Chapter 19 - Lecture

Chapter 19: Nutrient Cycling and Retention

Concept 19.1: Nutrient Cycles

• Nutrient Pools: Involve storage of chemical elements in various compartments within ecosystems.

• Nutrient Flux: Refers to the transfer of nutrients between these pools.

Concept 19.2: Factors Influencing Decomposition Rate

• Temperature, Moisture, and Chemical Composition: These environmental factors significantly affect the rate at which decomposition occurs.

Concept 19.3: Modification by Organisms

• Role of Plants and Animals: Both are capable of altering the distribution and cycling of nutrients within ecosystems.

Concept 19.4: Impact of Disturbance

• Nutrient Loss: Disturbances generally lead to increased nutrient loss from ecosystems.

Introduction to Nutrient Cycling

• Exchange of nutrients between organisms and the environment is crucial for ecosystem function.

• Energy Flow: Energy travels in a one-way path through ecosystems, while elements are recycled.

• Essential elements for development, maintenance, and reproduction are termed as nutrients.

• Nutrient Cycling: Involves the use, transformation, movement, and reuse of these nutrients.

19.1 Nutrient Cycles

• Storage and Movement: Nutrient cycles encompass the storage of nutrients in compartments and their movement within an ecosystem.

• Nutrient Sink vs. Nutrient Source:

  • A nutrient sink absorbs a nutrient faster than it releases it.

  • A nutrient source releases nutrients faster than they are absorbed.

19.2 Phosphorus Cycle

• Limited Availability: Phosphorus is scarce in the biosphere, lacking a significant atmospheric pool.

• Sources: Primarily found in mineral deposits and marine sediments. Its release into ecosystems occurs slowly through rock weathering.

• Mycorrhizae: Fungi that assist in phosphorus uptake by plants.

19.3 Nitrogen Cycle

• Major Atmospheric Pool: Nitrogen exists mainly in the form of N2 in the atmosphere.

• Nitrogen Fixation: This energy-intensive process is critical as only nitrogen fixers can utilize atmospheric nitrogen directly.

  • N2 is converted to ammonia (NH3) by nitrogen-fixing organisms and during lightning.

• Human Contribution: Industrial processes convert N2 to fertilizers like NH4.

Nitrogen Release and Cycling

• Decomposition by Microorganisms: Fungi and bacteria release nitrogen from dead organisms, primarily as ammonium (NH4).

• Conversion: Nitrogen is transformed into nitrate (NO3) by bacteria, which can be utilized by plants.

• Denitrification: Process where bacteria convert NO3 back to N2, returning nitrogen to the atmosphere.

19.4 Carbon Cycle

• Movement and Photosynthesis: Carbon cycles through ecosystems via photosynthesis and respiration.

• CO2 must dissolve in water for use by aquatic primary producers.

• Long-term Storage: Some carbon remains unavailable for extended periods, sequestered in soils, peat, fossil fuels, and carbonate rock.

Impacts of Fossil Fuels on the Carbon Cycle

• Increased Atmospheric CO2: Burning fossil fuels leads to a rise in atmospheric CO2 concentrations; however, the increase is slower than predicted.

• Carbon Sinks: Oceans, northern, and tropical forests serve as major carbon sinks.

19.5 Rates of Decomposition

• Nutrient availability to primary producers hinges on the rate of mineralization, mainly during decomposition.

• This involves breaking down organic matter and releasing CO2, significantly influenced by temperature, moisture, and litter composition.

Decomposition Studies in Various Ecosystems

Mediterranean Woodland Ecosystems

• Gallardo and Merino's study showed that chemical and physical factors affect decomposition rates in leaf litter, revealing that wetter conditions and certain leaf physical characteristics influence mass loss.

Temperate Forest Ecosystems

• Melillo et al. found decomposition rates were correlated with temperature, moisture, and nitrogen availability, emphasizing the impact of lignin content on mass loss.

Tropical Forests

• Higher decomposition rates are noted due to rich soil nutrient content, fostering aboveground net primary production and litter fall.

Aquatic Ecosystems

• Decomposition rates in aquatic environments depend on species, temperature, and nutrient concentrations, with higher lignin content slowing the process.

19.6 Organisms and Nutrients

Nutrient Cycling in Streams and Lakes

• Stream dynamics, termed nutrient spiraling, includes downstream transport and limited cycling in one location.

• The spiraling length relates to both nutrient cycling speed and water velocity.

Impact of Stream Invertebrates

• Research indicates that aquatic macroinvertebrates significantly enhance nitrogen cycling through high ingestion rates, thus affecting nutrient retentiveness and spiral length.

Influence of Vertebrate Species

• Different allocation patterns of nutrients by various organisms impact the nitrogen and phosphorus recycling ratios.

• Vanni et al. (2002) noted that excretion ratios can vary inversely to the nutrient ratios of fish and amphibians.

Effects of Disturbance on Nutrient Dynamics

• Studies show that deforestation drastically increases nitrogen losses, with ecological recovery leading to nitrogen sinks post-fire.

• Phosphorus Dynamics: Research from Meyer and Likens pointed out the episodic nature of phosphorus export linked to water flow, particularly during seasonal events like leaf fall and snowmelt.