Decomposition and Nutrient Cycling

Decomposition and Nutrient Cycling

  • Reminders:
    • Lecture assignment 9 and makeup lecture assignment due next Friday.
    • Final example on THURSDAY MAY 8 at 2:00pm.
    • Lab is in the stream again this week!

Top Hat Question

  • When will CO2CO_2 output from a deciduous forest be highest?
    • Spring
    • Summer
    • Fall
    • Winter

Decomposition and Mineralization

  • Plant litter → Soil organic matter
  • Decomposition and mineralization result in dark, homogenous organic matter = humus.
  • Soil organic matter = humus embedded in the matrix of the soil.
  • Decomposition over time leads to:
    • Continued mass loss.
    • Decline in carbon.
    • CO2CO_2 lost to the atmosphere with microbial respiration.

Long Term Decomposition Study

  • Nitrogen immobilized:
    • Taken up by microbes, not available for plant uptake.
  • Nitrogen mineralized:
    • Released into soil (microbe poop) as inorganic.
    • Suitable for plant uptake.

Plant Litter and Soil Organic Matter

  • Plant litter → Soil organic matter:
    • Fragmentation by soil invertebrates and chemical alterations convert the litter into soil organic matter.
  • As microbes die, chitin and other compounds that are difficult to break down are an increasing proportion of the residual organic matter.
    • This leads to the production of humus.
  • The quality of soil organic matter is gradually reduced as it ages. The C/N ratio continues to decline.
  • Decomposition of the remaining litter proceeds very slowly.

Rhizosphere

  • Region in the soil where plant roots function.
  • Active zone of root growth and death.
  • Intense microbial and fungal activity.
  • Roots alter rhizosphere chemistry by secreting carbohydrates into the soil (exudate).
    • High-quality energy source for bacteria.
  • Bacteria are limited by a lack of essential nutrients, which they obtain from soil organic matter.
  • Predation on bacteria by protists and nematodes remobilizes essential nutrients for plant uptake.

Soil Microbial Loop

  • Process:
    • Plants supplementing C to microbial decomposers in the rhizosphere.
    • Enhanced decomposition of soil organic material.
    • Predator remobilization of mineral nutrients for plant uptake.
  • Relationship between microbial decomposers and microbivores determines the rate of nutrient cycling in the rhizosphere.
    • Enhances availability of mineral nutrients to plants.

Rates of Nutrient Cycling

  • Cycling of nutrients within an ecosystem relies on photosynthesis and decomposition.
  • Primary productivity determines the rate of nutrient uptake.
  • Decomposition determines the net mineralization rate.

Nutrient Retention

  • Swidden agriculture:
    • Method of traditional subsistence farming, mainly in the tropics.
    • Burn forest → nutrient-rich soil → farmed and harvest extracted until nutrient poor.
    • Left to regenerate through succession (many years of recovery).
  • Fertilizers:
    • Substances that supplement soil nutrient supplies.
    • Contain chemical elements that improve soil fertility and enhance the growth and productivity of plants.
    • Organic (food for decomposers) and inorganic (ready for plant uptake but can easily runoff into aquatic ecosystems).

Nutrient Availability Feedback Systems

  • High nutrient availability:
    • High nutrient return to soil in litter.
    • High net mineralization rate.
    • High net primary productivity.
    • High leaf nutrient concentration.
    • High nutrient uptake.
  • Low nutrient availability:
    • Low nutrient return to soil in litter.
    • Low net mineralization rate.
    • Low net primary productivity.
    • Low leaf nutrient concentration.
    • Low nutrient uptake.

Decomposition Rates

  • Shows % original mass remaining over days for high marsh, creek bank, and submerged environments

Vertical Separation of Production and Decomposition

  • Terrestrial:
    • Zone of primary production (canopy).
    • Zone of decomposition (forest floor).
  • Aquatic:
    • Zone of primary production (photic zone/surface waters).
    • Zone of decomposition (benthic zone/bottom sediments).

Vertical Structure of Open-Water Ecosystems

  • Three distinct zones:
    • Epilimnion: Warm, low-density, low-nutrient waters
    • Thermocline
    • Hypolimnion: Cold, high-density, high-nutrient waters
  • In temperate zones, the thermocline breaks down in the fall and spring, and turnover occurs.

Nutrient Spiral

  • Uptake (incorporation of mineral nutrients into biomass).
  • Turnover (decomposition and mineralization of nutrients in dead organic matter).
  • Spiral length is influenced by flow, water column, and particulate component.

Nutrient Cycling in Coastal Ecosystems

  • Coastal ecosystems are very productive environments.
  • Water from rivers and streams eventually drains into the oceans.
  • An estuary is where freshwater and saltwater meet.
  • As rivers meet the ocean, current velocity drops.
  • Nutrient cycling in these ecosystems combines features of terrestrial, open-water, and stream ecosystems.
  • Estuary – where freshwater and saltwater meet.

Estuary Dynamics

  • Tidal subsidy: tidal cycle influence on nutrient movement in and out of the estuary.
  • Pycnocline: vertical change in salinity and density of water.
  • Anaerobic bacteria dominate.

Ocean Currents & Nutrients

  • Surface currents driven by the Coriolis effect move nutrients in the top 100m.
  • Upwelling (vertical circulation) of water at equators transports nutrients from benthic zones.

Global Scale Exchange of Nutrients

  • Requires viewing the biogeochemical processes on a broad spatial scale.

Top Hat Question: Components of Biogeochemical Cycles

  • Which of the following are components of biogeochemical cycles? Select all that apply.
    • A. Inputs
    • B. Internal cycles
    • C. Outputs
    • D. Synthesis

Nutrient Cycling Diagram

  • Atmospheric input
  • Ecosystem
  • Net primary productivity
  • Internal cycling
  • Incorporation into plant tissues
  • Plant uptake
  • Litterfall
  • Dead organic matter
  • Plant tissues
  • Decomposition/Mineralization
  • Soil nutrients
  • Output
  • Input from the weathering of rocks and minerals

The Carbon Cycle

  • Sunlight
  • Photosynthesis
  • Decay organisms
  • CO2CO_2 cycle
  • Auto and factory emissions
  • Organic carbon
  • Animal respiration
  • Dead organisms and waste products
  • Plant respiration
  • Root respiration
  • Ocean uptake
  • Fossils and fossil fuels