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

Nutrients must be in a specific form – specie - for use by organisms

leaching

+root uptake

microbial uptakesolid solution

Soil Organic Matter(CHONPS)

Nutrient Cycling: N, S, P, K

Primary Productivity

Leaves & Roots

SOM/Minerals

DecompositionHeterotrophic respiration

O

A

B

leaching of nutrients

Gas loss

plant nutrient uptake

Microbes

Minerals(P, S, K)

ions and molecules in solution (leaching)colloidal transporterosionrunoff

OU

T

gases and particulates to atmosphere (fossil fuel combustion (coal/oil); cycles; trees)

harvestingroot uptake

bedrock (1ry/2ry minerals in parent material)

Nutrients: natural & anthropogenic sources (IN) and outputs (OUT)

roots (exudates, biomass)

OU

T

IN

IN

N2fertilizers (chemical, manure, sludge)pesticideswet and dry deposition (acid rain, aerosols)plant tissue/residues

Fluxes (transformations)

Pools (sinks)(SOM/clays/oxides)

SOIL

The Nitrogen Cycle

+

Oxiczone

Suboxiczone

NO3-

Nitrogen evolution (gases)

NO

NO2-

N2O

N2

denitrification

nitr

ifica

tion

Organic-N NH3N-mineralization

immobilization

N o

xida

tion

stat

e

+5

-3

E h(m

V)

236

700

N-fixation

(-3)

(0)

(+1)(+2)

(+3)(+5)

imm

obili

zatio

n

Atmospheric N depositionFertilizer additions

(~90% of soil N) (-3)

Nitrification: NH3 to oxidized forms of N (NO2

-, NO3-)

(energy-releasing)

Oxidation of N2 does not occur except in a few

organisms (N-N triple bond)

N-fixation: enzyme-catalyzed reaction (occurs only in living organisms)

N2 (g) is prevalent in the air of soil pores

NO3- is the most stable

chemical form of N in aerated soil solutions

Denitrification: reduction of NO3

-

to reduced forms of N (N2) (energy-

consuming)

The Nitrogen Cycle

NH4+ sorption

The Sulfur Cycle

Oxiczone

Suboxiczone

Anoxiczone S2-

SO42-

oxid

atio

n/re

duct

ion

O2

A large variety of organic forms of S

M2+ + S2- = MS (pptation of metal sulfides)

metal sulfides tend to be “insoluble” (Hg, Ag, Cd, Cu, Pb, Zn, Fe(II))

Sulfate found in oxidized

environments

degradation of organic materials

(plant/animal)

H2S

The Sulfur Cycle

Oxidation states range from +6 to -2

Sulfur undergoes dramatic changes in redox cycling

reduced organic S (thiol)

sorption

In soil, SO42- originates from

• Organic matter mineralization

• Weathering of soil minerals

• Sulfur inputs from atmosphere

The Phosphorus Cycle

Mostly in organic P and inorganic P forms

PO43- is strongly retained by soils but can be transported in colloidal forms

P primarily as phosphate (PO43-), with an oxidation state of 5+

Al and Fe phosphates (acid mineral soils) Ca phosphates (non acid soils)

(variscite, AlPO4.2H2O ) (hydroxyapatite, Ca5(PO4)3OH )

PO43- sorption onto Al and Fe oxides is strong

Phosphate minerals (low solubility):

(strengite, FePO4.2H2O )

The Potassium Cycle

Sources: Primary and Secondary Minerals

Potassium as K+ with no changes in oxidation state

K+ flux controlled by CEC & chemical weathering ~ no organisms