CPMAS 13C NMR and IR spectra of spruce and pine litter and of the Klason lignin fraction at different stages of decomposition

Fresh and decomposed spruce and pine litter and the Klason lignin fraction of spruce needles at different stages of decomposition were studied by CPMAS ¹³C NMR and IR spectroscopy as well as by chemical methods. It was shown that decomposition of needles is accompanied by an increase in aliphatic substances and carboxyl group content; the amount of polysaccharides is reduced. It is assumed that stable aliphatic compounds like cutin and lipids of microbial origin will accumulate during litter decomposition and humification. Aromaticity is low and does not alter drastically. The NMR spectra of the Klason lignin fraction show pronounced peaks at 30, 55, 115, 130, 150 and 175 ppm. Obviously, this fraction contains appreciable amounts of aliphatic and carboxyl carbon besides the typical aromatic units of lignin. During decomposition aromaticity decreases whereas the relative amounts of aliphatic substances and carboxyl groups increase. This is probably due to splitting of aromatic ring structures and side chains. The findings agree with the results from chemical analyses.     

Degradation of hemicellulose, cellulose and lignin in decomposing spruce needle litter in relation to N

Decomposing needles from a Norway spruce forest in southern Sweden were studied for 559 days under laboratory conditions. Falling needles were collected in control (Co) plots and plots that had received 100 kg N ha⁻¹ yr⁻¹ as (NH₄)₂SO₄ for 9 years under field conditions. One of the aims was to determine whether the previously documented low decomposition rate of the N fertilized (NS) needles could be explained by a lower degradation degree of lignin. The lignin content was studied using the alkaline CuO oxidation method, the Klason lignin method and CPMAS ¹³C NMR spectroscopy. The amounts of cellulose and hemicellulose were also determined.The fertilized needle litters initially decomposed faster than the unfertilized, but later this reaction reversed, so that at the end the mass loss was 45% initial C in the control and 35% initial C in NS. Klason lignin decreased with time in both treatments and overall, the change of Klason lignin mirrored the litter mass loss. No major difference as regards the decomposition of hemicellulose occurred between the treatments, whereas significantly lower concentrations of cellulose were found in NS needles throughout the incubation. The CuO derived compounds (VSC) were somewhat lower in NS needles throughout the decomposition time. Initially, VSC increased slightly in both treatments, which contradicts the Klason lignin data. There was a weak positive relationship (p>0.05) between VSC and Klason lignin. Both vanillyls compounds (V) and cinnamyl compounds (Ci) increased slightly during decomposition, whereas syringyl compounds (S) vanished entirely. The lignin degradation degree, i.e. the acid-to-aldehyde ratio of the vanillyl compounds expressed as (Ac/Al)_v, showed no significant effect of treatment. The ¹³C NMR analyses of the combined samples showed increased content of aromatic C with increasing decomposition time. The carbohydrate content (O-alkyl C) was lower in the fertilized needle litter throughout the incubation time. The alkyl C content tended to increase with decomposition time and N fertilization. The alkyl C/O-alkyl C ratios increased in both treatments during the incubation. The NMR results were not tested statistically.In conclusion, no major difference concerning lignin degradation could be found between the unfertilized and N fertilized needle litter. Thus, the study contradicts the hypothesis that higher amounts of N reduce lignin degradation. The reduced biological activity is probably due to direct N effects on the microorganisms and their decomposing ability.     

Lignin degradation controls the production of dissolved organic matter in decomposing foliar litter

Lignin is considered to be a crucial component controlling litter decomposition but its role in the production of dissolved organic matter (DOM) from litter is not well understood. Our main objective therefore was to examine the amounts and properties of DOM produced in decomposing litter, with special emphasis on the role of lignin degradation. We exposed litter of five different tree species (Sycamore maple, Mountain ash, European beech, Norway spruce, Scots pine) in litterbags at the soil surface of two neighbouring sites to degradation under field conditions. Litterbags were sampled eight times during 27 months of exposure in the field. We determined mass loss and characterized the lignin fraction by two different methods (van Soest procedure, acid‐detergent lignin: ADL, CuO oxidation). Litter was irrigated in the laboratory and leachates were analysed for dissolved organic carbon (DOC) and characterized by UV and fluorescence spectroscopy. Litter decomposition followed a two‐stage model characterized by initially rapid and then decreasing degradation with time. In the initial phase of litter decomposition, leached amounts of DOM decreased with time and no effects of lignin degradation were found. The contents of ADL in the litter residues and CuO oxidation products suggest larger degradation and oxidation of lignin in beech, spruce and pine litter than in maple and ash litter. The production of DOM from litter with larger lignin degradation increased in the second phase of decomposition, when mass loss exceeded 10–20%. In contrast, DOM produced from litter showing weak lignin degradation (maple, ash) did not increase further in the second phase of decomposition. In the leachates of litter with large lignin degradation (beech, spruce, pine), UV absorbance and fluorescence spectroscopy indicated a larger increase in the contribution of lignin‐derived compounds to DOM with increasing mass loss than for litter species with relatively stable lignin. We conclude that degradation of lignin is an important control on DOM production during the second phase of litter decomposition.     

Microbial biomass and activity in soil and litter underPinus sylvestris, Picea abies andBetula pendula at originally similar field afforestation sites

Microbial biomass C and N, and activities related to C and N cycles, were compared in needle and leaf litter, and in the uppermost 10 cm of soil under the litter layer in Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies L.) and silver birch (Betula pendula L.) stands, planted on originally similar field afforestation sites 23–24 years ago. The ground vegetation was differentiated under different tree species, consisting of grasses and herbs under birch and pine, and mosses or no vegetation with a thick layer of needles under spruce. The C:N ratio of the soils was 13–21 and the soil pH_{CaCl 2} 3.8–5.2. Both showed little variation under different tree species. Microbial biomass C and N, C mineralization, net ammonification, reduction) did not differ significantly in soil under different tree species either. Birch leaf litter had a higher pH_{CaCl 2} (5.9) than spruce and pine needle litter (pH 5.0 and 4.8, respectively). The C:N ratio of spruce needles was 30, and was considerably higher in pine needles (69) and birch leaves (54). Birch leaves tended to have the highest microbial biomass C and C mineralization. Spruce needles appeared to have the highest microbial biomass N and net formation of mineral N, whereas formation of mineral N in pine needles and birch leaves was negligible. Microbial biomass C and N were of the same order of magnitude in the soil and litter samples but C mineralization was tenfold higher in the litter samples.     

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.     

Processes controlling the production of aromatic water-soluble organic matter during litter decomposition

Dissolved organic matter (DOM) plays a fundamental role for many soil processes. For instance, production, transport, and retention of DOM control properties and long-term storage of organic matter in mineral soils. Production of water-soluble compounds during the decomposition of plant litter is a major process providing DOM in soils. Herein, we examine processes causing the commonly observed increase in contribution of aromatic compounds to WSOM during litter decomposition, and unravel the relationship between lignin degradation and the production of aromatic WSOM. We analysed amounts and composition of water-soluble organic matter (WSOM) produced during 27 months of decomposition of leaves and needles (ash, beech, maple, spruce, pine). The contribution of aromatic compounds to WSOM, as indicated by the specific UV absorbance of WSOM, remained constant or increased during decomposition. However, the contribution of lignin-derived compounds to the total phenolic products of ¹³C-labelled tetramethylammonium hydroxide (¹³C-TMAH) thermochemolysis increased strongly (by >114%) within 27 months of decomposition. Simultaneous changes in contents of lignin phenols in solid litter residues (cupric oxide method as well as ¹³C-TMAH thermochemolysis) were comparably small (−39% to +21% within 27 months). This suggests that the increasing contribution of lignin-derived compounds to WSOM during decomposition does not reflect compositional changes of solid litter residues, but rather the course of decomposition processes. In the light of recently published findings, these processes include: (i) progressive oxidative alteration of lignin that results in increasing solubility of lignin, (ii) preferential degradation of soluble, non-lignin compounds that limits their contribution to WSOM during later phases of decomposition.     

Response of soil C and N transformations to tannin fractions originating from Scots pine and Norway spruce needles

Tannins are polyphenolic compounds that may influence litter decomposition, humus formation, nutrient (especially N) cycling and ultimately, plant nutrition and growth. The aim of this study was to determine the response of C and N transformations in soil to tannins of different molecular weight from Norway spruce (Picea abies (L.) Karst) and Scots pine (Pinus sylvestris L.) needles, tannic acid and cellulose. Arginine was added to test whether the soil microbial community was limited by the amount of N, and arginine+tannin treatments were used to test whether the effects of tannins could be counteracted by adding N. Soil and needle samples were taken from adjacent 70-year-old Scots pine and Norway spruce stands located in Kivalo, northern Finland. Tannins were extracted from needles and fractioned based on molecular weight; the fractions were then characterized by LC-MS and GC-MS. Light fractions contained tannin monomers and dimers as well as many other compounds, whereas heavy fractions consisted predominantly of polymerized condensed tannins. Spruce needles contained more procyanidin than prodelphinidin units, while in pine needles prodelphinidin units seemed to be dominant. The fractions were added to soil samples, pine fractions to pine soil and spruce fractions to spruce soil, and incubated at 14 °C for 6 weeks. CO₂ evolution was followed throughout the experiment, and the rates of net mineralization of N and net nitrification, concentration of dissolved organic N (DON) and amounts of microbial biomass C and N were measured at the end of the experiment. The main effects of the fractions were similar in both soils. Light fractions strongly enhanced respiration and decreased net N mineralization, indicating higher immobilization of N in the microbial biomass. On the contrary, heavy fractions reduced respiration and slightly increased net N mineralization, suggesting toxic or protein-precipitating effects. The effects of tannic acid and cellulose resembled those of light fractions. DON concentrations generally decreased during incubation and were lower with heavy fractions than with light fractions. No clear differences were detected between the effects of light and heavy fractions on microbial biomass C and N. Treatments that included addition of arginine generally showed trends similar to treatments without it, although some differences between light and heavy fractions became more obvious with arginine than without it. Overall, light fractions seemed to act as a labile source of C for microbes, while heavy fractions were inhibitors.     

Analytical pyrolysis of a soil profile under Scots pine

The chemical properties of pine needle litter cause slow decomposition, which results in an accumulation of highly lignocellulosic material on the forest floor. Decomposition of organic matter is important for the nutrient turnover in pine forests on nutrient-poor soils. We studied the biodegradation of needles in an organic layer focusing on the various stages of lignin degradation by fungi. Samples were obtained from pine needle litter and a stratified organic layer over nutrient-poor sand under a 60-year-old Scots pine (Pinus sylvestris forest stand. Pyrolysis mass spectrometry (PyMS) and pyrolysis gas chromatography mass spectrometry (PyGCMS) were used to characterize the chemical composition of the needles and the soil. The pyrolysis data show that diterpenoid acids are a main component in fresh needles, but rapidly decrease in the organic layer of the soil, as a result of decomposition. The chemical composition of the soil profile is dominated by guaiacyl lignin and polysaccharides from needle litter. The hexose/pentose ratio increases with depth in the soil profile. The partial preservation of hexose polymers is the result of the preferential decomposition of pentose polymers by white-rot fungi, and points to the input of microbially synthesized polysaccharides. Indications for the degradation of guaiacyl lignin are also found in the soil profile. Oxidative reactions by soil fungi result in a shortening of the side chain of the guaiacyl lignin derivatives and an increase of carbonyl and carboxyl groups. These degradational patterns of lignin in the soil profile under Scots pine are similar to those observed in lignin model compounds and wood lignin degraded by fungi under controlled laboratory conditions.     

Microbial biomass and activity in soil and litter underPinus sylvestris, Picea abies andBetula pendula at originally similar field afforestation sites

Microbial biomass C and N, and activities related to C and N cycles, were compared in needle and leaf litter, and in the uppermost 10 cm of soil under the litter layer in Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies L.) and silver birch (Betula pendula L.) stands, planted on originally similar field afforestation sites 23–24 years ago. The ground vegetation was differentiated under different tree species, consisting of grasses and herbs under birch and pine, and mosses or no vegetation with a thick layer of needles under spruce. The C:N ratio of the soils was 13–21 and the soil pH_{CaCl 2} 3.8–5.2. Both showed little variation under different tree species. Microbial biomass C and N, C mineralization, net ammonification, reduction) did not differ significantly in soil under different tree species either. Birch leaf litter had a higher pH_{CaCl 2} (5.9) than spruce and pine needle litter (pH 5.0 and 4.8, respectively). The C:N ratio of spruce needles was 30, and was considerably higher in pine needles (69) and birch leaves (54). Birch leaves tended to have the highest microbial biomass C and C mineralization. Spruce needles appeared to have the highest microbial biomass N and net formation of mineral N, whereas formation of mineral N in pine needles and birch leaves was negligible. Microbial biomass C and N were of the same order of magnitude in the soil and litter samples but C mineralization was tenfold higher in the litter samples.     

Amounts and degradability of dissolved organic carbon from foliar litter at different decomposition stages

Litter is one of the main sources of dissolved organic carbon (DOC) in forest soils and litter decomposition is an important control of carbon storage and DOC dynamics. The aim of our study was to evaluate (i) effects of tree species on DOC production and (ii) relationships between litter decomposition and the amount and quality of DOC. Five different types of leaves and needles were exposed in litterbags at two neighboring forest sites. Within 12 months we sampled the litterbags five times and leached aliquots of field moist litter in the laboratory. In the collected litter percolates we measured DOC concentrations and recorded UV and fluorescence spectra in order to estimate the aromaticity and complexity of the organic molecules. Furthermore, we investigated the biodegradability of DOC from fresh and decomposed litter during 6 weeks incubations. Fresh sycamore maple litter released the largest amounts of DOC reaching about 6.2% of litter C after applying precipitation of 94 mm. We leached 3.9, 1.6, 1.0 and 3.3% carbon from fresh mountain ash, beech, spruce and pine litter, respectively. In the initial phase of litter decomposition significantly decreasing DOC amounts were released with increasing litter mass loss. However, after mass loss exceeds 20% DOC production from needle litter tended to increase. UV and fluorescence spectra of percolates from pine and spruce litter indicated an increasing degree of aromaticity and complexity with increasing mass loss as often described for decomposing litter. However, for deciduous litter the relationship was less obvious. We assume that during litter decomposition the source of produced DOC in coniferous litter tended toward a larger contribution from lignin-derived compounds. Biodegradability of DOC from fresh litter was very high, ranging from 30 to 95% mineralized C. DOC from degraded litter was on average 34% less mineralizable than DOC from fresh litter. Taking into account the large DOC production from decomposed needles we can assume there is an important role for DOC in the accumulation of organic matter in soils during litter decomposition particularly in coniferous forests.     

Interactions between litter quality, decomposition and soil fertility: a laboratory study

Leaf litters from beech (Fagus sylvatica L.) and oak (Quercus robur L.) trees were collected from mixed, deciduous woodlands growing on three soil types that varied in mineral nutrient concentrations and N mineralisation potential. Litter quality, including %N, %Mn, %P, acid detergent fibre, cellulose, Klason lignin, phenylpropanoid constituents of lignin, hexose and pentose sugar (mainly from hemicelluloses) varied within species according to soil type. However, oak and beech showed the opposite responses to soil nutrient status for most of these variables. The litters were incubated in the laboratory for 12 months (at 18 °C and constant moisture) on beds of forest floor material from two soils of contrasting high nutrient material (HNM) or low nutrient material (LNM) nutrient status to investigate litter quality and substrate interactions. At 4, 8 and 12 months there were significant differences in mass losses from oak and beech litters from all sites, and for each litter type exposed to the HNM and LMN soils. At 12 months mean mass losses were higher for HNM treatment (38.7% oak, 27.8% beech) than for the LNM treatment (30.6% oak, 25.5% beech). However, the beech and oak litters from the different sites consistently responded in opposite ways on the same soil treatment reflecting site-related effects on litter quality. Initial concentration of Klason lignin was the best predictor for mass losses from litter species and litter types. Intra-specific variation in rates of litter decomposition of beech and oak litters from different sites, and differences in their interactions with the two forest floor materials, illustrate the complexities of proximate controls on decomposition that are often masked in system-level studies.     

Decomposition of beech leaves (Fagus sylvatica) and spruce needles (Picea abies) in pure and mixed stands of beech and spruce

The decomposition of spruce needles and beech leaves was investigated in a 30- and 120-yr-old beech, spruce and mixed (beech/spruce) forest using 1 mm mesh litterbags. The mass loss, content of C, N and water and microbial biomass, basal respiration and specific respiration of the litter materials were analyzed after exposure for 1.5, 3, 6, 9, 12, 18 and 24 months in the field. Decomposition of both types of litter was faster in beech than in spruce stands and after 24 months loss of C from litter materials was at a maximum in beech stands (>60%) and considerably less in the spruce and mixed stands (ca. 40%). Generally, spruce needles decomposed more rapidly than beech leaves, but the faster decay was not associated with higher N concentrations. Rather, N was accumulated more rapidly in beech leaves. Concomitantly, in beech stands microbial biomass of beech leaves exceeded that of spruce needles indicating that beech leaves consist of more favorable resources for microorganisms than spruce needles. Differences in decomposition between beech leaves and spruce needles were most pronounced in beech stands, intermediate in mixed stands and least pronounced in spruce stands. Decomposition, N content and microbial biomass in litter materials exposed in the 120-yr-old stand consistently exceeded that in the 30-yr-old stand indicating adverse conditions for litter decay in regrowing stands. Generally, mixed stands ranked intermediate between spruce and beech monocultures for most of the variables measured indicating that the adverse conditions for litter decay and microorganisms in spruce forest are effectively counteracted by admixture of beech to spruce monocultures. It is concluded that the accumulation of litter materials in spruce forests is not due to the recalcitrance of spruce needles to decay. Rather, adverse environmental conditions such as high polyphenol contents in the litter layer of spruce stands retard decomposition processes; spruce needles appear to be more sensitive to this retardation than beech leaves.     

Effects of fungal inocula on the decomposition of lignin and structural polysaccharides in Pinus sylvestris litter

 Litter bags containing sterile Scots pine (Pinus sylvestris) needles (19.8% lignin, 26.5% cellulose and 0.34% N) were inoculated with two species of fungi in the laboratory and then placed in the litter layer of a pine plantation. Marasmius androsaceus, which can degrade lignocellulose, was initially displaced by other fungal colonisers and was not detected in the litter after 2–3 months; but was re-isolated from the needles after 12 months. Trichoderma viride, which is a cellulolytic species and also antagonistic to other fungi, dominated the litter throughout the experiment. The control litter was naturally colonised by litter fungi. After 12 months, mass losses were similar at 52% for M. androsaceus and 48% for T. viride, compared with 36% for the control litter colonised by a more complex fungal community. Lignin concentrations increased with time in control litter and with T. viride because mass losses of carbohydrates were greater than those of lignin. Litter inoculated with M. androsaceus showed significant lignin decomposition throughout the experiment but cellulose concentrations showed a proportional increase in the first 6 months, suggesting that the fungus was preferentially exploiting hemicellulose and non-structural carbohydrates. Analysis of TFA-extractable sugars (mainly from hemicellulose) and CuO-derived phenylpropanoid moieties from lignin confirmed the differential patterns of resource decomposition which were not evident from total mass losses. During the initial stages of decomposition, T. viride was as effective in utilising structural polysaccharides as the complex fungal community in the control litter. Furthermore, M. androsaceus not only exhibited unexpectedly low cellulolytic activity but also facilitated lignin depolymerisation after the fungus was no longer detectable in the litter. The pre-inoculation of litter with these two fungal species therefore affected the overall dynamics of decomposition at a biochemical level. This study illustrates the importance of understanding the effects and interactions of specific fungi, rather than assumptions about the functional competence of diverse communities, on the processes of litter decomposition. Received: 5 July 2000     

Saprotrophic fungi transform organic phosphorus from spruce needle litter

Fungal decomposition of and phosphorus transformation from spruce litter needles (Picea abies) were simulated in systems containing litter needles inoculated with individual saprotrophic fungal strains and their mixtures. Fungal strains of Setulipes androsaceus (L.) Antonín, Chalara longipes (Preus) Cooke, Ceuthospora pinastri (Fr.) Höhn., Mollisia minutella (Sacc.) Rehm, Scleroconidioma sphagnicola Tsuneda, Currah & Thormann and an unknown strain NK11 were used as representatives of autochthonous mycoflora. Systems were incubated for 5.5 months in laboratory conditions. Fungal colonization in systems and competition among strains were assessed using the reisolation of fungi from individual needles. After incubation, needles were extracted with NaOH and extracts were analysed using ³¹P nuclear magnetic resonance spectroscopy (NMR). Needle decomposition was determined based on the decrease in C:N ratio. Systems inoculated with the basidiomycete S. androsaceus revealed substantial decrease in C:N ratio (from 25.8 to 11.3) while the effect of ascomycetes on the C:N ratio was negligible. We suppose that tested strains of saprotrophic ascomycetes did not participate substantially in litter decomposition, but were directly involved in phosphorus transformation and together with S. androsaceus could transform orthophosphate monoesters and diesters from spruce litter needles into diphosphates, polyphosphates and phosphonates. These transformations seem to be typical for saprotrophic fungi involved in litter needle decomposition, although the proportion of individual phosphorus forms differed among studied fungal strains. Phosphonate presence in needles after fungal inoculation is of special interest because no previous investigation recorded phosphonate synthesis and accumulation by fungi. Our results confirmed that the ³¹P NMR spectroscopy is an excellent instrumental method for studying transformations of soil organic phosphorus during plant litter decomposition. We suggest that polyphosphate production by S. androsaceus may contribute to the phosphorus cycle in forest ecosystems because this fungus is a frequent litter colonizer that substantially participates in decomposition.     

Elevated CO<Subscript>2</Subscript> concentration affected pine and oak litter chemistry and the respiration and microbial biomass of soils amended with these litters

Elevated atmospheric CO₂ concentration ([CO₂]) may change litter chemistry which affects litter decomposability. This study investigated respiration and microbial biomass of soils amended with litter of Pinus densiflora (a coniferous species; pine) and Quercus variabilis (a deciduous species; oak) that were grown under different atmospheric [CO₂] and thus had different chemistry. Elevated [CO₂] increased lignin/N through increased lignin concentration and decreased N concentration. The CO₂ emission from the soils amended with litter produced under the same [CO₂] regime was greater for oak than pine litter, confirming that broadleaf litter with lower lignin decomposes faster than needle leaf litter. Within each species, however, soils amended with high lignin/N litter grown under elevated [CO₂] emitted more CO₂ than those with low lignin/N litter grown under ambient [CO₂]. Such contrasting effects of lignin/N on inter- and intra-species variations in litter decomposition should be ascribed to the effects of other litter chemistry variables including nonstructural carbohydrate, calcium and manganese as well as inhibitory effect of N on lignin decomposition. The microbial biomass was also higher in the soils amended with high lignin/N litter than those with low lignin/N litter probably due to low substrate use efficiency of lignin by microbes. Our study suggests that elevated [CO₂] increases lignin/N for both species, but increased lignin/N does not always reduce soil respiration and microbial biomass. Further study investigating a variety of tree species is required for more comprehensive understanding of inter- and intra-species variations of litter decomposition under elevated [CO₂].     

Effects of tree species and topography on soil chemistry, litter quality, and decomposition in Northeast Turkey

Leaf litters from beech (Fagus orientalis Lipsky.) and oak (Quercus robur L.), and needle litters from fir (Abies nordmanniana Spach.) and pine (Pinus sylvestris L.) trees were collected from north-facing site and south-facing site and at three slope positions (top, middle and bottom) on each aspect that varied in soil chemical characteristics (soil pH, cation exchange capacity and base saturation). The litters were analysed for initial total carbon, nitrogen, acid detergent fibre, lignin and cellulose concentrations. Nitrogen, acid detergent fibre and lignin concentrations and carbon:nitrogen and lignin:nitrogen ratios varied significantly within and between species according to soil chemical characteristics on aspects and slope positions. Litter decomposition was studied in the field using the litterbag technique. The litters were placed on two aspects and at three slopes on each aspect in October 2001, and were sampled every 6-month for 2 years. The main effects of aspect, species and slope position on decomposition rates were all statistically significant. Oak leaf litter showed highest decomposition rates, followed by pine, fir and beech litter, and the litters placed on north-facing site decomposed faster than those on the south-facing site. The litters placed at the top slope position decomposed slower than at those at either the bottom or middle positions. Initial lignin concentrations explained most of the variation in decomposition rates between species, and within species for the aspects and the slope positions, but the explained variance showed differences between aspects and slope positions. This result illustrates the important point that litter quality may define the potential rates of microbial decomposition but these are significantly influenced by the biotic and abiotic environment in which decomposition takes place.     

Immobilization of avian excreta-derived nutrients and reduced lignin decomposition in needle and twig litter in a temperate coniferous forest

The possible effects of excreta of the Great Cormorant Phalacrocorax carbo on decomposition processes and dynamics of nutrients (N, P, Ca, K, Mg) and organic chemical components (lignin, total carbohydrates) were investigated in a temperate evergreen coniferous forest near Lake Biwa in central Japan. Two-year decomposition processes of needles and twigs of Chamaecyparis obtusa were examined at two sites, control site never colonized by the cormorants (site C) and colonizing site (site 2). Mass loss was faster in needles than in twigs. Mass loss of these litter types was faster at site C than at site 2, which was ascribed to the decreased mass loss rate of acid-insoluble ‘lignin’ at site 2. Net immobilization of N, P, and Ca occurred in needles and twigs at site 2; whereas at site C, mass of these elements decreased without immobilization during decomposition. Duration of immobilization phase of these nutrients at site 2 was estimated to be 1.6 to 2.5 years in needles and 19.6 to 23.5 years in twigs. Immobilization potential (maximum amount of exogenous nutrient immobilized per gram initial material) was similar between needles and twigs for N and Ca but was about 10 times higher in twigs than in needles for P. δ¹³C in needles was relatively constant during the first year and then increased during the second year, whereas δ¹³C in twigs was variable during decomposition. Acid-insoluble fraction was depleted in ¹³C compared to whole needles (1.6-2.1‰) and twigs (2.0-2.5‰). δ¹⁵N of needles and twigs and their acid-insoluble fractions approached to δ¹⁵N of excreta during decomposition at site 2. This result demonstrated the immobilization of excreta-derived N into litter due to the formation of acid-insoluble lignin-like substances complexed with excreta-derived N. No immobilization occurred in K and Mg and their mass decreased during decomposition at both sites. Based on these results of nutrient immobilization during decomposition and on the data of litter fall and excreta amount at site 2, we tentatively calculated stand-level immobilization potential of litter fall and its contribution to total amount of N and P deposited as excreta. Thus, the potential maximum amount immobilized into litter fall (needles and twigs) was estimated to account for 5-7% of total excreta-derived N and P.     

Decomposition of aspen leaf litter results in unique metabolomes when decomposed under different tree species

We examined whether the decomposition rate of trembling aspen (Populus tremuloides) leaf litter differed when decomposed for one year in litter bags placed within adjacent monotypic stands of trembling aspen, Engelmann spruce, and lodgepole pine trees in four replicate blocks in the San Juan mountains of Colorado, and whether they were metabolized into different metabolic byproducts. Mass loss was 6-8% lower in pine stands than in spruce or aspen stands, but this trend was not significant (p = 0.27). Water-soluble leaf litter metabolites were characterized using ultra-performance liquid chromatography coupled to a quadrapole time-of-flight mass spectrometer (UPLC-MS). Aspen leaf litter metabolomes were highly chemically complex; thousands of unique molecular features were identified in each sample. Although many of the molecular features were common to litter decomposed in all three forest types, we identified a subset of features that differed in abundance among the forest types. Our results suggest that the decomposer communities associated with each forest type not only affected the overall decomposition rate, but also produced many compounds in the diverse suite of metabolic byproducts at different rates, which could be an important control on the long-term sequestration of C in soil organic matter.     

Extraction and isolation of lignin for utilization as a standard to determine lignin concentration using the acetyl bromide spectrophotometric method.

Lignin extracted with acidic dioxane was investigated as a possible standard for quantitatively determining lignin content in plant samples using the spectrophotometric method employing acetyl bromide. Acidic dioxane lignins were analyzed for carbohydrate, total protein, nitrobenzene oxidation products, and UV spectral characteristics. Total carbohydrate content of isolated lignins ranged from 2.21 to 5.70%, while protein ranged from 0.95 to 6.06% depending upon the plant source of the original cell wall sample. Nitrobenzene analysis indicated differences in the amount of guaiacyl and syringyl units making up the lignins, but this did not alter the UV spectrum of lignin solubilized in acetyl bromide. Regression equations developed for the acetyl bromide method using the isolated lignins for all the plant samples were similar to each other. Lignin values obtained by the acetyl bromide method were similar to the lignin values obtained as acid insoluble residues following a Klason lignin procedure.     

Low levels of nitrogen addition stimulate decomposition by boreal forest fungi

Climate warming and associated increases in nutrient mineralization may increase the availability of soil nitrogen (N) in high latitude ecosystems, such as boreal forests. These changes in N availability could feed back to affect the decomposition of litter and organic matter by soil microbes. Since fungi are important decomposers in boreal forest ecosystems, we conducted a 69-day incubation study to examine N constraints on fungal decomposition of organic substrates common in boreal ecosystems, including cellulose, lignin, spruce wood, spruce needle litter, and moss litter. We added 0, 20, or 200 μg N to vials containing 200 mg substrate in factorial combination with five fungal species isolated from boreal soil, including an Ascomycete, a Zygomycete, and three Basidiomycetes. We hypothesized that N addition would increase CO₂ mineralization from the substrates, particularly those with low N concentrations. In addition we predicted that Basidiomycetes would be more effective decomposers than the other fungi, but would respond weakly or negatively to N additions. In support of the first hypothesis, cumulative CO₂ mineralization increased from 635 ± 117 to 806 + 108 μg C across all fungal species and substrates in response to 20 μg added N; however, there was no significant increase at the highest level of N addition. The positive effect of N addition was only significant on cellulose and wood substrates which contained very little N. We also observed clear differences in the substrate preferences of the fungal species. The Zygomycete mineralized little CO₂ from any of the substrates, while the Basidiomycetes mineralized all of the substrates except spruce needles. However, the Ascomycete (Penicillium) was surprisingly efficient at mineralizing spruce wood and was the only species that substantially mineralized spruce litter. The activities of β-glucosidase and N-acetyl-glucosaminidase were strongly correlated with cumulative respiration (r = 0.78 and 0.74, respectively), and Penicillium was particularly effective at producing these enzymes. On moss litter, the different fungal species produced enzymes that targeted different chemical components. Overall, our results suggest that fungal species specialize on different organic substrates, and only respond to N addition on low N substrates, such as wood. Furthermore, the response to N addition is non-linear, with the greatest substrate mineralization at intermediate N levels.