Title: Plant Nitrogen Speaker: Bill Pan

online.wsu.edu

Lesson 2.3 Plant Nitrogen •Nitrogen distribution in the soil-plant-

atmosphere

•Chemical N forms and oxidation states

• Biological roles of N in plants

•Seasonal N accumulation, N deficiencies

•Biological N fixation

Figure 4-1. The N cycle.

Test your command of

N terminology in

Assnmt 2.1

Table 4-1.

Approximate Distribution of N Throughout the

Soil-Plant/Animal-Atmosphere System

N pool Metric Tons

% of Total

Atmosphere 3.9 x 1015 99.3840

Sea (various) 2.4 x 1013 0.6116

Soil (nonliving) 1.5 x 1011 0.0038

Plants 1.5 x 1010 0.00038

Microbes in soil 6 x 109 0.00015

Animals (land) 2 x 108 0.000005

People 1 x 107 0.00000025

Nitrogen Oxidation States

NH4+ ,NH3 N2 N2O NO NO2

- NO3-

N Oxidation States: -3 0 + 1 + 2 + 3 + 5

Energy requiring reactions

Energy yielding reactions

Plant Nitrogen Uptake

Most plants are capable of absorbing

ammonium and nitrate.

Relatively little organic N is directly

absorbed by crop roots, it must first be

mineralized into these inorganic forms

before plant uptake can occur.

Since nitrate is the predominant inorganic

N form in soil solutions, it is typically the

primary form absorbed by crop roots.

Biological Role of N in Plants

Forms peptide bond linking amino acids in

proteins

Constituent of other important

biochemicals such as chlorophyll

Provides functional groups in enzymes for

reaction site or attachment of substrates,

cofactors

Nitrogen Forms the Peptide Bond in

Proteins

Nitrogen Forms a Coordination

Complex with Mg in Chlorophyll

Nitrogen Provides Functional Groups

at Reaction Sites of Enzymes Such as

ATPase

Plant N Accumulation

1 to 5% N

Seasonal uptake patterns often follow a

sigmoidal pattern:

Pla

nt

N

Time

100–450 lb N/ac

typical ranges for

crop plants over

season (see texts)

Nutrient uptake over time

Corn Dry Matter

Time

grain

leaves

stalk

http://maize.agron.iastate.edu/corngrows.html#nutrient

Corn P uptake

Time

N

P

N deficiency symptoms: General

patterns

1. Chlorosis (loss of green color, usually

seen as yellowing) of leaves

◦ Lack of chlorophyll

◦ Seen esp. in OLDER leaves, since N is very

mobile in plant

2. General stunting of plant

◦ Decreased carbohydrate production

◦ Decreased protein production

N Deficiency Symptoms: Examples

• Corn (maize): chlorosis of lower leaf, progresses up the midrib first

• Potatoes: Stunted plants, chlorosis of lower leaves, upward cupping

• Wheat: smaller leaves, less tillering, chlorosis of lower leaves

• Peach: reddish leaves with shothole appearance

Remember, by the time you see symptoms, plant health / quality / yield have ALREADY been diminished

N Deficiency in Greenhouse Corn

N Deficiency in

Field-grown Corn (Maize)

chlorosis of

lower leaves

necrosis of leaf

tips

yellowing of

midrib region

Field-view of N Deficient Corn

• General yellowing of canopy

• May be detectable by on-the-go sensors or by remote sensing

Some General N Deficiencies

Kale

Ru

tab

ag

a

Su

gar

beet

Po

tato

N Toxicities Cucumber

N toxicity – necrosis

of newer growth

regions

Biological Components of the

Nitrogen Cycle

Processes and Organisms of:

N fixation

Mineralizat ion

Immobilizat ion

Nitrif icat ion

Biological N fixation

Bacteria and algae

Symbiotic with

• legumes

• some trees & bushes

•water ferns

Non-symbiotic free-living or in loose associations

N fixation

Industrial Fixation reduces N2 to NH4+.

◦ Requires 300-400 C; absence of air and water, and 500 atm pressure.

Biological Nitrogen Fixation:

Nitrogenase

N2 + 8e- + 16ATP + 10 H+ 2NH4 + 16ADP + 16Pi + H2

Very energetically expensive. Most N fixation by symbioses where plants provide the energy.

a) Example of nodules

of Bradyrhizobia on

soybean

b) showing non-inoculated

alfalfa (left) and inoculated

(right) with proper Rhizobia

bacteria

Figure 4-5. Conversion of N2 to NH4+ by rhizobia

inside a legume root nodule.

Table 4-7. N2 Fixed by Legumes in Temperate Climates

N fixed (lb/a/yr)

Legume Range Typical

Alfalfa 50–300 200

Beans 20–80 40

Chickpeas 20–100 50

Clovers (general) 50–300 150

Cowpeas 60–120 90

Crimson clover 30–180 125

Fava beans 50–200 130

Hairy vetch 50–200 100

Kudzu 20–150 110

Ladino clover 60–240 180

Lentils 40–130 60

Peas 30–180 70

Red clover 70–160 115

Soybeans 40–260 100

Organisms General Properties Agricultural Importance

Azotobacter Aerobic; free fixers; live in soil,

water, rhizosphere (area

surrounding the roots), leaf

surfaces

Minor benefit to agriculture; found in

vascular tissue of sugarcane, with

abundant sucrose as a possible energy

source for N2 fixation

Azospirillum Microaerobic; free fixers; or

found in association with roots

of grasses

Inoculation benefits some nonlegume

crops, shown to increase root hair

development

Rhizobium Fix N in legume-Rhizobium

symbiosis

Legume crops are benefited by

inoculation with proper strains

Actinomycetes,

Frankia

Fix N in symbiosis with

nonlegume wood trees—alder,

Myrica, Casuarina

Potentially important in reforestation,

wood production

Blue-green algae,

Anabaena

Contain chlorophyll, as in

higher plants; aquatic and

terrestrial

Enhance rice in paddy soils; Azolla (a

water fern)—Anabaena-Azolla

symbiosis; used as green manure

Table 4-5. Economically Important Microorganisms

Involved in Biological N Fixation

Inoculation

Inoculation may be required.

Rhizobium only survive a

few years in the soil without

the proper host.

Commercial strains selected

for most N fixed on specific

crops.

Inoculate seeds themselves.

Mix bacteria with peat and

gum (sugar) to coat seeds.

Best survival if done just

before planting. Or

inoculate within or below

the seed row.

Figure 4-7. Soybean yield as influenced by P

availability and inoculation. (Singleton et al., 1990, Applied BNF Technology: A

Practical Guide for Extension Specialists, NifTAL, Paia, Hl.)

Leibig’s Law of the Minimum in action.

N fixation is pH-dependent

0

2

4

6

8

10

12

14

16

18

5 5.5 6 6.5 7

Non-legume

Low-pHsensitive

Low-pHtolerant

Soil pH

Multiple N Pathways in Legumes

Crop legumes typically assimilate

soil N like non-legumes, while

biologically fixing atmospheric N.

Higher soil N availability inhibits N

fixation.