ch 19: Nutrient Cycling and Retention: Carbon, Nitrogen, and Phosphorus Cycles

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Biogeochemical Cycles

Overview of Nutrient Cycling

Biogeochemical cycles describe the movement of nutrients between living organisms and the physical environment. These cycles are essential for maintaining ecosystem function and include the cycling of carbon (C), nitrogen (N), and phosphorus (P) through organisms, rocks and soils, oceans and lakes, and the atmosphere.

Organisms: Plants, animals, and microbes participate in nutrient uptake and release.
Abiotic reservoirs: Rocks, soils, water bodies, and the atmosphere act as storage and transfer points.

Carbon Cycle

Pathways and Reservoirs

The carbon cycle involves the exchange of carbon between the atmosphere, organisms, and the earth. Carbon is present as CO2 in the atmosphere and is cycled through photosynthesis and respiration.

Photosynthesis: Plants and some microbes convert atmospheric CO2 into organic matter.
Respiration: Organisms release CO2 back into the atmosphere.
Fossil fuels: Coal, oil, and natural gas store carbon underground.
Carbonate rocks: Large reservoirs of carbon in the earth's crust.

Key processes:

From atmosphere to earth: Photosynthesis, Dissolution into water
From earth to atmosphere: Respiration, Fuel burning, Deforestation

Human effects: Burning fossil fuels and deforestation increase atmospheric CO2, contributing to global warming.

Global Carbon Reservoirs and Fluxes

Carbon is stored in various reservoirs and moves between them via annual fluxes. Reservoirs include the atmosphere, terrestrial biosphere, oceans, and fossil fuels.

Reservoir	Carbon Content (GtC)	Annual Flux (GtC/year)
Atmosphere	730	+3.2
Terrestrial Biosphere	2,000	Variable
Oceans	39,000	Variable
Fossil Fuels	5,000	5.4

Additional info: 1 GtC = 1012 kg.

Nitrogen Cycle

Importance and Pathways

The nitrogen cycle is crucial for the synthesis of nucleic acids, amino acids, proteins, ATP, and other biomolecules. The major atmospheric pool is N2 gas, which is largely inaccessible to most organisms.

Nitrogen fixation: Conversion of atmospheric N2 to ammonia (NH3) by nitrogen-fixing bacteria (e.g., Rhizobium in legumes) or by industrial processes and lightning.
Ammonia fate: Absorbed directly by organisms or converted to nitrite (NO2-) and then nitrate (NO3-) via nitrification.
Nitrate fate: Absorbed as nutrients, leached into ecosystems, or converted back to N2 gas via denitrification under anaerobic conditions.

Key equations:

Human influences: Burning fossil fuels releases nitrous oxides (N2O), a greenhouse gas and acid rain source; deforestation causes nitrogen loss from soils; fertilizer use leads to nutrient pollution and eutrophication.

Nitrogen Cycle Diagram

Process	Direction	Key Agents
Fixation	Atmosphere → Earth	Bacteria, lightning, industry
Precipitation	Atmosphere → Earth	Rainfall
Denitrification	Earth → Atmosphere	Bacteria (anaerobic)
Fuel Emission	Earth → Atmosphere	Combustion

Phosphorus Cycle

Pathways and Limitation

The phosphorus cycle is essential for nucleic acids, ATP, and other biomolecules. Unlike carbon and nitrogen, phosphorus does not have a significant atmospheric pool and is mainly found in mineral deposits and marine sediments.

Release: Phosphorus is slowly released into ecosystems via weathering of rocks.
Limiting nutrient: Phosphorus often limits productivity, especially in aquatic ecosystems.

Human influences: Mining phosphorus-bearing rocks for fertilizer can harm local environments; excessive fertilizer use leads to nutrient pollution and eutrophication.

Phosphorus Cycle Diagram

Reservoir	Form	Availability
Mineral Deposits	Insoluble phosphate	Low
Soil Biomass	Soluble phosphate (HPO42-, H2PO4-)	High
Marine Sediments	Insoluble phosphate	Low
Plants	Absorbed phosphate	High

Rates of Decomposition

Factors Affecting Decomposition

The rate at which nutrients become available to primary producers is determined by mineralization during decomposition. In terrestrial systems, decomposition rate is influenced by temperature, moisture, and chemical composition of the material.

Temperature: Higher temperatures increase decomposition rate.
Moisture: Higher moisture increases decomposition rate.
Material composition:
- Higher nitrogen and phosphorus content → faster decomposition
- Lower carbon:nitrogen ratio → faster decomposition
- Lower lignin content → faster decomposition

Example: Annual leaf mass loss in tropical forests is about three times higher than in temperate forests due to higher temperature and moisture.

Decomposition in Aquatic Ecosystems

Leaves with higher lignin content decompose at a slower rate because lignin inhibits fungal colonization.
Nitrate content in streams accelerates decomposition rate.

Organisms and Nutrient Cycling

Role of Plants and Animals

Plants and animals can modify the distribution and cycling of nutrients within ecosystems.

Pocket gophers: By burrowing and bringing N-poor subsoil to the surface, they increase heterogeneity in soil nitrogen and light penetration.
Grazing intensity: Higher grazing intensity by large herbivores (e.g., Serengeti Plain) increases nutrient turnover in plant biomass.
Plant litter: Acacia species (legumes) contribute more nitrogen to litter than non-legumes, affecting nutrient dynamics.
Introduced species: Myrica faya, an N-fixing tree, alters nitrogen dynamics in Hawaiian ecosystems.

Disturbance and Nutrient Loss

Effects of Disturbance

Disturbances such as clear-cutting, flooding, and other ecosystem disruptions increase nutrient loss from ecosystems.

Clear-cutting: Increases nitrate loss from soil, as shown in the Hubbard Brook Forest experiment.
Flooding: High streamflow years result in greater export of phosphorus than input, leading to nutrient loss from aquatic systems.

Example: Bear Brook exports more phosphorus during high flow years than it receives, indicating the impact of flooding on nutrient export.