Download Nutrient Dynamics

Nutrient Dynamics
Essential Plant Nutrients
16 Elements are Essential for Plants
O, C, H,
Macronutrients
N, K, P, Ca, Mg, S,
Cl, Fe, B, Mn, Zn, Cu, Mo,
Si, Na
Micronutrients
Species dependent
GES175, Science of Soils
Lecture 8
NITROGEN
Oxidation States of Soil N
N Form
Name
Oxidation state
organic-N
-3
NH4+
ammonium
N2
dinitrogen gas
NO2-
nitrite
+3
NO3-
nitrate
+5
-3
0
Fertilizer: N (first of three digit rating)
Nitrogen Redox Processes
Oxidation: loss of eReduction: gain of e-
-3
+5
NH4+ ÍÎ NO38 e- transfer
N-cycle
plant & animal residues
(fixation)
(denitrification)
NO3-
4
5
N2
organic-N
2
(immobilization)
3
1
(nitrification)
NO2-
3
(nitrification)
(mineralization)
NH4+
Carbon Pools
atmosphere
CO2
Vegetation
Soil
Lakes and Oceans
O2
CO2
Detritus (Plant Debris)
Earthworms
Fungi
Bacteria
Soil Humus
Organic Matter Mineralization
C:N
> 25:1
Polysaccharides (cellulose…)
Organic Acids
Amino acids, Peptides, Proteins
Lipids
Nucleotides
Peptides
Lignin
Cellulose
Proteins
Lignin
NH3
C:N
10:1
CO2
Humus
NH3
Carbon to Nitrogen Ratios
Material
C:N
Maple Leaf
45-70
Pine Needle
60-70
Wood
130-400
Grass
20-80
Alfalfa
9-16
Bacteria
4-12
Mineralization vs. Immobilization
Fate of N if added to soil???
Low C:N (high N content)
Alfalfa, peas, grass
High C:N (low N)
straw, bark, sawdust
Ammonia Volatilization
- gaseous loss of N
Ammonia Volatilization
Urea:
CO(NH2)2 → NH3 +CO2 + H2O
urea
soil enzymes
& H2O
- Most volatilization when:
9 coarse or sandy-textured soils
9 low clay and low organic matter
(which adsorb NH4+)
9 dry alkaline surface
Nitrification
NH4+ → NO2- → NO3ammonium
nitrite
nitrate
- oxidation of N
* Autotrophic bacteria
• obtain energy from N oxidation
• Nitrosomonas
NH4+ → NO2- + energy
• Nitrobacter
NO2- → NO3- + energy
Nitrification (cont’d)
* Rapid in well-aerated,
warm, moist soils
• aerobic organisms
(O2 is required)
• little NO2- accumulation
* Acid-forming process
NH4+ +3/2O2→ NO2- + 2H+ + H2O
Nitrogen (nitrate?) Leaching
Æ Eutrification
Hypoxic Zone:
Gulf of Mexico/ MS River Delta
oxygen depleted zone
N Input Sources to Mississippi River
N Input (metric tons x 106)
Soil Mineralization
Fertilizer
Denitrification
N2
NO3-
Denitrification
Gaseous loss of N upon N reduction
NO3-
+ e+ e+ e+ e→ NO2- → NO → N2O → N2
nitric
oxide
nitrous
oxide
Denitrification (cont’d)
* Microorganisms:
• facultative anaerobes
- prefer O2 but will use NO3for a terminal e- acceptor
• heterotrophic
- use organic-C for energy source
* Denitrification enhanced by:
• low O2 (flooding)
• high O.M. (energy source)
• high NO3-
Denitrification (cont’d)
* Metabolic reduction: Denitrification?
(no N gas formation)
organisms
NO3-
NO3- → NH4+ → organic-N
- N is reduced for use in protein formation
Biological Nitrogen Fixation
N2 (organisms)→ NH4+
* Symbiotic relation between
bacteria and plants:
- legumes
+
- rhizobium
Nitrogen Fixation
Bacteria: Rhizobium genus
(species specific)
R. meliloti - alfalfa
R. trifolii - clover
R. phaseoli - beans
- bacteria require plant to function as Nfixers
Process:
nodule
Rhizobium
(b) Process:
organic-N
N2
Rhizobium
organic-C
C from plant photosynthesis Ô
N from fixation of N2
Ò
⇒ symbiosis
Quantity of N Fixed
Alfalfa and clover
≈ 100 - 250 kg N/ha/yr
(mature stand, good fertility & pH)
Beans and peas
less fixation but high protein food
with minimum N input
added N fertilizer
Æ lowered N fixation
Symbiotic Nodules - Nonlegumes
* Actinomycetes - Frankia +
Alders and other trees
Symbiotic - without nodules
* Azolla/Anabaena complex
blue-green algae (N-fixer) in leaves of
floating ferns
Nonsymbiotic N-fixation:
Free-living Organisms
* Bacteria (Azotobacter, Azospirillum) and
blue-green algae (Anabaena)
z
aerobic and anaerobic
z
small amounts: 5 - 50 kg/ha/yr
z
inhibited by available soil N
Microbial Immobilization
NH4+
NO3-
C-NH2
microbes