SOIL 120 Lab Exer 3 S Oxidation

8/4/2019 SOIL 120 Lab Exer 3 S Oxidation

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LAB EXERCISE 3LAB EXERCISE 3

Oxidation of

Sulfur in Soil


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Oxidation of Sulfur in SoilOxidation of Sulfur in Soil

y 1 OBJECTIVE: To determine the oxidation of elemental sulfur tosulfate in soil, along with a concomitant decrease in soil pH due to theproduction of protons.

y

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2 METHODS:a. Set up 3 soil incubations: i) unamended control soil; ii) soil of varyingOM contents and sulfur;

b. Adjust soil moisture of all incubations to a value close to field

capacity;

c. Measure initial soil-sulfate concentration and initial soil pH;

d. Incubate all treatments for 4 weeks;

e. Evaluate sulfur oxidation in all three treatments by analyzing for

soil-sulfate and soil pH at weekly intervals


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3. MECHANISM3. MECHANISM

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In addition to enumerations, soil microorganismscan also be analyzed through measurements ofphysiological activity, where the focus is onenzymes or their activity.

y Enzymatic reactions can occur in soil bothbiotically and abiotically. Thus, in addition tometabolism of substrate by enzymes in living cells,evidence exists that cell-free enzymes can alsoparticipate in chemical reactions in the soil. Some

enzymes are known to be active extracellularly,while others are believed to be active only inliving cells.


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y Sulfur oxidation can occur in soil throughthe activity of chemoautotrophic orheterotrophic organisms, and the outcomeof sulfur oxidation can be harmful or

beneficial.y

y During strip mining, exposure of previouslysubsurface sediments that contain sulfides to

more aerobic environments can cause sulfuroxidation, a pH decrease, and subsequentmobilization of heavy metals into miningeffluents.


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y But in the desert, fertilizers routinely

contain elemental sulfur in deliberate

attempts to cause sulfur oxidation and

concomitant pH decreases in alkalinedesert soils (Maier et al., 2000). As sulfur

oxidizing organisms oxidize elemental

sulfur in soil to sulfate, two by-productsare formed: SO4

2- and H+ (acid):

SO + 3/2 O2 + H2O H2S O 4


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Chemoautotrophic organisms, such as

Thiobacillus thiooxidans, use theenergy released by the given reaction

to fix carbon dioxide.

Chemoautotrophic S oxidizers may beinhibited by the presence of easily

degraded carbon sources . Thus, soils

high in oxidizable organic matter may

exhibit low levels of chemoautotrophicsulfur oxidation.


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y A number of heterotrophic organisms can alsooxidize elemental sulfur according to the abovereaction (Pepper and Miller, 1978).

y However, this oxidation occurs cometabolically,

and the organisms derive no energy from thereaction, although the products of the reactionare the same with sulfate and H+ being produced.Because these organisms are heterotrophic, no Soxidation occurs unless metabolizable carbon is

present.y Note that the effect of glucose on S oxidation

depends on whether oxidation is by autotrophicor heterotrophic organisms (Figures 7-2 and 7-3).


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y Yet, another outcome is possible given

glucose addition.

If other, non-sulfur oxidizing organisms are

also stimulated by the easily metabolized

substrate, a substantial amount of the

produced sulfate may be immobilized inmicrobial biomass, and the measured soil

sulfate concentrations will be reduced


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y Because sulfate is produced by oxidation ofsulfur, the rate of sulfur oxidation can bemonitored by measuring changes in the soil

sulfate over time.

y Sulfate is easily extracted from the soil as itis anionic and not retained well on negatively

charged soil particles. The use of an NaClextractant solution is to aid in dispersing thesoil particles, exposing their surface to thesolution.


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y After the extracted sulfate is separated

from suspension via filtration, the sulfate

is precipitated by the addition of BaCl2:

BaCl2 +SO42- BaSO4 +2Cl

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y Because BaSO4 is an opaque solid, the

solution becomes turbid.


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As acid is also produced during sulfur

oxidation, progress of the reaction insoil can also be measured by

following changes in the soil pH.

One may measure the soil pH over

time using a 1 : 2 suspension of soil in

0.01 M CaCl2, a solution commonlyused for this purpose.

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SOIL 120 Lab Exer 3 S Oxidation