Nutrient Cycles and Decomposition in Ecosystems: Nitrogen Fixation, Litter Decomposition, and Soil Fertility

Non-mineral nutrients

Carbon, hydrogen, and oxygen (unusable forms).

Mineral nutrients

Nutrients absorbed from the soil in usable forms.

Macronutrients

Nitrogen, phosphorus, potassium, and magnesium.

Micronutrients

Boron, copper, iron, chlorine, manganese, and zinc.

Energy flow vs nutrient cycle

Energy flows in one direction (sun → producers → consumers → heat), while nutrients cycle between organisms and the environment.

Percentage of atmospheric nitrogen

~78% of the atmosphere is nitrogen gas (N₂), what most N starts as

Why is atmospheric N₂ unusable?

It has a strong triple bond that most organisms cannot break.

Nitrogen fixation types

Biological fixation, lightning fixation, and industrial fixation.

Biological Fixation

- By bacteria (free-living and symbiotic) in plants like legumes

- Uses nitrogenase enzyme to convert N2 → NH3

- BNF is vital component of cycle (90% of fixation)

Lightning Fixation

- Lightning splits nitrogen molecules and they bind with oxygen in atmosphere to form nitrogen oxides

- Oxides dissolve in rainwater to form nitric acid, which forms NO3- and is carried to soil by rain and absorbed as fertilizer

Industrial Processes

- Human-made process that converts N2 → NH3

- Haber-Bosch process combines N2 and H2 under heat and pressure with iron-based catalyst

- Creates NH3, which is used in synthetic fertilizers

Nitrogenase

Enzyme that converts N₂ → NH₃ in biological fixation.

Nitrogen-fixing plants

Legumes (contain symbiotic bacteria in root nodules).

What happens to nutrients before leaves or other parts of a plant fall?

Nutrients are reabsorbed by the plant.

What is one benefit of plant nutrient uptake and incorporation?

Increases nutrient use efficiency and reduces nutrient loss. (mineral N -> organic N)

How do plants benefit from storing nutrients?

They can use them in environments with low nutrients.

What does maintaining nutrient uptake do for plants?

It maintains plant fitness and survival.

What is a benefit of reducing dependence on external nutrients?

It enhances the plant's self-sufficiency.

What is the relationship between retranslocation percentage and nutrient resorption efficiency?

Higher retranslocation percentage leads to higher nutrient resorption efficiency.

Retranslocation (resorption)

Reabsorption of nutrients by plants before parts like leaves fall off.

Benefits of retranslocation

Increases nutrient efficiency, reduces loss, helps survival in nutrient-poor soils.

Litterfall

Dead organic matter (leaves, branches) falling to the ground.

Decomposition/mineralization

Decomposers break down organic matter and transfer organic nitrogen to inorganic mineral form usable by plants. (organic N -> mineral N)

decomposition/mineralization

reversal of photosynthesis because it releases energy h2o and co2

Litterbag experiment purpose

Measures litter decomposition rate based on mass loss over time.

Litterbag Experiments

1. place known mass of dried leaf litter into mesh bags (litterbags) with known weight

2. place bags on forest floor where they are exposed to natural decomposition

3. retrieve bags over set period of time periodically (every few months)

4. dry and weigh remaining contents of bags to determine mass loss

5. calculate decomposition rate using mass loss data over time and express as percentage of mass loss per unit of time

Factors affecting decomposition rate

Leaf composition, temperature, precipitation.

Fastest-decomposing carbon compounds

Soluble sugars and proteins > (hemi)cellulose > lignin.

climatic factors affecting rate of decomposition

virginia > west virginia > new hampshire because there is less moisture for decomposers

relationships between temperature and decomposition rate

higher temp/moisture -> higher decomp rate

higher temp -> higher co2 rate

more prep -> less nitrogen remaining

Steps of litter decomposition

Leaching → Microbial colonization → Physical fragmentation → Humification → Mineralization.

leaching

rain washes out water-soluble nutrients in dead organic material

microbial colonization

bacteria/fungi colonize material and prepare for catabolism

physical fragmentation

detritivores (earthworms, mites, millipedes) break down larger organic material into smaller organic fragments; Increases surface area for microbial action

humification

forms humus after mixed with soil

Humus

Stable, nutrient-rich organic matter that improves soil structure and water retention.

mineralization

inorganic nutrients locked into organic matter are released into soil and are absorbed by plant

litter decomposition

begins with plant litter with lower C:N ratio

What happens to the C:N ratio during consumption?

C:N ratio increases

What do bacteria and fungi consume as a source of energy and nutrients?

Litter

What gas is released through microbial respiration?

CO2

What is the C:N ratio range for decomposers?

10:1-15:1

5 year experiment following decomposition of Scots pine leaf litter in sweden -> Time (days) vs % original remaining with nitrogen and mass

As litter is consumed, there is ↓ leaf biomass

Due to high 134:1 C:N ratio of pine needles, nitrogen is drawn from soil to support microbial growth

This leads to ↑ % nitrogen in litterbag (immobilization)

5 year experiment following decomposition of Scots pine leaf litter in Sweden -> Mass loss (%) vs % nitrogen in residual organic matter

As litter is consumed, significant ↓ C from microbial respiration

Rest of C is converted into microbial biomass

Net effect: [N] of residual organic matter ↑ with decomposition

Primary plant materials + secondary microbial tissues

↓ mass = ↑ % nitrogen (nitrogen retained in litter)

5 year experiment following decomposition of Scots pine leaf litter in Sweden -> Time (days) vs C:N ratio

Decomposition proceeds towards humus development

↓ C and ↑[N] = ↓ C:N ratio

5 year experiment following decomposition of Scots pine leaf litter in Sweden -> Time (days) vs % lignin in residual organic matter

Residual matter portion made of complex lignin-based compounds

N is bound in these resistant lignin compounds

↑ decomposition = ↑ % lignin in residual organic matter

Processes in litter decomposition

Nutrient mineralization and immobilization.

Mineralization

Conversion of organic compounds into mineral nutrients.

Immobilization

Uptake of mineral nutrients by decomposers.

Net mineralization rate

Mineralization rate - immobilization rate.

C:N ratio during decomposition

Decreases as carbon is lost and nitrogen concentration increases.

Lignin during decomposition

Increases in proportion because it resists breakdown.

Terrestrial vs aquatic nutrient cycling

Terrestrial: decomposition/recycling in soil; Aquatic: vertical water turnover.

aquatic ecosystems

vertical water turnover

summer: densest water at bottom of lake (hypolimnion)

Thermal stratification

Separation of lake layers by temperature and density.

epilimnion

warm, less dense, less nutrients

thermocline

middle, temp changes rapidly

hypolimnion

cold, more dense, more nutrients

winter

densest water is at bottom of lake (hypolimnion)

Ice forms on top (0°C)

Densest water (4°C) stays at bottom (hypolimnion)

Water is stratified again, but reversed (colder at top, slightly warmer at bottom)

fall/spring

densest water is same density throughout lake

Vertical turnover

Surface water cools (in fall) or warms (in spring) to match bottom water density

When densities equalize, mixing occurs in lake

This vertical turnover distributes oxygen downward and nutrients upward, replenishing lake ecosystem

Vertical turnover

Mixing of lake layers during fall/spring that redistributes nutrients and oxygen.

Forest vs farmland nutrient cycles

Forests are closed (recycle nutrients); farmlands are open (lose nutrients via harvest).

forests

NATURAL closed balanced system

Nutrients are recycled and dead leaves decompose and return nutrients to soil, which are taken up by plants

farmland

AGRICULTURAL open system with external input

Crops/nutrients are harvested and removed from field, which breaks natural cycle, so fertilizers are added for replenishment

Continuous harvesting = ↓ crop yield due to ↓ in soil quality

Strategies to maintain soil quality from harvest loss

Soil quality maintenance strategies

Natural recovery, manure application, crop rotation, agroforestry.

abandon and recovery

natural recovery

manure application

nitrogen is introduced

crop rotation

planting nitrogen-fixating crops

agroforestry

conserving trees

synthetic nitrogen fertilizers

- lead to ↑ in corn yield

- Synthetic fertilizers have nutrients in mineral form

- Mineral nutrient form leads to high mobility in soil

stats of synthetic fertilizer

⅔ nitrogen and ½ of phosphorus in fertilizers is immediately lost from runoff and leaching

Synthetic fertilizer consequences

Runoff and leaching cause ecosystem imbalances.

leaching

nutrients move downward through soil with water, contaminating groundwater

runoff

nutrients washed off land by rain/irrigation

This creates ecological imbalances

big picture of biogeochemical cycles

- Matter recycling: the big picture

- Earth's resources are finite: atoms are being recycling in time and space

- Essential chemicals: carbon, nitrogen, sulfur, phosphorus

- Essential molecules (DNA, RNA, proteins)/all living things require nitrogen

- 78% of atmosphere is nitrogen gas (N2) that is unusable

- N2 forms triple bond which is hard to break

Main recycled elements

Carbon, nitrogen, sulfur, phosphorus.

FANAD (nitrogen cycle)

Fixation, Assimilation, Nitrification, Ammonification, Denitrification.

What is nitrogen fixation?

The process of converting N₂ gas into NH₃/NH₄⁺ by bacteria or industry.

Where does nitrogen fixation primarily occur?

In the soil and root nodules of legumes.

What type of bacteria are primarily responsible for nitrogen fixation?

Nitrogen-fixing bacteria, specifically rhizobium.

- Rhizobia bacteria are symbiotically associated with some plants like clovers and soybeans

- They infect root systems and cause nodules to form and contain the bacteria

- Rhizobia provide up to 40-70% of soybean nitrogen requirements and can increase yield up to 25%

- Now, humans add synthetic nitrogen sources (fertilizer)

Which plants are symbiotically associated with rhizobia bacteria?

Plants like clovers and soybeans.

What do rhizobia bacteria do to plant root systems?

They infect the root systems and cause nodules to form that contain the bacteria.

How much nitrogen do rhizobia provide for soybean plants?

Up to 40-70% of soybean nitrogen requirements.

What is the potential yield increase for soybeans due to rhizobia?

Up to 25%.

How do humans contribute to nitrogen fixation?

By adding synthetic nitrogen sources, such as fertilizers.

Why is nitrogen fixation significant for life on Earth?

It makes atmospheric nitrogen available to life.

Nitrogen Fixation Experiment: design to test if adding synthetic fertilizer alters plant-bacteria interactions

- Control group: plants with no fertilizer

- Experimental group: plants with fertilizer

- Independent variable: presence/absence of fertilizer

- Dependent variable: number/size of nodules, growth of plant, nitrogen content of plant tissues, crop yield

- Control variable: plant type, soil type, water/light/temp

What is assimilation in the context of nitrogen compounds?

Uptake of nitrogen compounds by plants to build proteins/nucleic acids.

What is the two-step process of nitrogen assimilation?

NH3 (ammonia) → NO2- (nitrite) → NO3- (nitrate)

What type of bacteria are involved in the nitrification process?

Nitrifying bacteria such as nitrosomonas and nitrobacter.

What is the requirement for the nitrification process?

Oxygen (aerobic process).

Why is nitrogen assimilation significant for plants?

It creates the preferred nitrogen form for many plants.

Nitrification

NH₃ → NO₂⁻ → NO₃⁻ by nitrifying bacteria (aerobic process).

significance of assimilation

how nitrogen enters food web

What is ammonification?

The process of decomposition where nitrogen returns to the soil as NH4+.

When does ammonification occur?

At the death of organisms and through waste products.

What organisms are involved in ammonification?

Decomposer bacteria and fungi.

What does ammonification produce?

NH3/NH4+ that returns to the soil.

What is the significance of ammonification?

It recycles organic nitrogen.

Denitrification

NO₃⁻ → N₂ gas in anaerobic conditions.