The extraction and characterization of soil polysaccharide by whole soil methylation

A soil of the Countesswells series was repeatedly methylated by the Hakomori procedure and a chloroform-soluble product isolated after each methylation. Ninety-two per cent of the material engendered by seven methylations was released during the first four methylations. This had a methoxyl content of about 20% and contained 2% N. Later fractions had lower methoxyl and N contents. Residual carbohydrate in the soil had reducing sugar content on hydrolysis, equivalent to about 3% of the original value. The extracted material had the infrared spectrum of a methylated soil polysaccharide and, on hydrolysis by 2 m trifluoracetic acid, released methylated sugar derivatives of which 68 were characterised by GC-MS. Derivatives corresponding to (1→4) linked sugars predominated for both hexose and pentose sugars but there was also a large proportion of (1→3) linkages, (1→3) linkages predominated for the deoxyhexose sugars. There were more sugars with only one or two methylated hydroxyl groups than could be accounted for as branching points because of the relatively small numbers of end groups. Prior reduction of the soil with sodium borohydride had no measurable effect on the nature or yield of the methylated product. This indicates that amino acid sugar linkages susceptible to β-elimination reactions can have only a very small influence on the reaction. The isolated sugar derivatives accounted for 70% of the total soil sugars. The methylated material before hydrolysis had a low nominal molecular weight on diafiltration, with 68% < 10000. Some of the sugars unaccounted for were probably lost during the dialysis stage necessary to remove dimethyl sulphoxide.     

DECOMPOSITION OF SOIL POLYSACCHARIDE

Polysaccharide material was isolated by absorption on charcoal from the acidified, non-humic fraction extracted by alkali from three soils. The polysaccharides were used as substrates in soil incubation, perfusion, and suspension experiments. Concordant results were obtained with freely drained Countess-wells and Insch Association soils derived from acidic and basic igneous parent materials respectively. Polysaccharide material added to soil at low concentration (I per cent) was apparently totally decomposed after 8 weeks when the amounts of polysaccharide in control and amended soils were statistically indistinguishable. At higher concentrations (2-3 per cent) a significant difference in reducing sugar, equivalent to about 30 per cent of the substrate, remained after 32 weeks. Partial neutralization of the polysaccharide material with calcium hydroxide increased the rate of decomposition in Countesswells Association soil but had an opposite, smaller effect in Insch Association soil. Soil polysaccharide material was decomposed slightly faster in perfusion and suspension experiments than in moist soil. Only 20 per cent of the carbohydrate in the unfractionated alkali–soluble organic matter of soil was decomposed during incubation in soil for up to 133 weeks. There was usually little change in the carbohydrate content of soil incubated alone. The soil microbial population showed a marked increase in response to added polysaccharide material but only slight qualitative changes were detected. It is concluded that the persistence of naturally occurring polysaccharide in soil is related to inaccessibility caused by chemical combination, complexing or insolubility but not to a biologically-stable molecular structure.     

Structure of soil carbohydrates resistant to periodate oxidation

Permethylation has been applied to the polysaccharide in soil, which has been oxidized with 0.2 m NaIO₄ and treated with 0.1 m Na₂B₄O₇. The amounts and types of methylated sugar obtained on hydrolysis have been compared with those from unoxidized soil.
As well as a number of derivatives corresponding to 1 -3 linked glucose, arabinose and xylose, considerable proportions of 1-4 linked glucose and xylose also persisted. The polysaccharides appear to persist for both chemical and physical reasons.     

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 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.     

Alkaline cupric oxide oxidation of a methylated fulvic acid

Fulvic acid, (FA), extracted from the Bh horizon of a Podzol soil, was methylated and then oxidized with alkaline cupric oxide. The oxidation products were extracted into organic solvents, remethylated and separated by column-, thin layer-, and preparative gas chromatography into relatively pure components, which were identified by matching their mass and i.r. spectra and gas Chromatographic retention times with those of authentic specimens.The degradation products isolated and identified accounted for approximately 18 per cent of the weight of the initial methylated FA. Major oxidation products were: (a) methylated phenolic aldehydes and esters (66.9 per cent): (b) benzenecarboxylic acid methyl esters (14.6 per cent): (c) aliphatic dicarboxylic acid methyl esters (2.7 per cent): and (d) adsorbed materials such as n-alkanes (0.3 per cent), n-fatty acid methyl esters (0.3 per cent) and dioetyl adipate (15.2 per cent). The oxidalive degradation of methylated FA indicates the presence of two types of basic structural units: (1) those yielding phenolic aldehydes and esters, and (2) those producing benzenecarboxylic acids. Alkaline cupric oxide oxidation of methylated FA is relatively selective for the isolation of the phenolic components and appears to be a promising technique for structural investigations on humic substances.     

Extracellular (1 å 3),(1 → 6)-linked β-d-glucan produced by the soil fungus Ulocladium atrum

An extracellular β-d-glucan produced by Ulocladium atrum was studied after precipitation from culture liquid with acetone, and purification using polyvinyl pyrrolidone for soluble pigment elimination. The polysaccharide showed a high intrinsic viscosity and a large molecular size, estimated by gel filtration. Evidence of glucose epimerization and degradation during Saeman's hydrolysis was observed. The presence of glucosyl residues joined by β-(1 → 3) and β-(1 → 6) linkages in a 2:1 proportion was recognized after periodate oxidation, complete Smith degradation, and high-field ¹³C-NMR. Because this fungus also produces humic acid-like melanin, its contribution to soil properties is discussed.     

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.     

Effect of various procedures on the yield of methylated carbohydrate substances obtained from soil

Abstract

The molecular shape of a polysaccharide, which determines its cohesive and water retention properties, depends on the types of bonding present between sugar residues. These may be determined by permethylation and hydrolysis. Neither acetylation, nor reduction of soil prior to methylation, increased the overall yield of methylated product, although acetylation resulted in a greater proportion being released in the first methylation which may avoid some ß‐elimination reactions on remethylation. Reduction by NaBH₄ before each methylation step did not result in any large differences in the proportions of different sugar residues, suggesting that ß‐elimination reactions did not have a prominent effect in the remethylation of the soil residue. Grinding and freeze‐milling of the soil prior to methylation gave a greater release of methylated product in the first three methylations and the yield was about 15% greater overall.     

Effects of vertical hedge interval of vetiver grass on erosion on steep agricultural lands

Vetiver grass is widely used to reduce soil erosion and has been applied in many areas of the world. However, studies of the effect of vertical hedge intervals on runoff, soil loss and outflow sediment size distribution under a steep slope area are rare. The vetiver grass system (VGS) with three vertical hedge intervals (0·75, 1·5 and 3 m) and no hedgerow were tested at three land slopes (30, 40 and 50 per cent) under three simulated rainfall intensities (60, 85 and 110 mm h⁻¹). It has been observed that vetiver grass (Vetiveria nemoralis) has great potential for reducing runoff and soil loss by about 38·7–68·6 and 56·2–87·9 per cent, respectively. The vetiver strips delayed incipient runoff and reduced peak runoff rate and steady erosion rate. The land slope affected soil loss but did not have a significant effect on runoff. A narrow vetiver hedge interval slightly reduced runoff and soil loss more than a wider one. The soil loss equation obtained in this study revealed that runoff has a higher effect on soil loss. The median sediment size that passed through the vetiver strip increased with rainfall intensity and was mostly dominated by very fine sand, silt and clay. Copyright © 2009 John Wiley & Sons, Ltd.     

Decomposition and distribution of residual activity of some 13C-microbial polysaccharides and cells,glucose, cellulose and wheat straw in soil

Studies were made to determine the rate of decomposition of some ¹⁴C-labeled microbial polysaccharides, microbial cells, glucose, cellulose and wheat straw in soil, the distribution of the residual ¹⁴C in various humic fractions and the influence of the microbial products on the decomposition of plant residues in soil. During 16 weeks from 32 to 86 per cent of the C of added bacterial polysaccharides had evolved as ¹⁴CO₂. Chromobacterium violaceum polysaccharide was most resistant and Leuconostoc dextranicus polysaccharide least resistant. In general the polysaccharides, microbial cells, and glucose exerted little effect on the decomposition of the plant products. Upon incubation the ¹⁴C-activity was quickly distributed in the humic. fulvic and extracted soil fractions. The pattern of distribution depended upon the amendment and the degree of decomposition. The distribution was most uniform in the highly decomposed amendments. After 16 weeks the bulk of the residual activity from Azotobacter indicus polysaccharide remained in the NaOH extracted soil. From C. violaceum polysaccharide both the extracted soil and the humic acid fraction contained high activity. About 50–80 per cent of the residual activity from the ¹⁴C-glucose, cellulose and wheat straw amended soils could be removed by hydrolysis with 6 n HCl. The greater part of this activity in the humic acid fraction was associated with the amino acids and that from the fulvic acids and residual soils after NaOH extraction with the carbohydrates. About 8 16 per cent of the activity of the humic acid fraction was present in substances (probably aromatic) extracted by ether after reductive or oxidative degradation.     

Soil degradation by Erosion of a typic hapludalf in central Ohio and its rehabilitation

Soil erosion and runoff were monitored from 1988 to 1990 on a Miamian soil (Typic Hapludalf) of 5-6 per cent slope using field runoff plots. Four treatments were studied: (i) disk-plough up and down the slope to 0.3 m depth (DP); (ii) disk-plough up and down the slope followed by a protective netting (PN); (iii) uncultivated fallow without any vegetation followed by surface soil removal (R); (iv) uncultivated fallow with natural vegetation followed by ploughing (F). Mean annual runoff losses were 6, 114 and 128 mm, or 4, 20 and 18 per cent of the rainfall, and mean annual soil losses were 1.2, 85.0 and 64.0 Mg ha⁻¹ in 1988, 1989 and 1990, respectively. Mean runoff amounts were 26, 69, 116 and 118mm and mean annual soil losses were 0.4, 23.2, 58.6 and 118 Mg ha⁻¹ for the F, PN, DP and R treatments, respectively. In comparison with DP, PN decreased annual runoff by 40.3 per cent and annual soil loss by 79.5 per cent. The high mean soil loss for the R treatment was due to erosion following soil removal. An additional 2920 Mg ha⁻¹ of surface soil was removed from the R treatment in May 1990. The F treatment reduced runoff by 78, 77 and 62 per cent and reduced soil loss by 99.7, 99.4 and 98.4 per cent compared with the R, DP and PN treatments, respectively. Mean losses of K, Ca, Mg and P were 1.3, 4, 1 and 01 kg ha⁻¹, respectively for F, 3, 16, 5 and 0.3kg ha⁻¹, respectively, for PN, 5, 31, 1 and 0.6kg ha⁻¹, respectively, for DP, and 3, 32, 12 and 0.4 kg ha⁻¹, respectively, for R. Soil and nutrient losses for each treatment were in the order R > DP > PN > F. The soil organic carbon (SOC) content was significantly affected by soil erosion and management treatments, and ranged from 0.98 per cent for the R treatment to 2.3 per cent for the F treatment. Soil surface removal for the R treatment in 1990 reduced water-stable aggregates (WSA) by 9.0 per cent, SOC by 0.6 per cent, and clay content of the uppermost 0-50 mm depth by about 7.0 per cent. Mean total porosity (f_t) ranged from 0.43 for the F to 0.52 for the DP treatment. Cumulative infiltration for 3h ranged from 13 cm for R to 34cm for PN, with corresponding infiltration rates of 4 cm h⁻¹ and 13 cm h⁻¹, respectively. Regardless of the treatment, there were also temporal changes in soil properties. In comparison with 1988, measurements made in 1990 showed a significant decrease in WSA of 21.3 per cent, an increase in clay content of 2.8 per cent, and a decrease in SOC of 0.39 per cent. Runoff and soil losses were significantly correlated with the mean weight diameter (MWD), SOC, bulk density (p_b) and available water capacity (AWC). Plant height measured 8 weeks after planting (WAP) for the R treatment was reduced by 33.3 per cent, 33.0 per cent and 29.0 per cent compared withh DP, PN and F, respectively. Nitrogen uptake by maize plants (Zea mays L.) 10 WAP for the R treatment was lower by 15 per cent, 8 per cent, and 6 per cent compared with the DP, PN and F treatments, respectively, while P uptake was lower by 33 per cent, 32 per cent and 29 per cent, respectively, compared with the same treatments. Grain yield was 9.78 Mg ha⁻¹ for PN, 9.76 Mg ha⁻¹ for DP, 8.64 Mg ha⁻¹ for F and 6.60 Mg ha⁻¹ for R during the 1990 crop season. Grain yield was reduced by about 32.4 per cent in the R treatment compared with the PN treatment, representing a maize grain yield reduction of 158 kg ha⁻¹ for each centimeter of soil lost.     

Long Term Changes of Base Cation Pools in Soil and Biomass in a Beech and a Spruce Forest of Southern Sweden

During the past 60 years there has been a considerable decline in pH in mineral soil beneath spruce and beech stands at Tönnersjöheden Experimental Forest in south-west Sweden. In this report an attempt is made to estimate the corresponding declines in base cation pools. The exchangeable storage of Na, K, Ca and Mg in soil, down to 70 cm depth, is calculated to have decreased by 57–60 per cent for beech and by 56–74 per cent for the spruce stands during the period 1927–1984. The calculated cation depletions are compared with estimated nutrient uptake in biomass, base cation release by weathering and leaching losses due to percolation of strong mineral acids and organic anions during the period. The biological acidification may explain about 50–60 per cent of the total losses of base cations from soil, the cation accumulation in biomass then explain 41–43 per cent units for beech and 34–45 per cent units for spruce. The estimated losses of base cations due to acid rain correspond to an amount of cations similar to that accumulated in the spruce biomass during one generation.     

DISTRIBUTION OF FORMS OF NITROGEN IN A PODZOLIC SOIL PROFILE FROM GARPENBERG, CENTRAL SWEDEN

The chemical nature and distribution patterns of the forms of N in a podzolic soil profile from central Sweden, developed under a stand of Norway spruce, were studied. Total N, native fixed NH₄, acid hydrolysable-N, and the amounts of ammonia, hexosamine, and amino-acid N in the hydrolysate were determined. From 17 to 27 per cent of the N was insoluble in 6N HC1, the highest percentage being in the A₂ horizon. Amino-acids in the acid hydrolysates decreased from 50 per cent of the total N content of the soil in the humus layer (A₀) to 24 per cent in the B horizon. Amino-acid composition varied little in samples from different horizons. Hexosamine-N was 11–14 per cent of the total soil N, tending to increase with depth. Approximately 15 per cent of the total soil N was found in the soil hydrolysate as NH₄. Values for native fixed NH₄ extracted by N HF: N HCl were 34–80 ppm but were reduced to 10–17 ppm when corrected for the NH₄ released by N HCl only. The figures thus obtained are considerably lower than those reported by other workers.     

Further investigations on the chemistry of fungal “humic acids”

In view of the considerable interest in laboratory-prepared fungal “humic acids” as possible precursors or incorporated structural components of soil humic substances, we degraded four fungal “humic acids” by the relatively mild alkaline cupric oxide oxidation. The oxidation products were extracted into organic solvents, methylated, separated by thin-layer chromatography and identified on a gas chromatographic-mass spectrometric-computer system.Average yields of major degradation products were: (a) aliphatic compounds, 38 per cent; (b) benzene-carboxylic compounds, 25 per cent; and (c) phenolic compounds, 21 per cent. The remaining 16 per cent consisted of a number of dialkyl phthalates. Our data agree with those that we reported earlier when we degraded a number of fungal “humic acids” by the more drastic alkaline permanganate oxidation and show that fungal “humic acids” are enormously complex organic materials containing aliphatic and aromatic structures, (some of which contain N), but only a relatively small proportion of which is phenolic. Most of the aliphatics isolated consisted of alkanes and fatty acids, which are known to persist in soils over long periods of time and are frequently firmly retained by soil humic substances.     

Rationale and methods for compiling an atlas of desertification in Italy

This study presents the main results and the methodology used in the creation of the atlas of the risk of desertification in Italy. A desertified area was defined as an unproductive area for agricultural or forestry use, due to soil degradation processes. An area at risk of desertification was a tract of the earth's surface which is vulnerable or sensitive to the processes of desertification. In a vulnerable land, environmental characteristics are close to that of a desertified area, but some factors (e.g. vegetation cover or irrigation), successfully mitigate the desertification process. On the other hand, sensitive land is a surface where the process leading to desertification is active, although the land is not yet unproductive. The DPSIR (Driving force‐Pressure‐State‐Impact‐Response) framework has been adopted as a reference. Using the national soil information system and socio‐economic layers, an atlas of indicators of desertification risks was created, which was organized into different soil degradation systems. 51.8 per cent of Italy was considered to be at potential risk of desertification. Some 21.3 per cent of Italy (41.1 per cent of the area at potential risk) featured land degradation phenomena. Specifically, 4.3 per cent of Italy is already unproductive; 4.7 per cent is sensitive and 12.3 per cent is vulnerable. In the territory at potential risk of desertification, unproductive lands, plus areas vulnerable or sensitive to soil erosion, are at least the 19 per cent. Areas affected by aridity also sum up to 19 per cent. Salt‐affected soils in Italy are estimated to cover >1 million ha. Irrigation can mitigate soil aridity and salinization, nevertheless, only about 15 per cent of the sensitive and vulnerable lands of southern Italy are actually equipped with irrigation networks. Copyright © 2009 John Wiley & Sons, Ltd.     

MINERALOGY AND RADIOISOTOPE RETENTION PROPERTIES OF A CHRONOSEQUENCE OF SOILS DEVELOPED IN BASALTS OF VICTORIA, AUSTRALIA

The mineralogical composition and retention properties for radioisotopes (²⁰Sr and ¹³⁷Cs) of soils developed in five basalt flows of age varying from 6000 years to about four million years occurring in western Victoria were investigated. The trend of mineral weathering has been almost exclusively to amorphous material, kaolinite-plus-halloysite, and chlorite, the more soluble products of weathering having been removed. The most significant changes in clay mineralogical composition with time are the progressive decrease in the Si0₂/Al₂O₃ molar ratio of the amorphous material in the clay fraction of the surface horizons, from an initial value of approximately 4 to values of approximately 2, and the progressive increase in the amount of kaolinite-plus-halloysite, both in the topsoil and at depth, with age of the basalt flow. The amount of kaolinite plus halloysite increases from approximately 20 per cent of the clay of soil developed in the basalt flow 6000 years old to approximately 50 per cent of that of soil in basalt about four million years old. Evidence for the presence of halloysite was obtained by electron microscopy studies. The amorphous material and chlorite contents, each of which constitutes between 20 and 50 per cent of the clay fractions, decrease concurrently with the increase in kaolinite-plushalloysite content. Fixation of Sr by whole soil samples was controlled by the organic matter and free iron oxide contents rather than by the mineralogy of the samples. A high proportion of the added Cs was sorbed by whole soil samples. Much of the sorbed Cs was not readily replaced by CaCl, washings but was replaced in part by subsequent washing with NaCl of pH 5.3 and almost entirely by subsequent NH₄Cl washings. Much of the Sr and Cs deposited on these soils by rainfall and dry fall-out would be sorbed; the ease of replacement suggests that these elements would be available for further movement through the food chain.     

Soil compaction and sealing in Al‐Salmi area,western Kuwait

In the deserts of Kuwait in general and in Al‐Salmi area in particular, soil compaction and sealing are the most significant mechanisms of land degradation. In the present study, soil compaction and sealing in the Al‐Salmi area are assessed. The study is based on analysis of satellite images and aerial photographs, besides field measurements and laboratory investigations. Based on this study, a recent map is prepared for the concerned area. It shows three different soil classes. These are highly compacted (8\8 per cent), slightly compacted (1\7 per cent), and almost non‐compacted or natural (89\5 per cent). Soil compaction in Al‐Salmi area causes adverse changes in soil physical properties, e.g. infiltration rate, bulk density and soil strength. The infiltration capacity of the compacted soils has decreased by 18\46 to 91\96 per cent in comparison with non‐compacted soils. The bulk density for the compacted soil varies between 1\6 and 1\7 g cm⁻³, whereas it varies between 1\2 and 1\35 g cm⁻³ in the non‐compacted soils, that is an increased of 29\97 per cent. In some sites, the physical properties show small or no difference between compacted and natural soils due to the effect of soil sealing (crustation). Copyright © 2000 John Wiley & Sons, Ltd.     

Deforestation effects on soil degradation and rehabilitation in western Nigeria. IV. Hydrology and water quality

Hydrological and water-quality measurements were made on a 44·3 ha watershed under forest cover and following deforestation and conversion to an agricultural land-use. Under secondary tropical rainforest, water yield ranged from 2·2 per cent to 3·1 per cent of annual rainfall. Deforestation of 7 per cent of the watershed area increased water yield to 7·0 per cent of annual rainfall. Baseflow increased with deforestation, and increased progressively with time after deforestation. It was 5·1 per cent of annual rainfall in 1979, 15·1 per cent in 1980, 16·4 per cent in 1981 and 17·9 per cent in 1982. In comparison, surface flow was 4·5 per cent in 1979 and 6·2 per cent in 1980, but decreased to 2·3 per cent in 1981 and 2·4 per cent in 1982. Total water yield following deforestation and conversion to agricultural land-use ranged from 9·6 per cent to 21·3 per cent of the annual rainfall received. The dry season flow decreased with time as the dry season progressed, but increased over the years following deforestation. Surface runoff during the rainy season depended on ground cover and soil quality. The extent and severity of soil degradation affected the dynamics of surface flow. Because of actively growing crops, plant nutrient concentrations in surface runoff were low. Forested lysimeters had higher seepage losses than cropped lysimeters, and the water-use efficiency was 1·9–3·6 kg ha⁻¹ mm⁻¹ for cowpeas compared with 6·1–11·0 kg ha⁻¹ mm⁻¹ for maize. The delivery ratio was high immediately after deforestation and decreased to a steady value of about 3·2 per cent within 7 years. The data show five distinct phases of soil degradation in relation to generation of surface runoff. © 1997 John Wiley & Sons, Ltd.     

SULPHUR, CARBON, AND NITROGEN CONTENTS OF ORGANIC FRACTIONS FROM ACETYLACETONE EXTRACTS OF SOILS

Ultrasonic dispersion of acid-pretreated soils in aqueous acetylacetone at pH 8, and at a solvent: soil ratio of about 45: I, extracted from 61 to 97 per cent of the organic sulphur from five Scottish soils. Narrowing the ratio to about 9:1 reduced the proportion extracted but was more convenient for the subsequent isolation of the organic components. Gel permeation chromatography of the extracts yielded materials containing from 14 to 60 per cent of the soil organic sulphur. Most of the soils gave four distinct fractions which contained from 21 to 38 per cent carbon, 0.2 to 2.1 per cent nitrogen, and 0.1 to 0.8 per cent sulphur. There was no consistent similarity between corresponding fractions of the various soils. Considerable amounts of iron and aluminium were extracted, presumably incorporated in organic complexes. Since they were obtained under very mild conditions, the fractions should provide useful starting materials for qualitative investigations on the nature of soil organic sulphur.