes, restarting the cycle

Human alterations: Phosphorous cycle (Eutrophication)

Excess use of fertilizers

1. Increased nutrients causes algal bloom

2. Increased decomposition of dead and algae uses up o2 in water

3. Fish begin to die as oxygen levels fall

4. Increased growth of algae shading out other plants

Explain eutrophication

When a body of water becomes overly enriched with nutrients —> induce excessive growth of algae

• excessive growth due to over supply of nutrients

• excess algae dies; decomposition reduces o2 content of water —> death of fish and other aquatic animals

• algae shades out bottom rooting plants

cyanobacteria produce toxins to help them outgrow other algae - toxins can be poisonous to fish and may work their way up to the food chain of humans

• e.g eating oyster from eutrophic seawater —> food poisoning

Traditional agriculture: Hunter gatherers

Modern Homo sapiens have existed for about 300,000 yrs

• until approx 12,000 yrs ago all humans were hunter gatherers; food from the wild

• low food availability / high energy expended capturing the food —> low population densities —> typically low and small scale environmental impact

Agricultural revolution

Started 8000 - 12000 yrs ago: people domesticated wild animals for food and labour and began to cultivate wild plants

• gradual change in lifestyle from nomadic hunting and gathering to settled agricultural communities

• increased supplies of food, reduced amount of work needed to capture the food —> increased human population and division of labour
  e.g craftman, militarization

• Close contact w domestic animals facilitated evolution of diseases

Common method for traditional agriculture

Shifting (slash-and-burn) cultivation

1. Burning clears land for planting and ashes fertilize soil

2. Plant crops such as maize, cassava

3. After an area has been used for several years, nutrients in soil are depleted and farmers move to new plot

4. Abandoned patch is left fallow (no crops) for 10 - 30 yrs before soil becomes fertile enough to grow new crops

What are the two types of agriculture?

Traditional Subsistence Agriculture

• grow enough food to feed 1 family - little leftovers

• depends on human and animal muscle power and basic equipment

• planting decisions based on what the family need the coming year, secondarily market prices
  
  

Traditional Intensive Agriculture

• enough food to feed many families and sell surpluses

• increased demand of food and invention of new technologies

• farmer increase inputs of human and animal labour, organic fertilizer, irrigation of water

Traditional agriculture systems usually:

• grow several crops within the same plot —> polyculture
  -reduce pest outbreak - pest target one crop
  -remain soil fertility - diff crops diff nutrients
  -maximize ener

• crop rotation - different crops planted in different years
  -prevent nutrient depletion

Rise of intensive agriculture

Mechanization

• increased availability of machines has enabled mechanization of labour (ploughing, fertilizers)

• reduce labour cost and increase farm incomes

• decrease rural jobs increase urban populations

Traditional rice farming in Malaysia VS modern

Traditional

• Single crop planted per year, long maturation period

• Poor response to fertilizers (grow too tall no increased yield)

• Variation in planting seasons
  - West coast: planted with onset of rainy season in August and harvested in Feb
  - East coast: planted in december w onset of monsoon rain and harvested in june

• Farmers only grew rice for half a year and the soil was left to fallow

Modern (1940-70s)

• selective breeding —> new, high yielding crop strains (rice, wheat corn)

• Synthetic fertilizers (Haber process)

• Pesticides

• multiple cropping (no. of crops grown per yr on the same plot of land)

Traditional rice farming dry and wet seasons

Dry season

• Rice stalks, weeds, and vegetation would die and the soil would dry out

Wet season

• fields were flooded; freshwater fish entered the rice paddies —> source of protein for the farmers and natural pest control for rice crop

Green revolution in Malaysia

New rice varieties developed from cross breeding at the Int. Rice Research Institute in the Philippines (IRRI)

• ‘dwarf rice’ plants grow more grain / yield with fertilizer instead of tall and fall

• large amounts of fertilizer are required

Initially farmers were hesitant

• Large development was funded by gov. —> irrigation schemes to keep field flooded all year round

• Practise double cropping; 2 rice crops planted per year

• Rice yield have increased since 1960s

Environmental impacts of Food production

Land use and Deforestation

• habitat loss and reduce biodiversity and increase global warming

Slash and burn cultivation

• release large amount of GHG (Co2 and CO)

• air pollution from, black soot

• fire danger

Abandoned land

• soil exhaustion

• leaving behind degraded land w little ecological or economic value

• long term damage

Soil degredation

• top soil erosion without roots or plants

• without crop rotation and soil management, land loses fertility

• overuse and nutrient depletion

Nutrient depletion

• repeated farming on the same land

environmental impact of modern agriculture

1. Mechanization

2. Biodiversity loss

3. Agricultural runoff and Water quality

4. Monoculture

5. Modern rice varieties

6. Fertilizers

7. Pest control

Mechnization

• Fossil fuels (tractors use petrol and diesel)

• Ploughing land changes soil structure —> reduce water retention and aeration of roots for optimum plant growth

• Ploughed soils are prone to erosion, removing the rich top soil layer —> increased sediment runoff into waterways

• Promote monoculture

Biodiversity loss

During domestication and selective breeding for high yield, loss of genetic diversity

Agricultural runoff and Water Quality

Rain exceeds soil absorbtion capacity

• all water moves collects pollutants and fertilizers into nearby water bodies

Monoculture

Create ideal condition for pests and pathogens to spread

• increase pesticides increase water pollution

• Loss of biodiversity, pollinators —> lower crop productivity

Modern rice varieties are prone to blast disease

• traditional rice exhibit high genetic diversity, resist diseases

• now more susceptible to blast disease outbreaks

Fertilizers

Modern crop require high amount of nutrients (nitrogen, phosphorous etc..)

• efficiency of fertilizer used by rice plant is poor, much leaves the paddy by evaportion and run-off

• eutrophication, increase water treatment cost

Increase GHG through production of fertilizer

• nitrous oxide formed by microbes in soil from fertilizer

Pest control

Requires continuous selective breeding to generate new resistant crop varieties

• endless development of new pesticides

Challenges for the future production of food - Closing yield gaps

Yield gap is the difference between actual crops yields currently obtained and the maximum potential yields that could be achieved under ideal conditions

• improve farming practise, produce more food on the same land

Methods for closing yield gaps

1. Applying the right amount and type of fertilizer at the right time to crops
   - nutrients without excess use

2. Use integrated pest management
   - combine biological pest controls, crop rotation and targeted pesticides use to reduce pest damage minimizing chemical impact on environment

3. Implementing methods like drip or sprinkler irrigation that deliver water directly to roots
   - reduce water waste
   -improve crop growth in limited water supply

How to farm without harming the environment case study

Biological pest control in rice paddies (China)

• paddies with fish required 68% less pesticides and 24% less fertilizer to achieve similar yield to paddies without fish

• fish control pests and enhance water circulation

• fertilizer and fish feed contribute nitrogen and fish spread the nitrogen reducing fertilizer waste