Nutrients: Nitrogen (N), Sulfur (S), Phosphorus (P), Potassium (K)
Essential for all life, their availability (or lack thereof) controls the distribution of flora and fauna
Look into their:
Sources
Pools (sinks)
Fluxes
root uptake
+microbial uptake leaching
- solution
- solid
Nutrients must be in a specific form – specie - for use by organisms
Soil Organic Matter
Nutrient Cycling: N, S, P, K
(CHONPS)
Minerals
(P, S, K)
Primary Productivity
O
Leaves & Roots
A
Decomposition
Heterotrophic respiration
B
Gas loss
SOM/Minerals
Microbes
leaching of nutrients
plant nutrient uptake
Nutrients: natural & anthropogenic sources (IN) and outputs (OUT)
N2
fertilizers (chemical, manure, sludge) pesticides gases and particulates to atmosphere
(fossil fuel combustion (coal/oil); cycles; trees)
wet and dry deposition (acid rain, aerosols) plant tissue/residues harvesting root uptake
SOIL
Pools (sinks)
(SOM/clays/oxides)
Fluxes (transformations)
- ions and molecules in solution (leaching)
- roots (exudates, biomass)
colloidal transport erosion bedrock (1ry/2ry minerals in parent material) runoff
The Nitrogen Cycle
+
The Nitrogen Cycle
Denitrification:
reduction of NO3 to reduced forms of N (N2) (energyconsuming)
- -
- -
NO3 is the most stable chemical form of N in aerated soil solutions
N2 (g) is prevalent in the air of soil pores
Atmospheric N deposition
Fertilizer additions
Nitrogen evolution (gases)
700 236
(+5)
NO3
+5 -3
Oxic zone
-
(+3)
NO2
-
(+2)
NO
(+1)
N2O
(0)
N2
N-fixation
Suboxic zone
N-mineralization immobilization
NH3
(-3)
Organic-N
+ sorption
NH4
(-3)
(~90% of soil N)
N-fixation: enzyme-
catalyzed reaction (occurs only in living organisms)
Oxidation of N2 does not occur except in a few organisms (N-N triple bond)
Nitrification: NH3 to oxidized
- -
- -
forms of N (NO2 , NO3 )
(energy-releasing)
The Sulfur Cycle
The Sulfur Cycle
Sulfur undergoes dramatic changes in redox cycling
Oxidation states range from +6 to -2
O2
sorption
Sulfate found in oxidized environments
2-
Oxic zone
SO4
Suboxic zone
degradation of organic materials
(plant/animal)
A large variety of organic forms of S
reduced organic S
Anoxic zone
(thiol)
S2-
H2S
M2+ + S2- = MS
(pptation of metal sulfides)
metal sulfides tend to be “insoluble”
(Hg, Ag, Cd, Cu, Pb, Zn, Fe(II))
2-
In soil, SO4 originates from
• Organic matter mineralization
• Weathering of soil minerals
• Sulfur inputs from atmosphere
The Phosphorus Cycle
3-
P primarily as phosphate (PO4 ), with an oxidation state of 5+
Mostly in organic P and inorganic P forms
3-
PO4 is strongly retained by soils but can be transported in colloidal forms
3-
PO4 sorption onto Al and Fe oxides is strong
Phosphate minerals (low solubility):
Al and Fe phosphates (acid mineral soils)
(variscite, AlPO4.2H2O )
Ca phosphates (non acid soils)
(hydroxyapatite, Ca5(PO4)3OH )
(strengite, FePO4.2H2O )
The Potassium Cycle
Potassium as K+ with no changes in oxidation state
K+ flux controlled by CEC & chemical weathering ~ no organisms
Sources: Primary and Secondary Minerals