Chapter 13: Nitrogen Economy of Soils
Effects of Nitrogen on Plants
 Nitrogen is an integral component of many essential
plant compounds
 Stimulates root growth and development
 Plants take up dissolved nitrate NO3- and ammonium
NH4+ ions, and even soluble proteins and amino acids
Nitrogen Cycle
 Atmosphere = 78% N2 (an inert gas)
 Microbial nitrogen fixing and lighting convert N2 into
reactive forms of nitrogen
 Most of terrestrial nitrogen is found in the soil
 Horizon A contains 0.02 to 0.5% nitrogen, contained
mostly in organic molecules: soil OM contains about 5%
nitrogen.
 Mineralization is the microbial enzymatic process that
converts organic nitrogen to inorganic nitrogen (e.g. An
amino acid to nitrate)
 Immobilization does the reverse of mineralization, and
can be completed by either plants or microbes
Soluble Organic Nitrogen (SON)
 SON is the primary nitrogen source in acidic and
infertile soils (e.g. forest areas that lack mineral N)
 SON is composed mostly of the leached nitrogen which
contributes to environmental problems
Ammonia Fixation by Clay Minerals
 NH4+ is attracted to clays and humus; it can become
fixed in the rigid part of the crystal structure in 2:1 clays
because of its size.
Ammonia Volatilization
 Results in the production of NH3 gas
 Occurs more at higher pH levels, in soils with less clay
and humus as these colloids absorb NH3 gas
 Reversible; plants/soil can absorb NH3 from the air
Nitrification/Denitrification
 Nitrification: Autotrophs gain energy by oxidizing NH4+;
resulting in nitrite and increased soil acidity
 Denitrification: bacteria reduce nitrate to NO, N2O, N2
gases. NO, N2O contributes to acid rain, green house
effect and the formation of ground level ozone.
 Nitrogen loss by denitrification is highest in low lying,
organic rich, and moist, saturated or submerged areas
Nitrite Leaching Problem
 Negative charged NO3- ions are not adsorbed by
colloids, which causes them to leach from the soil
 Causes groundwater quality problems; can be managed
by limiting fertilizer and manure or planting nitrogendemanding winter plants
Nitrogen Fixation
 Biological nitrogen fixations is an important
biochemical reaction; Rhizobium converts N2 to reactive/
plant useable nitrogen using the enzyme nitrogenase
 Rhizobium and Bradyrhizobium species infect legume
roots producing root nodules that serve as a site of
nitrogen fixation. Symbiotic fixation also occurs in nonlegumes but with different bacteria
 Free living autotrophs and heterotrophs in soil and water
are also a source of nitrogen fixation
Nitrogen Deposition from the Atmosphere
 Usually in the form of NH4+ and NO3- through rain,
snow, dust, and gaseous absorption.
 Highest in areas with high rainfall down wind from cites
and areas with concentrated animal farming
 Leads to soil acidification and methane oxidization
 Disrupts forest soil ecosystems through N saturation
 High nitrogen in grassland leads to lower biological
diversity and productivity, as native plants thrive at low
nitrogen levels
Practical Management of Soil Nitrogen
 3 goals: maintain soil OM to ensure N supplies, regulate
soluble forms N needed by plants, and minimize
environment damages from the loss of soil N
 Fertilizers are applied moderately as most of the N is
lost before plants can use it
 Leaving lawn clippings in place results in little N loss
 Legume crop covers supply the main crop with N for
several months as it decays
 Including a legume crop rotation decreases the need for
N fertilizer for the next crop in the following year
 Nitrogen budgets need to be balanced; e.g. in a feedlot
the N imported in the form of feed should not exceed the
N exported in the form of manure
Meghan Nannt