Lecture 12 Nutrients: Nitrogen and Phosphorus

Lecture 12: Nutrients: Nitrogen and Phosphorus

1. Nutrients: Limiting Nutrients

  • Definition of Nutrients: Nutrients are mineral resources, in addition to carbon (C), oxygen (O), and water, required for life.

  • Limiting Nutrients: Essential resources that reduce the rate of a biological process (e.g., net primary productivity (NPP), growth, photosynthesis) due to an inadequate supply. This limitation can also arise from environmental conditions such as low temperature.

2. Nutrient Impacts

  • Context: Nutrient availability can dramatically influence ecosystem functions and population dynamics in plants and animals.

  • Example: CHONPS components are crucial for life, with average composition for land plants indicated by the ratio: C₁₂₀₀ H₁₉₀₀ O₉₀₀ N₃₅ P₂ S₁.

3. Nitrogen Cycle: Reservoirs and Fluxes

  • Nitrogen as an Element:

    • Percentage in Atmosphere: Earth's atmosphere consists of 78.1% nitrogen (N₂) in a gaseous, stable form.

    • Process of Nitrogen Fixation:

    • Natural: Lightning strikes can fix nitrogen in the atmosphere into usable forms (e.g. NO₃⁻).

    • Biological: Nitrogen-fixing bacteria in the root nodules of legumes convert atmospheric N₂ into NH₄⁺.

    • Anthropogenic: Human-induced processes like the Haber-Bosch process convert N₂ into ammonia (NH₃).

  • Nitrogen Forms in the Cycle:

    • N₂: Gaseous, tightly bound, very stable.

    • NH₄⁺ (Ammonium): Form available to plants in soil, produced by decomposers or nitrifying bacteria.

    • Norg: Organic nitrogen in tissues of plants, animals, and microbes.

    • NO₂⁻ (Nitrite): Form accessible to nitrifying bacteria.

    • NO₃⁻ (Nitrate): Available for soil microbes and plants; also produced from lightning.

    • NH₃ (Ammonia): Released into the atmosphere via processes like nitrification and denitrification.

    • NOₓ and N₂O: Produced by denitrifying bacteria and fossil fuel combustion.

4. Phosphorus Cycle: Reservoirs and Fluxes

  • Definition of Phosphorus: Phosphorus is a chemical element essential for forming organic molecules (e.g., ATP, DNA).

  • Forms of Phosphorus in the Cycle:

    • PO₄³⁻ (Phosphate): The form available to plants.

    • Porg: Bound to organic molecules.

    • Pbound: Bound to soil particles, making it unavailable to plants.

  • Processes in the Aquatic and Terrestrial Phosphorus Cycle:

    • Chemical weathering of rocks releases PO₄³⁻, which plants uptake and transform into Porg.

    • When plants die or are consumed, Porg returns to soil or sediment, where microbes convert it back to PO₄³⁻.

5. Nutrient Pollution

  • Nitrates in Water: Harter et al., 2012, indicated the population risks from nitrate contamination in areas like the Tulare Basin and Salinas Valley, where 96% of groundwater nitrates derive from fertilizers. The EPA standard for nitrate presence in drinking water is < 45 mg L⁻¹.

  • Eutrophication Impact: Excess nutrients can lead to eutrophication, where nutrient overload in water bodies causes algal blooms and oxygen depletion, creating “Dead Zones” (as illustrated by hypoxia levels noted in the Gulf of Mexico).

6. Statistical Data & Graphs

  • Nitrogen Use: Fertilizer use has increased dramatically since the 1960s, in parallel with population growth.

    • Estimated nitrogen use:

    • Fertilizers: 120 Tg/y

    • Population growth correlated with nitrogen use trends:

    • 1880 - 2020 analytics indicate a direct relationship.(Graph data extracted from Ronald Amundson et al., Science 2015).

  • Anthropogenic Phosphorus Use: Increased usage of fertilizers since the mid-1800s, coupled with manure recycling, is critical for maintaining agricultural productivity post crop harvesting.

7. Ethical and Practical Implications

  • Environmental Impact Awareness: The social ramifications of nutrient pollution are significant, disproportionately affecting low-income communities.

  • Management Strategies: Suggested strategies include careful monitoring and regulation of agricultural runoff, industrial emissions, and enhanced understanding of nutrient cycles to create sustainable ecosystems.

Key Concepts Connected Together:
The nitrogen and phosphorus cycles are interrelated in their impacts on ecosystems and the overarching concern of anthropogenic influences such as agriculture, which aim to enhance productivity but concurrently lead to harmful eutrophication and habitat degradation in waterways.