Comprehensive Study Guide on Nutrient Cycles

Introduction to Nutrients and Global Recycling

Nutrients are chemical substances required for plant and animal growth, as well as the maintenance of critical life processes. Within Earth’s biosphere, these nutrients are not depleted but are constantly recycled. The movement of nutrients occurs through various layers of the Earth, including the atmosphere, hydrosphere, and geosphere. These nutrients are stored in different reservoirs for varying lengths of time.

Stores are categorized into two primary types based on the duration of retention. Short-term stores include living organisms and the atmosphere, where nutrients move relatively quickly. In contrast, longer-term stores include Earth’s crust and sedimentary rock, where nutrients may remain trapped for millions of years. Nutrient cycles describe the continuous flow of these chemicals in and out of stores through a combination of biotic (living) and abiotic (non-living) processes. Without human interference, these nutrient cycles naturally maintain a state that is almost perfectly balanced. There are three primary cycles critical to terrestrial and aquatic ecosystems: the carbon cycle, the nitrogen cycle, and the phosphorus cycle.

The Carbon Cycle: Fundamental Processes and Storage

Carbon is an essential component of all living cells and is a requirement for life-sustaining chemical reactions. It cycles through living and decaying organisms, the atmosphere, bodies of water, soil, and rock. Six main processes facilitate the movement of carbon between different stores. Two of the most critical processes involve the exchange of energy and gases between plants and animals.

Photosynthesis is a chemical reaction occurring in plants and algae that converts solar energy, atmospheric carbon dioxide gas ($CO_2$), and water ($H_2O$) into chemical energy in the form of carbohydrates or glucose ($C_6H_{12}O_6$) and oxygen gas ($O_2$). The chemical reaction is expressed as:

$\text{energy (sunlight)} + 6 CO_2 + 6 H_2O \rightarrow C_6H_{12}O_6 + 6 O_2$

Cellular respiration is the complementary process where plants and animals obtain energy by converting chemical energy ($C_6H_{12}O_6$) and oxygen gas ($O_2$) back into carbon dioxide ($CO_2$) and water ($H_2O$), releasing energy in the form of adenosine triphosphate (ATP). The chemical reaction is expressed as:

$C_6H_{12}O_6 + 6 O_2 \rightarrow 6 CO_2 + 6 H_2O + \text{energy (ATP)}$

Additional processes include decomposition, where decomposers release $CO_2$ into the atmosphere by breaking down carbon-rich organic matter in the soil. Ocean processes play a massive role as dissolved carbon dioxide is stored in the water; marine organisms use carbon-rich carbonate ($CO_3^{2-}$) to build shells, which eventually settle and form sedimentary rock. Finally, volcanic eruptions and forest fires serve as natural mechanisms that release carbon from stores back into the atmosphere.

Quantitative Analysis of Carbon Stores and Flows

Carbon stores are measured in gigatonnes ($Gt$). The distribution of carbon across the planet is as follows:

• Marine sediments and sedimentary rock: $68,000,000$ to $100,000,000\,Gt$
• Intermediate and deep water: $38,000$ to $40,000\,Gt$
• Coal deposits: $3,000\,Gt$
• Soil and organic matter: $1,600\,Gt$
• Atmosphere: $750\,Gt$
• Terrestrial vegetation: $540$ to $610\,Gt$
• Oil and gas deposits: $300\,Gt$
• Marine life: $3\,Gt$

Carbon flows, or exchanges, are measured in gigatonnes per year ($Gt/y$). Natural processes include:

• Photosynthesis: $120\,Gt/y$
• Marine gas exchange (uptake and release): $90\,Gt/y$ and $92\,Gt/y$
• Plant respiration: $60\,Gt/y$
• Microbial respiration and decomposition: $60\,Gt/y$
• Dissolved organic carbon flow: $40\,Gt/y$
• Sedimentation to deep stores: $0.5\,Gt/y$ (this process is very slow, taking millions of years)

Human activities also contribute significantly to carbon flows:

• Fossil fuel combustion: $5.5\,Gt/y$ (rapid release from deep stores taking only tens or hundreds of years)
• Change in land use (e.g., agriculture and clearing): $1.5\,Gt/y$

Human activities such as fossil fuel combustion and land clearance quickly reintroduce carbon into the atmosphere from long-term stores. This increases levels of carbon dioxide, a greenhouse gas that contributes to global climate change.

The Nitrogen Cycle: Fixation, Nitrification, and Denitrification

Nitrogen is a vital component of DNA and proteins. The largest store of nitrogen is the atmosphere, where it exists as nitrogen gas ($N_2$). Other stores include bodies of water, living organisms, and decaying organic matter. Most organisms cannot use $N_2$ in its atmospheric form. The nitrogen cycle involves four main processes, three of which make nitrogen biologically available.

Nitrogen Fixation is the process where nitrogen gas ($N_2$) is converted into ammonium ($NH_4^+$), which is usable by plants. This occurs primarily through nitrogen-fixing bacteria in soil and root nodules of legumes, and through lightning strikes in the atmosphere. Nitrification follows, where nitrifying bacteria convert ammonium ($NH_4^+$) into nitrite ($NO_2^-$) and then into nitrate ($NO_3^-$). Uptake involves plants absorbing these usable forms (primarily nitrate and nitrite) through their roots to incorporate them into plant proteins. When herbivores and omnivores consume these plants, nitrogen is transferred to their tissues.

Finally, Denitrification occurs when denitrifying bacteria convert nitrite ($NO_2^-$) and nitrate ($NO_3^-$) back into atmospheric nitrogen gas ($N_2$). Human activities significantly impact this cycle through fossil fuel combustion and burning organic matter, which releases nitrogen oxides ($NO$, $NO_2$) into the atmosphere, leading to acid precipitation ($HNO_3$ or acid rain). The use of chemical fertilizers introduces excess nitrogen that can leach into waterways, causing eutrophication. This process leads to increased algal growth that deprives aquatic organisms of sunlight and oxygen.

The Phosphorus Cycle: Weathering and Marine Sedimentation

Phosphorus is essential for carrying energy to cells. Unlike carbon and nitrogen, phosphorus is not stored in the atmosphere as a gas. Instead, it is found in phosphate ($PO_4^{3-}$) rock and ocean floor sediments. The cycle begins with weathering, where chemical or physical processes break down rocks, releasing phosphate into the soil from long-term stores. Organisms take up this phosphorus, and when they die, decomposers return it to the soil.

Excess phosphorus often washes into lakes and oceans, settling on the floor to form sedimentary rock. It remains trapped in these deep stores for millions of years until it is eventually exposed through geologic uplift or mountain building. Quantitative data for phosphorus (measured in megatonnes, $Mt$) includes:

• Earth's crust: $20,000,000,000\,Mt$
• Ocean sediments: $100,000,000\,Mt$
• Mineable phosphate: $10,000$ to $60,000\,Mt$
• Terrestrial organisms: $3,000\,Mt$

Annual flows include:

• Weathering: $10\,Mt/y$
• Mining: $12\,Mt/y$
• Fertilizer application: $50\,Mt/y$
• Leaching and run-off: $7\,Mt/y$
• Fish harvest: $0.01\,Mt/y$

Human impacts on the phosphorus cycle include the use of commercial fertilizers and phosphate-containing detergents, which contribute additional phosphate to waterways. Furthermore, slash-and-burn forest clearance reduces phosphate levels in ecosystems; the phosphate from trees enters the soil as ash, where it rapidly leaches away and settles on lake and ocean bottoms, becoming unavailable to living organisms.