Plant Biopolymers Research Articles

Litter decomposition along a primary post-mining chronosequence

The aim of this study was to describe the decomposition of litter along a successive series of sites developed at a post-mining overburden deposit over 12, 21 and 45 years, representing the early, mid and late stages of succession. Litter decomposition was largely dependent on the initial composition of the litters. The tree litter of the mid and late stages decomposed faster than the grass litter of the early stage, with 64, 60 and 35 % of mass lost over 2 years, respectively. The contents of hot-water-soluble C and N, which were the best predictors of litter decay rates, were relatively stable over time in all litters. Neither the nutrient content nor the plant biopolymer composition exhibited convergence during decay, indicating that the litter-derived soil organic matter most likely carries a legacy of the original vegetation. In contrast to the litter chemistry, the development of the microbial community was largely specific to the decay stage and consistent among the litters, showing decreasing fungal dominance in time. Extracellular enzymes were found to be of limited value in the prediction of litter decay rates, chemical transformation or microbial community composition.

Controls on wood and leaf litter incorporation into soil fractions in forests at different successional stages

In this study, we examined the response of surface soils to increased leaf and wood litter input within adjacent successional forests recovering from agricultural disturbance at the Smithsonian Environmental Research Center (SERC), Maryland, USA. Previous studies at this site demonstrated an arrested development of O-horizon, even after 130 years of forest growth, and an annual loss of leaf litter in forests with the highest abundance of invasive earthworms. Biogeochemical indices of plant biopolymer dynamics, i.e. extractable lignin and substituted fatty acids (SFAs), were applied to soil physical fractions in order to assess the fate of 5 years of increased Tulip poplar (Liriodendron tulipifera L.) wood and leaf litter into O-horizon and mineral soil particles of purportedly different protection levels in this recovering forest system. Our results showed that in this continuously-disturbed recovering system the pattern of litter incorporation into soil varied with both litter type and forest age. For example, young successional forests, that also contained higher abundances of soil feeding endogeic earthworms, incorporated wood amendments deeper into soils and in a predominantly particulate organic matter (POM) form than older successional systems with predominantly litter and surface dwelling earthworms. Soil lignin concentration increased sharply with wood amendments in both forest stages, but young successional forests exhibited incorporation of fresher lignin into both POM and silt and clay (SC) fractions over 0–5 cm and 5–10 cm depths while old forests only increased in POM in the 0–5 cm depth. We attribute these differences to the higher rates of physical mixing from soil feeding endogeic species and potentially lower fungal activity in young successional forests. However, despite nearly 2.5 times of background annual leaf litter input over 5 years, neither total C content nor SFA concentration in soil fractions increased, a phenomenon we attribute to full decomposition of leaf litter amendments. These results demonstrate how the chemical trajectory of soils and litter layers in recovering forests can be a function of both legacy and current disturbance.

Assimilation of cellulose-derived carbon by microeukaryotes in oxic and anoxic slurries of an aerated soil.

Soil microeukaryotes may trophically benefit from plant biopolymers. However, carbon transfer from cellulose into soil microeukaryotes has not been demonstrated so far. Microeukaryotes assimilating cellulose-derived carbon in oxic and anoxic soil slurries were therefore examined by rRNA-based stable-isotope probing. Bacteriovorous flagellates and ciliates and, likely, mixotrophic algae and saprotrophic fungi incorporated carbon from supplemental [U-(13)C]cellulose under oxic conditions. A previous study using the same soil suggested that cellulolytic Bacteria assimilated (13)C of supplemental cellulose. Thus, it can be assumed that ciliates, cercozoa, and chrysophytes assimilated carbon by grazing upon and utilizing metabolic products of Bacteria that hydrolyzed cellulose in the soil slurries.

Biopolymer Stabilized Iron Sulphide Nanoparticles for Removal of Acid Black 1 Dye

India is one of the largest producer and exporter of dyestuff among developing countries. Large amount of dyes is lost in industrial effluents during production and industrial use. Persistence of dyes in the environment, their non-biodegradability and toxicity necessitate urgent steps to develop environment-friendly approaches to remove them from wastewaters. The present work describes attempts to use iron-based catalytic nanoparticles (NPs) for removal and degradation of Acid Black 1, one of the notoriously toxic dyes. A rapid screening assay was developed in order to select NPs possessing reducing properties. Methylene blue (MB) dye (0.1mg/L, 250µl) was exposed to different preparations of NPs (7mg) for 30 minute in a 96-well microtiter plate and read at 630 nm using an ELISA reader. Iron sulphide nanoparticles (FeSNPs) stabilized by a plant biopolymer (BP) could remove >95% MB and were selected for further studies. The FeSNPs could remove Acid Black 1 efficiently (73.8%) with specific dye removal capacity of 19.7 mg/g NPs. FeSNPs were immobilized in alginate beads (average diameter 3.45 mm) and packed in polypropylene columns (22.5 cm long, 3 cm i.d.) having bed volume of 42 ml. Acid black 1 solution (20 mg/L) was passed through the columns at predetermined flow rates in up-flow mode using peristaltic pump. The mass transfer kinetics were favourable at a flow rate of 2.2 ml/min (mass transfer coefficient 9.38X10-5 L.mg-1.min-1), and 120 bed volumes of the solution could be treated with efficiency exceeding 90%. Analysis of column effluent by UV-visible and FTIR spectroscopy revealed that removal of dye from solution was due to reductive degradation.

Chemical composition and molecular structure of polysaccharide-protein biopolymer from Durio zibethinusseed: extraction and purification process

BackgroundThe biological functions of natural biopolymers from plant sources depend on their chemical composition and molecular structure. In addition, the extraction and further processing conditions significantly influence the chemical and molecular structure of the plant biopolymer. The main objective of the present study was to characterize the chemical and molecular structure of a natural biopolymer from Durio zibethinus seed. A size-exclusion chromatography coupled to multi angle laser light-scattering (SEC-MALS) was applied to analyze the molecular weight (Mw), number average molecular weight (Mn), and polydispersity index (Mw/Mn).ResultsThe most abundant monosaccharide in the carbohydrate composition of durian seed gum were galactose (48.6-59.9%), glucose (37.1-45.1%), arabinose (0.58-3.41%), and xylose (0.3-3.21%). The predominant fatty acid of the lipid fraction from the durian seed gum were palmitic acid (C16:0), palmitoleic acid (C16:1), stearic acid (C18:0), oleic acid (C18:1), linoleic acid (C18:2), and linolenic acid (C18:2). The most abundant amino acids of durian seed gum were: leucine (30.9-37.3%), lysine (6.04-8.36%), aspartic acid (6.10-7.19%), glycine (6.07-7.42%), alanine (5.24-6.14%), glutamic acid (5.57-7.09%), valine (4.5-5.50%), proline (3.87-4.81%), serine (4.39-5.18%), threonine (3.44-6.50%), isoleucine (3.30-4.07%), and phenylalanine (3.11-9.04%).ConclusionThe presence of essential amino acids in the chemical structure of durian seed gum reinforces its nutritional value.

Aqueous extract of gum olibanum (Boswellia serrata): A reductant and stabilizer for the biosynthesis of antibacterial silver nanoparticles

A simple and ecofriendly biosynthetic process has been developed for silver nanoparticles using the aqueous extract of gum olibanum (Boswellia serrata), a renewable natural plant biopolymer. The water soluble compounds in the gum serve as dual functional reducing and stabilizing agents. The effect of concentration of gum and silver nitrate; and reaction time on nanoparticle synthesis was studied. The UV–visible spectroscopy, transmission electron microscopy and X-ray diffraction techniques were used to characterize the synthesized nanoparticles. By tuning the reaction conditions, size controlled spherical nanoparticles of around 7.5±3.8nm was achieved. Using Fourier transform infrared spectroscopy and Raman spectroscopy, a probable mechanism involved in reduction and stabilization of nanoparticles has been explained. The produced silver nanoparticles exhibited substantial antibacterial activity on both the Gram classes of bacteria. By virtue of being biogenic and encapsulated with proteins, these surface functionalized nanoparticles can be easily integrated for various biological applications.

Rationalization and In Vitro Modeling of the Chemical Mechanisms of the Enzymatic Oxidation of Phenolic Compounds in Planta: From Flavonols and Stilbenoids to Lignins

Enzymatic oxidation of phenolic compounds is a widespread phenomenon in plants. It is responsible for the formation of many oligomers and polymers, which are generally described as the result of a combinatorial coupling of the different radicals formed through oxidation of the phenol group and delocalization of the radical. We focused our interest on several phenolic compounds that are present in plants and known to form, under enzymatic oxidation, oligomers with different type of linkages between monomers. To explain this diversity of inter-monomer linkages and their variation according to the experimental procedure used for the enzymatic oxidation, we report an alternative mechanistic pathway involving dismutation of the radicals, leading to the formation of carbocations which, thereafter, react with nucleophilic species present in the medium. This alternative pathway allows the understanding of peculiar linkages between monomeric units in the oligomer and offers new insights for understanding the formation of phenolic biopolymers in plants.

Soil carbon pools, plant biomarkers and mean carbon residence time after afforestation of grassland with three tree species

Afforestation of grassland has been globally identified as being an important means for creating a sink for atmospheric carbon (C). However, the impact of afforestation on soil C is still poorly understood, due to the paucity of well designed long-term experiments and the lack of investigation into the response of different soil C fractions to afforestation. In addition, little is known about the origins of soil C and soil organic matter (SOM) stability after afforestation. In a retrospective study, we measured C mass in the soil light and heavy fractions in the first 10 years after afforestation of grassland with Eucalyptus nitens, Pinus radiata and Cupressus macrocarpa. The results suggest that C mass in the soil heavy fraction remained stable, but the C mass in the light fraction decreased at year 5 under three species. Soil δ 13C analysis showed that the decrease in the light fraction may be due to reduced C inputs from grassland species litter and low inputs from the still young trees. After the initial reduction, the recovery of soil C in the light fraction depended on tree species. At year 10, an increase of 33% in light fraction soil C was observed at the 0–30 cm depth under E. nitens, compared to that under the original grassland (year 0). Planting P. radiata restored light fraction soil C to the original level under grassland, whereas planting C. macrocarpa led to a decrease of 33%. We concluded that the increase of light fraction soil C between year 5 and 10 is most likely due to C input from tree residues. Most of the increased C was derived from root turnover under pine and from both root and leaf turnover under E. nitens, as indicated by plant C biomarkers such as lignin-derived phenols and suberin and cutin-derived compounds in the 0–5 cm soil layer. Modelling of soil ∆ 14C‰ suggested that SOM had a greater mean residence time at year 10 than year 0 and 5 due to increased relative abundance of recalcitrant plant biopolymers.

The strawberry transcription factor FaMYB1 inhibits the biosynthesis of proanthocyanidins in Lotus corniculatus leaves

Proanthocyanidins (PAs) are agronomically important biopolymers in higher plants composed primarily of catechin and epicatechin units. The biosynthesis of these natural products is regulated by transcription factors including proteins of the R2R3MYB class. To gain insight into the genetic control of the catechin and epicatechin branches of the PA pathway in forage legumes, here the effects of the expression of FaMYB1, a flavonoid R2R3MYB repressor from strawberry, in Lotus corniculatus (birdsfoot trefoil), were tested. It was found that in leaves of T(0) transgenic lines the degree of PA inhibition correlated with the level of FaMYB1 expression. These effects were heritable in the transgene-positive plant T(1) generation and were tissue specific as the suppression of proanthocyanidin biosynthesis was most pronounced in mesophyll cells within the leaf, whereas other flavonoid and phenolic compounds were substantially unaltered. The data suggest that FaMYB1 may counter-balance the activity of the endogenous transcriptional MYB-bHLH-WD40 (MBW) complex promoting proanthocyanidin biosynthesis via the catechin and epicatechin branches and that FaMYB1 does not interfere with the expression levels of a resident R2R3MYB activator of PAs. It is proposed that in forage legumes leaf cell commitment to synthesize proanthocyanidins relies on the balance between the activity of activator and repressor MYBs operating within the MBW complex.

Isolation and Identification of Triglycerides and Ester Oligomers from Partial Degradation of Potato Suberin

Suberized cell walls from wound-healing potato tubers ( Solanum tuberosum ) were depolymerized under mild conditions using methanolic potassium hydroxide in order to investigate the chemical linkages present in this protective plant biopolymer. Analysis of the resulting soluble oligomeric fragments with HPLC, 1D and 2D NMR, LC/MS, and MS(n) methods allowed identification of several novel compounds: a family of homologous triglycerides, a family of homologous aliphatic ester trimers, and an ether-linked phenylacetic acid dimer. These findings illustrate the diversity of rigid and flexible molecular linkages present in both poly(aliphatic) and poly(aromatic) domains of potato suberin, and they point toward architectures that may account for its function as a potent hydrophobic barrier to water, thermal equilibration, and microbial pathogens.

Isolation and characterization of glucuronoarabinoxylans from wheat bran obtained by classical and ultrasound-assisted extraction methods

Introduction Plant biopolymers like arabinoxylans possess valuable potentials as food additives with health-promoting properties or as basic products for the production of chemically functionalized polymers with unexploited technical or physiological properties. Wheat bran as an abundant milling by-product can serve as a cheap renewable source of cereal arabinoxylans. Isolation yields of xylans from plant materials are traditionally only moderate or low. Objectives A study was undertaken to investigate the effect of short-ultrasound application on the extractability of arabinoxylans from wheat bran suspended in aqueous alkaline solutions on a laboratory scale. The influence of the ultrasonic treatment on the overall extraction yield, purity and chemical structure of bran arabinoxylans under conditions of different sonication intensities and alkaline media was compared with conventional procedures not applying ultrasound. Results Extraction of xylans from bran using alkaline peroxide solutions without sonication for 240min at 601C and subsequent purification provided polymers of 83% purity of a total yield of about 14%. Products of the same purity could be isolated by ultrasonication of bran suspensions in alkaline peroxide solutions for 10min at 601C. Structural details of the recovered xylans were verified by high-pressure liquid chromatography-based monomer analysis, Fourier transform infrared, 13 Cnuclear magnetic resonance spectroscopy and molecular weight distribution analysis by high-performance size exclusion chromatography with triple detection. Ultrasound-assisted extraction lead to a partial depolymerization of the isolated arabinoxylan. Conclusion Applying ultrasound for the extraction of xylans from plant cell walls can reduce the extraction time considerably but has no influence on the maximum extraction yield.

Sources of plant‐derived carbon and stability of organic matter in soil: implications for global change

AbstractAlterations in forest productivity and changes in the relative proportion of above‐ and belowground biomass may have nonlinear effects on soil organic matter (SOM) storage. To study the influence of plant litter inputs on SOM accumulation, the Detritus Input Removal and Transfer (DIRT) Experiment continuously alters above‐ and belowground plant inputs to soil by a combination of trenching, screening, and litter addition. Here, we used biogeochemical indicators [i.e., cupric oxide extractable lignin‐derived phenols and suberin/cutin‐derived substituted fatty acids (SFA)] to identify the dominant sources of plant biopolymers in SOM and various measures [i.e., soil density fractionation, laboratory incubation, and radiocarbon‐based mean residence time (MRT)] to assess the stability of SOM in two contrasting forests within the DIRT Experiment: an aggrading deciduous forest and an old‐growth coniferous forest. In the deciduous forest, removal of both above‐ and belowground inputs increased the total amount of SFA over threefold compared with the control, and shifted the SFA signature towards a root‐dominated source. Concurrently, light fraction MRT increased by 101 years and C mineralization during incubation decreased compared with the control. Together, these data suggest that root‐derived aliphatic compounds are a source of SOM with greater relative stability than leaf inputs at this site. In the coniferous forest, roots were an important source of soil lignin‐derived phenols but needle‐derived, rather than root‐derived, aliphatic compounds were preferentially preserved in soil. Fresh wood additions elevated the amount of soil C recovered as light fraction material but also elevated mineralization during incubation compared with other DIRT treatments, suggesting that not all of the added soil C is directly stabilized. Aboveground needle litter additions, which are more N‐rich than wood debris, resulted in accelerated mineralization of previously stored soil carbon. In summary, our work demonstrates that the dominant plant sources of SOM differed substantially between forest types. Furthermore, inputs to and losses from soil C pools likely will not be altered uniformly by changes in litter input rates.

Diagenesis of plant biopolymers: Decay and macromolecular preservation of Metasequoia

Analysis of modern Metasequoia leaves revealed the presence of the structural polyester cutin, guaiacyl lignin units and polysaccharides. Analysis of environmentally decayed Metasequoia leaves revealed that guaiacyl lignin units and cellulose were degraded more than vinyl phenol (the last being the primary pyrolysis product of cutin and plant cuticles) suggesting that cutin is more stable than lignin and cellulose during degradation, contrary to some previous studies. This observation is supported by electron microscopy showing changes in the cellular structure and cuticle of modern, decayed and fossil Metasequoia leaves. Metasequoia fossils from the Eocene of Republic (Washington State) showed a significant aliphatic component, but biopolymeric lignin and polysaccharides were not detected. Fossils from the Eocene of Axel Heiberg revealed the presence of lignin and an aliphatic polymer up to C 29 with cellulose, and fossils from the Miocene Clarkia deposit (Idaho) revealed lignin and an aliphatic polymer up to C 27 without any polysaccharides. Modern Metasequoia needles heated experimentally in confined conditions generated a macromolecular composition with an aliphatic polymer up to C 32 and additional phenolic compounds similar to those present in the fossils. Experimental heating of cutin is known to generate an aliphatic polymer with carbon chain length units <C 20. Thus, the n-alkyl component with chain length units >C 20 in the heated Metasequoia needles is a product of incorporation of longer chain plant waxes, indicated by the odd/even predominance of the >C 27 n-alkanes. The resistant nature of cutin compared to lignin and polysaccharides explains the presence of an n-alkyl component (<C 20) in fossil leaves even when polysaccharides are absent and lignin has decayed; cutin and its diagenetically altered products contribute significantly to the presence of aliphatic components in terrestrially derived sedimentary organic matter.

Nanometer-scale structure of alkali-soluble bio-macromolecules of maize plant residues explains their recalcitrance in soil

The quantity and quality of plant litter in the soil play an important role in the soil organic matter balance. Besides other pedo-climatic aspects, the content of recalcitrant molecules of plant residues and their chemical composition play a major role in the preservation of plant residues. In this study, we report that intrinsically resistant alkali-soluble bio-macromolecules extracted from maize plant (plant-humic acid) (plant-HA) contribute directly to the soil organic matter (OM) by its addition and conservation in the soil. Furthermore, we also observed that a high syringyl/guaiacyl ( S/ G) ratio in the lignin residues comprising the plant tissue, which modifies the microscopic structure of the alkali-soluble plant biopolymers, enhances their recalcitrance because of lower accessibility of molecules to degrading enzymes. These results are in agreement with a recent study, which showed that the humic substance of soil consists of a mixture of identifiable biopolymers obtained directly from plant tissues that are added annually by maize plant residues.

Molecular changes in organic matter of a compost‐amended soil

SummaryThe molecular changes of organic matter in a cultivated soil after compost amendments was followed by off‐line‐pyrolysis‐TMAH GC‐MS. Thermochemolysis of soil and compost provided a detailed molecular characterization of soil organic matter (SOM) by releasing a large amount of different molecules mainly derived from plant biopolymers such as lignin, waxes and bio‐polyesters. No significant differences were found before and after 1 year of cultivation in the pyrolytic products released by control soil, which were mainly fatty acids, oxidized forms of lignins, and minor amounts of microbial bio‐products and biopolyesters derivatives. Conversely, significant qualitative and quantitative variations were found in the molecular characteristics of SOM between control and compost‐amended soils after 1 year of cultivation. Increasing amounts and diversified components of fatty acids, n‐alkanes and various biopolyesters derivatives such as hydroxy‐alkanoic and alkandioic acids were found in the compost‐amended soil. These results indicate that a significant amount of exogenous compost‐derived organic molecules were incorporated into SOM after 1 year of cultivation. The organic structural indexes derived from these results indicated direct inputs of undecomposed lignin residues and hydrocarbon waxes from compost material. When compared with the control soil, small but significant amounts of plant biomarkers, such as cyclic di‐ and triterpenes derivatives, were found only in the compost‐amended soil. These findings suggest that the molecular changes of SOM brought about by amendment with biomass residues can be followed by using thermochemolysis of bulk soil samples.

Earthworms, stand age, and species composition interact to influence particulate organic matter chemistry during forest succession

The landscapes colonized by invasive earthworms in the eastern U.S. are often patchworks of forest stands in various stages of successional development. We established six field sites in tulip poplar dominated forests in the Smithsonian Environmental Research Center in Edgewater, MD, that span mid (50–70 years-three plots) and late (120–150 years-three plots) successional stages where younger sites had greater earthworm density and biomass than older sites and were dominated by non-native lumbricid species. In particular Lumbricus rubellus, a litter-feeding species, was abundant in mid successional forests. Here, we separated particulate organic matter (POM) from the bulk soil by a combination of size and density fractionation and found that patterns in soil POM chemistry were similar to those found previously during litter decay: in younger forests with high abundance of earthworms, organic carbon normalized cutin- and suberin-derived substituted fatty acid (SFA) concentration was lower and lignin-derived phenols greater than in older forests where earthworms were less abundant. The chemistry of the dominant litter from mid versus late successional tree species did not fully explain the differences in POM chemistry between age classes. Instead, the differences in leaf body versus petiole and leaf versus root chemistry were the dominant drivers of POM chemistry in mid versus late successional stands, although aspects of stand age and tree species also impacted POM chemistry. Our results indicate that preferential ingestion of leaf body tissue by earthworms and the subsequent shifts in sources of plant biopolymers in soil influenced POM chemistry in mid successional forests. These results indicate that invasive earthworm activity in North American forests contributes to a shift in the aromatic and aliphatic composition of POM and thus potentially influences carbon stabilization in soil.

Production of renewable polymers from crop plants

Plants produce a range of biopolymers for purposes such as maintenance of structural integrity, carbon storage, and defense against pathogens and desiccation. Several of these natural polymers are used by humans as food and materials, and increasingly as an energy carrier. In this review, we focus on plant biopolymers that are used as materials in bulk applications, such as plastics and elastomers, in the context of depleting resources and climate change, and consider technical and scientific bottlenecks in the production of novel or improved materials in transgenic or alternative crop plants. The biopolymers discussed are natural rubber and several polymers that are not naturally produced in plants, such as polyhydroxyalkanoates, fibrous proteins and poly-amino acids. In addition, monomers or precursors for the chemical synthesis of biopolymers, such as 4-hydroxybenzoate, itaconic acid, fructose and sorbitol, are discussed briefly.

A new conceptual model for the structural properties of char produced during vegetation fires

The elemental and 13C nuclear magnetic resonance (NMR) analysis of laboratory prepared char samples from the main plant biopolymers (cellulose, lignin, casein), as well as grass and beech sawdust, indicated a greater chemical heterogeneity of plant derived chars than generally assumed from common black carbon (BC) models. The data support a recent concept proposing char as a heterogeneous mixture of thermally altered biomacromolecules with N, O and likely also S substitutions as common features. In contrast to soot, graphitic polyaromatic domains play a minor role in such chars. The high resistance of casein against thermal treatment and the low atomic N/C values for grass chars demonstrate the quantitative importance of pyrogenic N. A more detailed consideration is needed if a more realistic view of the role of pyrogenic organic matter (PyOM) in soils and sediments is required.

Sorption of 1-naphthol by plant cuticular fractions

Abstract The contribution of aliphatic-rich plant biopolymer to sorption of hydrophobic organic compounds is significantly important because of their preservation and accumulation in the soil environment, but sorption mechanism is still not fully understood. In this study, sorption of 1-naphthol by plant cuticular fractions was examined to better understand the contributions of respective fraction. Toward this end, cuticular materials were isolated from the fruits of tomato by chemical method. The tomato cuticle sheet consisted of waxes (6.5 wt%), cuticular monomer (69.5 wt%), and polysaccharide (24.0 wt%). Isotherms of 1-naphthol to the cuticular fractions were nonlinear ( N value (0.82 – 0.90)) at the whole tested concentration ranges. The K oc/ K ow ratios for bulk cuticle (TC1), dewaxed cuticle (TC2), cutin (TC4), and desugared cuticle (TC5) were larger than unity, suggested that tomato bulk cuticle and cutin are much powerful sorption medium. Sorption capability of cutin (TC4) was 2.4 times higher than the nonsaponifiable fraction (TC3). The 1-naphthol interactions with tomato cuticular materials were governed by both hydrophobic-type interactions and polar (H-bonding) interactions. Removal of the wax and polysaccharide materials from the bulk tomato cuticle caused a significant increase in the sorption ability of the cuticular material. There was a linear negative trend between K oc values and the amount of polysaccharides or fraction's polarities ((N+O)/C); while a linear positive relationship between K oc values and the content of cutin monomer (linear R 2 = 0.993) was observed for present in the cuticular fractions. Predominant sorbent of the hydrophobic organic compounds (HOCs) in the plant cuticular fraction was the cutin monomer, contributing to 91.7% of the total sorption of tomato bulk cuticle.

Humic Substances in Soils: Are They Really Chemically Distinct?

Humic substances (HS) are an operationally defined fraction of soil organic matter, and they represent the largest pool of recalcitrant organic carbon in the terrestrial environment. It has traditionally been thought that extractable HS consist of novel categories of cross-linked macromolecular structures. In this study, advanced nuclear magnetic resonance approaches were used to study the major components (proteins, carbohydrates, aliphatic biopolymers, and lignin) that are known to be present in HS, and to identify their fingerprints in humic mixtures. Theoretically, once all known components have been identified, the remaining signals should be from materials with novel structures, themselves forming a distinct chemical category of humic materials. Surprisingly, nearly all of the NMR signals in traditional HS fractions could be assigned to intact and degrading biopolymers. We therefore suggest that the vast majority of operationally defined humic material in soils is a very complex mixture of microbial and plant biopolymers and their degradation products but not a distinct chemical category. It is important to note this work in no way rules out the existence of a distinct category of humic macromolecules, either at low abundance in the soluble fraction from young soils, in diagenetically evolved samples (for example lignites, etc.), or in the nonextractable humin fraction.