Biogeochemical Cycles in Ecosystems (APES Unit 1)

Carbon Cycle

What carbon is (and why ecosystems care)

Carbon is an element that forms the backbone of organic molecules—carbohydrates, lipids, proteins, and nucleic acids. When AP Environmental Science talks about the carbon cycle, it means the movement of carbon through biotic components (living organisms) and abiotic components (atmosphere, oceans, rocks, and soils).

Carbon matters because it is simultaneously:

• A building block of life (stored in biomass)
• A driver of energy flow through food webs (because organic carbon contains chemical energy)
• A major climate regulator (because carbon dioxide and methane are greenhouse gases)

A useful way to think about biogeochemical cycles is: reservoirs store a chemical, and fluxes move it between reservoirs. In the carbon cycle, some reservoirs exchange carbon quickly (like the atmosphere and living biomass), while others store it for a long time (like sediments and fossil fuels).

Main reservoirs and fluxes (how carbon moves)

The biggest carbon reservoirs you’ll encounter in APES include:

• Atmosphere: mostly as carbon dioxide (CO2)
• Biosphere: carbon in living organisms and recently dead organic matter
• Oceans: dissolved inorganic carbon (including dissolved CO2, carbonic acid, bicarbonate, and carbonate)
• Lithosphere: carbonate rocks (like limestone) and fossil fuels

Key processes (fluxes) that move carbon:

1. Photosynthesis: producers take CO2 from air (or water) and convert it into organic molecules. This is the main biological “entry point” of carbon into food webs.
2. Cellular respiration: organisms break down organic molecules for energy and release CO2 back to the atmosphere or water.
3. Decomposition: decomposers break down dead matter; carbon returns as CO2 (and sometimes methane in low-oxygen conditions).
4. Combustion: burning biomass or fossil fuels rapidly converts stored carbon into CO2.
5. Ocean-atmosphere exchange: CO2 dissolves into the ocean and can also be released back to the air depending on temperature and concentration gradients.
6. Sedimentation and burial: over long timescales, carbon becomes stored in sediments, carbonate rocks, and fossil fuels.

A common misconception is that “carbon cycle = only CO2.” Carbon exists in many forms. In ecosystems, the key distinction is often organic carbon (in living or once-living matter) vs. inorganic carbon (CO2 and carbonate-related forms).

The fast (biological) carbon cycle vs. the slow (geologic) carbon cycle

It helps to separate carbon movement by timescale:

• Fast cycle (days to centuries): photosynthesis, respiration, decomposition, and exchange with surface oceans. This is the cycle most directly tied to ecosystems and food webs.
• Slow cycle (thousands to millions of years): burial, formation of fossil fuels, formation of carbonate rocks, and geologic processes that eventually return carbon to the atmosphere.

In APES, you often explain carbon changes by identifying which reservoir is being tapped and how quickly carbon is moved.

Human impacts: why the carbon cycle is central to environmental science

Humans change the carbon cycle mainly by moving carbon from long-term storage into the atmosphere faster than natural processes would.

1. Burning fossil fuels transfers carbon from geologic reservoirs to the atmosphere as CO2. This strengthens the greenhouse effect and contributes to global climate change.
2. Deforestation and land-use change reduce photosynthesis (fewer trees to pull CO2 from the air) and can release stored carbon from biomass and soils through burning and decomposition.
3. Agriculture and soil disturbance can reduce soil carbon storage by increasing decomposition and erosion.

A carbon-related issue that often shows up alongside climate is ocean acidification. When the ocean absorbs more CO2, seawater chemistry shifts in ways that can make it harder for organisms like corals and some shell-formers to build calcium carbonate structures. On APES-style questions, you’re typically not asked to do complex chemistry—just to connect “more atmospheric CO2” to “more dissolved CO2” to “changes in ocean chemistry” and biological impacts.

Carbon cycle in action: two concrete examples

Example 1: Forest clearing for agriculture

• When a forest is cut and burned, carbon stored in wood (biomass) is rapidly converted to CO2.
• After conversion to cropland, the new vegetation usually stores less carbon than a mature forest.
• Tilled soils often lose carbon faster because oxygen exposure and disturbance speed up decomposition.

Example 2: Warming and permafrost

• In cold regions, frozen soils store large amounts of organic carbon.
• If warming thaws permafrost, decomposition increases.
• In waterlogged, low-oxygen thaw conditions, decomposition can produce methane (CH4), a potent greenhouse gas.

Memory aid

A simple way to remember the “big four” biological carbon fluxes:

• Photo takes CO2 in
• Resp puts CO2 out
• Decomp puts CO2 out (and sometimes CH4)
• Combust puts CO2 out fast

Exam Focus

• Typical question patterns:
  • Trace carbon through a scenario (e.g., “Describe what happens to carbon when forests are converted to farmland”).
  • Explain how a human activity alters a reservoir/flux (e.g., fossil fuel combustion moving carbon from lithosphere to atmosphere).
  • Connect increased atmospheric CO2 to downstream effects (climate change; ocean chemistry impacts).
• Common mistakes:
  • Saying plants “get carbon from soil.” Plants get most of their carbon from atmospheric (or dissolved) CO2; soil mainly supplies water and nutrients.
  • Confusing energy flow with matter cycling. Energy flows one-way through ecosystems; carbon (matter) cycles.
  • Treating the ocean as only a carbon “sink.” It can absorb CO2, but it can also release it depending on conditions.

Nitrogen Cycle

What nitrogen is (and why life needs it)

Nitrogen is essential for life because it is a key component of amino acids (proteins) and nucleic acids (DNA and RNA). The atmosphere is about 78% nitrogen gas, but most organisms cannot use nitrogen in that form.

That apparent contradiction is the core idea of the nitrogen cycle: nitrogen is abundant, but often biologically unavailable.

The main atmospheric form, nitrogen gas (N2), is very stable because of the strong bond between the two nitrogen atoms. To become useful to plants, nitrogen must be converted into more reactive forms like ammonium (NH4+) or nitrate (NO3-).

Key transformations (how nitrogen changes form)

The nitrogen cycle is especially important in APES because it’s not just movement—it’s movement plus chemical transformations, largely driven by bacteria.

1) Nitrogen fixation

Nitrogen fixation converts N2 into ammonia-related forms that can be used by organisms.

• Biological fixation: certain bacteria (often associated with roots of legumes) convert N2 into ammonia, which in soil commonly becomes NH4+.
• Lightning can also fix nitrogen by providing enough energy to form nitrogen oxides that can become nitrates in rain.
• Industrial fixation: humans create ammonia for fertilizers (a major modern input of reactive nitrogen to ecosystems).

Why it matters: fixation is often the “gateway” step that brings atmospheric nitrogen into food webs.

2) Nitrification

Nitrification is a bacterial process that converts NH4+ into NO2- (nitrite) and then into NO3- (nitrate).

• Plants can take up both NH4+ and NO3-, but nitrate is especially important because it is common in soils.
• A big tradeoff: nitrate is highly water-soluble, so it’s easily leached (washed down through soil into groundwater) or carried into waterways.

3) Assimilation

Assimilation is when plants take up NH4+ or NO3- and incorporate the nitrogen into organic molecules (proteins, DNA). Animals then obtain nitrogen by eating plants or other animals.

A common misconception is that animals “absorb nitrate” directly from soil or water as their main nitrogen source. In most ecosystems, animals get nitrogen primarily through dietary organic nitrogen (protein).

4) Ammonification (mineralization)

Ammonification is when decomposers convert organic nitrogen (from dead organisms and wastes) back into NH4+.

This step connects the nitrogen cycle directly to decomposition and nutrient recycling—if decomposition is slowed (cold, dry, or oxygen-poor conditions), nitrogen recycling slows too.

5) Denitrification

Denitrification converts NO3- back into N2 gas (and sometimes nitrous oxide) via bacteria, typically in anaerobic (low-oxygen) conditions such as wetlands, waterlogged soils, or sediments.

Denitrification matters because it returns nitrogen to the atmosphere, reducing nitrate in soils and waters. Wetlands can therefore act like nutrient “filters” by promoting denitrification.

Human impacts: fertilizer, pollution, and eutrophication

Humans dramatically accelerate nitrogen inputs, especially through synthetic fertilizers and some combustion processes.

1. Fertilizer runoff

   • Excess NO3- and NH4+ from farms and lawns can wash into rivers, lakes, and coastal waters.
   • Nutrient enrichment can cause eutrophication: rapid algae growth followed by decomposition that consumes dissolved oxygen.
   • Low oxygen conditions can create hypoxic “dead zones,” where many aquatic organisms cannot survive.

2. Atmospheric nitrogen deposition

   • Burning fossil fuels at high temperatures can form nitrogen oxides (NOx).
   • These compounds contribute to air pollution and can deposit back to ecosystems, adding reactive nitrogen.

3. Groundwater contamination

   • Because nitrate leaches easily, areas with heavy fertilizer use can see increased nitrate in groundwater.

Nitrogen cycle in action: two concrete examples

Example 1: Why wetlands can reduce water pollution

• Water carrying nitrate enters a wetland.
• Saturated soils have low oxygen.
• Denitrifying bacteria convert nitrate to N2 gas.
• Result: less nitrate leaves the wetland downstream.

Example 2: Fertilizer and a lake algae bloom

• Spring rains wash nitrate/phosphate into a lake.
• Algae bloom increases.
• When algae die, decomposers use oxygen to break them down.
• Dissolved oxygen drops; fish kills may occur.

Memory aid

A classic sequence to remember the “microbial middle” of the nitrogen cycle:

• Fix (N2 to NH4+)
• Nitrify (NH4+ to NO3-)
• Denitrify (NO3- to N2)

Associate the “-fy” endings with bacteria doing chemical conversions.

Exam Focus

• Typical question patterns:
  • Identify which nitrogen process is occurring in a described environment (e.g., waterlogged soils pointing to denitrification).
  • Explain how fertilizer leads to eutrophication and hypoxia in aquatic systems.
  • Compare nitrogen forms and their movement (e.g., why nitrate leaches easily).
• Common mistakes:
  • Mixing up nitrogen fixation and nitrification. Fixation brings N2 into usable forms; nitrification converts ammonium to nitrate.
  • Ignoring oxygen conditions. Denitrification is associated with low-oxygen environments.
  • Claiming “plants use N2 directly.” Most plants cannot use atmospheric N2 without nitrogen-fixing microbes.

Phosphorus Cycle

What phosphorus is (and why it’s different)

Phosphorus is essential for life because it’s part of ATP (cellular energy transfer), DNA/RNA, and cell membranes (phospholipids). In ecosystems, phosphorus often limits plant growth, meaning if phosphorus is scarce, primary productivity can be constrained even if other resources are available.

The phosphorus cycle is distinct from carbon and nitrogen because it typically does not have a major atmospheric (gaseous) phase. Instead, it is mostly a sedimentary cycle, moving through rocks, soil, water, and organisms.

That absence of a big atmospheric reservoir matters: phosphorus tends to move more slowly across landscapes and is strongly influenced by geology and erosion.

Main reservoirs and fluxes (how phosphorus moves)

Key reservoirs:

• Phosphate in rocks and minerals (largest long-term reservoir)
• Soils (phosphate ions available for plant uptake, often bound to particles)
• Aquatic sediments
• Biomass (organic phosphorus in living tissue)

Key processes:

1. Weathering and erosion

   • Over time, rocks release phosphate (often present as PO4^3- in simplified form) into soils and waters.
   • This is typically a slow input but can be accelerated by disturbance and erosion.

2. Assimilation

   • Plants absorb phosphate from soil.
   • Animals obtain phosphorus by eating plants/other animals.

3. Decomposition and mineralization

   • When organisms excrete waste or die, decomposers return phosphorus to soil or water.

4. Sedimentation and burial

   • In water bodies, phosphorus can settle into sediments and become unavailable for long periods.

A common misconception is to treat phosphorus like nitrogen—something that cycles quickly through the atmosphere. Instead, think “rocks and sediments” first.

Why phosphorus is often a limiting nutrient

A limiting nutrient is the nutrient in shortest supply relative to demand, which constrains growth. Phosphorus is frequently limiting in freshwater ecosystems and many soils because:

• It binds to soil particles and is not very mobile compared with nitrate.
• It has no large atmospheric input comparable to nitrogen fixation.

This connects directly to eutrophication: adding even a modest amount of phosphate to a phosphorus-limited system can trigger strong algae growth.

Human impacts: mining, fertilizers, and eutrophication

Humans impact the phosphorus cycle mainly by moving phosphorus from geologic storage to ecosystems.

1. Phosphate mining and fertilizer production

   • Phosphate rock is mined and used in fertilizers.
   • This increases phosphorus availability beyond natural weathering rates.

2. Agricultural runoff and soil erosion

   • Phosphorus often travels attached to soil particles.
   • When soil erodes from fields or construction sites, phosphorus can be carried into waterways.

3. Wastewater and detergents (where applicable)

   • Human waste contains phosphorus; wastewater effluent can add phosphorus to rivers and lakes if not effectively treated.

Phosphorus cycle in action: two concrete examples

Example 1: Why controlling erosion helps water quality

• A farm field lacks vegetation cover.
• Rain causes soil erosion.
• Phosphorus-rich soil particles enter a stream.
• Downstream lake receives phosphorus and experiences algae blooms.

Example 2: Lake sediment as a long-term phosphorus sink

• Phosphorus enters a lake.
• Some is used by algae and moves through the food web.
• Some settles and becomes buried in sediments.
• Over time, burial can reduce available phosphorus—unless conditions disturb sediments and release it back into the water.

Memory aid

To remember the “no big atmosphere” feature:

• Phosphorus = ‘P’ for ‘Parent rock’

It’s not perfect, but it nudges you to start with rocks rather than air.

Exam Focus

• Typical question patterns:
  • Explain why phosphorus is often a limiting nutrient and how adding it affects primary productivity.
  • Trace a pathway from land disturbance (erosion) to aquatic eutrophication.
  • Compare phosphorus cycling to nitrogen cycling (especially the lack of an atmospheric phase).
• Common mistakes:
  • Saying phosphorus cycles mainly through the atmosphere. In APES, phosphorus is primarily rock/soil/water-based.
  • Assuming phosphorus behaves like nitrate in water. Phosphorus often binds to particles and is linked to erosion.
  • Describing eutrophication without mentioning decomposition-driven oxygen depletion (the “dead zone” mechanism).

Hydrologic (Water) Cycle

What the hydrologic cycle is (and why it links all the others)

The hydrologic (water) cycle is the movement of water through Earth’s systems: atmosphere, land, oceans, surface water, and groundwater. Water is not just “background”—it is the main transport medium for many nutrients and pollutants.

Water matters in Unit 1 ecosystems because:

• It controls habitat conditions (soil moisture, stream flow, wetland extent)
• It influences primary productivity and decomposition rates
• It transports nutrients like nitrate and phosphate, shaping eutrophication risk
• It connects local ecosystems to watershed-scale processes

Core processes (how water moves step by step)

You can understand the water cycle as a set of phase changes and movements.

1. Evaporation

   • Liquid water becomes water vapor from oceans, lakes, and soils.

2. Transpiration

   • Transpiration is water vapor released by plants through stomata.
   • Evaporation + transpiration is often discussed together as evapotranspiration.

3. Condensation and cloud formation

   • Water vapor cools, condenses onto particles, and forms clouds.

4. Precipitation

   • Water returns to Earth as rain, snow, sleet, or hail.

5. Infiltration and percolation

   • Infiltration is water soaking into the soil surface.
   • Percolation is deeper movement through soil and rock layers.

6. Runoff

   • When precipitation exceeds infiltration capacity, water flows over land into streams and rivers.

7. Groundwater flow and aquifers

   • Groundwater is water stored in pores and fractures underground.
   • An aquifer is a permeable rock or sediment layer that stores and transmits groundwater.

A common misconception is that groundwater is mainly underground “rivers.” In reality, groundwater is usually water moving slowly through pore spaces in sediment or fractures in rock.

Watersheds and why location matters

A watershed (drainage basin) is the land area where all water drains to a common outlet (a stream, river, lake, or ocean). Watersheds matter because what happens on land—farming, deforestation, urbanization—affects water quality and ecosystem health downstream.

If you’re asked to predict impacts, think in watershed logic: “Where will water (and dissolved nutrients) go next?”

Human impacts: changing flow, storage, and water quality

Humans alter the hydrologic cycle by changing land cover and by directly moving water.

1. Urbanization

   • Paved surfaces reduce infiltration and increase runoff.
   • Higher runoff can increase flooding, stream erosion, and pollutant transport.

2. Deforestation

   • Reduced transpiration and canopy interception can change local water balance.
   • Loss of roots and ground cover increases erosion, which can carry phosphorus into waterways.

3. Agriculture and irrigation

   • Irrigation increases evaporation and can reduce stream flow if water is diverted.
   • Over-irrigation can increase runoff and leaching (especially nitrate).

4. Groundwater pumping

   • If pumping exceeds recharge, the water table can drop.
   • This can reduce baseflow to streams (the groundwater contribution that helps streams flow during dry times).

Water cycle in action: two concrete examples

Example 1: Why cities often have flashier streams

• Rain falls on pavement.
• Infiltration is low.
• Runoff enters storm drains quickly.
• Streams experience rapid high flows that erode banks and degrade habitat.

Example 2: Nitrate leaching after heavy rain

• Fertilizer adds nitrate to soil.
• A storm brings heavy rainfall.
• Water infiltrates and carries nitrate downward.
• Nitrate enters groundwater and may later feed into streams or wells.

Connecting water to the other cycles (high-yield connections)

The hydrologic cycle acts like the “delivery system” for nutrients:

• Nitrogen: nitrate is highly soluble, so water movement strongly controls nitrate leaching and runoff.
• Phosphorus: often moves with eroded soil particles; water-driven erosion is a major transport pathway.
• Carbon: water affects carbon storage through plant growth (water availability controls photosynthesis) and through decomposition (waterlogged soils can slow oxygen supply and change carbon release forms).

Exam Focus

• Typical question patterns:
  • Predict how land-use change (urbanization, deforestation, agriculture) alters runoff vs. infiltration and affects water quality.
  • Use watershed reasoning to identify where pollutants travel after a storm.
  • Connect water movement to nutrient pollution and eutrophication risk.
• Common mistakes:
  • Treating infiltration and runoff as independent. In many scenarios, increased impervious surface decreases infiltration and increases runoff.
  • Forgetting groundwater. Many questions expect you to include leaching to groundwater and delayed impacts on streams.
  • Assuming phosphorus leaches like nitrate. Phosphorus is often particle-associated, so erosion control is especially important.