Bacterial secondary production on vascular plant detritus: relationships to detritus composition and degradation rate.

Bacterial production at the expense of vascular plant detritus was measured for three emergent plant species (Juncus effusus, Panicum hemitomon, and Typha latifolia) degrading in the littoral zone of a thermally impacted lake. Bacterial secondary production, measured as tritiated thymidine incorporation into DNA, ranged from 0.01 to 0.81 microgram of bacterial C mg of detritus-1 day-1. The three plant species differed with respect to the amount of bacterial productivity they supported per milligram of detritus, in accordance with the predicted biodegradability of the plant material based on initial nitrogen content, lignin content, and C/N ratio. Bacterial production also varied throughout the 22 weeks of in situ decomposition and was positively related to the nitrogen content and lignin content of the remaining detritus, as well as to the temperature of the lake water. Over time, production was negatively related to the C/N ratio and cellulose content of the degrading plant material. Bacterial production on degrading plant material was also calculated on the basis of plant surface area and ranged from 0.17 to 1.98 micrograms of bacterial C cm-2 day-1. Surface area-based calculations did not correlate well with either initial plant composition or changing composition of the remaining detritus during decomposition. The rate of bacterial detritus degradation, calculated from measured production of surface-attached bacteria, was much lower than the actual rate of weight loss of plant material. This discrepancy may be attributable to the importance of nonbacterial organisms in the degradation and loss of plant material from litterbags or to the microbially mediated solubilization of particulate material prior to bacterial utilization, or both.     

Patterns in decomposition rates among photosynthetic organisms: the importance of detritus C:N:P content

The strength and generality of the relationship between decomposition rates and detritus carbon, nitrogen, and phosphorus concentrations was assessed by comparing published reports of decomposition rates of detritus of photosynthetic organisms, from unicellular algae to trees. The results obtained demonstrated the existence of a general positive, linear relationship between plant decomposition rates and nitrogen and phosphorus concentrations. Differences in the carbon, nitrogen, and phosphorus concentrations of plant detritus accounted for 89% of the variance in plant decomposition rates of detritus orginating from photosynthetic organisms ranging from unicellular microalgae to trees. The results also demonstrate that moist plant material decomposes substantially faster than dry material with similar nutrient concentrations. Consideration of lignin, instead of carbon, concentrations did not improve the relationships obtained. These results reflect the coupling of phosphorus and nitrogen in the basic biochemical processes of both plants and their microbial decomposers, and stress the importance of this coupling for carbon and nutrient flow in ecosystems. This work was funded through a grant of CICYT (MAR91-0503) to C.M.D.     

Meiofauna promotes litter decomposition in stream ecosystems depending on leaf species

Litter decomposition, a fundamental process of nutrient cycling and energy flow in freshwater ecosystems, is driven by a diverse array of decomposers. As an important component of the heterotrophic food web, meiofauna can provide a trophic link between leaf‐associated microbes (i.e., bacteria and fungi)/plant detritus and macroinvertebrates, though their contribution to litter decomposition is not well understood. To investigate the role of different decomposer communities in litter decomposition, especially meiofauna, we compared the litter decomposition of three leaf species with different lignin to nitrogen ratios in litter bags with different mesh sizes (0.05, 0.25, and 2 mm) in a forested stream, in China for 78 days. The meiofauna significantly enhanced the decomposition of leaves of high‐and medium‐ quality, while decreasing (negative effect) or increasing (positive effect) the fungal biomass and diversity. Macrofauna and meiofauna together contributed to the decomposition of low‐quality leaf species. The presence of meiofauna and macrofauna triggered different aspects of the microbial community, with their effects on litter decomposition varying as a function of leaf quality. This study reveals that the meiofauna increased the trophic complexity and modulated their interactions with microbes, highlighting the important yet underestimated role of meiofauna in detritus‐based ecosystems.     

高寒森林溪流对凋落叶分解过程中木质素降解的影响

溪流广泛分布于高寒森林地表, 凋落于其中的林木凋落物的分解是整个森林生态系统物质循环的重要环节, 水体流动过程中的冲刷和淋洗作用及其他独特的环境条件可能显著影响凋落物中木质素的降解。该研究采用凋落袋法对比研究了岷江上游高寒森林4种典型且初始质量差异显著的凋落叶, 即康定柳(Salix paraplesia)、高山杜鹃(Rhododendron lapponicum)、方枝柏(Sabina saltuaria)和四川红杉(Larix mastersiana), 在不同生境(林下、溪流和河岸带)下分解过程中木质素残留质量和浓度(质量百分率)的动态变化特征。经过两年的分解, 发现溪流显著促进了凋落叶中木质素的降解; 同一物种凋落叶在不同生境下木质素残留质量差异显著(p < 0.05), 整体表现为溪流<河岸带<林下; 在凋落叶分解的初期木质素有明显的降解, 其浓度表现为先降低后升高, 但不同物种之间存在显著(p < 0.05)的差异; 在整个分解过程中, 木质素残留质量总体呈现出了降低的趋势。此外, 生境类型、分解时期和区域性环境因子(温度、pH值和营养元素的有效性)能显著影响木质素的降解率。这些结果表明, 传统上认为木质素在凋落叶分解初期相对稳定的观点可能并不准确, 其浓度很可能是先下降后升高, 这也与有关木质素动态的最新研究结果相一致。另一方面, 在不同分解时期和不同生境下, 凋落叶木质素降解率表现出了显著差异, 表明区域性环境因子在凋落叶分解和木质素降解过程中具有重要的作用。     

Characterization of organic matter in a forest soil of coastal British Columbia by NMR and pyrolysis-field ionization mass spectrometry

Organic matter in the soil profile under a young Douglas-fir stand in coastal British Columbia was characterized by examining intact samples of fresh litterfall and organic horizons (LF, H), and fractions (floatables, humic acid [HA], fulvic acid [FA], humin [HU]) from the three mineral horizons (Ae, Bm, BC). Some 30–40% of the carbon in the mineral horizons was found in poorly-decomposed plant material floatable in water, a fraction whose characteristics changed little with depth, and which contained over 1% Fe. The proportion of soil C in HA plus FA was approximately 8%, but the ratio of C in FA/HA increased with depth. Solid-state ¹³C NMR spectra of litterfall, LH and H samples showed effects of decomposition, in particular a decrease in 0-alkyl C from litterfall to LH to H, and degradation of resolution from LF to H. For the mineral soil fractions, both floatables and de-ashed HU (HU_d prepared by HCl/HF treatment) indicated high levels of the original plant biopolymers, including a large alkyl component. Solution ¹³C spectra of the HAs from mineral horizons showed little difference with depth, except that peaks due to lignin were more pronounced for the Bm HA. The NMR spectra of FAs were high in 0-alkyl and carboxyl C. Pyrolysis-field ionization mass spectrometry confirmed and extended the results from NMR and chemical analyses, in particular demonstrating the accumulation of suberin in some fractions and the leaching and decomposition of lignin components with increasing depth in the mineral horizons. The general features of the HA, FA and HU_d from this forest soil, and the effects of decomposition and pedogenesis were similar to those widely found for agricultural and forest soils. However, the accumulation of suberin, and the leaching and decomposition of lignin are particularly associated with forest soils. The low proportion of soil C in HA and FA, and the high proportion in poorly decomposed, iron-rich plant fragments suggest that decomposition is somewhat limited at this site, which is classified as having low fertility. The high accumulations of alkyl C from suberin may also indicate, or contribute to inhibition of decomposition.     

Shoot litter breakdown and zinc dynamics of an aquatic plant,Schoenoplectus californicus

Decomposition of plant debris is an important process in determining the structure and function of aquatic ecosystems. The aims were to find a mathematic model fitting the decomposition process of Schoenoplectus californicus shoots containing different Zn concentrations; compare the decomposition rates; and assess metal accumulation/mobilization during decomposition. A litterbag technique was applied with shoots containing three levels of Zn: collected from an unpolluted river (RIV) and from experimental populations at low (LoZn) and high (HiZn) Zn supply. The double exponential model explained S. californicus shoot decomposition, at first, higher initial proportion of refractory fraction in RIV detritus determined a lower decay rate and until 68 days, RIV and LoZn detritus behaved like a source of metal, releasing soluble/weakly bound zinc into the water; after 68 days, they became like a sink. However, HiZn detritus showed rapid release into the water during the first 8 days, changing to the sink condition up to 68 days, and then returning to the source condition up to 369 days. The knowledge of the role of detritus (sink/source) will allow defining a correct management of the vegetation used for zinc removal and providing a valuable tool for environmental remediation and rehabilitation planning.     

Prediction of in situ root decomposition rates in an interspecific context from chemical and morphological traits

Background and Aims

We quantitatively relate in situ root decomposition rates of a wide range of trees and herbs used in agroforestry to root chemical and morphological traits in order to better describe carbon fluxes from roots to the soil carbon pool across a diverse group of plant species.

Methods

In situ root decomposition rates were measured over an entire year by an intact core method on ten tree and seven herb species typical of agroforestry systems and were quantified using decay constants (k values) from Olson''s single exponential model. Decay constants were related to root chemical (total carbon, nitrogen, soluble carbon, cellulose, hemicellulose, lignin) and morphological (specific root length, specific root length) traits. Traits were measured for both absorbing and non-absorbing roots.

Key Results

From 61 to 77 % of the variation in the different root traits and 63 % of that in root decomposition rates was interspecific. N was positively correlated, but total carbon and lignin were negatively correlated with k values. Initial root traits accounted for 75 % of the variation in interspecific decomposition rates using partial least squares regressions; partial slopes attributed to each trait were consistent with functional ecology expectations.

Conclusions

Easily measured initial root traits can be used to predict rates of root decomposition in soils in an interspecific context.     

Characterization and chemodynamics of plant constituents during maturation,senescence and humus genesis in spruce ecosystems

Spruce needles of different age, litter materials and soil samples from the L-, O-and A-horizons of a mor profile were analysed by temperature-programmed pyrolysis (Py) in combination with field ionization mass spectrometry (FIMS). The integrated Py-FI mass spectra give characteristic fingerprints of the biomaterials investigated. The application of principal component analysis to the mass spectral data results in a clear discrimination and classification of the samples reflecting the chemical modifications and transformations of organic matter by biochemical and biogeochemical processes. The chemical compositions are determined by processes such as enrichment and/or translocation of plant constituents (e.g. carbohydrates, lignin, lipids, suberin, and aliphatic polymers) during maturation and senescence of needles; amendment of new components; decomposition; selective preservation and humification processes in the soil environment.During needle maturation, major chemical changes include the decrease of carbohydrate content, condensation of lignin, and crosslinking of waxes. Senescent needles are characterized by lower contents of carbohydrates and lower yields of monomeric pyrolysis products from lignin. The contribution of different litter materials to the humus layer can be estimated by differences in chemical composition. During litter decomposition and humification on the forest floor, carbohydrate content decreases rapidly. The lignin content remains almost constant but some subunits are continuously oxidized. Wax material accumulates until the mechnical disintegration of the needle occurs. In the O-horizons polymeric aliphatic materials are enriched in humified plant remains. A constant increase of aryl-alkyl esters from suberin in the O-horizons is due to both root input and selective preservation. In general, mainly aliphatic polymers and aryl-alkyl esters accumulate during the genesis of mor profiles under conifers.     

Direct uptake of detrital carbon by the deposit-feeding polychaete Euzonus mucronata (Treadwell)

Bacteria and other microbes colonizing detrital particles are assimilated efficiently by detritivores; in some cases the non-living detrital matter is relatively refractory. This comparison has led to the hypothesis that the bacterial component of detritus, rather than the non-living material, is the principal food resource of detritivores. This hypothesis was tested by feeding radiolabelled detritus to the deposit-feeding polychaete Euzonus mucronata (Treadwell), an abundant inhabitant of Pacific coastal sand beaches. The assimilation of carbon from sterile detritus at a concentration of 0.3 mg ash-free dry weight of detritus per g dry sand (0.03 % organic content) was sufficient to provide about one-half of the carbon requirements of E. mucronata. There was no significant difference in assimilation from chemically-extracted or water-extracted sterile detritus (representing relatively refractory and available material, respectively), or from the same materials after microbial colonization. Thus, the major source of carbon may have been the supposedly refractory component of the detritus. Despite a low estimated assimilation efficiency (0.76–1.72%), the carbon requirements of E. mucronata would be met entirely by direct assimilation of detrital carbon if only 10% of the total organic carbon in the sand is as available as the chemically-extracted detritus in this experiment. Low assimilation efficiency does not preclude the acquisition of nutritionally important quantities of carbon by direct assimilation from detritus.     

Characteristics of the biochemical composition of plant litter at different stages of decomposition (according to thermal analysis data)

The composition of samples of needles, leaves, sheaved cottongrass (Eriophorum vaginatum) tissues, and the L horizon of the forest floor of different degree of decomposition, isolated from the plant litter in southern taiga ecosystems, was studied by thermal analysis. It was established that plant litter decomposition is accompanied by structural changes in celluloses and that the decomposition rates of hemicellulose and structured cellulose vary at different stages of decomposition. The structural specificity and incongruent thermal decomposition of grass lignocellulose were observed in all samples of plant material. The rates at which the content of components of the plant litter decreased depended on the type and stage of decomposition of plant material. The decomposition rate of biochemical components tended to increase in better drained soils.     

Effects of streams on lignin degradation during foliar litter decomposition in an alpine forest

AimsStreams are widely distributed in alpine forests, and litter decomposition in which is an important component of material cycling across the forest landscape. The leaching and fragmenting effects as well as the unique environmental factors in streams may have significant impacts on lignin degradation during litter decomposition, but studies on this are lacking.
Methods Using litterbag methods, we investigated the dynamics of lignin mass remaining and concentration (percent litter mass, %) during the decomposition of four foliar litters, which varied significantly in the initial litter chemical traits, from the dominant species of Salix paraplesia, Rhododendron lapponicum, Sabina saltuaria, and Larix mastersiana under different habitats (forest floor, stream, and riparian zone) in the upper reaches of the Minjiang River.
Important findings After two year’s incubation, litter lignin mass remaining for a specific litter species varied significantly (p < 0.05) among habitats, with an order of stream < riparian zone < forest floor. Lignin was degraded substantially in the early stage of litter decomposition process, and the lignin concentration first decreased and then increased with the proceeding of litter decomposition, but varied significantly (p < 0.05) among different litter species. Lignin mass showed a general trend of decrease across the 2-year decomposition course. In addition, habitat type, decomposition period and microenvironmental factors (e.g., temperature, pH value and nutrient availability) showed substantial influences on lignin degradation rate. These results suggest that the traditional view that lignin was relatively recalcitrant with an increase of concentration in the early stage of litter decomposition is challenged, but the loss of lignin in the early phrase is in line with recent findings about the fate of lignin during litter decomposition. Moreover, the significant differences of lignin degradation rates among different decomposition period and habitat types indicated that local-scale environmental factors can play a significant role in litter decomposition and lignin degradation processes.     

Can the Biochemical Features and Histology of Wheat Residues Explain their Decomposition in Soil?

The biochemical characteristics or quality of crop residues is an important factor governing soil residue decomposition. To improve C and N biotransformation models the process underlying this decomposition needs to be better understood and new quality criteria found to describe it. The aims of this explorative study were to (i) improve our understanding of residue decomposition from detailed studies of cell wall biochemical compositions and tissue architecture (ii) find new ways of exploring generic indicators of organic matter quality. To do this, the cell wall composition and tissue architecture of wheat leaves, internodes and roots, before and after their incorporation into soil were determined. Results showed that leaves which were poorly lignified decomposed faster in soil than internodes and roots. Cellulose was the most degraded polysaccharide irrespective of wheat residue. However, cellulose was much more degraded in the case of leaves as compared to internodes and roots. Leaves also presented a highly condensed lignin structure and the extent to which uncondensed leaf lignin was affected by soil decomposition suggests that the contribution of leaf lignin to C mineralization during incubation was very low. Roots which contained similar amounts of lignin than the internodes decomposed more slowly. Roots were enriched in phenolic acids, and more particularly p-coumaric acid (pCA) and presented a more condensed lignin structure than internodes. Phenolic acids are involved in the formation of lignin–polysaccharide complexes known to be recalcitrant to enzymatic attack. Microscopic investigations confirmed that the vessels were the most resistant tissues to decomposition in soil and this could be related either to their lignin content or to the quality of this lignin (condensed-like type lignin). Therefore, cell wall biochemical analyses have revealed that phenolic acids, which in their esterified form represent only 0.1–1% of plant dry matter, have cross link functions within the cell walls that could be of major interest in estimating soil residue degradability. Lignin quality (monomers, level of condensation) was another crucial criterion that could explain why residues with similar amounts of lignin decomposed at different rates in soil (roots vs. aerial parts). Visualization of residue cell walls before and after decomposition in soil underlined the interest of a microscopic approach coupled with image analysis. This study, corroborated by the extensive literature on forage digestibility, confirmed that the proportions of vascular tissue and sclerenchyma cells in plant material are determinant factors affecting plant degradability. In the future, classification of plant material based on these criteria could lead to the definition of new quality parameters for models of C and N biotransformation in soil.     

The Effects on Lignin Structure of Overexpression of Ferulate 5-Hydroxylase in Hybrid Poplar1

Poplar (Populus tremula × alba) lignins with exceedingly high syringyl monomer levels are produced by overexpression of the ferulate 5-hydroxylase (F5H) gene driven by a cinnamate 4-hydroxylase (C4H) promoter. Compositional data derived from both standard degradative methods and NMR analyses of the entire lignin component (as well as isolated lignin fraction) indicated that the C4H∷F5H transgenic''s lignin was comprised of as much as 97.5% syringyl units (derived from sinapyl alcohol), the remainder being guaiacyl units (derived from coniferyl alcohol); the syringyl level in the wild-type control was 68%. The resultant transgenic lignins are more linear and display a lower degree of polymerization. Although the crucial β-ether content is similar, the distribution of other interunit linkages in the lignin polymer is markedly different, with higher resinol (β-β) and spirodienone (β-1) contents, but with virtually no phenylcoumarans (β-5, which can only be formed from guaiacyl units). p-Hydroxybenzoates, acylating the γ-positions of lignin side chains, were reduced by >50%, suggesting consequent impacts on related pathways. A model depicting the putative structure of the transgenic lignin resulting from the overexpression of F5H is presented. The altered structural features in the transgenic lignin polymer, as revealed here, support the contention that there are significant opportunities to improve biomass utilization by exploiting the malleability of plant lignification processes.In the continuing search for improved biomass utilization in processes including chemical pulping, natural ruminant digestibility, and biomass conversion to ethanol, considerable attention has focused on improving lignocellulosic feedstocks through genetic engineering. Perturbing plant biomass deposition by misregulating key genes/enzymes integral to major cell wall pathways can provide rich insights into cell wall development and architecture and also create significant opportunities for improved lignocellulosic utilization (Baucher et al., 2003; Boerjan et al., 2003; Ralph et al., 2004). Lignins comprise the second most abundant polymer class in the biosphere; however, their combinatorial biosynthesis renders them among the more complex biomacromolecules synthesized by plants. Alterations in plant cell wall chemistry or ultrastructure, including lignin content or structure, can have a profound effect on chemical or enzymatic degradability. For example, in chemical pulping, lignin structure and content have been shown to significantly impact delignification efficiency (both pulp yield and residual lignin [RL] content) and pulp bleachability (Chang and Sarkanen, 1973; Huntley et al., 2003; Stewart et al., 2006). Similarly, improvements in fermentable sugar yields have been reported in lignin-engineered Medicago sativa (Chen and Dixon, 2007).In recent years, the genes encoding enzymes specific to the lignin branch of the phenylpropanoid pathway have been cloned and their roles evaluated using a combination of forward and reverse genetics (Franke et al., 2002; Baucher et al., 2003; Boerjan et al., 2003). The down-regulation of genes early in the pathway may limit the overall flux of metabolites to lignin synthesis. In contrast, the genes common to the latter part of the pathway generally affect the distribution of p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) lignin units resulting from the primary monomers (the three monolignols p-coumaryl, coniferyl, and sinapyl alcohols). And, in several cases when the biosynthesis of a normal monolignol is severely curtailed, lignins appear to incorporate unique phenolics (e.g. 5-hydroxyconiferyl alcohol, hydroxycinnamaldehydes, and ferulic acid) that have not traditionally been considered lignin monomers (Sederoff et al., 1999; Boerjan et al., 2003; Ralph et al., 2004, 2008a, 2008b).Ferulate 5-hydroxylase (F5H), also referred to coniferaldehyde 5-hydroxylase to reflect one of its preferred substrate (Humphreys et al., 1999; Osakabe et al., 1999), is a key enzyme involved in synthesizing the monolignol sinapyl alcohol and, ultimately, S lignin moieties. F5H therefore affects the partitioning between the two major traditional monolignols, coniferyl and sinapyl alcohols, and is fundamental to the evolutionary differences between gymnosperms (with no S components) and angiosperms (with compositions favoring the S monomers). For example, the fah1 Arabidopsis (Arabidopsis thaliana) mutant, deficient in F5H, has little to no S lignin (Meyer et al., 1998; Marita et al., 1999). Like gymnosperms, it produces G-rich lignins, derived almost exclusively from coniferyl alcohol. In contrast, the overexpression of F5H in the mutant background produces plants displaying substantially higher than normal sinapyl alcohol-derived S units and is consequentially severely depleted in coniferyl alcohol-derived G units. Wet chemical analyses of cell wall lignins estimated S contents of up to about 92% in F5H-up-regulated Arabidopsis (Meyer et al., 1998), up to 84% in tobacco (Nicotiana tabacum; Franke et al. 2000), and as high as 93.5% in hybrid poplar (Populus tremula × Populus alba; Huntley et al., 2003; Li et al., 2003). These engineered cell walls, rich in S units, exceed the highest reported in nature to date (Baucher et al., 1998), with kenaf (Hibiscus cannabinus) bast fiber lignin, at 85% S, being among the highest (Ralph, 1996; Morrison et al., 1999). In poplars, the final methylation step, catalyzed by caffeic acid 3-O-methyl transferase (COMT), appears to adequately accommodate the increased flux from coniferaldehyde to 5-hydroxyconiferaldehyde to produce sinapaldehyde and ultimately sinapyl alcohol (Li et al., 2000, 2003). In Arabidopsis, however, evidence suggests that the COMT is not able to keep pace with the increased 5-hydroxyconiferaldehyde generated by F5H overexpression, since the ensuing lignins comprise a significant component derived from 5-hydroxyconiferyl alcohol (Ralph et al., 2001a). Novel 5-hydroxyguaiacyl benzodioxane structures, which result from incorporation of 5-hydroxyconiferyl alcohol into the lignification scheme, were the same as those noted in COMT-deficient plants (Ralph et al., 2001a, 2001b; Marita et al., 2001, 2003).The availability of woody plant material possessing an extreme S lignin concentration affords unique opportunities to explore the mechanisms and consequences of pushing plants toward compositional limits and has both major fundamental and industrial consequences. Engineering lignin composition to extreme levels provides rare and novel insights into the ramifications on the structure of the lignin component and on other cell wall characteristics. Here, we investigate the nature of the chemical modifications to the lignin polymer in up-regulated cinnamate 4-hydroxylase (C4H)∷F5H poplar and propose a model for how such gene perturbation impacts lignin biosynthesis.     

Stratospheric ozone reduction and ecosystem processes: enhanced UV-B radiation affects chemical quality and decomposition of leaves of the dune grassland species Calamagrostis epigeios

This study reports changes in the plant's chemical composition and the decomposition of this plant material under enhanced solar UV-B radiation. Calamagrostis epigeios, a dominant grass species in the dune grassland in The Netherlands, was grown outdoor on an experimental field under ambient and enhanced solar UV-B (5 and 7.5 kJ m^-2 day^-1 UV-B_BE, respectively), corresponding to about 15% stratospheric ozone depletion. After one growing season aerial plant parts were harvested. The decomposition of this harvested leaf material was studied in a dune grassland and on the above mentioned experimental field under ambient (5 kJ m^-2 day^-1 UV-B_BE) and enhanced (7.5 kJ m^-2 day^-1 UV-B_BE) radiation, using litter bags. The chemical quality of the leaves grown under enhanced solar UV-B changed. There was an increase in the leaf content of lignin, while no significant changes occurred for the content of -cellulose, hemicellulose and tannins under enhanced UV-B. In the field, the rate of decomposition of leaf material grown under enhanced UV-B (with an increased content of lignin) was reduced. The content of lignin of the decomposing leaf material increased, but less under exposure to enhanced UV-B. The latter may be explained by photodegradation of the lignin. The consequences of enhanced UV-B radiation for carbon fluxes in the dune grassland ecosystem are discussed.     

Kinetic study of corn straw pyrolysis: comparison of two different three-pseudocomponent models

With heating rates of 20, 50 and 100 K min(-1), the thermal decomposition of corn straw samples (corn stalks skins, corn stalks cores, corn bracts and corn leaves) were studied using thermogravimetric analysis. The maximum pyrolysis rates increased with the heating rate increasing and the temperature at the peak pyrolysis rate also increased. Assuming the addition of three independent parallel reactions, corresponding to three pseudocomponents linked to the hemicellulose, cellulose and lignin, two different three-pseudocomponent models were used to simulate the corn straw pyrolysis. Model parameters of pyrolysis were given. It was found that the three-pseudocomponent model with n-order kinetics was more accurate than the model with first-order kinetics at most cases. It showed that the model with n-order kinetics was more accurate to describe the pyrolysis of the hemicellulose.     

Decomposition of macroalgae,vascular plants and sediment detritus in seawater: Use of stepwise thermogravimetry

The applicability of a recently presented method (Stepwise Thermogravimetry, STG) to characterize biogenic organic matter (Kristensen 1990) was tested in comparative decomposition experiments. The initial microbial decay of pre-dried, fresh detritus from 6 different plant materials (2 macroalgae, 2 seagrasses, and 2 tree leaves) was examined for 70 days in aerobic seawater slurries. In addition, slurries of sediment detritus of low reactivity, representing the late stage of plant decay, were allowed to decompose aerobically and anaerobically for 200 days. Macroalgae lost 40–44% carbon over 70 days, whereas seagrasses lost 29–33% and tree leaves lost 0–8%. After a 3–5 days leaching phase, the temporal pattern of POC and PON loss from the plant detritus was exponential with higher rates for the former resulting in a 5–28% reduced C:N ratio. The Rp index decreased (<20%) during the initial leaching phase followed by a 30–40% increase to the end. Initial Rp was directly proportional to decay rate. POC loss in the anaerobic sediment slurry was 10% over the 200 day period (the aerobic was hampered by low pH). Preferential loss of PON caused a 30% increase in C:N ratio. The Rp index of sediment detritus also increased with 30%. Although the present laboratory experiments not fully simulate the natural environment, the Rp-C:N relationship obtained from the two slurry experiments can illustrate the general pattern of plant decay from fresh to refractory (humic) detritus. During initial aerobic decay, rapid leaching and microbial growth causes a decrease in both Rp and C:N (Rapid growth, phase 1). When all labile substrates have been consumed and the slower decay is controlled by enzymatic attack on particles with an associated production of humic compounds and accumulation of nitrogen rich bacterial cell remains, Rp increases and C:N decreases (Slow growth, phase 2). Later, when condensed humic compounds have accumulated, decay ceases. Most carbon is now bound in forms which are of low availability to bacteria and a preferential mineralization of nitrogen occurs; both Rp and C:N increases (Condenzation, phase 3).     

氮、磷养分有效性对森林凋落物分解的影响研究进展

通过对相关研究文献的综述结果表明,氮(N)和磷(P)是构成蛋白质和遗传物质的两种重要组成元素,限制森林生产力和其他生态系统过程,对凋落物分解产生深刻影响。大量的凋落物分解试验发现在土壤N有效性较低的温带和北方森林,凋落物分解速率常与底物初始N浓度、木质素/N比等有很好的相关关系,也受外源N输入的影响;而在土壤高度风化的热带亚热带森林生态系统中,P可能是比N更为重要的分解限制因子。然而控制试验表明,N、P添加对凋落物分解速率的影响并不一致,既有促进效应也有抑制效应。为了深入揭示N、P养分有效性对凋落物分解的调控机制,"底物的C、N化学计量学"假说、"微生物的N开采"假说以及养分平衡的理论都常被用于解释凋落物分解速率的变化。由于微生物分解者具有较为稳定的C、N、P等养分需求比例,在不同的养分供应的周围环境中会体现出不同的活性,某种最缺乏的养分可能就是分解的最重要限制因子。未来的凋落物分解研究,应延长实验时间、加强室内和野外不同条件下的N、P等养分添加控制试验,探讨驱动分解进程的微生物群落结构和酶活性的变化。     

Estimating the molecular composition of a diverse range of natural organic materials from solid-state <Superscript>13</Superscript>C NMR and elemental analyses

Most techniques for determining the chemical nature of natural organic matter in soil, sediment and water require prior extraction or concentration steps that are not quantitative and that create artifacts. ¹³C nuclear magnetic resonance (NMR) analysis can avoid these problems, but it gives little information at the scale of molecules. Here we show that the molecular composition of a diverse range of natural organic materials could be inferred from ¹³C NMR analysis combined with C and N analysis. Forty-six different organic materials including undecomposed and decomposed plant materials, soil organic matter, phytoplankton, and the organic matter found in freshwater, estuarine and marine sediments were examined. A mixing model simultaneously solved a series of equations to estimate the content of four biomolecule components representing the organic materials produced in greatest abundance by plants and other organisms (carbohydrate, protein, lignin and aliphatic material) and two additional components (char and pure carbonyl). Based on defined molecular structures for each component, signal intensities for ¹³C NMR spectra were predicted and compared with measured values. The sum of the absolute differences in signal intensity between the measured and predicted spectral regions was <7% for the terrestrial materials. For aquatic materials the fit of the predicted to measured signal intensities was not as good. Predicted molecular compositions correlated well with independent analyses of cellulose, protein and lignin contents of plant samples and char contents of soil samples. Across all samples, carbohydrates accounted for 10-76% of the sample C (40-76% in plants and 10-42% in soils, sediments and phytoplankton), protein for 2-80% (21-80% in phytoplankton and marine water column samples and 2-36% in plants, soils and sediments), lignin for 0-36%, aliphatic materials for 2-44%, char for 0-38% and carbonyl for 0-22%. For the soils, sediments and decomposed plant materials, the close correspondence between actual signal intensities and those predicted using known biomolecular components, suggested that either8humic structures can be approximated by mixtures of common biologically derived molecules or that humic structures did not exist in significant amounts.     

Decomposition in salt marsh ecosystems of the S.W. Netherlands: the effects of biotic and abiotic factors

Decomposition rates, determined with the litterbag technique in salt marshes of the S.W. Netherlands during the past decade are compared; the biotic and abiotic factors influencing these rates are identified and discussed.Tissue composition is the main variable affecting decay rates of halophytes, particularly variations in lignin content between plant parts and between species.Experiments in which the loss of the tensile strength of cotton strips was used as an index of cellulolytic decay, show that there is a conspicuous variation in decay rates on different sites in a salt marsh. Nonetheless, the locally varying environmental conditions within salt marshes of the S.W. Netherlands have less impact on the variation in decomposition rates of halophyte litter than the chemical make-up of the plant material.Larger fauna elements (> 300 m) may increase decomposition rates, but this effect is only limited and depends on location and litter type. The role of small fauna elements such as nematodes, which occur abundantly in association with halophyte litter, remains largely unknown.