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23 Cards in this Set

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  • Back

Optimum value of N

The constant level of fixed N that is contained within the biosphere since the amount of biomass present in the biosphere is also constant

Fixation of N2 occurs to maintain optimum value

Nitrogen fixation

The conversion of atmospheric nitrogen into biologically active nitrogen compounds by bacteria that contain a specialised enzyme capable of cleaning the triple bond in nitrogen gas

Biologically active nitrogen

Nitrogen that can be used by organisms


Process in the N cycle carried out by microbes in anaerobic conditions where nitrate acts as an oxidising agent

This results in the loss of nitrogen as it is converted to nitrous oxide and nitrogen gas

how can nitrogen fixation in nature occur

Via biological nitrogen fixation, producing NH4+, or during lightening strikes, producing NO3-

Only biological nitrogen fixation has any true ecological significance since only N-fixing organisms can respond rapidly to local needs

Ammonia volatilisation

The loss of NH3 from terrestrial ecosystems to the atmosphere

Nitrate leaching

The loss of nitrogen as nitrate ions dissolved in water

Nitrogen fixation by lightning

Energy from lightening breaks up N2 and O2 to form NO

This oxidises to NO2

Both nitric oxide and nitrogen dioxide are soluble and form nitrous and nitric acids which are deposited and dissociate into nitrite and nitrate

Industrialisation, the rise in population and political considerations led to what

The development of the Haber-Bosch process for the production of N-rich fertilisers

The Haber-Bosch basic process

Hydrogen gas is derived from a fossil fuel such as methane:

CH4 + 2H2O = CO2 + 4H2

Needs high temps and pressure and a nickel catalyst

Then the H is combined with atmospheric dinitrogen to form ammonia:

N2 + 3H2 = 2NH3

Requires high temps, very high pressure and iron catalyst

In a temperate grassland ecosystem what may be the main process which balances the losses of fixed nitrogen and how is this N lost

Biological nitrogen fixation by plants such as clover (legumes) balanced N losses

These losses are nitrate leaching, denitrification and ammonia volatilisation

What is the most important store of N in terrestrial systems

Organic matter


The conversion of an organic form of an element to an inorganic (mineral) form

Nitrogen in organic matter is recycled through

Mineralisation: organic matter releases into environment as NH4+

Ammonia volatilisation: urea breakdown and loss of ammonia

Nitrification: nitrifying bacteria oxidise NH4+ to nitrite and nitrate in oxidising environments

What is assimilation in the context of the N cycle

The process whereby soil NH4+ and nitrate are incorporated into plants

Agricultural impacts on the N cycle

Agricultural practices disrupt all aspects of the N cycle of a temperate grassland, enhancing the loss of fixed N

Sulfur occurs naturally in the / main reservoirs and most rapid fluxes

Atmosphere, oceans, soils, rocks and living organisms

Major reservoirs are rocks and seawater

Most rapid fluxes occur between the atmosphere biosphere and hydrosphere

Reduced sulfur in the earths crust

Sulfur in the Earths crust occurs mainly in mineral pyrite

It can be released by natural weathering and erosion as sulfate (SO4 2-)

Reduced sulfur in the earths crust

Sulfur in the Earths crust occurs mainly in mineral pyrite

It can be released by natural weathering and erosion as sulfate (SO4 2-)

Sulfate can be assimilated by plants

What is the largest natural flux of sulfur gas and what is it produced by

Dimethyl sulfide, (CH3)2S, which is produced by the decay of marine organisms such as phytoplankton

natural sources of atmospheric sulfur

Oceans - dissolved salts enter as sea spray

decomp of plankton in oceans to produce dimethyl sulfide gas

Decay of organic matter in anaerobic marine sediments


Weathering of rock that contains sulfur, mainly pyrite

Changes in the amount of SO2 in the atmosphere over time during twentieth and 21st centuries reflect what

Human economic development and the influence of pollution control technologies in Europe and N America

Largest and second largest S flux out of the ocean is

The precipitation of metallic sulfides at hydrothermal vents

Second is production of biogenic pyrite in marine sediments