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II. The Significance of Soil Polysaccharides

II. The Significance of Soil Polysaccharides

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SOIL POLYSACCHARIDES



I97



1958b; Webley et al., 1965) suggests that these gums will also occur in

soils, and so would be expected to increase the stability of natural soil

aggregates. As discussed below, mixtures of polysaccharides with properties which suggest that such microbial gums are included among them,

have been isolated from a wide range of soils. In some instances it has

been shown that the extracted polysaccharides are able to stabilize soil

aggregates (Rennie et al., 1954; Dubach et ul., 1955; Whistler and Kirby,

1956; Mehta et al., 1960).

Statistically significant correlations have also been demonstrated to

exist between estimates of polysaccharide content and the degree of

aggegation of the soil (Rennie et al., 1954; Chesters et al., 1957; Toogood

and Lynch, 1959; Acton et al., 1963b; Watson and Stojanovic, 1965;

Webber, 1965). The correlations obtained were not particularly close,

but this is not unexpected since the estimates of polysaccharide content

were somewhat crude (Acton et al., 1963a; Griffiths, 1965). In fact

Griffiths ( I965), who reviewed this subject critically, made the point that

satisfactory methods for the quantitative estimation of microbial polysaccharides in soils have not been developed, and consequently it has not

been possible to evaluate accurately the contribution of these materials

to aggregation. I t is also probable that only certain of the polysaccharides

present in soils are involved in aggregate stabilization, since Oades

( 1967b), who developed and used quantitative methods of quite high precision, found that the correlation of aggregate stability with both composition and total amount of neutral sugar constituents was no better than

that with other organic materials. This is consistent with the observation

that bacterially produced gums differ considerably in their effectiveness

(Clapp et al., 1962; J. P. Martin and Richards, 1963; J . P. Martin er al.,

1965) and some plant products such as cellulose exert no direct influence

on aggregate stability (Griffiths and Jones, 1965). It has also been shown

that although the composition of polysaccharides in a soil under old

pasture was similar to the composition of polysaccharides in the same

soil type which had been under a wheat-fallow rotation for 40 years, there

were differences between the treatments with respect to the amounts of

carbohydrates present in the soil and the ease with which these materials

could be removed from the soil (Swincer et al., 1968b). Thus the distribution of the polysaccharides within and around aggregates is probably

important (Williams et al., 1967), so that only a portion of an effective

polymer may actually be controlling stability.

Mehta et al. (1 960) showed that artificial aggregates stabilized by adding dextrans or soil polysaccharides to dispersed soils lost their stability



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G . D. SWINCER, J . M. OADES, AND D. J. GREENLAND



when treated with dilute (0.01 M ) sodium periodate and sodium borate

(pH 9.6). The periodate oxidizes sugars containing cisglycol groups

(Bobbit, 1956), and the partly oxidized polymers so produced are readily

degraded in alkaline solution into various nonpolymeric fragments

(Whistler and BeMiller, 1958). Polymers cleaved in this way can no

longer act as bridges between the soil particles forming an aggregate. The

natural aggregates examined by Mehta et al. (1960) did not lose their

stability when treated in the same way, thus indicating that other agents

were stabilizing the aggregates of these particular soils, possibly in addition to the polysaccharides. In soils of lower organic matter content,

treatment with dilute periodate has been shown to produce a marked decrease in aggregate stability (Greenland er al., 1961, 1962; Clapp and

Emerson, 1965; Deshpande et al., 1968). Furthermore, Harris er al.

(1963) and Watson and Stojanovic (1965) have shown that the increase in

the stability of aggregates in certain soils incubated with added organic

materials is largely associated with the production of periodate-sensitive

materials.

This evidence together with the other data discussed above leaves little

room for doubt that polysaccharides in soils exert an important influence

on the stability of their physical structure. It is, however, also clear that

other organic and inorganic materials can stabilize soil aggregates, and,

where such materials are present, the polysaccharides may be of little

additional benefit. The polysaccharides are probably of greatest importance in cultivated soils of relatively low total organic matter content

(Greenland er al., 1962).

In most of the studies discussed above, the relationship considered was

that between polysaccharides and aggregate stability as determined by

wet sieving. Aggregate stability is important in most soils except very

sandy ones, since it is only by virtue of aggregate formation that a

satisfactory continuity of pores in the soils is maintained, whereby adequate air and water movement can occur for optimum plant growth. In

some instances more direct measurements have been made of the changes

that occur in the physical properties of soils when polysaccharides are

added or removed. Greenland er al. ( 1 962) showed that the permeability

of beds of aggregates could be reduced by periodate treatment; and Clapp

and Emerson ( 1965) showed that clays slaked and dispersed more readily

after this treatment. Allison (1947), however, found that production of

microbial gums in submerged soils could lead to an undesirable reduction

in permeability because the microbial products were blocking some of the

more important coarse pores.



SOIL POLYSACCHARIDES



199



Ill. Studies on Soil Polysaccharides



A. INTRODUCTION



The polysaccharide materials that have been extracted from soils,

purified, and analyzed, often in considerable detail, have usually represented only a small proportion of the whole carbohydrate fraction of the

soil organic matter. Thus, although the main stimulus for these investigations was derived from the observation that polysaccharides exert a

favorable influence on soil physical properties, the polysaccharides exerting this influence could very well be those most firmly held by the soil

colloids, and which were therefore neither extracted nor analyzed.

However, the techniques that have been used and the results obtained

undoubtedly form an important basis for understanding the nature, origin,

and function of the majority, if not all, of the carbohydrate polymers

present in soils.



B. EXTRACTION

OF POLYSACCHARIDES

FROM SOILS

1 . Introduction



Ideally an extractant for soil polysaccharides should, in order of

priority: (a) be nondegradative, (b) give a sufficiently complete extraction

for the materials extracted to be representative of the total, (c) be equally

effective for all soils, and (d) extract selectively carbohydrate materials.

None of the many extractants used fulfils these requirements. The main

aim has often been simply to isolate from the soil a sample of relatively

pure polysaccharide material that can be used for chemical and physicochemical characterization.

2. Assessment of Extraction Yield and Polymer Degradation

a. Extraction Yield. Both colorimetric and gravimetric methods have

been used to measure the proportion of the soil carbohydrates brought

into solution under different sets of extraction conditions. Gravimetric

methods (Rennie et a f . , 1954; Chesters et al., 1957; Salomon, 1962;

Acton et af., 1963a,b) are of very limited value because they can be applied only to purified polysaccharide materials; there is a distinct possibility both of losses of polysaccharide materials during purification and of

incomplete removal of contaminants (Acton er a f . , 1963a; Dormaar,

1967). In any case, for determination of the proportion extracted, the

results must be related to values of “total soil carbohydrate” obtained

by another method.



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G. D. SWINCER, J. M. OADES, A N D D. J . GREENLAND



When preceded by complete hydrolysis into monosaccharides and removal of interfering compounds from the hydrolyzate, colorimetric

methods can give useful estimates of the carbohydrate content of the soil,

either before or after extraction, and of crude extracts. The colorimetric

methods are normally applicable only to single classes of monosaccharides, such as the hexoses, pentoses, uronic acids, or hexosamines, and

even within a particular class none of the methods gives the same color intensity for equimolar concentrations of the different individual monosaccharides. Moreover, optimum hydrolysis conditions vary at least from

one class of sugars to another, and probably also from one individual

sugar to another. Clearly, therefore, the most precise determinations of

the proportion of the total carbohydrates extracted would require the

measurement of the amount of each individual sugar in both the soil and

the extract.

Individual sugars, or the different classes of sugars, have been measured in soils and purified soil extracts (Graveland and Lynch, 1961;

Thomas and Lynch, 196 1; Ivarson and Sowden, 1962; Sowden and Ivarson, 1962; Gupta et al., 1963; Gupta and Sowden, 1965; Cheshire and

Mundie, 1966). Only Parsons and Tinsley (1961), Lynch et al. (1957,

1958), and Swincer et al. (1 968a,b) have attempted to relate the amounts

extracted with the amounts originally present in the soil or left in the

soil residue after extraction.

The results of Parsons and Tinsley are probably not accurate, particularly with respect to uronic acids, as the authors themselves admit. Although Lynch et al. (1 957, 1958) claimed to have measured the recovery

of the original carbohydrates in various extracts by separating and estimating seven different sugars, the yields reported were undoubtedly too

high as a result of the low values for “total soil carbohydrate” that must

have accompanied the very mild hydrolysis conditions used.

b. Polymer Degradation. The detection and evaluation of damage to

the polysaccharide molecules during the extraction process is by no

means easy. Positive and conclusive evidence against changes in the

carbohydrate polymers during extraction is virtually unobtainable because it is not yet possible to know the properties of these molecules before isolating them from the soil. Any information that can be obtained

must be either indirect or of a negative kind.

Whitehead and Tinsley ( 1964) made a useful indirect assessment of the

likely degradative effect of their extraction procedure on soil polysaccharides by subjecting several other natural polymers (starch, alginic

acid, chitin, cellulose, gluten) to the same treatment.

Bernier (1 958a) compared the viscosities of polysaccharides extracted



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