Acid-unhydrolyzable Residue Research Articles

Foliar litter decomposition in an alpine forest meta-ecosystem on the eastern Tibetan Plateau

Litter decomposition is a biological process fundamental to element cycling and a main nutrient source within forest meta-ecosystems, but few studies have looked into this process simultaneously in individual ecosystems, where environmental factors can vary substantially. A two-year field study conducted in an alpine forest meta-ecosystem with four litter species (i.e., willow: Salix paraplesia, azalea: Rhododendron lapponicum, cypress: Sabina saltuaria, and larch: Larix mastersiana) that varied widely in chemical traits showed that both litter species and ecosystem type (i.e., forest floor, stream and riparian zone) are important factors affecting litter decomposition, and their effects can be moderated by local-scale environmental factors such as temperature and nutrient availability. Litter decomposed fastest in the streams followed by the riparian zone and forest floor regardless of species. For a given litter species, both the k value and limit value varied significantly among ecosystems, indicating that the litter decomposition rate and extent (i.e., reaching a limit value) can be substantially affected by ecosystem type and the local-scale environmental factors. Apart from litter initial acid unhydrolyzable residue (AUR) concentration and its ratio to nitrogen concentration (i.e., AUR/N ratio), the initial nutrient concentrations of phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg) were also important litter traits that affected decomposition depending on the ecosystem type.

A comparison of decomposition dynamics among green tree leaves, partially decomposed tree leaf litter and their mixture in a warm temperate forest ecosystem

Decomposition dynamics were compared among green tree leaves, partially decomposed tree leaf litter (i.e., decayed tree leaf litter on forest floor) and a mixture of the two in a warm temperate forest ecosystem in central China to test the influence of litter chemical quality on the degree of decomposition. The study was conducted in situ at two contrasting forest sites, an oak forest dominated by Quercus aliena var. acuteserrata Maxim., and a mixed pine and oak forest dominated by Pinus armandii Franch. and Q. aliena var. acuteserrata. We found marked differences in the rate of decomposition among litter types at both forest sites; the litter decomposition constant, k, was about 39 % greater at the oak forest site and more than 70 % greater at the pine-oak forest site, for green leaves than for partially decomposed leaf litter. The decomposition dynamics and temporal changes in litter chemistry of the three litter types also greatly differed between the two forest sites. At both forest sites, the higher rate of decomposition for the green leaves was associated with a higher nitrogen (N) content and lower carbon to N ratio (C/N) and acid-unhydrolyzable residue to N ratio (AUR/N). We did not find any non-additive effects when mixing green leaves and partially decomposed leaf litter. Our findings support the contention that litter chemical quality is one of the most important determinants of litter decomposition in forest ecosystems at the local or regional scale, but the effect of litter chemical quality on decomposition differs between the contrasting forest types and may vary with the stage of decomposition.

An integrated spectroscopic and wet chemical approach to investigate grass litter decomposition chemistry

The chemical transformations that occur during litter decomposition are key processes for soil organic matter formation and terrestrial biogeochemistry; yet we still lack complete understanding of these chemical processes. Thus, we monitored the chemical composition of Andropogon gerardii (big bluestem grass) litter residue over a 36 month decomposition experiment in a prairie ecosystem using: traditional wet chemical fractionation based upon digestibility, solid state 13C nuclear magnetic resonance (NMR) spectroscopy and Fourier transform infrared (FTIR) spectroscopy. The goals of this study were to (1) determine the chemical changes occurring during A. gerardii litter decomposition, and (2) compare the information obtained from each method to assess agreement. Overall, we observed a 97 % mass loss of the original litter, through a two-stage decomposition process. In the first stage, within 12 months, non-structural, cellulose and hemicellulose fractions not encrusted in lignin were preferentially and rapidly lost, while the acid unhydrolyzable residue (AUR) and microbial components increased. During the second stage, 12–36 months, all wet chemical fraction masses decreased equivalently and slowly with time, and the AUR and the lignin-encrusted cellulose fractions decomposition rates were comparable to each other. Method comparisons revealed that wet chemical fractionation did not accurately follow the initial litter structures, particularly lignin, likely because of chemical transformations and accumulation of microbial biomass. FTIR and NMR were able to determine bulk structural characteristics, and aid in elucidating chemical transformations but lacked the ability to measure absolute quantities of structural groups. As a result, we warn from the sole use of wet chemical methods, and strongly encourage coupling them with spectroscopic methods. Our results overall support the traditional chemical model of selective preservation of lignin, but shows that this is limited to the early stages of decomposition, while lignin is not selectively preserved at subsequent stages. Our study also provides important evidence regarding the impact of chemically different litter structures on decomposition rates and pathways.

The chemistry of some foliar litters and their sequential proximate analysis fractions

Proximate analysis (PA) is widely used to assess foliar, litter, and wood quality. The acid-unhydrolyzable residue (AUR) of PA, originally known as Klason lignin from wood analysis, is often assumed to be entirely lignin-derived, but the AUR of much plant material also includes contributions from condensed tannins (CT) and cutin or suberin. To improve understanding of chemical changes throughout the PA procedure, we characterized seven foliar litters and their sequential PA fractions (nonpolar and hot-water extracted, AUR). Changes in total C and N, extractable and insoluble CT (as detected by butanol/HCl hydrolysis), δ13C values and solid-state 13C NMR spectra were consistent with loss of carbohydrates and protein after acid hydrolysis, and support previous studies that the AUR residue includes lignin, cutin and CT, all of which are depleted in δ13C. Hot-water extraction removed the bulk of extractable plus insoluble CT. Only trace levels were detected in the AUR, although 13C NMR shows that these are likely underestimates. The assumption of lignin-AUR equivalence still causes misinterpretation of PA results for many sample categories. It is time for the scientific community to limit use of “lignin” to chemically meaningful contexts?

Effects of nitrogen addition on litter decomposition and nutrient release in two tropical plantations with N2-fixing vs. non-N2-fixing tree species

Atmospheric nitrogen (N) deposition has elevated rapidly in tropical regions where N2-fixing tree species are widespread. However, the effect of N deposition on litter decomposition in forests with N2-fixing tree species remains unclear. We examined the effect of N addition on litter decomposition and nutrient release in two tropical plantations with Acacia auriculiformis (AA, N2-fixing) and Eucalyptus urophylla (EU, non-N2-fixing) in South China. Three levels of N additions were conducted: control, medium-N (50 kg N ha−1 yr.−1) and high-N (100 kg N ha−1 yr.−1) in each plantation. Initial decomposition rate (k a ) for the control plots was faster in the AA plantation than in the EU plantation, but later in decomposition, larger fraction of slowly decomposing litter (A) remained in the former. N addition increased the slow fraction (A), decreasing soil microbial biomass and reducing acid-unhydrolyzable residue (AUR) degradation in the AA plantation. In the EU plantation, however, N additions significantly increased initial decomposition rate (k a ) and soil N availability. Furthermore, N addition decreased litter carbon and N release (in the AA plantation), while litter phosphorus release also decreased in both plantations. With ongoing N deposition in future, tropical plantations with N2-fixing tree species would potentially increase carbon accumulation and nutrient retention in forest floor by slowing litter decomposition.

Late stage pine litter decomposition: Relationship to litter N, Mn, and acid unhydrolyzable residue (AUR) concentrations and climatic factors

The aim of this paper is to evaluate relationships between decomposition rates of Scots pine (Pinus sylvestris) and lodgepole pine (Pinus contorta var. contorta) needle litter in the late stage of decomposition (>30% accumulated mass loss), and the progressively changing concentrations of manganese (Mn), nitrogen (N), and acid unhydrolyzable residue (AUR), as well as mean annual temperature (MAT) and mean annual precipitation (MAP). Using available long-term decomposition studies on pine needle litter in a climate gradient in Sweden, we calculated annual mass loss and related to concentrations of Mn, N, and AUR at the start of each one-year period as well as to MAT and MAP. We investigated these relationships for (i) all data on annual mass loss combined and (ii) annual mass loss for five different decomposition categories as defined by accumulated mass loss.We found highly significant, negative, and dominant relationships between annual mass loss and N (R2=0.39) and AUR (R2=0.39), a slight but significant positive relationship to Mn (R2=0.08) and a significant negative relationship to MAT (R2=0.06). The relationships were dynamic, and changed with accumulated mass loss. The rate-dampening effect of N decreased to be a rate-enhancing effect at c. 60–80% accumulated mass loss. A similar trend was found for AUR, becoming rate-enhancing at 70–80% accumulated mass loss. For Scots pine needle litter the effect of MAT on mass loss decreased with increasing accumulated mass loss and changed to a rate-dampening effect at c. 50–70% accumulated mass loss. Mn showed a stimulating effect on mass loss rate in all categories whereas MAP showed no effect in this mainly boreal climatic gradient.The current approach indicates a method for detailed studies of rate-regulating factors for litter decomposition.

Influence of manganese on decomposition of common beech (Fagus sylvatica L.) leaf litter during field incubation

Litter decomposition is of crucial importance for sustainable production in forest ecosystems with pedoclimatic conditions and nutrients being the main factors controlling litter decomposition. In particular, manganese (Mn) could accelerate decomposition of litter in the lignin-dominated (late) stage. Correlations between Mn concentration and litter decay rate were previously reported and explained by the role of Mn2+ in lignin degradation as a cofactor of the enzyme manganese peroxidase. However, the role of Mn in litter decay has been little experimentally tested yet. This study aims to assess the increased decomposition rate of common beech (Fagus sylvatica L.) leaf litter experimentally enriched in Mn (0.9–17.0 mg g−1) using a litterbag experiment (500 days of in situ incubation). Mass loss and acid unhydrolyzable residue, Mn, carbon (C), and dissolved organic carbon were determined in remaining materials and litter leachates. Our results showed a positive influence of Mn on litter and C decay and on the release of hydrophobic dissolved organic C. We explained these results by an enhanced ligninolysis leading to an increase in dissolved polyphenolic compounds.

Effects of mixture of branch order-based roots and nitrogen addition on root decay in a subtropical pine plantation

Non-additive effects may occur when branching order-based fine roots decompose in mixture, due to their distinct chemical composition. However, these effects and how they respond to increased nitrogen (N) availability are poorly known. We conducted a 524-day field litterbag experiment to examine the potential non-additive effects of order-based fine root decomposition and their responses to different rates of N addition in a Pinus elliottii (slash pine) plantation in China. The intact fine roots of slash pine trees were hierarchically dissected and split into two groups (lower-order roots (the first and second order roots) and higher-order roots (the third to fifth order roots)). The decomposition of these two root groups was monitored either separately or in 1:1 mixture. The non-additive effects did not occur when the two root groups decomposed in mixture. However, high rate of N addition shifted the root group interactions from additive to synergistic effects through contrasting effects on individual root group (lower-order roots). High rate of N addition retarded the decomposition of lower-order roots more in isolation than in mixture, due to the decreased binding of acid-unhydrolyzable residue to inorganic N ions in mixture. Therefore, we suggest that although stoichiometric heterogeneity holds within the branching hierarchy of fine roots, the tight coupling of N with acid-unhydrolyzable residue of root tissues may limit N use by microdecomposers, and thereby causing the additive effect. Since the mixing effect of order-based root decomposition is affected by N addition, understanding the chemical heterogeneity within fine root systems and their linkage with ambient N availability during decomposition either in isolation or in mixture may facilitate the accurate prediction of root decomposition responses to N loading.

Bleaching of leaf litter and associated microfungi in subboreal and subalpine forests.

Fungal decomposition of lignin leads to the whitening, or bleaching, of leaf litter, especially in temperate and tropical forests, but less is known about such bleaching in forests of cooler regions, such as boreal and subalpine forests. The purposes of the present study were to examine the extent of bleached area on the surface of leaf litter and its variation with environmental conditions in subboreal and subalpine forests in Japan and to examine the microfungi associated with the bleaching of leaf litter by isolating fungi from the bleached portions of the litter. Bleached area accounted for 21.7%-32.7% and 2.0%-10.0% of total leaf area of Quercus crispula and Betula ermanii, respectively, in subboreal forests, and for 6.3% and 18.6% of total leaf area of B. ermanii and Picea jezoensis var. hondoensis, respectively, in a subalpine forest. In subboreal forests, elevation, C/N ratio and pH of the FH layer, and slope aspect were selected as predictor variables for the bleached leaf area. Leaf mass per area and lignin content were consistently lower in the bleached area than in the nonbleached area of the same leaves, indicating that the selective decomposition of acid unhydrolyzable residue (recalcitrant compounds such as lignin, tannins, and cutins) enhanced the mass loss of leaf tissues in the bleached portions. Isolates of a total of 11 fungal species (6 species of Ascomycota and 5 of Basidiomycota) exhibited leaf-litter-bleaching activity under pure culture conditions. Two fungal species (Coccomyces sp. and Mycena sp.) occurred in both subboreal and subalpine forests, which were separated from each other by approximately 1100 km.

Differential responses of needle and branch order-based root decay to nitrogen addition: dominant effects of acid-unhydrolyzable residue and microbial enzymes

Both chemical differences between foliage and different orders of fine roots and their contrasting decomposing microenvironments may affect their decomposition. However, little is known about how foliage and branch order-based root decomposition responds to increased N availability and the response mechanisms behind. The effects of different doses of N addition on the decomposition of needles and order-based roots of Pinus elliottii (slash pine) were monitored using the litterbag method for 524 days in a subtropical slash pine plantation in south China. The acid-unhydrolyzable residue (AUR) concentration and microbial extracellular enzymatic activities (EEA) in decomposing needles and roots were also determined. Our results indicate that the responses of needle and order-based root decomposition were N-dose-specific. The decomposition of both needles and lower-order roots was inhibited under the high N dose rate. The retarded decomposition of lower-order roots could be explained more by the increased binding of AUR to inorganic N ions, while the retarded decomposition of needles could be explained more by the reduced microbial EEA. Further, in contrast to lower-order roots, N addition had no effect on the decomposition of higher-order roots. We conclude that the decomposition of foliage and fine roots may fail to mirror each other at ambient conditions or in response to N deposition due to their contrasting decomposition microenvironments and tissue chemistry. Given the differential effects of N addition on order-based roots, our findings highlight the need to consider the tissue chemistry heterogeneity within branching fine root systems when predicting the responses of root decomposition to N loading.

Decomposing ability of diverse litter-decomposer macrofungi in subtropical, temperate, and subalpine forests

An integrative survey was conducted on the ability of litter-decomposing macrofungi (LDM) from forests of different climatic regions to decompose litter materials and recalcitrant compounds in the litter under pure culture conditions. A total of 75 isolates in six families of LDM from subtropical, cool temperate (CT), and subalpine (SA) forests in Japan were tested for their ability to decompose a total of eight litter types that are major substrates for macrofungi at each site. The mass loss of the litter (% original mass) during incubation for 12 weeks at 20 °C ranged from −3.1 % to 54.5 %. Macrofungi originated from forests of different climatic regions exhibited similar decomposing abilities, but the SA isolates caused negligible mass loss of Abies needles, possibly due to inhibitory compounds. Decomposing activity for recalcitrant compounds (as acid-unhydrolyzable residues, AUR) was found in many macrofungal isolates. The isolates of Marasmiaceae were generally more able to cause selective decomposition of AUR than those of Mycenaceae and to decompose AUR in partly decomposed materials. The isolates of Xylariaceae had lower ligninolytic activity than those of Basidiomycetes. The AUR mass loss caused by CT isolates was significantly lower in nitrogen-rich beech litter than in its nitrogen-poor counterpart, suggesting a retarding effect of nitrogen on AUR decomposition, which was obvious for Mycenaceae. The effect of fungal family was generally more significant than that of litter type, suggesting that possible changes in the composition of fungal assemblages influence their functioning more than changes in the quality of substrates.

Application of 13C NMR spectroscopy to characterize organic chemical components of decomposing coarse woody debris from different climatic regions

Solid-state 13C nuclear magnetic resonance (NMR) spectroscopy was applied to coarse woody debris (CWD) in different stages of decomposition and collected from forest floor of a subtropical, a cool temperate, and a subalpine forest in Japan. The purpose was to test its applicability to characterize organic chemical composition of CWD of broad-leaved and coniferous trees from different climatic conditions. O-alkyl-C, mainly representing carbohydrates, was the predominant component of CWD at the three sites, accounting for 43.5-58.1% of the NMR spectra. Generally, the relative area under the signals for aromatic-C and phenolic-C, mainly representing lignin, increased, whereas the relative area for O-alkyl-C decreased, as the decay class advanced. The relative area under NMR chemical shift regions was significantly correlated with the chemical properties examined with proximate analyses. That is, O-alkyl-C and di-O-alkyl-C NMR signal areas were positively correlated with the volumetric density of CWD and the content of total carbohydrates. Methoxyl-C, aromatic-C, phenolic-C, carboxyl-C, and carbonyl-C were positively correlated with the contents of acid-unhydrolyzable residues (lignin, tannins, and cutin) and nitrogen. Lignin-C calculated from NMR signals increased, and polysaccharide-C decreased, with the decay class of CWD at the three study sites. A review of previous studies on 13C NMR spectroscopy for decomposing CWD suggested further needs of its application to broad-leaved trees from tropical and subtropical regions.

Effects of litter type, origin of isolate, and temperature on decomposition of leaf litter by macrofungi

The dependence of hyphal growth and litter decomposition on litter type and incubation temperature used as substratum were compared for litter-decomposing macrofungi originating from subtropical (ST), cool temperate (CT), and subalpine forests (SA) in Japan. In the first series of pure culture decomposition tests using a total of 39 litter types as substrata inoculated with six fungal isolates from the three climatic regions, the fungal decomposition of litter was negatively affected by the content of acid-unhydrolyzable residues (AUR) or extractives and positively by N content in the litter. Secondly, cross-inoculation tests were performed to examine the mass loss of leaf litter of broad-leaved trees from ST, CT, and SA, each inoculated with three Mycena species from the three climates and incubated at seven temperatures between 5 and 35 °C. Fungal isolate, litter type, incubation temperature, and their interactions significantly affected the mass loss of litter during the incubation. The greatest values of mass loss were found at 20 or 25 °C, and were generally consistent with the optimum temperatures of colony diameter growth rate of these isolates. Isolates from cooler regions were more sensitive to higher temperature than isolates from warmer regions. The decomposition of recalcitrant compounds (as acid-unhydrolyzable residues, AUR) by Mycena sp. from ST was also affected by litter type and incubation temperature, but the degree of selective decomposition of AUR relative to other components, such as cellulose, was insensitive to the range of temperature tested.

Variations in leaf litter decomposition across contrasting forest stands and controlling factors at local scale

Aims Litter decomposition is a critical pathway linking the above- and belowground processes. However, factors underlying the local spatial variations in forest litter decomposition are still not fully addressed. We investigated leaf litter decomposition across contrasting forest stands in central China, with objective to determine the spatial variations and controlling factors in forest floor leaf litter decomposition in relation to changes in forest stands in a temperate forest ecosystem. Methods Leaf litter decomposition was studied by using litterbag method across several typical forest stand types in Baotianman Nature Reserve, central China, including pure stands of Quercus aliena var. acuteserrata, Q. glandulifera var. brevipetiolata and Q. variabilis, respectively, and mixed pine/oak stands dominated by armandii and Q. aliena var. acuteserrata, as well as stands of pure Q. aliena var. acuteserrata trees ranging in stand age from ~40 to >160 years. Measurements were made on litter mass remaining and changes in litter chemistry during decomposition over a 2-year period, along with data collections on selective biotic and environmental factors. A reciprocal transplant experiment involving Q. aliena var. acuteserrata and Q. variabilis was concurrently carried out to test the occurrence of ‘home-field advantage (HFA)’ in local forests when only considering contrasting oak tree species. Correlation analyses and path analyses were performed to identify the dominant drivers and their relative contributions to variations in leaf litter decomposition. Important findings Significant variations were found in the rate of leaf litter decomposition among stands of different tree species but not among stand age classes. The values of decay constant, k , varied from 0.62 in Q. aliena var. acuteserrata stands to 0.56 in Q. variabilis stands. The reciprocal litter transplant experiment showed that the rate of leaf litter decomposition was on average 5% slower in home-fields than on reciprocal sites. Path analysis identified litter acid-unhydrolyzable residue (AUR) to N ratio, soil microbial biomass carbon (MBC), soil pH and soil organic carbon (SOC) as most prominent factors controlling the rate of leaf litter decomposition, collectively accounting for 57.8% of the variations; AUR/N had the greatest negative effect on k value, followed by weaker positive effects of SOC and MBC. Our findings suggest that tree species plays a primary role in affecting forest floor leaf litter decomposition by determining the litter quality, with site environment being a secondary factor contributing to the local variations in leaf litter decomposition in this temperate forest ecosystem.

Interaction of initial litter quality and thinning intensity on litter decomposition rate, nitrogen accumulation and release in a pine plantation

Thinning alters litter quality and microclimate under forests. Both of these two changes after thinning induce alterations of litter decomposition rates and nutrient cycling. However, a possible interaction between these two changes remains unclear. We placed two types of litter (LN, low N concentration litter; HN, high N concentration litter) in a Chinese pine ( Carrière) plantation under four thinning treatments to test the impacts of litter quality, thinning or their combination on decomposition rate and N cycling. In our study, N was accumulated to approach an underlying critical acid-unhydrolyzable residue to nitrogen ratio (approximately 57–69) in litter. Moreover, an interaction between litter quality and thinning on decomposition rates, N accumulation and net release did exist. On one hand, one year decomposition rate of LN was elevated after thinning while that of HN remained the same or even lower (under light thinning); N accumulation of LN declined with light thinning and was restored with the increase of thinning intensity whereas that of HN did not decline with thinning and increased under heavy thinning; Net N release from LN was only found in light and heavy thinning while that from HN was found in all treatments, moreover net N release from LN and HN were both elevated under heavy thinning. On the other hand, HN decomposed faster, accumulated less and released more N than LN did under all treatments. Generally, high N concentration in litter and high-intensity thinning can lead to rapid litter decomposition and N cycling in coniferous plantations.Pinus tabulaeformis

Plant species effect on the decomposition and chemical changes of leaf litter in grassland and pine and oak forest soils

The aim of this study was to examine the effect of plant species differing in functional and phylogenetic traits on the decomposition processes of leaf litter in a grassland of Japanese pampas grass (Miscanthus sinensis) and adjacent forests of Japanese red pine (Pinus densiflora) and Japanese oak (Quercus crispula), representing sequential stages of secondary succession. The litterbag experiments were carried out for 3 years in a temperate region of central Japan. The decomposition constant (Olson’s k) was 0.49, 0.39, and 0.56/year for grass, pine, and oak, respectively. Nitrogen mass decreased in grass leaf litter during decomposition, whereas the absolute amount of nitrogen increased in leaf litter of pine and oak during the first year. Holocellulose in grass leaf litter decomposed selectively over acid-unhydrolyzable residues more markedly than in leaf litter of pine and oak. 13C nuclear magnetic resonance analysis also revealed a decrease in the relative area of O-alkyl-C in grass. The different decomposition among the three litter species implied that the secondary succession from grassland to pine forest and from pine to oak forests could decrease and increase, respectively, the rate of accumulation and turnover of organic materials and N in soils.

Relationships between nitrogen, acid-unhydrolyzable residue, and climate among tree foliar litters

Using literature data, we investigated coniferous and broadleaf litter from 58 tree species using a database encompassing concentrations of N and acid-unhydrolyzable residue (AUR) (gravimetric lignin) in newly shed litter, mean annual temperature, and mean annual precipitation. Our aims were to (i) demonstrate any large-scale relationships between concentrations of N and AUR in foliar litter and (ii) determine differences in this respect among litter from Pinus and Quercus. To this end, we had collected foliar litter data for Asia and Europe, forming a climate gradient. Litter from broadleaf and coniferous trees differed significantly in concentrations of N (p < 0.0001, 9.64 versus 5.50 mg/g, respectively) and AUR (p < 0.0001, 219 versus 292 mg/g, respectively). There were highly significant positive linear relationships between concentrations of N and AUR for broadleaf (p < 0.0001) and coniferous litter (p < 0.0001). There were also significant positive relationships for AUR as a function of N concentration for the genera Pinus and Quercus but not within species. That for Scots pine (Pinus sylvestris L.) was negative and that for common oak (Quercus robur L.) not significant.

Fungal succession and decomposition of beech cupule litter

AbstractBeech cupule litter is the second largest (next to leaf litter) component of total annual litterfall in mast years, and makes an important contribution to carbon budgets in beech forest soils. We investigated the decomposition processes of beech cupule litter over a 30‐month period with reference to the role of fungal succession in the decomposition of acid‐unhydrolyzable residue (AUR) and holocellulose. During the study period, weight loss of holocellulose occurred, while there was little weight loss of AUR, and 77 % of the original cupule weight remained at the end of the study period. Xylaria sp.1, Geniculosporium sp. and Nigrospora sp. that can attack holocellulose selectively caused mass loss of holocellulose and were responsible for the cupule weight loss. Although the beech cupule is a woody phyllome and its lignocellulose composition is similar to that of coarse woody debris (CWD) rather than leaf litter of beech, the selective decomposition of holocellulose by fungi was similar to the decay process of leaf litter rather than CWD.

Decomposition of black locust and black pine leaf litter in two coeval forest stands on Mount Vesuvius and dynamics of organic components assessed through proximate analysis and NMR spectroscopy

Litter quality is an important determinant of soil organic matter formation. Changes of organic components were investigated along decomposition of black locust (Robinia pseudoacacia L.) leaf litter and black pine (Pinus nigra Arn.) needle litter in the native adjacent coeval forest stands. To this purpose, data from proximate analyses were compared with those from CPMAS 13C NMR. Newly shed leaf litter of black locust had significantly higher concentrations of ADSS (acid detergent soluble substances) as well as lower concentrations of cellulose and AUR (acid unhydrolyzable residues that include lignin) and higher AUR-to-Cellulose ratio than that of black pine. The 13C CPMAS NMR spectra of newly shed leaf litter of black locust and black pine revealed that O-Alkyl-C components (including cellulose and hemicelluloses) accounted, respectively, for 53.8% and 61.4% of the total area of the spectra. All other C fractions were relatively more abundant in black locust than in black pine. Within individual sampling periods, relationships between residual litter mass and concentrations of ADSS, cellulose and AUR were examined, as were relationships between residual litter C and NMR fractions. Four periods were defined based on the slopes of the decomposition curve, with the length of period I defined by the start of a net decrease of AUR. Proximate analyses and NMR data showed changes in chemical composition over the decomposition process, as well as changes in decay rates of the residues, following different paths in the two litters. ADSS decayed faster in black locust litter; in contrast cellulose and AUR decayed faster in that of black pine. AUR concentration increased in both litters during decomposition; however, compared to black pine, the remaining litter of black locust was richer in AUR, despite the lower initial concentration, and had a higher AUR-to-Cellulose ratio. Phenol-C and Aryl-C decayed faster in black locust litter, while Alkyl-C decayed faster in that of black pine. In both litters, mass loss in periods was negatively correlated to concentration of AUR at the start of the periods. C loss in periods was negatively correlated to the concentration at the start of the periods of MC-to-PC (an index of lignin content) in black locust litter and positively correlated to Alkyl-C and O-Alkyl-C in that of black pine. Phenol-C, O-Alkyl-C and Aryl-C were the most decomposable C fractions in black locust. O-Alkyl-C and Alkyl-C were the most decomposable C fractions in black pine. Limit value was lower in black pine than in black locust. Consequently the different pattern of litter decomposition can affect the size of C sequestration in the forest floor and the quality of accumulated organic carbon.

Influence of litter chemistry and stoichiometry on glucan depolymerization during decomposition of beech (Fagus sylvatica L.) litter

Glucans like cellulose and starch are a major source of carbon for decomposer food webs, especially during early- and intermediate-stages of decomposition. Litter quality has previously been suggested to notably influence decomposition processes as it determines the decomposability of organic material and the nutrient availability to the decomposer community. To study the impact of chemical and elemental composition of resources on glucan decomposition, a laboratory experiment was carried out using beech (Fagus sylvatica, L.) litter from four different locations in Austria, differing in composition (concentration of starch, cellulose and acid unhydrolyzable residue or AUR fraction) and elemental stoichiometry (C:N:P ratio). Leaf litter was incubated in mesocosms for six months in the laboratory under controlled conditions. To investigate the process of glucan decomposition and its controls, we developed an isotope pool dilution (IPD) assay using 13C-glucose to label the pool of free glucose in the litter, and subsequently measured the dilution of label over time. This enabled us to calculate gross rates of glucose production through glucan depolymerization, and glucose consumption by the microbial community. In addition, potential activities of extracellular cellulases and ligninases (peroxidases and phenoloxidases) were measured to identify effects of resource chemistry and stoichiometry on microbial enzyme production. Gross rates of glucan depolymerization and glucose consumption were highly correlated, indicating that both processes are co-regulated and intrinsically linked by the microbial demand for C and energy and thereby to resource allocation to enzymes that depolymerize glucans. At early stages of decomposition, glucan depolymerization rates were correlated with starch content, indicating that starch was the primary source for glucose. With progressing litter decomposition, the correlation with starch diminished and glucan depolymerization rates were highly correlated to cellulase activities, suggesting that cellulose was the primary substrate for glucan depolymerization at this stage of decomposition. Litter stoichiometry did not affect glucan depolymerization or glucose consumption rates early in decomposition. At later stages, however, we found significant negative relationships between glucan depolymerization and litter C:N and AUR:N ratio and a positive relationship between glucan depolymerization and litter N concentration. Litter C:N and C:P ratios were negatively related to cellulase, peroxidase and phenoloxidase activities three and six months after incubation, further corroborating the importance of resource stoichiometry for glucan depolymerization after the initial pulse of starch degradation.