M. V. Cheshire

Determination of neutral and acidic sugars in soil by capillary gas-liquid chromatography after trifluoroacetic acid hydrolysis

Abstract A new method for the simultaneous, quantitative determination of neutral and acidic sugars liberated from non-cellulosic soil carbohydrates is described. A single hydrolytic step with 4 M trifluoroacetic acid (TFA) at 105°C for 4 h is suggested to be more effective in releasing sugar monomers from soil than other recommended hydrolysis procedures (2.5 M H 2 SO 4 reflux for 20 min followed by 12 M H 2 SO 4 for total sugar hydrolysis; 1 M HCl for 5 h at 105°C or for 7 h at 100°C; 4 M TFA at 125°C for 1 h or 2 M TFA at 105°C for 2 h). Different materials were tested for purification (cation exchange resin, activated carbon, C-18, XAD-4, XAD-7), and the combination of the cation exchange resin and of XAD-7 is recommended. Analysis of purified monomers involved separation of O-methyl-oxime-trimethylsilyl derivatives by capillary gas-liquid chromatography in 21 min. Results were compared with those obtained from spiked soil samples, resulting in a high recovery of standard mixtures (70–109% for individual sugars) processed in the way proposed here. The method was sensitive and characterized by simple, accurate and rapid work-up and, therefore, is suitable for routine use.

Metal distribution and nature of some Cu, Mn and V complexes in humic and fulvic acid fractions of soil organic matter

Organic matter from an arable soil derived from base rich parent material was extracted by alkali and fractionated on the basis of solubility in 0.1 N HCl, hot water and hot 6 N HCl and by selective adsorption on charcoal. The distribution of associated metals was determined and Cu had the largest proportion, 15%, associated with the organic matter. Moderate proportions of the total Al, Co, Ni, and V (3–8%) but only small amounts (⩽1%) of the Mn, Fe, Ti, Cr, Ba and Sr were extracted from the soil by alkali. The Fe and Ti were concentrated mainly in the humic fraction whereas Mn and V were both found largely in the fulvic acid. Electron paramagnetic resonance spectra of the various fractions were examined and attempts made to relate the spectra to the forms of some of the metals present. In the humic acid fraction Cu was present partly as a copper porphyrin-type complex but in the fulvic acid it was in some other complexed form. VO2+ occurred in complexed forms in the fulvic acid which were more covalent than VO2+ humic acid complexes, whereas the Mn2+ components of the humic and fulvic acids all had a high degree of ionicity.

Distribution of radiocaesium in organic soils

Abstract Radiocaesium added to organic soils remains plant-available far longer than might be predicted from the behaviour of the element in mineral soils. To investigate this, the distributions of radiocaesium added to a low-ash peat and a peaty podzol were studied using sequential extraction with ammonium acetate, sodium pyrophosphate, sodium hydroxide, and ammonium acetate after peroxidation. Comparisons were made with the distribution reported for mineral soils. Changes in distribution between soil phases with time were measured on a sequence of samples treated with radiocaesium over a period of 4 years. Extractions were performed on soils pretreated with HF to remove mineral matter, on soil fractions enriched or depleted in mineral matter isolated physically by means of wet sieving, and on soil fractions obtained by density separation with chloroform and ‘Nemagon’. These experiments showed that, even in a highly organic peat, the mineral components were important for the eventual fixation of radiocaesium. Oxidizable organic matter corresponding to humin also strongly retained caesium.

Use of 31P-NMR to study the forms of phosphorus in peat soils

31P-NMR has been used to characterise the major forms of organic and inorganic phosphorus in soil derived alkali extracts and humic acids. Blanket peat, which had been under different fertilisation treatments for 10 years was compared with an agricultural mineral soil. In the mineral soil the phosphorus was shown to be in the form of inorganic orthophosphate and phosphate monoesters whereas the peat soil also contained phosphate diesters and other inorganic forms of phosphorus. Changes in various phosphorus pools in the peat soils were shown to correlate with differences in fertilisation regime and land management practice. Fertilisation with superphosphate increased phosphate monoester content in the peat, although vegetation type appeared to have greater impact on the phosphate monoester concentration. The degree of microbial activity, as indicated by the presence of polyphosphates and absence of phosphonates, correlated positively with vegetational improvement and fertilisation of the peat soils.

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

Summary 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 14C glucose and starch in soil

Abstract To gain information about the breakdown and synthesis of carbohydrate, incubations of soil with 14C-stareh and 14C-glucose were studied under laboratory conditions. Carbon dioxide was liberated equivalent to 60–80 per cent of the substrate during incubation of soil with glucose (14 days) and with starch (84 days). The carbohydrate content of the hydrolysate of the substrate-soil mixture returned almost to the initial soil value within 35 days of incubation with 0.5 per cent starch, and within 7 days with 1 per cent glucose. The total glucose in the mixtures rapidly decreased but the amounts of other sugars present showed little change during incubation. To obtain the specific activities of sugars present in the hydrolysate various methods of separation were used including charcoal column, Celite column, and paper chromatography of the free sugars, and resin column chromatography of their borate complexes. With both substrates there was a rapid redistribution of 14C amongst the sugars. Galactose and mannose acquired considerable activity in all cases, rhamnose and fucose became labelled in one experiment, but arabinose and xylose were not labelled. This pattern of distribution remained unchanged with further incubation and glucose remained the most highly labelled sugar (70–80 per cent of total sugar radioactivity) for as long as the incubations were studied.

Influence of soil type, crop and air drying on residual carbohydrate content and aggregate stability after treatment with periodate and tetraborate

The relationship between the water stability of microaggregates and the residual carbohydrate content of soil was examined in 15 soils from 7 soil series under various cultivations. The carbohydrate was progressively removed by increasing the time of treatment with 0.02 M periodate and 0.1 M tetraborate. The resulting decrease in reducing sugar content was significantly correlated with an increased disruption of microaggregates (> 45 μm) as determined by a turbidimetric method. The most effective treatment removed about 80% of the soil carbohydrate and caused an increase of about 75% in the fraction of microaggregates (< 45 μm) compared to untreated soil.

COMPARISON OF SALT MARSH HUMIC ACID WITH HUMIC-LIKE SUBSTANCES FROM THE INDIGENOUS PLANT SPECIES SPARTINA ALTERNIFLORA (LOISEL)

Substances resembling humic acids were extracted from fresh plant material (Spartina alterniflora) and from dead plant debris. Their elemental composition, spectral characteristics in the UV, visible and IR regions, and electron spin resonance were compared with those of humic acid from mud in which the plants were growing. The results show the plant humic substances to consist of a higher portion of aliphatic compounds in their complex structures than that from mud. However, the overall similarities between the extracted material from the different sources indicate that S. alterniflora makes a direct contribution to the humic substances in the salt marsh ecosystems.

Solid-phase 31P NMR spectra of peat and mineral soils, humic acids and soil solution components: influence of iron and manganese

Solid-phase31P nuclear magnetic resonance (NMR) offers a direct means to determine the chemical environment of P present in soil and soil fractions. Iron is often a major component in soil and it has been thought that the presence of paramagnetic Fe and Mn in soil components is responsible for loss of resolution in NMR spectra. We have found that the resolution of signals in the solid-phase 31P NMR spectra of a Fe- and Mn-rich mineral soil was no worse than that for a series of four peat soils with a comparable concentration of P. Similarly, the resolution in the solid-phase 31P NMR spectra of the humic acid from the mineral soil was not much changed by extraction of the humic acid with acetylacetone in diethyl ether which removed around 40% of its Fe and 30% of its Mn. Removal of up to 50% of the Fe from organic rich, freeze-dried soil solutions from a soil catena with different land uses produced little change in spectral resolution. It was concluded that the limitations to resolution in solid-phase 31P NMR spectroscopy of soil humic substances do not stem from the presence of paramagnetic substances, but from the variable way P species are physically held in the amorphous milieu of the organic phase.

Translocation and plant availability of radio caesium in an organic soil

Abstract134Caesium chloride solution was injected into the surface of a peaty podzol at 3 cm depth with 5 cm spacing over a M2 at two upland sites, one of which had been fertilized and reseeded. The movement of radio caesium in the soil was subsequently followed by coring and/or taking out 10-cm square blocks at monthly intervals over a period of 2 years. There was very little movement of the caesium down the profile, with more than 95% remaining at the point of application. Lateral movement was also minimal. The caesium did not concentrate in the roots. Samples of herbage collected from the surface during the growing season showed a progressively decreasing concentration of caesium over the period. Total caesium removed in herbage amounted to 3.5 and 0.6% of the original present for the improved and umimproved soils, respectively.The extractability of the caesium from the 134Cs-amended peaty soil was compared with that from a low-ash peat which had been treated with the radionuclide for 2 months. Initially 1 M ammonium acetate (pH 7.2) extracted 30 and 56% of the Cs from 0–3 and 3–6 horizous of the improved peaty podzol, respectively. After 5 months only about 12 and 7% of the Cs was extracted by 1 M ammonium acetate and 4 and 1% by 0.1 M sodium pyrophosphate from the 0–3 and 3–6 cm horizons, respectively. After 24 months 1 M ammonium acetate extracted 1.6 and 3.3% of the Cs from the 0–3 and 3–6 cm horizons, respectively. In contrast, the values for the peat after 2 months were 100 and 80% for 1 M ammonium acetate and 0.1 M pyrophosphate, respectively.