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.     

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.     

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 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.     

TRANSFORMATION OF SUGARS WHEN RYE HEMICELLULOSE LABELLED WITH 14C DECOMPOSES IN SOIL

Incubation of soil with ¹⁴C hemicellulose from rye straw for 448 days resulted in the evolution of about 70 per cent of the substrate as CO₂. The two major sugar components of the hemicellulose, xylose (50 per cent) and arabinose (5 per cent), were almost completely decomposed. After 56 days only 5 per cent of the xylose remained and after 448 days only 1-2 per cent. Similar results were obtained for soil derived from either granitic or basic igneous parent material. Almost 4 per cent of the hemicellulose was transformed to glucose and I per cent to mannose during the first 14 days of incubation. Fine grinding of 14C rye straw increased the extent of its decomposition on incubation but after 448 days 20 per cent of both its xylose and arabinose remained. It is suggested that the isolated hemicellulose is decomposed faster because it has been made water soluble.     

THE HYDROLYTIC EXTRACTION OF CARBOHYDRATES FROM SOIL BY SULPHURIC ACID

The parameters of the extraction and hydrolysis of soil carbohydrates by methods involving 24N H₂SO₄ and N H₂SO₄ were studied for a sandy granitic loam. Sugars were measured by alkaline ferricyanide, orcinol, anthrone (hexoses), orcinol-ferric chloride (pentoses), cysteine-sulphuric acid (methyl pentoses), glucose oxidase (glucose), and also by analysis of the individual sugars by paper chromatography. After shaking the soil with 24N H₂SO₄ at 20° C a further period of treatment with N H₂SO₄ at 100° C was required to obtain maximum hydrolysis of the soil carbohydrates. This period decreased from 17 h to 5 h as the time with 24N H₂SO₄ increased from 2 to 16 h. NH₂SO₄ at 100° C alone was less efficient. The extraction of pentoses by 24N H₂SO₄ reached a maximum within 8 h, and methyl pentoses within 4 h, both declining thereafter. The release of hexoses was continuing after 40 h. The best compromise involved extraction with 24N H₂SO₄ for 16 h followed by hydrolysis with N H₂SO₄ for 5 h. By this treatment yields of pentoses and methyl pentoses were respectively 99 and 92 per cent of the maximum obtainable.     

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.     

Glucose oxidase activity in soil and its possible interference in assays of cellulase activity

The occurrence of naturally-occurring glucose oxidase activity in a soil collected at a depth of 0–8 cm under tussock has been indicated. This activity was probably responsible for utilization of some of the glucose added to a system of soil, buffer and toluene that was incubated at 30°C; no loss of added glucose was apparent at 50°C.The cellulase activity of this soil was assessed by measuring rates of hydrolysis of added cellulose with Somogyi and anthrone reagents in systems of soil, buffer and toluene at 30 and 50°C. The production of reducing sugars was approximately linear over the first 48 or 72 h of incubation but appeared to decline subsequently at both temperatures. Interference by the activity of the naturally-occurring glucose oxidase at 30°C is suggested. Interference by glucose oxidase appears less likely at 50°C because of the apparent stability of added glucose at this temperature and the instability of added glucose oxidase. An incubation period of 48 h at 30°C appears generally satisfactory for assaying the cellulase activity of this soil.The possible role of glucose oxidase in interfering in assays of other soil enzymes in which the production of glucose is measured is discussed.     

THE OCCURRENCE OF ETHYLENE IN ANAEROBIC SOIL

The production of ethylene and other hydrocarbon gases by soils under anaerobic conditions was measured by gas chromatography. Ethylene was the only hydrocarbon gas which occurred in physiologically significant concentrations; more than 20 ppm was found in several soils after 10 days at 20°C. These concentrations were considerably higher than those which were known to cause severe reductions in the extension of root axes of some plant species. Experiments with sterilized and unsterilized soil indicated that ethylene was produced by enzyme activity and not by chemical action. The gas was found in soil when the oxygen concentration fell below 2 per cent; total evolution was correlated with organic matter content, and was affected by drying and rewetting and by the growth of plant roots. The rate of production was increased by raising the temperature and by addition of glucose or peptone; high concentrations of nitrate depressed the rate, but sulphate and phosphate had little effect. It is concluded that ethylene may be a significant factor in causing injury to crop plants under waterlogged conditions and also in situations where anaerobic pockets occur within a mainly aerobic soil structure, provided that escape of the gas from the soil is impeded sufficiently to allow inhibitory concentrations to build up in the vicinity of plant roots.     

RESPONSE OF GROUNDNUTS DEPENDENT ON SYMBIOTIC AND INORGANIC NITROGEN TO HIGH AIR AND SOIL TEMPERATURES

Groundnuts (Arachis hypogaea L.) are frequently exposed to high temperatures in the semi-arid tropics. The objectives of the present research were: (i) to determine the response of groundnuts to different nitrogen sources; (ii) to quantify the effects of high air and soil temperatures on nodulation, dry matter production, partitioning and pod yields; and (iii) to discover whether plants dependent on symbiotic dinitrogen are more sensitive to heat stress than those dependent on inorganic nitrogen (N). Plants were grown at optimum air and ambient soil temperatures from sowing until the first flowering. Thereafter, plants were exposed to a factorial combination of two air temperatures [optimum: 28°/22°C (day/night) and high: 38°/22°C], two soil temperatures (ambient: 26°/24°C and high: 37°/30°C) and three N-sources [inoculated with Bradyrhizobium strain NC 92 (symbiotic N₂); inoculated and supplied with 20 ppm inorganic N (symbiotic N₂ plus 20 N); or not inoculated and supplied with 100 ppm inorganic N (inorganic N)]. At optimum air and ambient soil temperature dry matter and pod yields were greatest in plants dependent on inorganic N, intermediate in symbiotic N₂ plus 20 N and least in symbiotic N₂. High air or high soil temperatures significantly (P < 0.001) reduced pod yield to a similar extent and their effects were additive and without interaction. High soil, but not high air temperature, significantly (P < 0.001) reduced nodule numbers, nodule dry weight and 100 seed weight. High air and/or high soil temperature had no effect on pod yield in plants dependent on symbiotic N₂ or symbiotic N₂ plus 20 N, but significantly (P < 0.05) reduced pod yield in plants dependent on inorganic N. This suggest that effectively nodulated plants with small quantities of inorganic N are potentially more adaptable to hot environments than those relying on large quantities of inorganic N.     

THE EFFECTS OF TILLAGE, DIRECT DRILLING AND NITROGEN FERTILISER ON SOIL TEMPERATURE UNDER A BARLEY CROP

Continuous hourly records of soil temperature were collected at 1, 5 and 20 cm, throughout two growing seasons, 1973 and 1974, under crops of spring barley in the east of Scotland. Measurements were obtained from three cultivation treatments, deep ploughing, normal ploughing and direct drilling, at two nitrogen fertiliser levels, 0 and 150kg/ha. Compared with ploughed soil, direct-drilled soil had a higher surface reflection coefficient and a higher thermal diffusivity between 5 and 20 cm; overall, this resulted in lower heat sums (°C hours/day over 5 °C and 10 °C) in direct-drilled soil at 1 cm and 5 cm during the first 20 days after sowing. The thermal regimes of normally and deeply ploughed plots were very similar. Nitrogen fertiliser caused significant lowering of soil temperature at all three depths in the latter half of the growing season, due to increased shading of the soil surface by the crop canopy. Some implications of these findings for cultivations for cereals are discussed.     

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.     

FORMATION OF MICROBIAL BIOMASS DURING THE DECOMPOSITION OF 14C LABELLED RYEGRASS IN SOIL

Ryegrass uniformly labelled with ^I4C was incubated aerobically at 25°C for 62 days in two contrasting soils, a near-neutral (pH 6.8) palcudalf from England and a strongly acid (pH 3.6) haplorthox from Brazil. Decomposition of the labelled plant material was faster in the near-neutral soil throughout the whole of the incubation period. In neither soil did the addition of fresh plant material significantly accelerate the evolution of CO₂ from organic matter already in the soil, i.e. there was no priming action. In the near-neutral soil there was a rapid build up of labelled microbial biomass in the first 6 days, followed by a much slower increase that continued throughout the whole incubation period. After 62 days 22.5% of the labelled C remaining in the near-neutral soil was in the biomass. The yield coefficient (the fraction of the incoming plant C converted to microbial C) of this stabilized or ‘resting’ biomass was 0.15. Much less labelled microbial biomass was formed in the acid soil than in the near-neutral soil. By the end of 62 days only 6.2% of the labelled carbon remaining in the acid soil was in the biomass. Biomass C measurements in strongly acid soils must however be treated with caution as the technique used has not yet been adequately validated for such soils.     

无机氮素加入量对玉米秸秆分解过程中棕壤氨基糖含量的影响

在室内恒温(25℃)培养条件下,通过气相色谱法研究高C/N比玉米秸秆降解过程中微生物来源的氨基糖含量及其占有机质比例的变化及其对无机氮素添加水平(0, 60.3, 167.2, 701.9 mg•N•kg-1土,依次标记为N0, Nlow, Nmed, Nhigh)的响应情况。结果表明：在玉米秸秆分解过程中,土壤中的氨基糖含量及其对有机质贡献的比例随着无机氮素供应水平的增加而增加,即以微生物代谢物形式截获的有机碳/氮相应增多。Nmed和Nhigh处理中氨基糖积累量显著高于Nlow和N0处理。不同微生物来源的氨基糖受外源氮素的影响情况不同,胞壁酸比氨基葡萄糖更易于受到土壤中碳氮供给的影响,具有相对较快的转化速率;而在数量上氨基葡萄糖对土壤有机质的贡献比例显著高于前者;氨基半乳糖在土壤中的积累过程较为缓慢,受外源无机氮素添加水平的影响并不明显。可见,在高C/N比作物残体分解过程中,无机氮素的供应水平是影响土壤中氨基糖积累转化的重要因素之一。但是,过多的无机氮素施入并不能被微生物完全同化利用,因此秸秆还田的土壤中必须要考虑有效氮素的水平问题。     

Polyethylene mulching of soil to reduce viability of sclerotia of Sclerotium oryzae

Mulching of Sclerotium oryzae infested soil (moist or dry) with polyethylene sheets during hot summer days of May and June increased the soil temperature at 5 cm from 36°C (unmulched) to 48°C (wet) and from 44 to 52°C (dry) and at 20cm from 32 to 38°C (wet) and from 35 to 39°C (dry). In artificially-infested soil, the sclerotia were not eradicated but 95–100% loss in viability was observed at 5 cm by a mulch treatment for 1 week and at 20 cm by mulching for 8 weeks. Mulching effects were not influenced by moisture content of soil or by amendments with lucerne or wheat straw. Mulching of naturally-infested soil at a second site did not eradicate S. oryzae but reduced sclerotial viability by 93%.     

REACTIONS OF AMMONIA WITH SOIL II. SORPTION OF NH3 ON ENGLISH SOILS AND ON WYOMING BENTONITE

Sorption isotherms and heats of sorption of NH₃ gas on soils, Wyoming bentonite and silica gel were measured at 27 °C after preliminary drying at 22°-290 °C. Sorption of NMe₃ was also measured on some of the materials. Sorption on soils was not very sensitive to change in drying temperature, even though soil water content was markedly affected. Sorption on bentonite was dependent on drying temperature because this affected ease of lamellar separation. The results suggest that under the conditions employed, NH₃ sorption on the English soils studied is predominantly due to its protonation at pH-dependent charge sites and partly to co-ordination with exchangeable Mg or Ca (but not Na or K). Sorption through mechanisms involving hydrogen bonding is apparently negligible.     

Repeated freeze–thaw cycles changed organic matter quality in a temperate forest soil

Under temperate climate, the frequency of extreme weather events such as intensive freezing or frequent thawing periods during winter might increase in the future. It was shown that frost and subsequent thawing may affect the fluxes of C and N in soils. In a laboratory study, we investigated the effect of frost intensity and repeated freeze–thaw cycles on the quality and quantity of soil organic matter (SOM) in a Haplic Podzol from a Norway spruce forest. Undisturbed soil columns comprising O layer and top mineral soil were treated as followed: control (+5°C), frost at –3°C, –8°C, and –13°C. After a 2‐week freezing period, frozen soils were thawed at +5°C and irrigated with 80 mm water at a rate of 4 mm d^–1. Lignin contents were not significantly affected by repeated freeze–thaw cycles. Phospholipid fatty acid (PLFA) contents decreased in the mineral soil, and PLFA patterns indicate that fungi are more susceptible to soil frost than bacteria. Amounts of both plant and microbial sugars generally decreased with increasing frost intensity. These changes cannot be explained by increased mineralization of sugars or by leaching with DOM nor by a decreased microbial activity and, thus, sugar production with increasing frost intensity. Also physical stabilization of sugars due to frost‐induced changes in soil structure can be ruled out as sugar extraction was carried out on ground bulk soil. Therefore, the only possible explanation for the disappearance of plant and microbial sugars upon soil freezing are chemical alterations of sugar molecules leading to SOM stabilization.     

Reduction in viability of sclerotia of Macrophomina phaseolina with polyethylene mulching of soil

Mulching of Macrophomina phaseolina-inksted soil (moist or dry) with transparent polyethylene sheets during the hot days of May increased temperature of wet soil at 5 cm from 37°C (unmulched) to 52°C (mulched) and of dry soil from 52°C (unmulched) to 65°C (mulched). At 20 cm mulching increased temperature from 30°C to 41°C (wet) and from 38°C to 42°C (dry). In artificially-infested soil. the sclerotia of M. phaseolina were eradicated at 5 cm by a mulch treatment for 1 week and at 20 cm depth 50% sclerotia lost viability in wet soil but were not affected in dry soil. In a naturally infested soil (5–7 sclerotia g^?1), which gave 20% infection on Vigna, the sclerotia were reduced to such an extent that after 1 week mulching no disease was observed on Vigna.     

The effect of oxidation by periodate on soil carbohydrate derived from plants and microorganisms

It has previously been shown that treatment of soil with periodate and tetraborate releases much of the carbohydrate and destroys an equivalent proportion of the soil aggregates. The residual carbohydrate is proportionately richer in glucose, arabinose and xylose, sugars characteristic of plant remains, than the whole soil. The effect of sodium periodate (0.02 M, 6–168 h) and sodium tetraborate (0.1 M, 6 h) treatment of soil on carbohydrates of different origin was examined using ¹⁴C-labelled soil in which the label was present in microbial products arising from 7 and 28 day incubations of ¹⁴C-glucose in soil, or in both plant and microbial materials resulting from 12 week incubations of ¹⁴C-labelled barley leaf and 1 year incubations of ¹⁴C-labelled ryegrass in soil. Arabinose and xylose were the sugars most resistant to periodate in the glucose incubated soil; in the ryegrass incubation arabinose, xylose and glucose were more persistent than galactose, mannose and rhamnose. In the barley leaf incubation arabinose was more persistent than galactose and rhamnose. Thus periodate oxidation did not distinguish between sugars of different origin in soil and it was concluded that in the case of arabinose and xylose the persistence related to differences in chemical structures rather than to physical factors such as particle size of the plant fragments. The composition of the more stable residue can therefore not be used as an indication of polysaccharide origin in any comparison of the relative effects of plant and microbially derived material as aggregating agents.     

Effect of pH and temperature on the formation of volatile compounds in cysteine/reducing sugar/starch mixtures during extrusion cooking

Mixtures of cysteine, reducing sugar (xylose or glucose), and starch were extrusion cooked using feed pH values of 5.5, 6.5, and 7.5 and target die temperatures of 120, 150, and 180 degrees C. Volatile compounds were isolated by headspace trapping onto Tenax and analyzed by gas chromatography--mass spectrometry. Eighty and 38 compounds, respectively, were identified from extrudates prepared using glucose and xylose. Amounts of most compounds increased with temperature and pH. Aliphatic sulfur compounds, thiophenes, pyrazines, and thiazoles were the most abundant chemical classes for the glucose samples, whereas for xylose extrudates highest levels were obtained for non-sulfur-containing furans, thiophenes, sulfur-containing furans, and pyrazines. 2-Furanmethanethiol and 2-methyl-3-furanthiol were present in extrudates prepared using both sugars, but levels were higher in xylose samples. The profiles of reaction products were different from those obtained from aqueous or reduced-moisture systems based on cysteine and either glucose or ribose.