Saccharides of ectomycorrhizal fungal sclerotia as sources of forest soil polysaccharides

ABSTRACT

The neutral monosaccharide composition of forest soils differs from that of non-forest soils suggesting there is an accumulation of microbial saccharides. Ectomycorrhizal (ECM) fungi can be responsible as the fungi are typical in forest soils. We investigated neutral saccharides of ECM fungal sclerotia to determine what part it might play in the origin of forest soil polysaccarides. Sclerotial grain (SG) was collected from the O, A1 and A2 horizons of a soil of subalpine forest of Mt. Ontake, central Japan. Neutral saccharides in soil and SG were analyzed by two step hydrolysis with sulfuric acid and gas-chromatography of alditol acetate derivatives. Saccharides accounted for 6.0?16% of the SG by carbon content. The SG contained predominantly easily hydrolysable (EH)-glucose, which accounted for 75–85% of the composition depending on grain size and the soil horizon, followed by mannose (7.7?15%), galactose (2.2?4.8%) and non-easily hydrolysable (NEH)-glucose (1.7?6.1%). The SG contained all of these sugars irrespective of its size. The SG collected from the A1 and A2 horizons contained all sugar components found in that from the O horizon, except for fucose in that from A2 horizon. The monosaccharide composition of SG indicates that accumulation of ECM fungal sclerotial polysaccharides might have been responsible for enlarging the molar ratios of (galactose + mannose) /(arabinose + xylose) and EH-glucose/NEH-glucose of forest soils. The proportions of SG saccharides relative to soil saccharides were 3.6, 1.2, and 0.83% for the O, A1 and A2 horizons, respectively. These levels of the proportion are considerable as ECM fugal sclerotia are the products of a limited species among hundreds and thousands of microbial species inhabiting forest soils. The sclerotia forming ECM fungal species such as Cenococcum geophilum may be key sources of forest soil polysaccharides.     

Klimaeinfluß auf Lignin und Polysaccharide in Partikelgrößen-Fraktionen zonaler Steppenböden Rußlands

Climatic effect on lignin and polysaccharides in particle-size fractions of zonal steppe soils, Russia Zonal soils derived from similar parent materials are suitable for investigating the question, whether and how climate affects soil organic matter properties. For this reason we sampled 10 native surface (0—10 cm) and subsurface (ca. 50—60 cm) soil horizons in the native steppe and forest steppe of Russia. Polysaccharides and the vanillyl, syringyl and cinnamyl structural units of lignin (VSC) were determined in the fine earth (< 2 mm) as well as in clay (< 2 μm) and silt (2—20 μm) fractions. As the ratio of mean annual precipitation to potential evaporation (N/V) decreased, the concentrations of polysaccharides tended to decrease in the subsoil horizons. This was indicated most clearly for the silt fractions (r = 0.98**). In contrast, the VSC contents (in g kg^—1 organic C) of the subsoils increased as N/V decreased (r = —0.92*), resulting in increasing VSC/polysaccharide ratios of the subsoil horizons with decreasing N/V ratio (r = —0.94*). It is suggested that production of polysaccharides or their transport into the mineral subsoil or both is favored at sites with wide rather than narrow N/V ratio, whereas lignin might be selectively enriched during intense soil organic matter decay at the Southern sites.     

Chronic N deposition does not apparently alter the biochemical composition of forest floor and soil organic matter

Future rates of atmospheric N deposition have the potential to slow litter decay and increase the accumulation of soil organic matter by repressing the activity of lignolytic soil microorganisms. We investigated the relationship between soil biochemical characteristics and enzymatic responses in a series of sugar maple (Acer saccharum)-dominated forests that have been subjected to 16 yrs of chronic N deposition (ambient + 3 g NO₃⁻–N m⁻² yr⁻¹), in which litter decay has slowed and soil organic matter has accumulated in sandy spodosols. Cupric-oxide-extractable lignin-derived phenols were quantified to determine the presence, source, and relative oxidation state of lignin-like compounds under ambient and experimental N deposition. Pools of respired C and mineralized N, along with rate constants for these processes, were used to quantify biochemically labile substrate pools during a 16-week laboratory incubation. Extracellular enzymes mediating cellulose and lignin metabolism also were measured under ambient and experimental N deposition, and these values were compared with proxies for the relative oxidation of lignin in forest floor and surface mineral soil. Chronic N deposition had no influence on the pools or rate constants for respired C and mineralized N. Moreover, neither the total amount of extractable lignin (forest floor, P = 0.260; mineral soil, P = 0.479), nor the relative degree of lignin oxidation in the forest floor or mineral soil (forest floor P = 0.680; mineral soil P = 0.934) was influenced by experimental N deposition. Given their biochemical attributes, lignin-derived molecules in forest floor and mineral soil appear to originate from fine roots, rather than leaf litter. Under none of the studied circumstances was the presence or relative oxidation of lignin correlated with the activity of cellulolytic and lignolytic extracellular enzymes. Although chronic atmospheric N deposition has slowed litter decay and increased organic matter in our experiment, it had little effect on biochemical composition of lignin-derived molecules in forest floor and surface mineral soil suggesting organic matter has accumulated by other means. Moreover, the specific dynamics of lignin phenol decay is decoupled from short-term organic matter accumulation under chronic N deposition in this ecosystem.     

Soil organic matter quantity and quality in mountain soils of the Alay Range, Kyrgyzia, affected by land use change

 Changes in soil management practices influence the amount, quality and turnover of soil organic matter (SOM). Our objective was to study the effects of deforestation followed by pasture establishment on SOM quantity, quality and turnover in mountain soils of the Sui Checti valley in the Alay Range, Kyrgyzia. This objective was approached by analysis of total organic C (TOC), N, lignin-derived phenols, and neutral sugars in soil samples and primary particle-size soil fractions. Pasture installation led to a loss of about 30% TOC compared with the native Juniperus turkestanica forests. The pasture soils accumulated about 20% N, due to inputs via animal excrement. A change in land use from forest to pasture mainly affected the SOM bound to the silt fraction; there was more microbial decomposition in the pasture than in the forest silt fraction, as indicated by lower yields of lignin and carbohydrates, and also by a more advanced oxidative lignin side-chain oxidation and higher values of plant : microbial sugar ratios. The ratio of arabinose : xylose was indicative of the removal of carbohydrates when the original forest was replaced by pasture, and we conclude that this can be used as an indicator of deforestation. The accumulation of lignin and its low humification within the forest floor could be due to the extremely cold winter and dry summer climate. Received: 10 March 1999     

Chemistry of soil organic matter as related to C : N in Norway spruce forest (Picea abies(L.) Karst.) floors and mineral soils

Due to high nitrogen deposition in central Europe, the C : N ratio of litter and the forest floor has narrowed in the past. This may cause changes in the chemical composition of the soil organic matter. Here we investigate the composition of organic matter in Oh and A horizons of 15 Norway spruce soils with a wide range of C : N ratios. Samples are analyzed with solid‐state ¹³C nuclear magnetic resonance (NMR) spectroscopy, along with chemolytic analyses of lignin, polysaccharides, and amino acid‐N. The data are investigated for functional relationships between C, N contents and C : N ratios by structural analysis. With increasing N content, the concentration of lignin decreases in the Oh horizons, but increases in the A horizons. A negative effect of N on lignin degradation is observed in the mineral soil, but not in the humus layer. In the A horizons non‐phenolic aromatic C compounds accumulate, especially at low N values. At high N levels, N is preferentially incorporated into the amino acid fraction and only to a smaller extent into the non‐hydrolyzable N fraction. High total N concentrations are associated with a higher relative contribution of organic matter of microbial origin.     

Contribution of lignin and polysaccharides to the refractory carbon pool in C-depleted arable soils

We assessed the contribution of polysaccharides and lignin, major components of plant residues, to the refractory pool of soil organic carbon (SOC) in arable soils. Soil samples from two contrasting treatment types of European long-term agroecosystem experiments, i.e. conventionally managed (fertilized) and C-depleted plots, enriched in refractory compounds, were compared. Bulk samples from eight experimental sites and particle-size fractions of two of the sites were investigated. The CuO oxidation technique was used as a relative measure of lignin and its degree of structural alteration. The contents and composition of polysaccharides were determined following hydrolysis with trifluoroacetic acid (TFA). For the bulk samples, the amount of lignin phenols declined more than the total OC in the course of C-depletion. The contribution of lignin phenols to total OC was thus lower in the C-depleted versus the fertilized plots. A greater lignin biodegradation was found in the bulk samples of the depleted plots compared with the fertilized plots. The analysis of size fractions revealed lower OC-normalized contents of lignin phenols and a higher degree of lignin alteration in fractions <63 μm of the depleted versus the fertilized plots. These findings indicate that lignin does not accumulate within the refractory C pool of arable soils. The refractory SOC pool shows a lower contribution of lignin as compared with more labile fractions of SOC. If lignin-derived carbon is present in the stable pool it has been extensively modified so that it can no longer be identified as phenolic CuO oxidation products. OC-normalized contents of polysaccharides (neutral sugars and galacturonic acid) were similar in bulk samples of the C-depleted and fertilized plots. The contrasting treatments showed similar polysaccharide contents especially in separates <6 μm. The separates <6 μm in the C-depleted plots retained between 50 and 100% of the polysaccharide amounts in the fertilized plots. The mass ratio of (galactose+mannose)-to-(arabinose+xylose) (GM/AX) was higher in bulk samples of the C-depleted versus the fertilized plots, indicating a higher relative contribution of microbial sugars. Within a particular soil, the fine separates were those with the highest GM/AX ratio. These results indicate that the refractory C pool has a similar proportion of polysaccharides as the labile C pool, but refractory polysaccharides are mainly associated with fine separates and show a dominant contribution of microbial sugars. Our results provide evidence that polysaccharides, mainly those of microbial origin, are stabilized over the long-term within fine separates of arable soils. In contrast, CuO lignin is associated mainly with the coarse fractions and does not contribute to the refractory C pool.     

Carbon structure and enzyme activities in alpine and forest ecosystems

The chemical structure of soil organic matter fractions and its relationship to biological processes remains uncertain. We used pyrolysis-gas chromatography/mass spectrometry to analyze the molecular structure of light and heavy fraction C from soils in the San Juan Mountains, Colorado. The soil samples, each replicated three times, were from two elevations (alpine and low forest) within two geochemically distinct basins (igneous and sedimentary). We also analyzed whether variation in the activity of nine enzymes that mediate soil organic matter turnover and nutrient cycling could explain differences in C structure. We found that, across basins and elevation, light fraction and heavy fraction C had distinct chemistries. The light fraction was characterized by an abundance of plant lignin biomarkers, including phenol, 2-methoxy-4-vinyl-(vinylguaiacol) and phenol, 2-methoxy-(guaiacol); in contrast heavy fraction had very little unaltered lignin but an abundance of polysaccharides, such as furfural, and proteins such as pyrrole. In alpine sites, light fraction was less abundant (4.27 versus 31.79 g kg⁻¹) and had a lower C/N ratio (17.25 versus 32.01) than in forests. The alpine sites also had higher activities of phosphatase, β-d-1,4-cellobiosidase, β-1,4-glucosidase, l-leucine aminopeptidase, and β-1,4-xylosidase. Protein abundance in the heavy fraction was correlated with peptidase, β-1,4-glucosidase, and phosphatase activities; in the light fraction, protein abundance was correlated with peptidase, xylosidase, and β-d-1,4-cellobiosidase activities. β-1,4-N-acetyl-glucosaminidase was negatively correlated with polysaccharides in the light and heavy fractions and positively correlated with lignin in the light fraction. However, there were not always significant correlations between enzymes and substrates. We suggest that this is likely because soil organic matter chemistry reflects long-term decomposition processes while enzyme dynamics fluctuate with current conditions or due to the presence of a pool of sorbed enzymes in the heavy fraction. While alpine and forest ecosystem C distribution and enzyme activities varied, substantial depletion of lignin derivatives in the heavy fraction across sites suggest that these compounds do not persist in stable soil C pools.     

Land-use effects on the composition of organic matter in particle-size separates of soil: I. Lignin and carbohydrate signature

Soil from Eutrochrept A horizons under long-term spruce forest (Sf), mixed deciduous forest (Df), permanent grassland (Gp) and arable rotation (Ar) was fractionated according to particle size and analysed for contents of C, N, lignin-derived phenols and carbohydrates. Whole soil from Sf, Df, Gp and Ar contained 84, 59, 73 and 25 g C kg^?1 soil, respectively. For all sites, the C content declined and C/N ratio increased in the order: clay (<2 μm), silt (2–20 μm), sand (20–2000 μm). Clay and silt were significantly lower in C in Ar than in Sf, Df and Gp, C associated with sand being substantially lower under arable rotation. The yield of lignin-derived phenols decreased and carboxyl functionality and methoxyl demethylation of lignin derivatives increased with decreasing particle size, indicating a progressive lignin alteration. Whole soil from Sf and Gp was substantially higher in vanillyl (V), syringyl (S) and cinnamyl (C) units (VSC) than soil from Df and Ar. Compared to whole soil, clay was depleted and sand enriched in VSC. Only sand appeared to be affected significantly by land use. Sand from Ar and Df was more enriched in VSC than sand from Gp and Sf. Whole soil carbohydrates decreased in the order: Gp>Ar>Df>Sf. Sand- and clay-sized separates were enriched in carbohydrates compared to silt. Carbohydrates in sand were mainly of plant origin whereas microbially-derived sugars accounted for a larger proportion in the clay. Compared to Sf, Df and Gp, clay from Ar was enriched and sand depleted in microbial sugars. Lignin and carbohydrate distribution patterns indicate that organic matter was in a more advanced stage of decomposition in the sand separates from forest than from agricultural A horizons. The forest soils also show a higher degree of oxidative changes in lignin associated with clay. In contrast, differences between silt from the four A horizons were small.     

Evolution of composition and content of soil carbohydrates following forest wildfires

The time evolution of the content and composition of carbohydrates was studied in the surface layer of forest soils non-affected and affected by wildfires. The low- and high-severity fires caused an immediate reduction of the C present as carbohydrates of 34% and 47–55%, respectively, which was due to the decrease of both hexoses and pentoses in two hydrolysis fractions (hydrolysate-A, non-cellulosic polysaccharides; hydrolysate-B, cellulosic polysaccharides). Carbohydrates tended to recover with time; however, values had still not reached the amounts found in the corresponding unburnt samples after 12–15 months. No difference between the unburnt and burnt samples was observed in the distribution of the neutral sugars in the hydrolysates over time. On a percentage basis, 72–92% of the total neutral sugars was extracted in hydrolysate-A (59 ± 7% hexoses; 24 ± 7% pentoses) and the rest, 8–28%, in hydrolysate-B (15 ± 5% hexoses; 2 ± 1% pentoses). The contribution of hexoses and pentoses to the neutral sugar pool was different between the two hydrolysis fractions being the hexoses/pentoses ratio higher for hydrolysate-B than for hydrolysate-A. The results also showed that the proportion of soil C present as carbohydrates-C rather than the total carbohydrates content should be used for monitoring short- and medium-term changes induced by fire in soil organic matter quality.     

Hexosamines in the organic layer of two beech forest soils: effects of mesofauna exclusion

Summary In December 1988, litter bags (mesh size: 45 and 1000 m) were exposed in the organic layer of unlimed and limed moder soil under beech forest in the Solling area of Germany. Bags were retrieved on three sampling dates (May, September, and November 1989) and amounts (g m^-2) of glucosamine and galactosamine were determined. Horizon-specific differences generally explained by far the largest part of the treatment variance. In the substrate sampled in December 1988 from the unlimed area glucosamine increased by a factor of 11.8 from the L 1 layer to the H layer and galactosamine by a factor of 15.9. With the exception of the F2 layer, the hexosamine amounts found in the limed substrate sampled in December 1988 were higher in all horizons than in the corresponding horizons sampled from the unlimed area. Exclusion of mesofauna from the 45-m litter bags generally reduced the level of amino sugars at both sites. The difference between the two mesh sizes was most pronounced in the lowermost horizons (F2 and H) and quite small in the upper horizons. The exclusion of mesofauna significantly increased the glucosamine: galactosamine ratio in the two L layers of the unlimed soil on all sampling dates. Seasonal fluctuations suggested that the actual amino sugar content of the organic layer was essentially the function of two components, the first reflecting long-term accumulation of microbial metabo-lites, and the second reflecting short term fluctuations in the microbial colonization of various C sources together with spatial and temporal differences in the ability of the microflora to produce and to decompose hexosamines. The results of this study show that the mesofauna in the beech forest soils investigated significantly affected both the amino sugar components measured, and thus stimulated the accumulation of one of the most important N pools in forest soils.     

Organic matter dynamics in a temperate forest soil following enhanced drying

Climate models predict an increase in global surface temperature and a change in precipitation intensity during this century. For Europe, extended drought periods followed by heavy rainfall are expected. The consequences for soil organic matter (SOM) dynamics are poorly understood. In this study, we investigated the effect of changing soil moisture regime on SOM quality under field conditions. For this purpose, a throughfall exclusion (TE) experiment was conducted in the summers 2006 and 2007 on a Haplic Podzol under a 140 years old Norway spruce stand using a roof installation followed by re-wetting compared to non-manipulated control plots. Total organic carbon, lignin (stable carbon pool), plant and microbial sugars (labile carbon pool) and microbial biomass (phospholipid fatty acids) were determined before, during and after the experiment in the L, O, A and B horizons. No significant treatment effects could be observed for SOM quantity. Amounts of lignin and soil microbial biomass were also not affected by the moisture regime but structure of soil microbial community. In the L and organic layers, gram + bacteria and actinomycetes were reduced during water stress, while gram- bacteria, fungi and protozoa increased during drought. Warmer and drier weather led to a dominance of fungi while a cooler and moister regime favoured bacteria, at least in the L horizon. An increasing PLFA (cy17:0 + cy19:0)/(16:1ω7c + 18:1ω7c) ratio in the O layer and A horizon suggests that the microbes suffered from water stress in these horizons. This agrees with a decreasing contribution of microbial sugars to SOM with decreasing water content in the O and A horizons. Although the original plant material exhibited increasing plant sugar content with increasing dryness, the contribution of the plant sugars to total soil organic carbon (SOC) generally decreased with decreasing water content. Physical-chemical changes of soil structure can theoretically change the sugar extractability from soils and/or chemical changes of sugars structure can probably affect the analysis. Therefore, chemical alteration and stabilization could be responsible for sugar decrease in soil with increasing dryness explaining the contrast compared to the original plant material.     

Soil organic matter transformation in Argentinian Hapludolls

Distribution and transformation of SOM in an Argentinian Hapludoll under arable land use and afforested with Pinus radiata was investigated by a combined approach using particle-size fractionation, wet-chemical analysis and ¹³C NMR spectroscopy. The soils showed thick mollic A horizons and had high organic carbon (OC) contents even in the subsoil, clay-sized separates having the highest OC concentrations. Under pine, a thick forest floor was built up. CuO oxidation data indicated low transformation of lignin in the forest floor, but advanced oxidative decomposition in the mineral soil horizon. In contrast, non-cellulosic carbohydrates, appeared to be stabilized in the mineral soil horizon against mineralization. Humic acids extracted from the mineral soil horizons showed an extremely high aromaticity. We assume that this was due to the production of pyrogenic aromatic moieties (black carbon) as a result of frequent fires in this ecosystem. No clear profile differentiation with respect to SOM quality was obtained. Composition of SOM in the mineral soil appeared not yet influenced from land use.     

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

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

Determination of neutral sugars in soil by capillary gas chromatography after derivatization to aldononitrile acetates

We have quantified ribose, rhamnose, arabinose, xylose, fucose, mannose, glucose, and galactose in soil by gas chromatography (GC) simultaneously after converting to aldononitrile acetate derivatives. A recommended single-hydrolytic step by 4 M trifluoroacetic acid (TFA) at 105 °C for 4 h was more effective for releasing soil neutral sugars from non-cellulosic carbohydrates and better suited to our purification procedure compared with the sulphuric acid hydrolysis. Linearity of the GC detection for each neutral sugar was in the range of 10-640 μg ml⁻¹ and the recovery of neutral sugars from the spiked soil samples ranged from 76% to 109.7%. The coefficients of variation of the neutral sugars in four soils were lower than 2.0% for the instrument and 4.6-7.6% for the whole determination procedures. Compared with the trimethylsilyl (TMS) derivatization, the recovery of our newly modified method was more satisfactory and the reproducibility of ribose was improved significantly. Moreover, the aldononitrile acetate derivative was more stable than TMS derivative. Therefore, it is a promising approach suitable for a routine use in the quantitative analysis of soil neutral sugars, since it is fast, sensitive, and reproducible.     

Studies on the properties of organic matter in buried humic horizon derived from volcanic ash

To investigate the relationship between age and the sugar composition in hydrolysates of the surface horizon and buried humic horizons with age up to 28,000 years B.P., the neutral sugars and amino sugars in soil hydrolysates were determined.

The ratios of total sugar carbon content to total carbon content of soil ranged from 2.68 to 4.13 percent. These values showed no distinct relationship with age.

Rhamnose, fucose, arabinose, xylose, mannose, galactose, glucose, g1ucosamine and galactosamine were present in the hydrolysates of all soil samples.

The polysaccharides of soil samples which have been buried for shorter periods were dominated by glucose, while those of soil samples buried for longer periods were dominated by mannose.

The proportion of hexoses showed a tendency to increase with age, while that of pentoses showed a tendency to decrease with age.     

Soil organic matter stabilization in acidic forest soils is preferential and soil type-specific

Minerals with large specific surface areas promote the stabilization of soil organic matter (SOM). We analysed three acidic soils (dystric, skeletic Leptic Cambisol; dystric, laxic Leptic Cambisol; skeletic Leptic Entic Podzol) under Norway spruce (Picea abies) forest with different mineral compositions to determine the effects of soil type on carbon (C) stabilization in soil. The relationship between the amount and chemical composition of soil organic matter (SOM), clay content, oxalate‐extractable Fe and Al (Fe_o; Al_o), and dithionite‐extractable Fe (Fe_d) before and after treatment with 10% hydrofluoric acid (HF) in topsoil and subsoil horizons was analysed. Radiocarbon age, ¹³C CPMAS NMR spectra, lignin phenol content and neutral sugar content in the soils before and after HF‐treatment were determined and compared for bulk soil samples and particle size separates. Changes in the chemical composition of SOM after HF‐treatment were small for the A‐horizons. In contrast, for B‐horizons, HF‐soluble (mineral‐associated) and HF‐resistant (non‐mineral‐associated) SOM showed systematic differences in functional C groups. The non‐mineral associated SOM in the B‐horizons was significantly depleted in microbially‐derived sugars, and the contribution of O/N‐alkyl C to total organic C was less after HF‐treatment. The radiocarbon age of the mineral‐associated SOM was younger than that of the HF‐resistant SOM in subsoil horizons with small amounts of oxalate‐extractable Al and Fe. However, in horizons with large amounts of oxalate‐extractable Al and Fe the HF‐soluble SOM was considerably older than the HF‐resistant SOM. In acid subsoils a specific fraction of the organic C pool (O/N‐alkyl C; microbially‐derived sugars) is preferentially stabilized by association with Fe and Al minerals. Stabilization of SOM with the mineral matrix in soils with large amounts of oxalate‐extractable Al_o and Fe_o results in a particularly stable and relatively old C pool, which is potentially stable for thousands of years.     

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.     

Litter decomposition and nitrogen mineralization in newly formed gaps in a Danish beech (Fagus sylvatica) forest

Gap formation is suggested as an alternative forest management approach to avoid extreme changes in the N cycle of forest ecosystems caused by traditional management practises. The present study aimed to investigate the effect of gap formation on N availability in beech litter and mineral soil on sites, which experienced only little soil disturbance during tree harvest. N pools, litter decomposition, and N mineralization rates in mineral soil were studied in two gaps (17 and 30 m in diameter) in a 75-year-old managed European beech (Fagus sylvatica L.) forest in Denmark and related to soil temperature (5 cm depth) and soil moisture (15 cm depth). Investigations were carried out during the first 2 years after gap formation in measurement plots located along the north-south transect running through the centre of each gap and into the surrounding forest.An effect of gap size was found only for soil temperatures and litter mass loss: soil temperatures were significantly increased in the northern part of the large gap during the first year after gap formation, and litter mass loss was significantly higher in the smaller gap. All other parameters investigated revealed no effect of gap size. Nitrification, net mineralization, and soil N concentrations tended to be increased in the gaps. Cumulative rates of net mineralization were two fold higher in the gaps during the growing season (June-October), but a statistically significant increase was found only for soil NH₄-N concentrations during this period. Forest floor parameters (C:N ratios, mass loss, N release) were not significantly modified during the first year after gap formation, neither were the total C content nor the C:N ratio in mineral soil at 0-10 cm depth.     

Soil properties and tree growth along an altitudinal transect in Ecuadorian tropical montane forest

In tropical montane forests, soil properties change with increasing altitude, and tree‐growth decreases. In a tropical montane forest in Ecuador, we determined soil and tree properties along an altitudinal transect between 1960 and 2450 m asl. In different vegetation units, all horizons of three replicate profiles at each of eight sites were sampled and height, basal area, and diameter growth of trees were recorded. We determined pH and total concentrations of Al, C, Ca, K, Mg, Mn, N, Na, P, S, Zn, polyphenols, and lignin in all soil horizons and in the mineral soil additionally the effective cation‐exchange capacity (CEC). The soils were Cambisols, Planosols, and Histosols. The concentrations of Mg, Mn, N, P, and S in the O horizons and of Al, C, and all nutrients except Ca in the A horizons correlated significantly negatively with altitude. The C : N, C : P, and C : S ratios increased, and the lignin concentrations decreased in O and A horizons with increasing altitude. Forest stature, tree basal area, and tree growth decreased with altitude. An ANOVA analysis indicated that macronutrients (e.g., N, P, Ca) and micronutrients (e.g., Mn) in the O layer and in the soil mineral A horizon were correlated with tree growth. Furthermore, lignin concentrations in the O layer and the C : N ratio in soil affected tree growth. These effects were consistent, even if the effect of altitude was accounted for in a hierarchical statistical model. This suggests a contribution of nutrient deficiencies to reduced tree growth possibly caused by reduced organic‐matter turnover at higher altitudes.     

Rapid soil organic matter loss from forest dieback in a subalpine coniferous ecosystem

Forest dieback caused by climate-change associated stresses and insect outbreaks has emerged as a global concern, and the biogeochemical consequences of this phenomenon need to be elucidated. We measured biological and chemical traits of soil beneath live trees or trees recently killed by a mountain-pine-beetle outbreak in a subalpine coniferous forest in the Front Range of Colorado. We focused on the top 5 cm of mineral soil just beneath the O horizon and measured microbial biomass, soil invertebrate abundance and composition, and soil chemical characteristics. With the termination of inputs from rhizodeposition, mycorrhizal fungal turnover and fine root turnover, soil total carbon (C) and total nitrogen (N) in the mineral soil at three sites decreased by 38–49% and 26–45%, respectively. Tree mortality was associated with reduced soil microbial biomass but soil nematode and microarthropod densities were unchanged. Nematode trophic structure was altered with an increased proportion of bacterial feeders. Soil inorganic N concentrations were inversely correlated to microbial C:N ratios. Tree death was associated with increased soil pH, a possible loss of calcium (Ca²⁺), but an accumulation of soil inorganic N, largely as NH₄⁺. Our results suggest that forest dieback results in rapid C and N loss from surface mineral soils and that the accumulation of soil inorganic N, the reduction in microbial biomass, and the more bacterial-based soil food web increase the potential of enhanced N loss from affected ecosystems.