Effect of periodate treatment of soil on carbohydrate constituents and soil aggregation

Soil from a field under long-term grass was treated with 0.02m sodium periodate for various periods up to 1176 h, followed by 0.1 d sodium tetraborate for 6 h. This destroyed an increasing proportion of microaggregates >45 μm and carbohydrate. After periodate treatment for 6 h about 70% of the soil sugars remained in the residue as measured by reducing sugar content and about 67% as individual sugars measured by gas-liquid chromatography. After 48 h the reducing sugar content was about 45%. An inverse linear relationship was established between the proportion by weight of microaggregates >45 μm and residual carbohydrate. The residual carbohydrate showed an enrichment in sugars commonly found in plant materials; glucose, arabinose and xylose, suggesting that the microbial carbohydrate had been preferentially destroyed. When the concentration of the periodate was increased to 0.05 m the residue contained about 50% of the original carbohydrate after 6 h treatment, and 25% after 48 h and an additional increase of about 10% in the proportion by weight of particles in the <45 μm range. These results throw doubt on the validity of assumptions made in a number of studies about the limited extent to which soil polysaccharide is involved in aggregation.     

THE ORIGIN OF SOIL POLYSACCHARIDE: TRANSFORMATION OF SUGARS DURING THE DECOMPOSITION IN SOIL OF PLANT MATERIAL LABELLED WITH 14C

Incubation of soil with ¹⁴C-rye straw for 448 days resulted in the evolution of about 50 per cent of the carbon of the substrate as CO₂ The two main sugars of the straw, glucose and xylose, were degraded to approximately the same extent (70 per cent). The same results were obtained whether the soil was derived from granitic or basic igneous parent material. There was very little transformation of the substrate to galactose, mannose, arabinose, rhamnose, or fucose, and a much slower rate of degradation than with soil incubated with ¹⁴C-glucose over a similar period. Hydrolysis of the soil samples by a preliminary treatment with 5 N H₂SO₄, before treatment with 24 N H₂SO₄, followed by heating with N H₂SO₄ did not release significantly greater amounts of sugar than treatment with 24 N H₂SO₄ and N H₂SO₄ alone. Separate analysis of the hydrolysates showed that 90 per cent of each of galactose, mannose, arabinose, xylose, rhamnose, or fucose had been extracted by 5 N H₂SO₄, but only 50 per cent of the glucose. Fractionation of the straw-soil mixture after 224 days incubation showed that the specific activity of the glucose was higher in the humin fraction than in the fulvic acid, as would be expected if the remaining ¹⁴C were still in the form of unchanged plant material. This evidence that plant polysaccharide persists in soil could explain the presence of much of the xylose in the soil organic matter.     

Saccharides in some Japanese paddy soils

Monosaccharides released by acid hydrolysis from paddy field soil, from the light and the heavy fraction of soil, front some plant fragment were determined using automated anio-exchange chromatography.

Between 5 and 12 per cent of the organic carbon was present as saccharides.

The monosaccharide composition of the different soils was very similar, in spite of differences in the absolute amount of saccharides present. The amount of the various monosaccharide in the whole soil was found to be in the order glucose»xylose galactose, mannose, arabinose rhamnose ribose.

The monoccharide composition of the soils showed a marked contrast to that of the rice ra8ment, and partially decomposed plant remains taken from the soil. Glucose, xylose, arabi-the predominant saccharides in the rice fragments and the plant remains, while the amounts of galactose, mannose, rhamnose were negligibly small.

It was found that the proportion of galactose, mannose, rhamnose and ribose in the heavy fraction Of soil was greater than that of glucose, xylose, and arabinose

The present observation was in agreement with the view that soil sauharides comprised Pentoses originates in plant materials.

The molar ratio of xylose to mannose was calculated to show the characteristics of the mono-saccharide composition of soils and some plant muerials.     

On the effect of tetraborate ions in the generation of colored products in thermally processed glycine-carbohydrate solutions

The effect of tetraborate ions on Maillard browning was investigated in a series of monosaccharide-glycine reactions in aqueous bis-tris buffer at pH 7.2. Addition of borax (sodium tetraborate) in catalytic amounts led to enhanced browning measured by absorbance at 420 nm in the order xylose > arabinose > galactose approximately = fructose > ribose > mannose > rhamnose, and the degree of browning with borax was uniformly greater than that produced by phosphate on an equimolar basis. A mechanism is proposed for borax catalysis in which monosaccharide-borate complexation shifts carbohydrate equilibria to favor open-chain (carbonyl) forms, thereby enhancing the rate of the Maillard reaction.     

Carbohydrate composition of hay and maize soils and their possible importance in soil structure

The soils of three fields under continuous maize culture for 2,4 and 15 yr, and the soil of an adjacent hayfield were analyzed for carbohydrates by gas chromatography. A sharp decrease (30%) in total carbohydrates, paralleled by a smaller decrease in organic matter (9%), was observed after 2 yr of maize cultivation. The 4-yr maize soil showed an upward trend in both organic matter and carbohydrate content but this was subsequently reversed. The 15-yr highly-compacted maize soil had 40% less organic matter and 40% less carbohydrates than the hay soil. When compared to the well structured 2-yr maize soil, this compacted soil had less arabinose, xylose, glucose, galactose and mannose; carbohydrates which are probably of plant root origin. These sugars are likely to be involved in the structural stability of soil aggregates. Conversely, neither the amino sugars nor the uronic acids seemed to contribute to good soil structure.     

Changes in the amount and distribution of neutral monosaccharides of savanna soils after plantation of Pinus and Eucalyptus in the Congo

In the Congo, near Pointe-Noire, Pinus and Eucalyptus were planted on the savanna for 30 years. We have characterized the effects of this change on land-use on the composition of carbohydrates in whole soil and particle-size fractions of the soil. Carbohydrates represent variable proportions of the total soil organic carbon (TOC) of various particle size fractions. The largest proportions of sugar-C were found in the savanna soil with as much as 250 mg g⁻¹ TOC in the coarsest plant remains and approximately 190 mg g⁻¹ TOC in the finest organo-mineral fractions, whereas there was always less sugar in plantation soils. The monosaccharide xylose and mannose have different distributions: xylose appears to be the marker of the vegetal inheritance, whereas the dominance of mannose in the clay fraction bears the signature of current microbial sugar synthesis. The quantitative and qualitative evolution of the whole soil carbohydrates was studied as a function of plantation age. Carbohydrate-C represents 131 mg g⁻¹ of the soil organic carbon in the savanna soil, but decreases to an average value of 75 mg g⁻¹ in plantations more than 6 years old. This appears to be due mainly to the stimulation of the mineralization of the glucose, which represented 60% of the total sugars in savanna soil and only 45–48% in tree plantations. The ratio [arabinose + galactose + fucose]/[rhamnose + xylose], which is the largest in the oldest plantations, is significant for evaluating the replacement of carbohydrates of the original grass savanna by those of the trees.     

THE DISTRIBUTION OF LABELLED SUGARS IN SOIL PARTICLE SIZE FRACTIONS AS A MEANS OF DISTINGUISHING PLANT AND MICROBIAL CARBOHYDRATE RESIDUES

Soils of the Countesswells and Insch series incubated with ¹⁴C labelled glucose or plant materials have been separated into clay (< 2 μm), silt, (2–20 μm), fine sand (20–250 μm) and coarse sand (>250μm) fractions and the distribution of individual labelled and unlabelled sugars was determined in each fraction. Both soils contained about 10–15 per cent clay, 18–23 per cent silt and about 60 per cent fine and coarse sand. For all soil samples the concentrations of sugars were usually greatest in the clay, slightly less in the silt, with values in the sand fractions being five or ten times lower, except when fresh plant material was present. In ¹⁴C glucose amended Insch soil, 55 per cent of the radioactivity in sugars (predominantly hexoses) occurred in the clay, 36 per cent in the silt, 3 per cent in the fine sand and 6 per cent in the coarse sand after 28 days incubation. For the Countesswells soil the values were 55, 42, 2 and 1 per cent respectively. In ¹⁴C ryegrass amended soil before incubation. 77 per cent of the radioactivity in sugars (predominantly glucose, arabinose and xylose) was in the coarse sand. After one year's incubation this had fallen to 59 per cent. In soil amended with ¹⁴C cereal rye straw the distribution of radioactivity in sugars after four years incubation was: clay, 21 per cent; silt, 43 per cent; fine sand, 21 per cent; coarse sand, 4 per cent. These distributions were compared with that of the naturally occurring sugars: clay, 31–42 per cent; silt, 40–43 per cent; fine sand, 3–11 per cent; coarse sand, 12–20 per cent.     

THE ORIGIN OF THE PENTOSE FRACTION OF SOIL POLYSACCHARIDE

Incubation of soil with monosaccharide for 224 days resulted in the evolution of about 80 per cent of the substrate carbon as CO₂ and the transformation of 3 per cent to soil sugars whether the substrate was ¹⁴C-glucose or xylose and whether the soil was pH 7.4 or pH 5.0. There was no detectable change in the total amounts of individual sugars in the soil during incubation. ¹⁴C-glucose and xylose gave the same distribution of radioactivity among the soil sugars : hexoses and 6-deoxy-hexoses were initially well labelled, with glucose having twice the specific activity of the other sugars. As the incubation progressed some activity appeared in the pentoses (the activity in xylose became very low within the first 14 days of the ¹⁴C-xylose incubation) and that in the hexoses slowly declined, with glucose no longer predominant. Nevertheless after 448 days the hexoses were still 3–4 times more radioactive than the pentoses. The activity in rhamnose did not decline with time so that eventually it became the most strongly labelled sugar. Incubation of soil with glucose and ¹⁴C-acetate showed very little transformation of the acetate to sugars indicating that glucose is not metabolized to C₂ compounds before it is transformed to other sugars. Ammo-acids in soil incubated for 7 days with ¹⁴C-glucose had much lower levels of radioactivity than hexoses or 6-deoxy-hexoses. It is concluded that if soil pentose originates by microbial synthesis it must accumulate slowly by a long process of selective decomposition of a mixture of polysaccharides.     

Persistency and monosaccharide composition of polysaccharides of soil which received no plant materials for a certain period under field conditions

Abstract

The monosaccharides in acid hydrolysates were monitored in soil which was packed in a bag made of glass microfiber paper and buried in upland and paddy fields for up to 36 months. During the initial flush of decomposition, all constituent monosaccharides except for non cellulosic glucose were found to decrease. The amounts which disappeared were greater than the water extractable saccharides of the air dried soil or ground sample of the air dried soil. After the flush of decomposition, the changes in mannose, galactose, fucose and rhamnose were small, whereas cellulosic glucose, arabinose and xylose continued to decline in the upland field soil. The soil saccharides are classified into six groups and their relative persistency is discussed.

The monosaccharide composition did not change markedly, but the proportion of monosaccharides relating to plant materials declined with time after incubation. The molar ratios of hexoses to pentoses, deoxyhexoses to pentoses, and non-cellulosic glucose to cellulosic glucose increased gradually, whereas a gradual decline in the ratio of xylose to mannose was observed when the soil received no plant materials under field conditions.     

Changes in microbial biomass C, soil carbohydrate composition and aggregate stability induced by growth of selected crop and forage species under field conditions

The effects of growth of various crop and forage species on microbial biomass C, soil carbohydrate content and monosaccharide composition, and mean weight diameter (MWD) were investigated in two field experiments. One experiment was conducted over one growing season (4 months) whereas the other had been conducted for three consecutive growing seasons (32 months). In the four-month experiment, aggregate stability (estimated as MWD) of soil from experimental plots followed the order Italian ryegrass > prairie grass > phacelia = pea = maize. At the 32-month site the order was perennial ryegrass > annual ryegrass > perennial white clover = barley. At both sites crops with the greatest root mass and root length density had the greatest effect on increasing MWD. In all cases, rhizosphere soil had a significantly higher microbial biomass and MWD than non-rhizosphere soil. However, organic C, total content of acid-hydrolysable carbohydrate and content of individual monosaccharides in acid hydrolysates were similar in rhizosphere and non-rhizosphere soil. The fraction of soil carbohydrate extractable with hot water (representing about 6-8% of the total carbohydrate content) was significantly higher in the rhizosphere soil. This fraction has a galactose plus mannose over arabinose plus xylose ratio of 2.1–2.3 indicating that it was predominantly of microbial origin. It is suggested that the carbohydrate fraction extractable with hot water is made up of exocellular microbial polysaccharides that are involved in stabilizing soil aggregates in the rhizosphere. By comparison with arable crop species, grass species have a larger root mass and root length density, and therefore a higher microbial biomass and larger production of carbohydrate extractable with hot water. As a result they have a more marked effect on improving soil aggregate stability.     

THE EFFECT OF TEMPERATURE ON THE MICROBIAL TRANSFORMATION OF [14C] GLUCOSE DURING INCUBATION IN SOIL

When two different soils were incubated after the addition of [¹⁴C] glucose in the dark at winter temperatures or at 5°C in the laboratory and then hydrolysed, radioactivity was detected in all seven common soil sugars except arabinose. In contrast, in incubations at 20°C, little radioactivity was found in the xylose. Examination of the microflora showed that the number of viable bacteria was one-tenth at the lower temperatures, whereas the numbers of yeasts, fungi, and actinomycetes were unaffected. Analysis of cultures of representative microbial isolates showed that none of the fungi or actinomycetes and only 3 per cent of the bacteria synthesized xylose, compared with 85 per cent of the yeasts. It is concluded that when these soils are incubated at low temperatures xylose is synthesized principally by yeasts.     

Determination of neutral sugars in soil by capillary gas chromatography after derivatization to aldononitrile acetates

We have quantified ribose, rhamnose, arabinose, xylose, fucose, mannose, glucose, and galactose in soil by gas chromatography (GC) simultaneously after converting to aldononitrile acetate derivatives. A recommended single-hydrolytic step by 4 M trifluoroacetic acid (TFA) at 105 °C for 4 h was more effective for releasing soil neutral sugars from non-cellulosic carbohydrates and better suited to our purification procedure compared with the sulphuric acid hydrolysis. Linearity of the GC detection for each neutral sugar was in the range of 10-640 μg ml⁻¹ and the recovery of neutral sugars from the spiked soil samples ranged from 76% to 109.7%. The coefficients of variation of the neutral sugars in four soils were lower than 2.0% for the instrument and 4.6-7.6% for the whole determination procedures. Compared with the trimethylsilyl (TMS) derivatization, the recovery of our newly modified method was more satisfactory and the reproducibility of ribose was improved significantly. Moreover, the aldononitrile acetate derivative was more stable than TMS derivative. Therefore, it is a promising approach suitable for a routine use in the quantitative analysis of soil neutral sugars, since it is fast, sensitive, and reproducible.     

Distribution of 14C between particle size fractions and carbohydrates separated from a peat incubated with 14C-glycine

Samples of fresh Sphagnum peat from a raised bog were amended with ¹⁴C-labelled glycine. The distribution of ¹⁴C between particle size fractions obtained by wet sieving (sieve sizes 1, 0.5, 0.25, 0.15 and 0.05mm) was determined immediately on control (unincubated) samples and after 1, 6 and 12 months incubation at 10°C. The recovery of glycine in solution was almost 100%. During the incubation with ¹⁴C-glycine, ¹⁴CO₂ was released within the first 20 weeks, equivalent to 51.5% of the added ¹⁴C, but thereafter very little ¹⁴CO₂ was evolved. After 26 weeks a substantial amount of ¹⁴C was distributed amongst all the fractions, but the greatest incorporation (4.47%) occurred in the finest fraction (0.005–0.05 mm). Labelling of the other particle size fractions was <2.3% of added ¹⁴C. Carbohydrate accounted for 23% of ¹⁴C in the finest fraction and the sugars, rhamnose, arabinose, xylose, mannose, galactose and glucose all became labelled. Rhamnose showed the greatest, and arabinose, galactose and xylose the least, increase in specific activity; glucose and mannose had intermediate values. It was concluded that the finest fraction in peat contains a significant proportion of the microbially-synthesized material.     

An automated anion-exchange chromatographic procedure for the estimation of saccharides in acid hydrolysates of soil

It is well known that the major neutral monosaccharide components released from soil by acid hydrolysis are glucose, galactose, mannose, arabinose, xylose, ribose, rhamnose and fucose. A colorimetric determination of the saccharide mixture released from soil is unsatisfactory for determining an accurate figure for soil saccharides. More precise information can be obtained by the determination of monosaccharides after separation by chromatography. Paper and thin-layer chromatography for quantitative analysis are rather time consuming and laborious. The gas chromatographic procedure was applied successfully for the analysis of sugars in soil hydrolysates by OADES et at. (7). Preparation of the various derivatives for gas chromatography still requires many steps, much handling of the sample, and considerable time, although final analysis of the product derivative is accomplished in an hour or two.     

STRUCTURAL STUDIES ON SOIL POLYSACCHARIDE

The polysaccharide extracted by alkali from a Countesswells series soil has been fully methylated and the hydrolysis products identified by GC-MS. The parent neutral sugars are galactose, glucose, mannose, arabinose, xylose, fucose and rhamnose and these constitute about 40 per cent of the polysaccharide. The analysis shows that hexose components are predominantly present in 1 → 3 and 1 → 4 linkages and pentose sugar in 1 → 4 linkages. About 20 per cent of the residues were in branching positions. From the number of non-reducing terminal groups present the average molecular weight of the methylated material has been calculated to be about 1460 compared with a value of 2700 obtained by vapour pressure osmometry. This contrasts with much higher values reported for unmethylated soil polysaccharides. The mixture of derivatives obtained supports the concept that soil polysaccharide originates in both plants and microorganisms.     

Suppression of decomposition of 14C-labelled plant roots in the presence of living roots of maize and perennial ryegrass

The rates of decomposition of barley roots labelled with ¹⁴C were investigated in soil planted with maize or perennial ryegrass and in fallow controls. Evolution of ¹⁴CO₂ was significantly less from the planted soils than from fallow controls. Roots of maize and ryegrass appeared to compete substantially with soil microbes for ¹⁴C-labelled materials. Simple competitive effects were, however, insufficient to explain all of the observed effects of root growth on soil organic matter decomposition. There was no indication that the detrimental effects of maize roots on aggregate stability could be associated with increased degradation of native soil organic materials; the broader significance of the results is also discussed.     

Electrochemical study of the Maillard reaction

Electrochemical properties of beta-alanine/carbohydrate Maillard reaction products were measured using a combination platinum/Ag-AgCl (Cl(-)) redox electrode. Changes toward more negative voltages were observed, which were consistent with reductone formation during the course of the Maillard reaction. Using voltage change as a guide, the propensity for reductone formation among various sugars was ribose > xylose approximately arabinose > glucose approximately rhamnose approximately mannose approximately lactose > fructose. Similar electrochemical behavior indicative of reductone formation was observed in the decomposition products of a model Amadori compound, N-(1-deoxyfructos-1-yl)piperidine (1).     

土壤碳水化合物的研究

碳水化合物是土壤有机质的组成分之一.一般认为,它在土壤良好结构的形成中起着重要的作用^[1].根据Waksman和Stevens的“近似分析法",各种土壤的有机质中,“半纤维”的含量约在5-20%间,“纤维”的含量在0-5%间^[2].但是这种方法并不能提供关于土壤碳水化合物本性方面的资料.近年来,借助于分离的方法,已经证明各种土壤中都有微生物来源的多糖存在.     

Effect of barley plants on the decomposition of 14C-labelled soil organic matter

The effect of barley plants on the rate of decomposition of soil organic matter over a 6-week period was studied using soil that had been previously labelled by incubation with ¹⁴C-labelled ryegrass for 1 year. The plants reduced the loss of ¹⁴CO₂, from soil by 70 per cent over 42 days. About half of the reduction was accounted for by the uptake of labelled C by the plant roots, very little ¹⁴C label being associated with the shoot. Chemical fractionation of the root showed that the ¹⁴C was chemically incorporated into cell wall materials such as cellulose and holocellulose. The reduction in organic matter decomposition in the presence of plants has been explained by earlier workers in terms ofa reduction in microbial activity as a result of a soil moisture deficit caused by plant transpiration. This explanation does not account for all the reduction in decomposition noted in the present experiments. Control soil (without a plant, but amended with glucose or yeast extract to simulate the effect of root exudates) showed a small positive priming effect, the release of ¹⁴CO₂, being increased. Thus the mechanism by which plants conserve organic matter is complex and cannot be explained merely by analogy to an increased level of nutrients available for microbial metabolism.     

Neutral sugars in melanins synthesized by actinomycetes from Brazilian soils

 Nine actinomycete melanins synthesized under various culture conditions, eight of them by actinomycete samples isolated from Brazilian topsoils under savanna (cerrado) vegetation and one from an ATCC sample, were subjected to a two-step hydrolysis procedure and the sugars released qualitatively and quantitatively determined by capillary gas-liquid chromatography (GLC). Humic acids (HAs) extracted from these soils, analysed previously, were used for comparison. The neutral sugars glucose, galactose, mannose, xylose, arabinose, ribose, rhamnose and fucose and the alcohol sugar inositol were present in varying amounts in most of the melanins analysed. The same sugars were present in the HAs used for comparison, except for ribose. Some qualitative and quantitative differences observed in the two types of macromolecules would be expected, considering their origins. The results indicate that the actinomycete melanins have a qualitative sugar distribution pattern similar to that of the HAs from Brazilian tropical soils and of HAs reported for soils from other climatic regions. The possible participation of actinomycete melanins in the formation of soil humic substances is discussed. Received: 4 April 1997