AEC 360  Ecology

The nitrogen cycle and pollution
(note that with a guest lecture, this is largely, but not exactly what was presented in lecture. Regardless, it will provide a good guide for studying for the exam)

I.  Background:  effects of pollution

        Biological and geological processes that affect nutrient cycles

II.  The Nitrogen cycle
    A.  Pools of nitrogen include the atmosphere and living organisms

    B.  Flows of nitrogen
        1.  Nitrogen fixation

            - nitrogen fixing bacteria convert atmospheric nitrogen (N2) to ammonia (NH3)
            - some bacteria are free-living in soils; when they die, ammonia is released
            - free-living bacteria utilize energy from decomposition of detritus
            - ammonia is also converted to nitrite and then to nitrate (NO3-) by soil bacteria (Nitrification)


            - other bacteria live in symbiotic relationship with plants, occurring in nodules in roots

          2.  Decomposition
            - bacterial nitrogen fixation accounts for 12% of nitrogen assimilated by terrestrial plants
            - remainder comes from decomposition
            - when bacteria die, they release ammonia into soil

          4.  Loss of nitrate

            - nitrate does not bond to clay particles
            - if plants do not use nitrate, it is lost through run-off
            - clear cuts, disturbed habitats, fallow fields will all lose nitrogen through time
            - can lead to eutrophication of aquatic systems

            -- rare isotopes of N can be used to track the fate of N in ecosystems

          5.  Denitrification
            - in soils without oxygen (anaerobic soils), bacteria can obtain energy by reducing nitrate to nitric oxide (NO), nitrous oxide (N2O), or nitrogen gas (N2)
            - these gases return to the atmosphere

III.  A few words about the phosphorus cycle
        - weathering of rocks introduces phosphorus into terrestrial systems
        - plants uptake phosphate ions from soil
        - decomposers break down organic phosphorus
        - in aquatic systems, most is bound to soil particles and is biologically unavailable

IV.  Impacts of nitrogen fixation:  effects of species on nutrient cycling

        A.  Fungi help uptake nutrients like phosphorous 

         B. Myrica faya, an invasive species in Hawai’i
                - Nitrogen-fixing, and grows on lava flows
                - Increases N inputs to the ecosystem
                - Facilitates colonization of other invasive species


V.  Long-term impacts of nutrient loading

-- Humans now fix more nitrogen than is fixed through biological processes

    A.  Case study from terrestrial systems

        1. Decreases plant diversity
        2.  Increases plant production
        3.  Shifts plant composition to weedy species
        4.  Decreases consumer species richness
        5.  Increases consumer abundance
        6.  Affects N and C cycling

    B.  Case study from aquatic ecosystems
        1.  Increased primary production
        2.  Impacts on consumers
        3.  Increased rate of succession