Transformation of Sulphur in Soils

After reading this article you will learn about the transformation of organic and inorganic sulphur in soils.

Transformation of Inorganic Sulphur:

This form of sulphur typically occurs as its three groups: water soluble sulphate sulphur; adsorbed sulphate sulphur; and elemental and sulphide sulphur.

The amount of water soluble sulphate sulphur varies with seasons, moisture content, soils types, rate of mineralisation of organic sulphur and other environmental conditions etc. Sulphate sulphur undergoes rapid leaching losses from the surface soil because of its anionic nature.

However, such leaching losses of sulphate sulphur varies with nature of cations present in the soil solution, being greater loss due to leaching when soil solution contains an adequate amount of monovalent cation like Na⁺ , K⁺ etc. as compared to divalent cations. In addition, in acid soils such leaching loss of sulphate sulphur is less which may be due to presence of an adequate amount of exchangeable Al and Fe in soils.

Adsorption of sulphate sulphur depends on various factors like nature of soils, organic matter content, oxides and hydrous oxides of Fe and Al, CaCO₃ content, temperature, clay minerals, cations, anions, soil pH, concentration of sulphate, time of contact etc.

opilThis fraction of sulphur also plays an important role in contributing sulphur to the crops because it is protected from various harmful processes like leaching, other losses etc.

Soils containing higher amount of 1: 1 type of clay minerals adsorb more sulphate sulphur than that of 2: 1 type of clay minerals. Besides, the amount of sulphate adsorption is usually high in soils containing higher amount of oxides and hydrous oxides of Fe and Al and organic matter. Soil reaction affects the sulphate adsorption, being greater in strong acid soils.

Soils, containing other anions like phosphate, nitrate, chloride, molybdate etc., exhibit very little sulphate adsorption. Sulphate is usually considered to be weakly held with the force of retention decreasing in the order of: Hydroxyl > Phosphate > Sulphate = Acetate > Nitrate = Chloride

A simple scheme for the sulphate adsorption as affected by different factors is depicted below:

The adsorption of sulphate sulphur has been found to be increased with the decrease in soil pH (i.e. in acid soils) which may be due to effective neutralisation of replaced OH^– ions. Elemental sulphur (S⁰) and sulphides are not found in well-drained soils.

These are found in waterlogged soils where bacterial reduction takes place. Elemental sulphur is not soluble in water and it is a crystalline solid.

When this elemental sulphur is applied to soils, it is oxidised to sulphate form by soil micro-organisms as follows:

Elemental sulphur provides the highest amount of sulphur which may be available to the plant after transformation to its sulphate form. Of course, its usefulness as a plant nutrient depends on the rate of its oxidation.

Oxidation of elemental sulphur, sulphides and other inorganic sulphur compounds takes place both chemical and biological processes. The chemical process is very slow and hence it is little importance in sulphur oxidation as compared to microbiological process of sulphur oxidation.

However, the rate of sulphur oxidation due to microbiological process depends upon the following factors:

(i) Microbial population in the soil

(ii) Nature of sulphur source

(iii) Other environmental conditions in the soil e.g. temperature, moisture, organic matter etc.

There are usually two types of bacteria involved in the oxidation of sulphur: Chemolithotrophic—utilise energy released from the oxidation of inorganic sulphur compounds (I) and photo-lithotrophic—carry out photosynthetic carbon fixation using sulphide and other sulphur compounds as “oxidant sinks” (II).

The rate of sulphur oxidation, however, varies with soils having differences in types and numbers of sulphur oxidising micro-organisms. The most numerous sulphur oxidisers are heterotrophic bacteria followed by facultative, then obligate autotrophic Thiobacilli, and finally green (Chlorobium) and purple (Chromatium) bacteria, both are photo lithotrophic.

Transformation of Organic Sulphur:

This fraction of sulphur constitutes about 80-90 per cent of the total sulphur present in most Indian soils. Realising its occurrence in soils, the transformation of this form of sulphur is considered as the most important mechanism in supplying sulphur to the plants. Biological sulphur cycle showing major chemical pools of sulphur proposed by Trudinger (1979) is depicted in Fig. 21.17.

Transformation of different paths of sulphur cycle is carried out by variety of micro­organisms as follows:

Path 2: Carried out by dissimilatory reducers e.g. Desulphovibrio, Desulphotomaculum.

Path 1 and 3: Assimilatory reducers like bacteria, fungi, algae and plants.

Path 5: Carried out by Desulphuromonas

Path 4, 6 and 8: Carried out by chemolithotrophs (Thiobacillus, Beggiatoa) and photo lithotrophs (Chlorobium and chromatium).

Path 7 and 9: Carried out by heterotrophic micro-organisms, and chemo and photolithotrophs.

When different organic waste materials like plant and animal residues are incorporated into the soil, they undergo transformation by different micro-organisms releasing small amount of sulphate sulphur into the soil. However, majority of the total sulphur in these residues are present as its organic form and eventually becomes part of soil humus. It is evident that the release of sulphur from humus fraction is very slow.

Some simple organic sulphur compounds like sulphur containing amino acids are released and subsequently assimilated by plant roots. But it has less importance in plant nutrition because of its instability in soils. The process involved for the conversion of organic fractions of sulphur into inorganic forms by some micro-organisms, is known as “mineralisation”.

Mineralisation:

For the decomposition of a wide variety of organic waste materials (mainly plant and animal residues) various micro-organisms are involved in the oxidation of organic sulphur compounds to inorganic sulphate sulphur. Since the process of such mineralisation is a microbial and hence any factor which can affect the growth and activity of concerned micro-organisms ultimately modify the mineralisation of sulphur.

However, various factors like mineral content of organic materials undergoing decomposition, temperature, moisture, pH, presence or absence of plants, availability of food supply etc. are known to affect the rate of mineralisation and subsequent release of inorganic sulphur in the soil solution.

It is evident that smaller amount of inorganic sulphate sulphur released from the organic materials containing low amount of initial sulphur content and it may so happen that organic material containing less than 0.15% sulphur incorporated into the soil, the amount of sulphur content in the soil reduces which may be due to immobilisation of sulphur.

Immobilisation of sulphur takes place in soils when the ratio of either C or N to S is very wide. If C/S weight ratio is at or below 200: 1, then mineralisation of sulphur takes place and above this ratio, immobilisation occurs that means most of the sulphate-sulphur will be associated with organic forms like soil humus, microbial cells and by-products of microbial synthesis.

Immobilisation occurs with wide C/S ratios because of conversion of a larger amount of carbon into microbial biomass with a resultant higher requirement for sulphur. Fresh organic materials usually have C/S ratio of about 50: 1.

Temperature and moisture affect the rate of mineralisation, being higher with increase in temperature up to 40°C. The rate of mineralisation, however, has been found to be more at 60% of water holding capacity. Soil pH also influences the rate of mineralisation which is directly proportional to pH up to 7.5.

At pH values in soils, more than 7.5, the rate of mineralisation enhanced more rapidly suggesting an involvement of chemical hydrolysis. Neutral soil pH is believed to be more useful in encouraging microbial activity which results mineralisation of sulphur.