BIOFERTILIZERS AS COMPLEMENTS TO SYNTHETIC AND ORGANIC FERTILIZATION - Fertilizers: Components, Uses in Agriculture and Environmental Impacts - page 164

Sulfur Solubilization
Sulfur is the fourth most important element for plant growth after nitrogen, phosphorus
and potassium. Sulfur importance is equal to that of the nitrogen in terms of protein synthesis,
while in terms of assimilation by crops is greater than that of the phosphorus (Vidyalakshmi
& Sridar, 2007). The original source of sulfur in the earth was igneous rocks, primarily
igneous pyrite (FeS 2 ). Since then, the amount of sulfur in the environment has increased due
to volcanic activity and weathering of the earth´s crust in an oxygen atmosphere (Hoffman et
al. , 1998). Current sulfur sources come from the weathering of soil minerals, atmosphere and
sulfur already fixed in the organism's biomass.
The essential processes of the sulfur cycle in nature are:
1) Mineralization of organic sulfur into inorganic forms, such as hydrogen sulfide
(H 2 S), elemental sulfur, as well as sulfide minerals.
2) Oxidation of hydrogen sulfide, sulfide, and elemental sulfur (S) to sulfate (SO 4 2- ).
3) Reduction of sulfate to sulfide.
4) Incorporation of sulfide into organic compounds (including metal-containing
derivatives).
Transfer of sulfur between organic and inorganic sources within the sulfur cycle is caused
entirely by the activity of soil biota, particularly by the microbial biomass, which has the
greatest potential for mineralization and the subsequent transformation of the oxidation state
of sulfur.
Sulphate is the main form of sulfur assimilated by plants, so this element has to be first
converted into this salt to be assimilated by plants (Mahendra, 1988). Sulfur oxidation leading
to the formation of sulfate is the most important process in the sulfur cycle that leads to
increased soil fertility. Additionally, soil acidification resulting from this oxidation process,
help solubilizing nutrients and improve fertility in alkaline soils (Wainwright, 1984). The
reduced inorganic sulfur compounds are oxidized exclusively by prokaryotes, although fungi
such as Alternaria tenius, Aureobasidium pullulans, Epicoccum nigrum, Scolecobasidium
constrictum, Myrothecium cinctum, Aspergillus and a number of species of the genus
Penicillium are capable of oxidizing elemental sulfur and thiosulfate (Vidyalakshmi et al.,
2009). On the other hand, the oxidation of sulfur in members of the genus Eukarya is
conducted by bacterial lithoautotrophic endosymbionts (Nelson & Fisher, 1995).
Prokaryotes have the ability to oxidize hydrogen sulfide, sulfur, sulfite, thiosulfate and
different polythionates under alkaline, neutral or acidic conditions (Harrison, 1984; Sorokin et
al., 2001). The sulfur-oxidizing aerobic prokaryotes belongs to genera Acidianus,
Acidithiobacillus, Aquaspirillum, Aquifex, Bacillus, Beggiatoa, Methylobacterium,
Paracoccus, Pseudomonas, Starkeya, Sulfolobus, Thermithiobacillus, Thiobacillus and
Xanthobacter, which are basically mesophilic microorganisms. Photoautotrophic, anaerobic,
sulfur-oxidizing bacteria are primarily neutrophilic and mesophilic and belong to genera such
as Allochromatium (formerly Chromatium ), Chlorobium , Rhodobacter , Rhodopseudomonas ,
Rhodovulum and Thiocapsa (Friedrich et al., 2001).
Sulfur-oxidizing microorganisms are primarily Gram-negative bacteria of the genera
Thiobacillus , Thiomicrospira and Thiosphaera , although some heterotrophic bacteria such as