Plant-derived Sugars Research Articles

Microbial carbon recycling: an underestimated process controlling soil carbon dynamics – Part 2: A C<sub>3</sub>-C<sub>4</sub> vegetation change field labelling experiment

Abstract. The mean residence times (MRT) of different compound classes of soil organic matter (SOM) do not match their inherent recalcitrance to decomposition. One reason for this is the stabilization within the soil matrix, but recycling, i.e. the reuse of "old" organic material to form new biomass may also play a role as it uncouples the residence times of organic matter from the lifetime of discrete molecules in soil. We analysed soil sugar dynamics in a natural 30-year old labelling experiment after a wheat-maize vegetation change to determine the extent of recycling and stabilization by assessing differences in turnover dynamics between plant and microbial-derived sugars: while plant-derived sugars are only affected by stabilization processes, microbial sugars may be subject to both, stabilization and recycling. To disentangle the dynamics of soil sugars, we separated different density fractions (free particulate organic matter (fPOM), light occluded particulate organic matter (≤ 1.6 g cm−3; oPOM1.6), dense occluded particulate organic matter (≤ 2 g cm−3; oPOM2) and mineral-associated organic matter (> 2 g cm−3; mineral)) of a silty loam under long-term wheat and maize cultivation. The isotopic signature of neutral sugars was measured by high pressure liquid chromatography coupled to isotope ratio mass spectrometry (HPLC/IRMS), after hydrolysis with 4 M Trifluoroacetic acid. While apparent MRT of sugars were comparable to total organic carbon in the bulk soil and mineral fraction, the apparent MRT of sugar carbon in the oPOM fractions were considerably lower than those of the total carbon of these fractions. This indicates that oPOM formation was fuelled by microbial activity feeding on new plant input. In the bulk soil, MRT of the mainly plant-derived xylose were significantly lower than those of mainly microbial-derived sugars like galactose, rhamnose, fucose, indicating that recycling of organic matter is an important factor regulating organic matter dynamics in soil.

Organic matter composition and the protist and nematode communities around anecic earthworm burrows

By living in permanent burrows and incorporating organic detritus from the soil surface, anecic earthworms contribute to soil heterogeneity, but their impact is still under-studied in natural field conditions. We investigated the effects of the anecic earthworm Lumbricus centralis on fresh carbon (C) incorporation, soil organic matter composition, protists, and nematodes of a Cambisol under grassland. We used plant material labelled with stable isotope tracers to detect fresh C input around earthworm-occupied burrows or around burrows from which the earthworm had been removed. After 50 days, we sampled soil (0–10 cm depth) in concentric layers around the burrows, distinguishing between drilosphere (0–8 mm) and bulk soil (50–75 mm). L. centralis effectively incorporated fresh C into the drilosphere, and this shifted soil organic matter amount and chemistry: total soil sugar content was increased compared to unoccupied drilosphere and bulk soil, and the contribution of plant-derived sugars to soil organic matter was enhanced. Earthworms also shifted the spatial distribution of soil C towards the drilosphere. The total abundance of protists and nematodes was only slightly higher in earthworm-occupied drilosphere, but strong positive effects were found for some protist clades (e.g. Stenamoeba spp.). Additional data for the co-occurring anecic earthworm species Aporrectodea longa showed that it incorporated fresh C less than L. centralis, suggesting that the two species may have different effects on soil C distribution and organic matter quality.

Butanol Fermentation by Clostridium saccharobutylicum Based on Poplar Wood

As a potential source of liquid fuels, lignocellulosic material is an alternative to plant-derived starch and sugar, which are urgently needed to meet global demands for food. The utilization of wood as feedstock for bioconversion to biobutanol fuel not only could reduce production costs, but also could increase raw material supply. However, little is known about biobutanol fermentation based on lignocellulosic material from wood. In this paper, biobutanol fermentation from poplar wood hydrolysates by Clostridium saccharobutylicum was investigated under three different conditions. The desired biobutanol and ABE yields reached 6.98 and 9.64 g/L, respectively, and 69.8 g biobutanol and 96.4 g ABE per kg of poplar wood were achieved. Fermentation of hydrolysates with no additives and with extra mixed carbon sources to biobutanol was also studied. The predicted results were confirmed: in the former, the production of biobutanol and ABE were 4.88 and 6.63 g/L, respectively; in the latter, the biobutanol and ABE yields reached 7.28 and 10.18 g/L, respectively. The results indicated that poplar wood is a potential renewable raw material suitable for biobutanol production, and that Clostridium saccharobutylicum BAA-117 is a promising biobutanol producer for such conversion.

Coupling δ<sup>2</sup>H and δ<sup>18</sup>O biomarker results yields information on relative humidity and isotopic composition of precipitation – a climate transect validation study

Abstract. The hydrogen isotopic composition (δ2H) of leaf waxes, especially of n-alkanes (δ2Hn-alkanes), is increasingly used for paleohydrological and paleoclimate reconstructions. However, it is challenging to disentangle past changes in the isotopic composition of precipitation and changes in evapotranspirative enrichment of leaf water, which are both recorded in leaf wax δ2H values. In order to overcome this limitation, Zech M. et al. (2013) proposed a coupled δ2Hn-alkanes–δ18Osugar biomarker approach. This coupled approach allows for calculating (i) biomarker-based "reconstructed" δ2Hδ18O values of leaf water (δ2Hδ18Oleaf water), (ii) biomarker-based reconstructed deuterium excess (d-excess) of leaf water, which mainly reflects evapotranspirative enrichment and which can be used to reconstruct relative air humidity (RH) and (iii) biomarker-based reconstructed δ2Hδ18Oprecipitation values. Here we present a climate transect validation study by coupling new results from δ2H analyses of n-alkanes and fatty acids in topsoils along a climate transect in Argentina with previously measured δ18O results obtained for plant-derived sugars. Accordingly, both the reconstructed RH and δ2Hδ18Oprecipitation values correlate highly significantly with actual RH and δ2Hδ18Oprecipitation values. We conclude that compared to single δ2Hn-alkane or δ18Osugar records, the proposed coupled δ2Hn-alkane–δ18Osugar biomarker approach will allow more robust δ2Hδ18Oprecipitation reconstructions in future paleoclimate research. Additionally, the proposed coupled δ2Hn-alkane–δ18Osugar biomarker approach allows for the establishment of a "paleohygrometer", more specifically, the reconstruction of mean summer daytime RH changes/history.

Sugars as hydroxyl radical scavengers: proof-of-concept by studying the fate of sucralose in Arabidopsis.

Substantial formation of reactive oxygen species (ROS) is inevitable in aerobic life forms. Due to their extremely high reactivity and short lifetime, hydroxyl radicals are a special case, because cells have not developed enzymes to detoxify these most dangerous ROS. Thus, scavenging of hydroxyl radicals may only occur by accumulation of higher levels of simple organic compounds. Previous studies have demonstrated that plant-derived sugars show hydroxyl radical scavenging capabilities during Fenton reactions with Fe(2+) and hydrogen peroxide in vitro, leading to formation of less detrimental sugar radicals that may be subject of regeneration to non-radical carbohydrates in vivo. Here, we provide further evidence for the occurrence of such radical reactions with sugars in planta, by following the fate of sucralose, an artificial analog of sucrose, in Arabidopsis tissues. The expected sucralose recombination and degradation products were detected in both normal and stressed plant tissues. Oxidation products of endogenous sugars were also assessed in planta for Arabidopsis and barley, and were shown to increase in abundance relative to the non-oxidized precursor during oxidative stress conditions. We concluded that such non-enzymatic reactions with hydroxyl radicals form an integral part of plant antioxidant mechanisms contributing to cellular ROS homeostasis, and may be more important than generally assumed. This is discussed in relation to the recently proposed roles for Fe(2+) and hydrogen peroxide in processes leading to the origin of metabolism and the origin of life.

Sugar-source preference, sugar intake and relative nutritional benefits in Anopheles arabiensis males

Plant-derived sugar is the only source of dietary carbohydrate for males of most mosquito species. Male resource acquisition and utilization remain an under-researched area of behavior in vectors of human diseases. However, the renewed interest in the use of sterile males against disease vector mosquitoes reinforces the urgent need for studies on the behavioral and ecological processes that underpin male fitness and reproductive success. Here an attempt was made first to characterize the conditions and modes of resource acquisition (plant derived sugar meals) early in the life of An. arabiensis males, and second to test the hypothesis that the plants chosen for their sugar meals are those which maximize their fitness in terms of energy gains (i.e. amount of lipids, proteins, glycogen and glucose). Olfactometry assays demonstrated the ability of An. arabiensis males to discriminate among a sample of ten abundant flowering plants present in their natural habitats. In further experiments, we observed significant variations in the sugar intake rates that matched their olfactory preferences, with the most attractive plants eliciting significantly higher sugar intake rates. Consistent with our expectations, analyses of the whole-body free sugars, lipids and glycogen unequivocally showed that the energy reserve accumulated post-feeding is dependent on the diet of the adult males, with the preferred plants providing more energy reserves than the less preferred ones, despite mosquitoes actively feeding on both. Taken together, these results show that An. Arabiensis males are able to discern between food sources, preferentially feeding on those species of plant that provide the highest metabolic payoff. Ensuring or somehow heightening the ability to detect and obtain rewarding sugar meals by male mosquitoes reared for field release could enhance their competitive ability in the field.

Xylitol prevents NEFA-induced insulin resistance in rats.

Increased NEFA levels, characteristic of type 2 diabetes mellitus, contribute to skeletal muscle insulin resistance. While NEFA-induced insulin resistance was formerly attributed to decreased glycolysis, it is likely that glucose transport is the rate-limiting defect. Recently, the plant-derived sugar alcohol xylitol has been shown to have favourable metabolic effects in various animal models. Furthermore, its derivative xylulose 5-phosphate may prevent NEFA-induced suppression of glycolysis. We therefore examined whether and how xylitol might prevent NEFA-induced insulin resistance. We examined the ability of xylitol to prevent NEFA-induced insulin resistance. Sustained ~1.5-fold elevations in NEFA levels were induced with Intralipid/heparin infusions during 5 h euglycaemic-hyperinsulinaemic clamp studies in 24 conscious non-diabetic Sprague-Dawley rats, with or without infusion of xylitol. Intralipid infusion reduced peripheral glucose uptake by ~25%, predominantly through suppression of glycogen synthesis. Co-infusion of xylitol prevented the NEFA-induced decreases in both glucose uptake and glycogen synthesis. Although glycolysis was increased by xylitol infusion alone, there was minimal NEFA-induced suppression of glycolysis, which was not affected by co-infusion of xylitol. We conclude that xylitol prevented NEFA-induced insulin resistance, with favourable effects on glycogen synthesis accompanying the improved insulin-mediated glucose uptake. This suggests that this pentose sweetener has beneficial insulin-sensitising effects.

Forest Succession and Harvesting of Hemipteran Honeydew by Boreal Ants

Ants are important harvesters of plant-derived sugars, but little is known about how anthropogenic disturbances influence this behaviour. We investigated factors related to honeydew harvesting by red wood ants, Formica aquilonia, in managed boreal forests. Ant activity was lowest in middle-age stands (30–40 years old), which had the fewest and smallest ant mounds. It was best predicted by a model containing an interaction among tree species, basal diameter and stand age. Individual workers collected similar masses of honeydew from the different aged stands, which suggests that colonies optimise foraging efforts by adjusting the number of active foragers to match resource availability. The rate of honeydew harvesting from recently clear-cut stands during the survey was similar to that in old stands, although significantly faster than in middle-aged stands. This may be a result of high aphid loads on clear-cuts and recent changes in forest management that improve the temporal continuity of forests for red w...

Non-cellulosic neutral sugar contribution to mineral associated organic matter in top- and subsoil horizons of two acid forest soils

We investigated the polysaccharide composition of bulk and mineral-bound (density fractions >2 g cm −3) organic matter in topsoil and subsoil horizons of a Podzol and a Cambisol. Total sugar contents were generally higher in the Cambisol than in the Podzol. For most horizons of both soils, the sugars were enriched in the mineral-bound organic matter fraction. This fraction showed a monosaccharide distribution typical for microbial sugars, whereas in bulk soil horizons higher contributions of plant-derived sugars were observed. A strong relationship with the 14C activity of the dense fraction suggests that microbial-derived polysaccharides are most likely stabilised preferentially by mineral interactions compared to plant-derived polysaccharides.

Nectar Feeding by Wandering Spiders on Cotton Plants

Spiders are assumed to be strictly carnivorous in assessments of their nutritional and energetic requirements, their habitat preferences, and their potential as biological control agents. However, members of Salticidae (jumping spiders), Thomisidae (crab spiders), and the fast-moving Miturgidae, Anyphaenidae, and Corinnidae, all non-webbuilding wandering spiders, have been observed at floral and extrafloral nectaries of plants, presumably feeding on nectar. To test spiders in the field for nectar feeding, we used a cold anthrone test to detect the presence of ingested fructose, a plant-derived sugar, in wandering spiders occupying cotton plants (Gossypium hirsutum L.), which have floral and extrafloral nectaries. Field collections focused on three ecologically similar, highly active nocturnal spiders: Cheiracanthium inclusum (Hentz) (Miturgidae), Hibana futilis (Banks), and H. arunda (Platnick) (Anyphaenidae). During 2002 and 2003, 27 and 21%, respectively, of all field-collected adults and subadults tested positive for fructose, indicating consumption of extrafloral nectar. In both years, significantly more females were positive than males (38 versus 11% in 2002; 26 versus 12% in 2003). Immatures tested positive at a lower rate than adults (3 and 13%, respectively). Smaller numbers of spiders in the Lycosidae, Oxyopidae, and Thomisidae were also tested. Among the thomisids, 38% in 2002 and 41% in 2003 tested positive for fructose. None of the lycosids (wolf spiders) tested positive; two of nine oxyopids (lynx spiders) did test positive. Oxyopidae is new to the list of nectarivorous spiders. These results suggest that nectarivory is common for foliage wandering spiders and may contribute to fitness.

Nectar Feeding by Wandering Spiders on Cotton Plants

Spiders are assumed to be strictly carnivorous in assessments of their nutritional and energetic requirements, their habitat preferences, and their potential as biological control agents. However, members of Salticidae (jumping spiders), Thomisidae (crab spiders), and the fast-moving Miturgidae, Anyphaenidae, and Corinnidae, all non-webbuilding wandering spiders, have been observed at floral and extrafloral nectaries of plants, presumably feeding on nectar. To test spiders in the field for nectar feeding, we used a cold anthrone test to detect the presence of ingested fructose, a plant-derived sugar, in wandering spiders occupying cotton plants (Gossypium hirsutum L.), which have floral and extrafloral nectaries. Field collections focused on three ecologically similar, highly active nocturnal spiders: Cheiracanthium inclusum (Hentz) (Miturgidae), Hibana futilis (Banks), and H. arunda (Platnick) (Anyphaenidae). During 2002 and 2003, 27 and 21%, respectively, of all field-collected adults and subadults tested positive for fructose, indicating consumption of extrafloral nectar. In both years, significantly more females were positive than males (38 versus 11% in 2002; 26 versus 12% in 2003). Immatures tested positive at a lower rate than adults (3 and 13%, respectively). Smaller numbers of spiders in the Lycosidae, Oxyopidae, and Thomisidae were also tested. Among the thomisids, 38% in 2002 and 41% in 2003 tested positive for fructose. None of the lycosids (wolf spiders) tested positive; two of nine oxyopids (lynx spiders) did test positive. Oxyopidae is new to the list of nectarivorous spiders. These results suggest that nectarivory is common for foliage wandering spiders and may contribute to fitness.

Microbial conversion of sugars from plant biomass to lactic acid or ethanol

Concerns for our environment and unease with our dependence on foreign oil have renewed interest in converting plant biomass into fuels and 'green' chemicals. The volume of plant matter available makes lignocellulose conversion desirable, although no single isolated organism has been shown to depolymerize lignocellulose and efficiently metabolize the resulting sugars into a specific product. This work reviews selected chemicals and fuels that can be produced from microbial fermentation of plant-derived cell-wall sugars and directed engineering for improvement of microbial biocatalysts. Lactic acid and ethanol production are highlighted, with a focus on engineered Escherichia coli.

The nutritional value of aphid honeydew for non-aphid parasitoids

Intake of sugar-rich foods by adult parasitoids is crucial for their reproductive success. Hence, the availability of suitable foods should enhance the efficacy of parasitoids as biological control agents. In situations where nectar is not readily available, homopteran honeydew can be a key alternative food source. We studied the impact of honeydew feeding on the longevity of the larval endoparasitoids Cotesia marginiventris, Campoletis sonorensis and Microplitis rufiventris, all natural enemies of important lepidopteran pests. Females of these wasps lived longer when feeding on honeydew produced by the aphid Rhopalosiphum maidis on barley compared to control females provided with water only. However, they lived shorter than females fed with a sucrose solution. Further investigations with C. marginiventris showed that access to honeydew also increases the number of offspring produced, but less so than access to a sucrose solution. Moreover, it was found that females of this species need to feed several times throughout their life in order to reach optimal longevity and reproductive output. Analyses of the sugars in the honeydew produced by R. maidis on barley revealed that it contains mainly plant-derived sugars, but also several aphid-synthesized sugars. The sugar composition of the honeydew changed as a function of aphid colony size and time a colony had been feeding on a plant. In general, the higher the aphid infestation, the smaller the percentage of aphid-synthesized sugars in the honeydew. Experiments with honeydew sugar mimics allowed us to reject the hypothesis that the relatively poor performance of the parasitoid on a honeydew diet was due to the sugar composition. Instead, the results from additional feeding experiments with diluted honeydew showed that the nutritional value of pure honeydew is primarily restricted by its high viscosity. The possible consequences of these findings for biological pest control are discussed.

Natural nitrogen-15 abundance and carbohydrate content and composition of organic matter particle-size fractions from a sandy soil

Sandy soil samples collected from under a woody/grass savanna in the Lamto experimental area (6°13N, 5°20W; Cote d’Ivoire, West Africa), were fractionated according to particle size with the aim of measuring the natural abundance of 15N and determining the contents and composition of hydrolysable carbohydrates of soil organo-mineral particles for a better understanding of the contribution of each individual fraction to the soil function. The contributions of the fractions <20 μm to the total pool of organic matter were 77% for C and 84% for N. Larger amounts of carbohydrates were found in the clay and silt fractions (3,784–6,043 μg g−1 soil). The carbohydrate composition indicated that microbe-derived carbohydrates [e.g. galactose (Gal) and mannose (Man)] accumulated preferentially in the fine fractions while plant-derived sugars [e.g. arabinose (Ara) and xylose (Xyl)] were dominant in coarse fractions. A negative relationship was observed between C:N ratio and 15N natural abundance on the one hand, and on the other hand between C:N and (Gal+Man):(Ara+Xyl), Man:(Ara+Xyl) and Man:Xyl ratios, clearly indicating that the chemistry of the organic materials of the particle-size fractions reflects a change from soil chemistry dominated by plant materials to that dominated by microbial biomass and metabolites. The contribution of a given fraction to soil microbial activity is controlled by the quality or quantity of associated soil organic matter, its microbial biomass and also by the accumulation of microbial-derived carbohydrates which can be resynthesized or recycled.

BIOMASS OR BUST

ONE DAY, COMMODITY CHEMIcals to make pharmaceuticals, agricultural adjuvants, plastics, and transportation fuels will be produced nearly exclusively from plant-derived sugars rather than from fossil-based raw materials. How soon? Proponents of industrial biotechnology believe the needed technologies are at hand and that a biobased world economy is insight. Industry observers tend to agree with this vision, and they are supportive of industrial biotech. But they caution that the future growth and success of producing commodity chemicals and consumer products through novel bioprocesses will depend on the fate of crude-oil supplies and prices and the quality and availability of feedstock crops. The hyperbole and the reality surrounding industrial biotech were highlights of the inaugural World Congress on Industrial Biotechnology & Bioprocessing, held April 21-23 in Orlando, Fla. This conference, organized by the Biotechnology Industry Organization (BIO), American Chemical Society and National Agricultural Bi...

Plant Residue and Bacteria as Bases for Increased Stool Weight Accompanying Consumption of Higher Dietary Fiber Diets

Objective: Stool diluting effects of relatively inert material, such as unfermentable dietary fiber, has been proposed as an effect of fiber beneficial to the colon. Stool dilution by increasing bacterial mass may be beneficial or deleterious, depending on bacterial metabolic products. The purpose of this study was to determine the basis for stool weight when two stepwise increases of fiber from all classes of fiber-containing foods were consumed.Methods: Stool from five men consuming three constant diets containing 15, 30 and 42 g/d of dietary fiber were fractionated into plant material and bacteria and analyzed for neutral and amino sugar content. Fecal nitrogen, fat and ash were measured.Results: Daily gravimetric yield and sugar content of the plant fraction from stool increased with each fiber addition. Compared to the low fiber diet, the medium fiber diet decreased the concentration of the bacterial mass in wet stool by 11% and the high fiber diet by an additional 32%. The high fiber diet decreased stool fat concentration; the medium and high fiber diets decreased stool nitrogen concentration to the same extent. Apparent digestibility of plant-derived neutral sugars decreased with each fiber addition.Conclusions: Inherently less fermentable plant material modulates the colon environment in three beneficial ways: it is a relatively unreactive diluent of lumenal contents; it adds mass to promote distal movement of waste; it does not promote a large bacterial mass.

Mechanisms by which wheat bran and oat bran increase stool weight in humans

Generally, stool weight is significantly increased by adding sources of insoluble fiber to the diet. Comparable amounts of fiber provided by wheat and oat brans have the same effect on daily stool output, even though > 90% of wheat bran fiber but only 50-60% of oat bran fiber is insoluble. To determine the bases for these increases in stool weight, stool samples collected from 5 men in 2 constant diet studies that determined the effects of wheat and oat brans on large-bowel physiology were fractionated by using a physicochemical procedure into plant, bacterial, and soluble fractions, which were weighed and analyzed for sugar content and composition. Nitrogen, crude fat, and ash outputs were also determined. Wheat bran increased the fecal concentration of sugars and mass of plant material more than did oat bran, whereas oat bran increased fecal bacterial mass more. Each fiber source increased nitrogen, ash, and fat excretion, but excretion of fat was greater with oat bran. The apparent digestibility of plant-derived neutral sugars decreased significantly when wheat but not oat bran was consumed. The apparent digestibility of neutral sugars provided by wheat bran was 56%; the apparent digestibility of those provided by oat bran was 96%. We conclude that bacteria and lipids are major contributors to the increase in stool weight with oat bran consumption, whereas undigested plant fiber is responsible for much of the increase in stool weight with wheat bran consumption. Results are consistent with the hypothesis that oat bran increases stool weight by providing rapidly fermented soluble fiber in the proximal colon for bacterial growth, which is sustained until excretion by fermentation of the insoluble fiber.

Influences of mycelial fungi on soil aggregation and organic matter storage in conventional and no-tillage soils

Soil microbial community composition, aggregation and organic matter (SOM) content can be markedly influenced by tillage and crop management practices. This study was undertaken to determine to what extent differences in populations of mycelial fungi in conventional (CT) and no-tillage (NT) soils contribute to soil aggregation and soil organic matter storage. Fungicide (Captan) and control treatments were established in long-term CT and NT plots on a well-drained Hiwassee sandy clay loam soil (clayey kaolinitc thermic Rhodic Kanhapludult). The effects of these treatments on total and FDA-active fungal hyphal lengths, total bacteria (0–15 cm) and in situ soil respiration rates were measured at approximately monthly intervals. Soil carbohydrates and water-stable aggregate (WSA) distributions were quantified on the final sample date. Surface soil (0–5 cm) of NT had more macroaggregates (> 250 μm diam) and 1.30 to 1.46 times higher densities of fungal mycelia as compared to CT soils. The higher populations of fungal mycelia corresponded to a nearly two-fold higher concentration of acid-hydrolysable carbohydrates, which were composed of proportionally more microbial- than plant-derived sugars. Treatment with the fungicide resulted in a 40% reduction in > 2000 μm WSA and lower concentrations of carbohydrates in NT surface soils, but had no significant effects in CT soils. The contributions of fungi to aggregate stability may represent an important biotically-regulated mechanism for the protection of soil organic matter and may help to explain the greater retention of soil organic carbon in NT than in CT soils.

Lignin and carbohydrate alteration in particle-size separates of an oxisol under tropical pastures following native savanna

Establishment of improved tropical pastures on acid savanna soils of low fertility can increase the soil organic matter (SOM) content and soil fertility. Our objective was to follow the quantitative and qualitative changes of SOM associated with sand-, silt-and clay-sized separates after establishment of a grass-alone ( Brachiaria decumbens) and a grass-legume ( Brachiaria decumbens + Pueraria phaseoloides) pasture on isohyperthermic savanna soils of Colombia. Additionally, earthworm casts of anecic and endo-anecic species derived from the grass-legume pasture were investigated. Whole soil and size separates were analyzed for C, N, lignin-derived phenols (alkaline CuO-oxidation) and carbohydrates (4 m trifluoroacetic acid hydrolysis). For native savanna, grass-alone and grass-legume pasture the C content increased in the order sand (1.6–3.3 g C kg −1) < silt (25.8–32.1 g C kg −1) < clay (38.8–43.0 g C kg −1); the C-to-N ratio decreased in the same order. Fifteen years of pasture establishment resulted in an increase of C by factors of ca. 2 (sand), 1.3 (silt) and 1.1 (clay) compared to native savanna. Decreasing yields of phenolic CuO-oxidation products and increasing side-chain oxidation of the remnant lignin indicated progressive lignin alteration with decreasing particle size. Soil under savanna was substantially lower in intact lignin structures than soil under grass-alone and grass-legume pastures. Enrichment factors, E = (mg kg −1 soil under pasture/mg kg −1 soil under savanna), for lignin-derived phenolics after establishment of B. decumbens and B. decumbens + P. phaseoloides were 3.9 and 2.7, 4.3 and 2.4, and 2.6 and 2.6 in sand-, silt-and clay-sized separates, respectively. Clay-bound SOM was rich in microbially-derived sugars, whereas the SOM of the sand-sized separates contained large amounts of plant-derived sugars. Silt-associated SOM had low concentrations of both. Establishment of grass-alone and grass-legume pasture increased the amount of easily-degradable plant-derived sugars by factors of 3.0 and 3.5 (sand), 1.9 and 1.3 (silt), and 1.1 and 1.1 (clay). SOM distribution across size separates suggested an increasing amount of unprotected macro-OM after establishment of grass-alone and grass-legume pastures. Concurrently, lignin and carbohydrate signature indicated higher contents of relatively undergraded plant-derived SOM, notably in the sand-sized separates. It was concluded that the increase of this SOM fraction has beneficial effects on the soil internal N cycle as far as N mineralization and N immobilization is concerned. Earthworm casts were characterized by a tremendous enrichment of C, intact lignin and plant-derived sugars in the sand-sized separates, compared with the surrounding soil. This suggested a pronounced incorporation of fresh litter into the mineral soil (anecic effect), that decomposition processes were dramatically slowed in the casts, and a pool of relatively labile, but physically-protected SOM was built up.