NRES 201 Lectures 35-37 (Fall 2014): Biotic Cycling of Nitrogen & Sulfur1NRES 201Biotic CyclingofNitrogen & Sulfur12Today’s lecture topics- Nitrogen in plants- Natural occurrence of nitrogen- Nitrogen in soils- The nitrogen cycle- Biological fixation- Atmospheric deposition- Mineralization- ImmobilizationBiotic Cycling of Nitrogen & SulfurNRES 201 Lectures 35-37 (Fall 2014): Biotic Cycling of Nitrogen & Sulfur2Plant nutrient concentrationsSource: Havlin et al. (1999)3Functions of nitrogen in plantsProtein constituent- Structural- Enzymes- Required for all metabolic processesInheritance- Constituent of DNA and RNAEnergy storage and transfer- Constituent of ATP and related compoundsPhotosynthesis- Constituent of chlorophyll4NRES 201 Lectures 35-37 (Fall 2014): Biotic Cycling of Nitrogen & Sulfur3Functions of nitrogen in plantsProtein constituent- Structural- Enzymes- Required for all metabolic processesInheritance- Constituent of DNA and RNAEnergy storage and transfer- Constituent of ATP and related compoundsPhotosynthesis- Constituent of chlorophyllNo life without N5Natural occurrence of nitrogenN occurs in all four spheres of the earth- Lithosphere- Atmosphere- Hydrosphere- BiosphereSource: Stevenson (1982)6NRES 201 Lectures 35-37 (Fall 2014): Biotic Cycling of Nitrogen & Sulfur4Natural occurrence of nitrogenN occurs in all four spheres of the earth- Lithosphere- Atmosphere- Hydrosphere- BiosphereAbout 98% is inthe lithosphere- As fixed NH4+in igneous rocksThe rest is mainly in the atmosphere- Air is 78% N2(by volume)- The ultimate source of N for living systemsSource: Stevenson (1982)7Total content- Surface soil: 0.06-0.3% N (1200-6000 lb/A-6”)- Subsoil: <0.02% N- Organic soil: up to 3.5% NOrganic forms of N- >95% of total N in most surface soils- Derived from living organismsInorganic forms of N- Typically 2-5% of soil N- Major formsNitrogen in soils8NRES 201 Lectures 35-37 (Fall 2014): Biotic Cycling of Nitrogen & Sulfur5The nitrogen cycleSource: Havlin et al. (2005)9Overall reaction- N2 NH4+ organic NHigh energy requirement- A triple bond (NN)must be brokenTwo forms- Symbiotic N2fixation- By legumes- And a few nonlegumes- Nonsymbiotic N2fixation- By free-living bacteriaBiological nitrogen fixation10Source: http://www.slideshare.netNRES 201 Lectures 35-37 (Fall 2014): Biotic Cycling of Nitrogen & Sulfur6Symbiotic N2fixationLegumes- Associations with Rhizobium or Bradyrhizobium- Fixation occurs in root nodulesSource: Brady and Weil (2004)11Symbiotic N2fixationLegumes- Associations with Rhizobiumor Bradyrhizobium- Fixation occurs in rootnodules- Amount of N fixed varieswidely- Fixation reduced by:- Soil acidity- Plant stress-NO3-availability- Legumes take up soil NSource: Tisdale et al. (1993)12NRES 201 Lectures 35-37 (Fall 2014): Biotic Cycling of Nitrogen & Sulfur7Nonlegumes- Observed for 100 plant species from 7 families- Many are trees or shrubs that form nodules withthe actinomycete, Frankia- Colonize infertile or disturbed soils- Lower fixation than with nodulated legumes13Source: http://en.wikipedia.orgAn alder root noduleNonsymbiotic N2fixationMajor organisms- Photoautotrophs- Mainly blue-green algae- Important in rice paddies- Heterotrophs- Such as AzotobacterSignificance- Very limited becauseof the need for:- Light-Organic C- Fertilizer sales14Source: http://www.kvkphek.nic.inNRES 201 Lectures 35-37 (Fall 2014): Biotic Cycling of Nitrogen & Sulfur8Atmospheric nitrogen depositionForms of deposition- As NH4+or NO3-- In particulates (dry deposition)- In rain or snowfall (wet deposition)Sources of deposition- Manure- Combustion of fossil fuel- Natural fires- LightningMagnitude of deposition- Typically 1-20 lb/A per yr- Greatest near animal feedlots and urban areas15Source: http://www.fws.govMineralizationDefined as- Organic N  NH4+Significance- The most important N-cycleprocess for plant growth- Without N2fixation- In the absence of Nfertilization- On fertilized soils- Opens access to soil reserves of organic N- Provides an ongoing supply of mineral N16Source: http://filebox.vt.eduNRES 201 Lectures 35-37 (Fall 2014): Biotic Cycling of Nitrogen & Sulfur9“During the first decade of cultivation, the prairie soil was apparently too rich for wheat: the wheat tended to grow too tall and then fall over, or lodge, thus reducing grain yields.”L. F. Welch (1979)17A process of decomposition- Decomposition occursin stages- C and N are liberatedas waste products- C as CO2- N as NH4+- Carried out by a widevariety of heterotrophs:- Fungi- Actinomycetes- BacteriaSource: Stevenson (1986)18NRES 201 Lectures 35-37 (Fall 2014): Biotic Cycling of Nitrogen & Sulfur10- Key fraction of soil N is alkali-labile- Decomposable by NaOH- Probably derived in part from amino sugars,which occur:• In bacterial cell walls• In actinomycete spores• In fungal hyphae (as chitin)- Estimated by the Illinois soil N test (ISNT)19Source: http://www.australianscience.com.auPromoted by- Favorable environmental conditions- Adequate soil moisture- Warm temperatures (100°F optimal)- Good aeration- Absence of soil acidity- Wetting and drying cycles- Drying kills some soil microbes- Upon rewetting, the survivors utilize the deadbiomass- The result is a flush of mineralization20NRES 201 Lectures 35-37 (Fall 2014): Biotic Cycling of Nitrogen & Sulfur11- Cultivation- Improves aeration- Exposes fresh organicmatter to microbialattack- N fertilization- Accelerates residuedecomposition- Unless P or S is limiting21Source: https://www.pioneer.comImmobilizationDefined as- organic NA process of assimilation- The reverse of mineralization, except that eitherNH4+or NO3-may be immobilized- NH4+is strongly preferred over NO3-- Both processes may operate simultaneouslyNH4+NO3-22NRES 201 Lectures 35-37 (Fall 2014): Biotic Cycling of Nitrogen & Sulfur12- Carried out by heterotrophic microbes- The same ones that carry out mineralization- Promoted by:- Warm temperatures- Adequate soil moisture- Absence of soil acidity- Presence of carbonaceous residues- Temperature effects on immobilization muchgreater for NO3-than for NH4+- Because of the need for mineralizable C,which is required for immobilization of NO3-but not NH4+23Effect of the C/N ratio of organic amendmentsSource: Havlin et al. (2005)24NRES 201 Lectures 35-37 (Fall 2014): Biotic Cycling of Nitrogen & Sulfur13- The C/N ratiodecreases duringdecompositionbecause:-CO2is lost-NH4+is not lostSource: Havlin et al. (2005)2526Today’s