Soils 205- General Soils

Lecture 25- The Nitrogen cycle

I.  importance of N

     -most limiting nutrient in many ecosystems

     -excess N can adversely affects plants and human health

     -essential component of protein

     -ozone destruction

    a.  deficiency symptoms

        -pale yellowish-green color

        -stunted appearance

        -low protein content

    b.  oversupply

        -excessive vegetative growth

        -plant quality

        -environmental damage

II.  inputs to the N cycle

    -plant/animal material

    -biological N fixation

    -lightening

    -fertilizers

    a.  Types of N in the soil-  Two main types of N, or pools of N are found in the soil.

        ii.  mineralization/immobilization

        iii.  The soil N cycle

        iv.  Human alteration of the N cycle- The following numbers were published by Vitousek et al 1997

       Natural N-fixation:

                lightening     5 - 10 Tg/yr

                biological    90 - 140 Tg/yr

                    total        95 - 140 Tg/yr

        Additional N-fixation:

                  fertilizers                             80 Tg/yr

            leguminous crops                      32-53 Tg/yr

            combustion of fossil fuels           20 Tg/yr

            Land clearing, burning of

            forests, draining of wetlands        60 Tg/yr

                       total                        192 - 213 Tg/yr

        v.  impacts

           -acidification

           -decreased biological diversity?

           -atmospheric chemistry