Chapter 48.3: Human Influence on the Nitrogen and Phosphorus Cycles

Nitrogen Cycle

• Nitrogen-fixing bacteria and archaea convert N2 to biologically available forms of nitrogen

• SInce N2 is abundant in the atmosphere but can’t be used by organisms

• Fixed nitrogen: nitrogen in biologically available forms - ammonia and nitrate 

  • Found in soil and food 

Impacts on Nitrogen Cycle: 

• Farmers artificially add fixed nitrogen to the soil in order to fertilize their plants as the same soil being used over and over loses its natural nitrogen availability 

  • The nitrate is artificially produced (a process that also uses fossil fuels) and then spread across fields as a fertilizer that can can runoff fields and travel by rivers to lakes or oceans 

    • Some denitrifying bacteria take nitrate for respiration reutrning N2 to the atmosphere as N2O which is another greenhouse gas

Eutrophication

• excessive richness of nutrients in a lake or other body of water, frequently due to runoff from the land, which causes a dense growth of plant life and death of animal life from lack of oxygen

Dead zone

• regions with completely depleted oxygen

  • This dead zones seasonally expand and also expand by the year becoming bigger and bigger

Phosphate in Agriculture:

• Phosphate (PO₄³⁻) is one of the major nutrients required for plant growth

  • Unlike nitrogen fertilizer that can be industrially made with the haber bosch process phosphate can not

  • Phosphate fertilizer comes only from mining phosphate rocks 

    • Finite and geographically limited 

Phosphate Eutrophication

• It can also run off from field into water sources 

• Again excessive nutrients contributed to eutrophication 

• Artificial Addition of Phosphate: 

• Seen through a study that regions with low-phosphate ecosystems have plants evolved to survive off the low availability 

• When phosphate runoff was introduced increasing phosphate levels in the ecosystems…

  • Invasive species increased 

  • Native plants declined due to competition and messed up environmental balance leading to the loss of biodiversity 

• Phosphate as a limited resource: NON RENEWABLE 

• Only a few regions with high phosphate sedimentary rock deposits 

• We are depleting high quality reserves 

  • We now mine poor quality deposits since the good stuff is gone 

  • Coasts are increasing due to demand and scarcity 

  • Long-term ability to support global agriculture is threatened

• Similar to the depletion of oil….. 

  • Both rely on finite geological sources

  • Both become more expensive as high-quality sources run out

  • Both raise sustainability concerns.

Challenge

The world population and demand for food are growing rapidly with rising consumption

Conventional Methods to support the demands of the population

1. Convert more land to agriculture

2. Raise yields on existing farmland: Use better crop varieties via traditional breeding

Alternative Methodsto support the demands of the population

1. Improve Post-Harvest Losses (Storage, Transport, Packaging)

2. Shift Dietary Habits → less meat

3. Use More Efficient, Sustainable Agriculture Practices

4. Reduce Dependence on Nonrenewable Nutrient Sources

5. Political, Economic, and Social Strategies

1. Convert more land to agriculture

• Tradeoff: 

  • Reducing Carbon storage (increasing atmospheric CO₂), 

  • Reduced Biodiversity (loss of native species / habitats)

Raise yields on existing farmland:

Use better crop varieties via traditional breeding

• Helps prevent the need to have more land for agriculture 

• agriculture improved via selective breeding

• Genetic Engineering: Take advantage of genetic engineering to introduce traits such as:

  • Higher yield

  • Drought tolerance

  • Disease-resistance

  • Better nutritional quality

• Tradeoff: 

  • Heavy fertilizer use (nitrate & phosphate)

    • Causes eutrophication and nutrient runoff, harming aquatic & natural ecosystems.

    • Relying on finite resources 

  • Energy-intensive farm machinery

  • Contributes to climate change (through fossil fuel use and greenhouse gas emissions).

Green revolution

• high-yield, disease-resistant strains of wheat, rice, and corn dramatically boosted global food production → agriculture improved via selective breeding

Red Queen hypothesis

• that species must constantly adapt, evolve, and proliferate in order to survive while pitted against ever-evolving opposing species.

Improve Post-Harvest Losses (Storage, Transport, Packaging)

• Spoilage by fungi, bacteria during storage, transport, supermarket, home.

• Solution: 

  • Better storage, transportation, packaging systems

  • Cold chains, improved technology to reduce spoilage

  • This effectively increases net yield without expanding farmland or increasing fertilizer use

Shift Dietary Habits → less meat

•  feeding grain directly to humans feeds more people than feeding it to livestock, then eating the meat (because energy is lost at each trophic transfer).

• Eating locally grown produce where possible:

  • Helps reduce energy use and decrease spoilage risks 

Ecological principle

• feeding grain directly to humans feeds more people than feeding it to livestock, then eating the meat (because energy is lost at each trophic transfer).

Use More Efficient, Sustainable Agriculture Practices

• Develop crops that require less fertilizer

• Use precision agriculture: optimize fertilizer application (right amount, right place, right time) to minimize runoff/pollution and maximize yield.

• Implement crop rotations, cover cropping, soil conservation, organic farming

  • maintain soil fertility, reduce erosion, and minimize environmental damage

• Use microbial and biological solutions: Reduce dependence on synthetic fertilizers

Reduce Dependence on Nonrenewable Nutrient Sources

• Recycling nutrients from waste (human/animal waste, food waste, wastewater).

• Recovering phosphorus from agricultural runoff, sewage sludge, and manure.

• Developing alternative fertilizers or more efficient nutrient delivery systems.

Political, Economic, and Social Strategies

• promote sustainable agricultural policies that balance food production with conservation

• Support research & development

• Encourage consumer choices that reduce wastage