What are the 18 essential elements plants need?

Nitrogen and Sulfur

Chapter 13

Nitrogen

Deficiency chlorosis and poor growth

Oversupply rank, abnormal growth

Importance of N to Plants

Taken up as NH4+ or NO3

-

Component of biomolecules

→

→

Most N in atmosphere

Some in vegetation but 20 x as much in soil

Most soil N in organic matter Only 1 to 2 % as inorganic ions

Nitrogen Cycle

Plants can use

Immobilization and Mineralization

N mineralization refers to the conversionof organic-N to inorganic-N (True / False)

And what is N immobilization?

True, and immobilization is the reverse, incorporation into biomass.

The upper 6 inches of an acre of soilcontains 3 % organic matter. If 2 % ofthe organic matter is mineralizedannually and it contains 5 % N, howmuch N is made available? Assume2,000,000 lbs per AFS.

If you do the multiplication, I think you will arrive at 60 # N per acre per year,which is a fair amount. Of course, soil microbes are competing for it and thereare several other processes going on in the N cycle that affect the fate of mineralized N. Nevertheless, N mineralization is an important source of plant-available N.

N mineralization is an importantsource of N for plant growth (True / False)

NH4+ Fixation

NH4+ trapped between units of 2:1 minerals

Vermiculite > illite > smectite

NH4+ slowly released

The green box is biomass, the brown box organicmaterials subject to mineralization and the firstbaby blue box, the pool of mineralized N (NH4

+).We are at the box with star, NH4

+ fixation.

NH4+ is about the same

size as K+ and so issubject to interlayerentrapment.

The effect isgreatest withvermiculitebecause adjacentcrystals are notalready collapsed onto one another as with illite withits interlayer K+. Some NH4

+ fixation about interlayeredges in illite.

NH3 Volatilization

NH4+ + OH- ↔ H2O + NH3 ↑

High pH Increase volatility?High CECIncorporationDry soil

This is a spot where some appreciation ofchemical equilibrium is useful. The notion isthat if OH- concentration is high, the reactiontends to go to the right, with production of NH3,which can be lost from solution to soil air anddiffuse out of the soil. As for CEC, since NH4

+ is a cation, it is adsorbed by – charged colloids andthe greater the density of – charge, the greater isits adsorption. So, more adsorbed, less in solutionsubject to reaction going to the right.

If you use an NH4+

type fertilizer, the deeper it’s put, theless likely it’s to go.

Nitrification

Microbial oxidation of NH4+

2NH4+ + 3O2 → 2NO2

- + 4H+ + 2H2O + E

2NO2- + O2 → 2NO3

- + E

So, a two-step process with nitriteproduced first, then nitrite oxidizedto nitrate. The microbes carrying outthe nitrification process derive energy (E)from it.

Carried out by autotrophic bacteria

Step 1 Nitrosomonas

Step 2 Nitrobacter

Nitrification acidifies soil

Doesn’t the first reaction indicate so?

Rapidly in warm,moist, well-aeratedand fertile soils

Chemical inhibitorsthat reduce Nitrosomonasactivity can be used

If you had your choice, would you ratherinhibit the first or second of the two reaction?

If you inhibited the second, you would build up nitrite, no?

NO3- Leaching

Loss of nutrient from soil

Contaminates ground and surface waters

These are the negative consequences. Sincenitrate is a anion, it is not subject to muchadsorption, at least not in soils (like around herein the sub-tropics or in the temperate region) withmostly negative charge. So, it leaches quicklyif water is percolating.

May lead to

Eutrophication, especially marine systems

Methemoglobinemia, blue baby syndrome,from reduction of NO3

- to NO2-

This is, basically, fertilization of the water body, leading to increased algal and aquatic plant growth.In turn, there is more organic matter in the waterbody, and if there is more organic matter, there ismore microbial activity, perhaps, tending to depleteO2. Other negatives, too.

N is considered more limiting in salt water, P in freshwater. Think, Gulf of Mexico hypoxia.

Happens in GI tract by bugs. Nitrite tends to bind to hemoglobin, reducing O2 capacity.

Denitrification

Reduction of NO3- to NO, N2O or N2

NO3- → NO2

- → NO → N2O → N2

volatile lossesAnaerobic respiration process. Go checkback with chapter on soil air.

Reaction kinetics, anyone?

So, nitrate is reduced,leading to the productionfirst of nitrite, which is reducedto a gaseous form of N, principallyeither nitrous oxide or N2.

Factors affecting denitrification

NO3-

Oxidizable substrates

Anaerobic conditions

How affect rate of denitrification?

You gotta have all three, and the more nitrate and organic matter, the moredenitrification you get. Make sense?

Where does denitrification occur?

Riparian zones?Look at figure. MWD = moderately well-drained,SPD = somewhat poorly, etc. How comemore where developed?

Wetlands and rice fields?

Sure, youwouldn’t usenitrate with rice,but if you usedan ammoniumlike fertilizer andleft it at thesurface, it wouldnitrify, moveinto the loweranaerobic soil,and denitrify.

So, put it deeperwhere it won’tnitrify, no?

Even upland agricultural soils?

Spatially and temporally variable but up to60 kg / ha annually

Sure, even uplands andall the time, like deep inaggregates where anoxicconditions may exist.

+ / - environmental effects

Acid deposition due to HNO3 formed fromNO and N2O

N2O is a greenhouse gas

NO3- removed so less potential for nutrient

enrichment Of water bodies, that is, and less potential of babies going blue.

Biological N Fixation

N2 + 6H+ + 6e → NH3

NH3 into amino acids

Certain bacteria, actinomycetes and cyanobacteria carry out N fixation

~ 140,000,000 Mg N fixed annuallyVery, very important process for the biological well-being of the Earth aswe know it. Most think it’s next most important to photosynthesis.

Then various pathways.

This is the major way N gets into the soilnaturally.

A

B

B is whathappenswhen theN-fixersin the soilhave beenautoclavedaway. A is livinglarge andsymbioticallywith N-fixers.

Nitrogenase enzyme complex involved

Reaction requires energy

Therefore, biological N fixation is aided byassociation with plants which supplyphotosynthetic products (True / False).

Biological N Fixation

Answer: True. The Ns in N2 are triple bonded. Strong bond that needs a lotof energy to break.

Inhibited by NH4+ and NO3

-

So, do you fertilize N fixing plants with N?

Biological N Fixation

Zero response of N-fixingclover to N fertilization.

Symbiotic Fixation with Legumes

Rhizobium and Bradyrhizobium genera ofbacteria involved

Form nodules on roots of legumes

OK, these guys chemicallyreduce N2. You can’tdo that in an oxidizingenvironment, yet theseguys are aerobes.

They maintain an anoxiczone, by excluding O2. Cut a nodule open andyou get this.

Symbiosis specific between legume andbacteria species

Can inoculate if right species absent

Biological N Fixation

That is, maybe plant N-fixing plant if it’s been grown there before and the soilhas a good population of Rhizobia to infect the seedling root. If not, coat theseed with inoculum to best ensure nodule development.

There is also

Symbiotic Fixation with Nonlegumes

Nodulated / nonnodulated associations

Actinomycetes and others as well asbacteria

External (rhizosphere)Wouldn’t you say that the next bestthing to being in the plant would bebeing in the rhizosphere with all thatrhizo-deposition going on, you knowenergy for the N-fixing bugs?

Sulfur

Component of certain amino acids andvitamins

Deficiencies result in chlorosis andstunted growth

A BWhereas if a plant is deficient in N(low available soil N), N is trans-located from older to newer tissue,S is less mobile within the plant.

So, if you had to choose which plant, A or B, was suffering a Sdeficiency, which would it be?

Sources of S

Organic S

Soil mineralsCaSO4, FeS and adsorbed SO4

2-

Atmospheric S

Focus on

Sulfur Cycle

Mineralized S may beeither in oxidized form,SO4

2-, or reduced, S2-.In an oxidizing (aerated)environment, the reducedform(s) are oxidized tosulfate, and in a reducing environment, the oxidized form is reduced.

Oxidation / Reduction

Mineralization of organic-S releasesincompletely oxidized forms of S

Oxidation to SO42- is largely biological

H2S + 2O2 → 2H2O + 2H2SO4

Carried out by autotrophic Thiobacillus

Reduction under anaerobic conditions

SO42- + 8H+ + 8e → S2- + 4H2O

This is coupled with oxidation of organic matter

Recall this matter from the chapter on soil air.

Acidification Problems

Acid sulfate soilsMined soilsAcid deposition on forest soils

Environmental problems due to S pertain to acidity. Wetland soils, dredgedmaterials, etc. may contain an appreciable amount of chemically reduced S. Whenthese soils / materials become aerated, the reduced S undergoes oxidation,producing high levels of H2SO4. Some mined waste similarly contains mineralswith reduced S that, when exposed to air or in contact with aerated soil, behave the same way, releasing H2SO4. Then there is also the matter of acid rain.

Acid sulfate soilsMined soils

Oxidation of FeS and FeS2 creates very low pH

Soil with reduced S is drained or reduced S is excavated

4FeS + 9O2 + 4H2O → 2Fe2O3 + 4H2SO4 Acidic mine drainage.

Acid deposition on forest soils

H2SO4 + HNO3

Adds H+ in addition to natural acidification processes

Accelerates natural leaching loss of nutrients