Forest Humus Layers Research Articles

Estimating Forest Soil Properties for Humus Assessment—Is Vis-NIR the Way to Go?

Recently, forest management faces new challenges resulting from increasing temperatures and drought occurrences. For sustainable, site-specific management strategies, the availability of up to date soil information is crucial. Proximal soil sensing techniques are a promising approach for rapid and inexpensive collection of data, and could facilitate the provision of the necessary information. This study evaluates the potential of visual and near-infrared spectroscopy (vis-NIRS) for estimating soil parameters relevant for humus mapping in Saxon forests. Therefore, soil samples from the organic layer are included. So far there is little knowledge about the applicability of vis-NIRS in the humus layer of forests. We investigate the spectral behaviour of samples from organic (Oh) and mineral (0–5 cm, Ah) horizons, pointing out differences in the occurring absorption features. Further, we identify and assess the accuracy of selected soil properties based on vis-NIRS for forest sites, compare the outcome of different regression methods, investigate the implications for forest soils due to the presence and different composition of the humus layer and organic horizons and interpret the results regarding their usefulness for soil mapping and monitoring purposes. For this, we used retained humus soil samples of forests from Saxony. Regression models were built with Partial Least Squares Regression, Support Vector Machine and Cubist. Investigated properties were carbon (C) and nitrogen (N) content, C/N ratio, pH value, cation exchange capacity (CEC) and base saturation (BS) due to their importance for assessing humus conditions in forests. In organic Oh horizons, prediction results for C and N content achieved R2 values between 0.44 and 0.58, with corresponding RPIQ ranging from 1.58 to 2.06 depending on the used algorithm. Estimations of C/N ratio were more precise with R2 = 0.65 and RMSE = 2.16. Best results were reported for pH value, with R2 = 0.90 and RMSE = 0.20. Regarding BS, the best model accuracy was R2 = 0.71, with RMSE = 13.97. In mineral topsoil, C and N content models achieved higher values of R2 = 0.59 to 0.72, with RPIQ values between 2.22 and 2.54. However, prediction accuracy was lower for C/N ratio (R2 = 0.50, RMSE = 3.52) and pH values (R2 = 0.62, RMSE = 0.29). Models for CEC achieved R2 = 0.65, with RPIQ = 2.81. In general, prediction precision varied dependent on the used algorithm, without showing clear tendencies. Classification into pH classes was exemplified since this offers a new perspective for humus mapping on forest soils. Balanced accuracy for the defined classes ranged from 0.50 to 0.87. We show that vis-NIR spectroscopy is suitable for assessing humus conditions in Saxon forests (Germany), in particular not only for mineral horizons but also for organic Oh horizons.

Chlorination of soil organic matter: The role of humus type and land use

The levels of natural organic chlorine (Clorg) typically exceed levels of chloride in most soils and is therefore clearly of high importance for continental chlorine cycling. The high spatial variability raises questions on soil organic matter (SOM) chlorination rates among topsoils with different types of organic matter. We measured Clorg formation rates along depth profiles in six French temperate soils with similar Cl deposition using 36Cl tracer experiments. Three forest sites with different humus types and soils from grassland and arable land were studied. The highest specific chlorination rates (fraction of chlorine pool transformed to Clorg per time unit) among the forest soils were found in the humus layers. Comparing the forest sites, specific chlorination was highest in mull-type humus, characterized by high microbial activity and fast degradation of the organic matter. Considering non-humus soil layers, grassland and forest soils had similar specific chlorination rates in the uppermost layer (0–10 cm below humus layer). Below this depth the specific chlorination rate decreased slightly in forests, and drastically in the grassland soil. The agricultural soil exhibited the lowest specific chlorination rates, similar along the depth profile. Across all sites, specific chlorination rates were correlated with soil moisture and in combination with the patterns on organic matter types, the results suggest an extensive Cl cycling where humus types and soil moisture provided best conditions for microbial activity. Clorg accumulation and theoretical residence times were not clearly linked to chlorination rates. This indicates intensive Cl cycling between organic and inorganic forms in forest humus layers, regulated by humic matter reactivity and soil moisture, while long-term Clorg accumulation seems more linked with overall deep soil organic carbon stabilization. Thus, humus types and factors affecting soil carbon storage, including vegetation land use, could be used as indicators of potential Clorg formation and accumulation in soils.

Stump removal and tree species composition promote a bacterial microbiome that may be beneficial in the suppression of root disease.

Stumping is an effective forest management practice for reducing the incidence of Armillaria root-rot in regenerating trees, but its impact on the soil bacterial community has not been ascertained. This study investigated the long-term impact of stumping and tree species composition in a 48-year-old trial at Skimikin, British Columbia, on the relative abundance, diversity and taxonomic composition of bacterial communities by sequencing the v4 region of 16S rRNA gene using the Illumina Miseq platform. A total of 108 samples were collected from the forest floor (fermented (F) and humus (H) layers) and mineral soil (A (0-10cm) and B (10-20cm) horizons) of 36 plots (half each stumped or unstumped) that were planted with pure stands and admixtures of Douglas-fir, western redcedar and paper birch. Bacterial α-diversity in the B horizon declined with stumping whereas β-diversity was affected both by tree species and stumping treatments, with fir and birch supporting distinct bacterial communities. All horizons of stumped plots of birch and its admixtures were significantly enriched with potential plant growth-promoting bacteria. In conclusion, stumping along with planting birch alone or in admixture with other species promotes a bacterial microbiome that appears beneficial in the suppression of root disease.

Gas Diffusivity in the Forest Humus Layer

While several diffusivity models have been developed for mineral soils, there are few data for the forest humus layers that overlie, and are distinct from, the mineral soil. We measured soil gas diffusivity of the organic layer of spruce and beech stands. Total pore space and relative soil gas diffusivity were much higher in the humus layer than in mineral soil. An exponential diffusivity function was used to describe relative soil gas diffusivity in the humus layer of all spruce sites using one parameter set. The best-fit parameters exceeded the range recommended for mineral soil, indicating the distinctness in pore structure of mineral soil and the organic layer. Different model parameter sets were required to describe the different types of humus at the beech sites. As a consequence, traditional predictive models developed for mineral soil should not be used to derive diffusivity values to interpret the aeration status of the forest humus layer. Specific parameter sets should be used for calculating soil gas fluxes through these layers using the gradient flux approach.

Signatures of chlorinated dioxins and furans along the exposure path – The relation between vegetation and soil

Polychlorinated dibenzo-p-dioxins and dibenzo-furans (PCDD/Fs) belong to the most toxic persistent organic environmental pollutants. Therefore, knowledge about their fate along the exposure path is of special concern. In order to comprehend the transfer of PCDD/Fs across different environmental compartments, PCDD/F concentrations in plants and in soil were evaluated. Pairs of soil and plant samples were selected according to the shortest distance between sampling points. At 15 sites PCDD/F concentrations in spruce needles and in forest humus layers where compared. Summer conditions were evaluated on the basis of 8 sites with ryegrass- and grassland topsoil samples. Autumn conditions were addressed using 7 sites with curly kale and topsoil samples under grassland. Correlation analyses of the PCDD/F congener profiles for plant- and soil samples were conducted. The correlations were compared to influencing site (e.g. local temperature) and spatial as well as temporal offset parameters. No governing parameter that decisively influenced the similarity between plant and soil signature became evident. By means of the toxicity factors of TEQ-WHO, tetra- and penta-PCDD/F homologues were assigned to the group of low-chlorinated homologues (lcH), and hexa-, hepta- and octa-PCDD/F homologues to the high-chlorinated homologues (hcH). lcH and hcH are presumed to differ in solubility, volatility and rate of degradation. The ratio of lcH/hcH revealed characteristic fingerprints that enabled the differentiation of the individual PCDD/F-plant and -soil pairs. Spruce-humus pairs showed a close relation during winter exposure times, while the lower summer concentrations were not reflected in the humus layer. Kale was exposed at the beginning of the season with elevated PCDD/F immissions, and showed a closer relation to grassland topsoil than did ryegrass. lcH/hcH proved as a simple criterion that can reveal related PCDD/F fingerprints of different environmental compartments despite signal attenuation due to decomposition, volatilization and particulate transport.

Manganese availability is negatively associated with carbon storage in northern coniferous forest humus layers

Carbon sequestration below ground depends on organic matter input and decomposition, but regulatory bottlenecks remain unclear. The relative importance of plant production, climate and edaphic factors has to be elucidated to better predict carbon storage in forests. In Swedish forest soil inventory data from across the entire boreal latitudinal range (n = 2378), the concentration of exchangeable manganese was singled out as the strongest predictor (R2 = 0.26) of carbon storage in the extensive organic horizon (mor layer), which accounts for one third of the total below ground carbon. In comparison, established ecosystem models applied on the same data have failed to predict carbon stocks (R2 < 0.05), and in our study manganese availability overshadowed both litter production and climatic factors. We also identified exchangeable potassium as an additional strong predictor, however strongly correlated with manganese. The negative correlation between manganese and carbon highlights the importance of Mn-peroxidases in oxidative decomposition of recalcitrant organic matter. The results support the idea that the fungus-driven decomposition could be a critical factor regulating humus carbon accumulation in boreal forests, as Mn-peroxidases are specifically produced by basidiomycetes.

Responses of fungal and plant communities to partial humus removal in mid-boreal N-enriched forests

Partial removal of the forest humus layer was performed in nitrogen-enriched urban Scots pine forest stands in the northern Finland in order to improve soil conditions for ectomycorrhizal (ECM) fungi, important symbionts of trees. Aboveground part of understory vegetation and the uppermost half of the humus layer were removed (REMOVAL treatment) from sample plots in six urban and eight rural reference forest sites at the beginning of the 2001 growing season. During the seasons 2001–2005, we inventoried sporocarp production of ECM and saprophytic fungi, and in 2003 the recovery of understory vegetation. The REMOVAL treatment resulted in a higher number of fruiting ECM species and sporocarps than controls at the rural, but not at urban sites. The sporocarp number of saprophytic fungi declined in the REMOVAL subplots at the urban sites. The recovery of bryophytes and lichens in the REMOVAL treatment was slow at both the urban and rural sites, whereas Vaccinium dwarf shrub cover, and herb and grass cover returned rapidly at the urban sites. We conclude that the partial vegetation and humus layer removal as a tool to promote the reproduction of ECM fungal species is limited in the boreal urban forests.

Contribution of charcoal to short-term nutrient dynamics after surface fire in the humus layer of a dwarf bamboo-dominated forest

Ecological function of charcoal has been mainly investigated by adding charcoal to soil, which is not fully adequate to understand in situ the role in fire-prone forest ecosystem. To determine in situ effects of charcoal on ecosystem functions, such as nutrient availability, we conducted an experimental burning in a Japanese white birch forest with dense coverage of dwarf bamboo in the understory with or without removal of charcoal. Ammonium-N in the remaining humus layer increased immediately after the burning, but decreased to the level of unburnt plots within 1 month of the burning. Removal of charcoal had no significant effect on the NH4+-N dynamics. Although burning did not affect NO3−-N dynamics during the sampling period, charcoal removal led to a slight increase in NO3−-N. The available P increased immediately after the burning, but then fell at 1 month after burning. Charcoal inhibited the available P depletion and prolonged the high availability of P. Greater availability of P might be due to the adsorption of phosphate in charcoal pores. Exchangeable Ca and Mg increased gradually; charcoal appeared to extend the period of higher concentration of exchangeable Ca and Mg. Charcoal deriving from fire is a key factor in influencing available nutrient in the humus layer of post-fire forests.

Sequestration of carbon in the humus layer of Swedish forests — direct measurements

To determine sequestration rates of carbon dioxide (CO2) we calculated the carbon (C) storage rate in humus layers of Swedish forests with Podsolic soils, which account for 14.2 × 106ha of the 22.7 × 106ha of forested land in Sweden. Our data set covered 41 years of humus inventories and mean humus layer thickness in 82 513 plots. We analysed three forest types: (i) all combinations of tree species, (ii) forests dominated (&gt;70%) by Norway spruce ( Picea abies (L.) Karst.), and (iii) forests dominated (&gt;70%) by Scots pine ( Pinus sylvestris L.). To relate changes in humus layer thickness to land area we used the intersections in 25 km × 25 km grids and used kriging interpolation, permitting calculations for each forest type. For each intersection mean humus thickness for each year was calculated and regressed against time to obtain the rate of change. This rate, humus bulk density, and humus C concentration were used to calculate sequestration rates. The mean sequestration rate was 251 kg C·ha–1·year–1, which is higher than theoretical values. The sequestration rate was positively related to temperature sum, albeit including effects of forest management. The pine-dominated forest type had a mean rate of 283 kg C·ha–1·year–1, and the spruce-dominated had a mean rate of 239 kg C·ha–1·year–1. Under similar site conditions, pine sequestered more C than spruce (difference of 71 kg C·ha–1·year–1; p &lt; 0.0001), showing the importance of this type of ecosystem for C sequestration.

Retention of available P in acid soils of tropical and subtropical evergreen broad-leaved forests

Precipitation of mineral phosphate is often recognized as a factor of limiting the availability of P in acidic soils of tropical and subtropical forests. For this paper, we studied the extractable P pools and their transformation rates in soils of a tropical evergreen forest at Xishuangbanna and a subtropical montane wet forest at the Ailao Mountains in order to understand the biogeochemical processes regulating P availability in acidic soils. The two forests differ in forest humus layer; it is deep in the Ailao forest while little is present in the Xishuangbanna forest. The extractable P pools by resin and sodium-bicarbonate decreased when soil organic carbon content was reduced. The lowest levels of extractable P pools occurred in the surface (0–10 cm) mineral soils of the Xishuangbanna forest. However, microbial P in the mineral soil of the Xishuangbanna forest was twice that in the Ailao forest. Potential rates of microbial P immobilization were greater than those of organic P mineralization in mineral soils for both forests. We suggest that microbial P immobilization plays an essential role in avoiding mineral P precipitation and retaining available P of plant in tropical acidic soils, whereas both floor mass accumulation and microbial P immobilization function benefit retaining plant available P in subtropical montane wet forests.

Exceedance of critical loads of nitrogen and sulphur and its relation to forest conditions

The calculation of critical loads and their exceedance is one method to describe the vulnerability of forests to environmental stress caused by anthropogenic impact. Exceedance of critical loads for acidifying inputs and nitrogen was compared to different indicators of the soil and forest conditions in the German part of the extensive forest monitoring (ICP Forests/EU Level I), including more than 1,800 plots. In addition, an empirical relationship between the C/N ratio of the forest floor humus layer (C/NHumus) and the estimated nitrogen output for ten plots of the intensive monitoring (ICP Forests/EU Level II) was established in order to estimate the potential nitrogen output on Level I plots dominated by Norway spruce. Regarding all tree species assessed, the exceedance of critical loads for nitrogen and sulphur is negatively correlated with pH and base saturation up to 30 cm soil depth. The sulphur deposition and the exceedance of critical loads are highly correlated with the sulphur content of leaves and needles, whereas the respective relations for nitrogen were lower. The crown condition was weakly positively related to the sulphur content in tree leaves and needles. For Norway spruce sites, high exceedance of critical loads for nitrogen and nitrogen deposition corresponded well with low C/NHumus. In regions with high nitrogen load and low C/N ratios in the humus layer, the calculated nitrogen output was high. The results support the concept of critical thresholds in that way that their exceedance can impair forest ecosystem functions like nitrogen retention.

Element concentrations in boreal, coniferous forest humus layers in relation to moss chemistry and soil factors

Concentrations of 15 elements (Al, Ca, Cd, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, P, Pb, S and Zn) in the current annual growth increments of the ectohydric feather moss, Hylocomium splendens, were compared with those in the humus layer (Of+Oh) of coniferous forests at 353 sites throughout Finland. Moss concentrations in the moss increments were considered measures of atmospheric deposition of these elements. Concentrations in both moss and humus layer samples were negatively correlated with latitude, except Mn (moss) and Cr and Ni (humus layer), indicating a direct effect of deposition. Spearman correlations between concentrations in moss and humus layer samples were significant (P<0.05) for all elements; that for Pb concentrations was the highest (rs=0.60) while those for the other elements were ≤0.48. Using thickness and organic matter content of the humus layer, clay content of the mineral soil (5–20 cm layer) and the concentration in the moss explained between 8% (Cd) and 44% (Cr, Mg) of the variation in the concentrations in the humus layer samples. It was concluded that, for most elements, concentration in the humus layer are a poor indicator of current atmospheric deposition levels, at least in relatively unpolluted regions such as Finland.

Upward mobilization of 137Cs in surface soils of Chamaecyparis obtusa Sieb. et Zucc. (hinoki) plantation in Japan

The use of 137Cs has recently been adopted to estimate erosion in hinoki plantations in Japan. However, there have been several reports of the upward mobilization of 137Cs in forest humus layers. In this study, the vertical distribution of 137Cs within the soil profile was measured in a hinoki plantation. In order to confirm the upward migration of 137Cs from mineral soil to fresh surface litter and to identify mechanisms of the transfer, changes in 137Cs specific activity in the contents of litterbags were examined in a hinoki plantation. A controlled laboratory experiment was also conducted to assess the effect of microbial activity on the upward migration of 137Cs. As a result, the higher 137Cs activities in the surface organic layer of a hinoki plantation than in fresh litter and the increasing 137Cs total content of litterbags with time demonstrated the upward mobilization of 137Cs from mineral soil to the surface organic layer. Physical movement of soil particles by raindrop splash was considered an important process in 137Cs upward migration. The results of our laboratory experiment indicate an influence from soil microbial activity on the upward mobilization of 137Cs. Thus, upward migration of 137Cs and constant litter removal by runoff may induce 137Cs loss from steep forested catchments and underestimation of the 137Cs inventory leading to the overestimation of soil redistribution rates.

A mechanism of abiotic immobilization of nitrate in forest ecosystems: the ferrous wheel hypothesis

AbstractForest soils, rather than woody biomass, are the dominant long‐term sink for N in forest fertilization studies and, by inference, for N from atmospheric deposition. Recent evidence of significant abiotic immobilization of inorganic‐N in forest humus layers challenges a previously widely held view that microbial processes are the dominant pathways for N immobilization in soil. Understanding the plant, microbial, and abiotic mechanisms of N immobilization in forest soils has important implications for understanding current and future carbon budgets. Abiotic immobilization of nitrate is particularly perplexing because the thermodynamics of nitrate reduction in soils are not generally favorable under oxic conditions. Here we present preliminary evidence for a testable hypothesis that explains abiotic immobilization of nitrate in forest soils. Because iron (and perhaps manganese) plays a key role as a catalyst, with Fe(II) reducing nitrate and reduced forms of carbon then regenerating Fe(II), we call this ‘the ferrous wheel hypothesis’. After nitrate is reduced to nitrite, we hypothesize that nitrite reacts with dissolved organic matter through nitration and nitrosation of aromatic ring structures, thus producing dissolved organic nitrogen (DON). In addition to ignorance about mechanisms of DON production, little is known about DON dynamics in soil and its fate within ecosystems. Evidence from leaching and watershed studies suggests that DON production and consumption may be largely uncoupled from seasonal biological processes, although biological processes ultimately produce the DOC and reducing power that affect DON formation and the entire N cycle. The ferrous wheel hypothesis includes both biological and abiological processes, but the reducing power of plant‐derived organic matter may build up over seasons and years while the abiotic reduction of nitrate and reaction of organic matter with nitrite may occur in a matter of seconds after nitrate enters the soil solution.

Short and long-term effects of wood ash on the boreal forest humus microbial community

The short-term effects of loose and hardened wood ash on the coniferous forest humus layer microbes were studied 1–3 years after fertilization. The experiment was performed using two fertilization levels (3 and 9 t ash ha −1) and repeated in two coniferous forest stands of different site fertility. It was hypothesized that the effects of hardened wood ash on soil microbes are of less magnitude when compared to loose ash due to the slower dissolution rates. The long-term effects of loose ash (3 t ash ha −1) were studied in four forest stands of different site fertility 18 years after ash application. In order to study ash effects, the microbial activity (basal respiration- and thymidine incorporation rates) and microbial community structure (PLFA pattern) were determined. The results showed that irrespective of the forest site fertility, ash fertilization induced the same responses in the humus layer. It raised the microbial activity and changed the community structure. The changes were related to the dose and form of ash applied. Applying the same fertilization rate induced comparatively more changes to the loose wood ash sites than hardened wood ash sites, due to the detected slower dissolution of hardened ash into the humus. The effects of wood ash were long-term. Changes in the humus microbial activity and PLFA pattern were still detectable after 18 years.

A microcosmos study on the effects of cd-containing wood ash on the coniferous humus fungal community and the cd bioavailability

The use of wood ash in forestry has been questioned because the cadmium (Cd) concentration of ash, which varies between 1 and 20 mg kg-1 ash, exceeds the level allowed for fertilizers (3 mg kg-1) used in agriculture. To investigate the effects of Cd and ash on the fungal community composition and Cd bioavailability of the humus layer of boreal, coniferous forests, pumice or wood ash, spiked with a water soluble (CdCl2) or insoluble (CdO) form of Cd at three levels (0, 400 and 1000 mg kg-1), were applied at a fertilization level of 5000 kg ha-1 in a laboratory microcosm study. After 2 months, the humus in the microcosms was sampled and extracted for total DNA to detect changes in the fungal community by using polymerase chain reaction (PCR) and denaturing gradient gel electrophoresis (DGGE) techniques. PCR was performed using the fungal 18S rDNA primers FR1 + FF390 and FR1 + NS1. The bioavailability of Cd was measured with a bacterial biosensor(Bacillus subtilis BR 151/pT0024) emitting light in the presence of Cd. Using the primer pairs FR1 + FF390 and FR1 + NS1, resulted in over 35 and 15 DGGE bands, respectively. Both primer pairs detected an ash, but no Cd effect. When using FR1 + FF390, a higher fluorescence was observed in one DGGE band of all ashed samples compared to the pumiced samples. With the primer pair FR1 + NS1, the ashed samples had a DGGE band which was not visible or only faintly visible in the pumiced samples. In addition, one DGGE band disappeared from the ashed samples. Humus layer water extracts showed that the Cd added with the pumice was in a bioavailable form. The luminescence intensity of the biosensor was dependent on the form and level of the Cd added. No luminescence was detected when the Cd was added with the ash. Ash fertilization altered the humus layer fungal community, whereas the level and form of additional Cd in the ash had no influence because it was not bioavailable.

Diversity of endophytic fungi of single Norway spruce needles and their role as pioneer decomposers.

The diversity of endophytic fungi within single symptomless Norway spruce needles is described and their possible role as pioneer decomposers after needle detachment is investigated. The majority (90%) of all 182 isolates from green intact needles were identified as Lophodermium piceae. Up to 34 isolates were obtained from single needles. Generally, all isolates within single needles had distinct randomly amplified microsatellite (RAMS) patterns. Single trees may thus contain a higher number of L. piceae individuals than the number of their needles. To investigate the ability of needle endophytes to act as pioneer decomposers, surface-sterilized needles were incubated on sterile sand inoculated with autoclaved or live spruce forest humus layer. The dry weight loss of 13-17% found in needles after a 20-week incubation did not significantly differ between the sterilized and live treatments. Hence, fungi surviving the surface sterilization of needles can act as pioneer decomposers. A considerable portion of the needles remained green during the incubation. Brown and black needles, in which the weight loss had presumably taken place, were invaded throughout by single haplotypes different from L. piceae. Instead, Tiarasporella parca, a less common needle endophyte, occurred among these invaders of brown needles. Needle endophytes of Norway spruce seem thus to have different abilities to decompose host tissues after needle cast. L. piceae is obviously not an important pioneer decomposer of Norway spruce needles. The diversity of fungal individuals drops sharply when needles start to decompose. Thus, in single needles the decomposing mycota is considerably less diverse than the endophytic mycota.

Lowest Effect Levels of Lead and Mercury on Decomposition of Mor Layer Samples in a Long-Term Experiment

Indications of possible negative effects of lead (Pb) and mercury(Hg) on microbial respiration in Southern Swedish forest humus layers led to experiments on dose-response relationships by additions of metal salts in the laboratory. Respiration rates andweight loss due to decomposition of organic material were measured. For relevance to field situations metal doses were low,the time span was long, 550 days including freeze storage, and microbial activity was kept up by plant litter additions. We looked for effects of Pb and Hg at levels moderately elevated above the Southern Swedish reference, as well as combined effectsof Pb + Hg. A reduction in respiration and decomposition of10% was found at about 225–245 μg g-1 of total Pb, i.e. ata Pb level elevated 3.5 times. Although small effects of Hg werefound even at the lowest dose level, 10% inhibition of microbialactivity appeared temporarily at about 2–3 μg g-1 of total Hg, i. e. at 6–8 times the reference level. There were nolong-term additive effects of Pb and Hg on decomposition. Type of anion had a strong influence on the test, chlorides of Pbbeing more toxic than nitrates. Long-term monitoring and maintenance of microbial activity during the experiment were prerequisites for the occurrence of effects at low metal levels.

Chemical characterization of the organic matter in forest soils by Curie point pyrolysis-GC/MS and pyrolysis-field ionization mass spectrometry

The chemical composition of three different forest humus layers (mull, moder, mor) was studied by flash pyrolysis-gas chromatography/electron impact mass spectrometry (Py-GC/EIMS) and by temperature-programmed, direct pyrolysis-field ionization mass spectrometry (Py-FIMS) in order to evaluate the relevance of individual humus constituents for geoecological and geochemical processes. Pyrolysis products with a molecular weight up to 430 Dalton that derive from carbohydrates, intact and degraded lignins, proteins, lipids, polyphenols, and aliphatic polymers were identified by accurate mass measurements in the FI mode and by GC/EIMS. However, the pyrolysis products that caused the main differences between the Py-FI mass spectra of the three humus types in the mass range above m/z 300 were not recorded by Py-GC/EIMS. Their assignments are based on in-source pyrolysis and high resolution FIMS. They derive mainly from biphenyl-, diarylpropane-, phenylcoumaran-, and resinol-type subunits from lignin, and phytosterols and their dehydration products. Furthermore, free and bound lipids such as homologous series of n-fatty acids (C 22-C 32), alkyl monoesters (C 40-C 52), and aromatic esters (C 27-C 34) are found.

Changes of lipid content and lipid composition in forest humus layers derived from Norway spruce

Forest humus layers derived from Norway spruce [ Picea abies (L.) Karst.] were fractionated into unbound lipid (UL), unbound polar compounds (UPC), hydrolyzable compounds (HY), bound lipid (BL). bound polar compounds (BPC) and residue (RES) by a combination of solvent extraction and acid hydrolysis. On a dry weight basis the UL and BL fractions totalled 7.7 and 3.0%, respectively, of the spruce litter material (L layer), but decreased markedly to 0.7 and 0.3% in the mineral soil (Aeh horizon). Unbound and bound lipid C together amounted to 14.8% of total organic C in the L and 13.6% in the Aeh horizon, showing a minimum in the Of (11.6%) and Oh (11.4%) layers. This chemically-detectable alkyl-C (sum of UL- and BL-C) accounted for two-thirds of the spruce litter total alkyl-C estimated from solid state (CPMAS) 13C-NMR spectra, but only for one-third of the mineral soil alkyl-C. Spruce litter UL contained higher amounts of C, O and H compared to BL, and with increasing depth significant changes in C, O and H contents were observed for both lipid fractions. C-to-H atomic ratios were consistently lower for UL (7.3–7.7) than BL (8.2–9.0), whereas unbound and bound lipid C-to-O ratios were nearly identical in spruce litter (4.7–4.8), but differed greatly in the Aeh horizon (UL: 5.5; BL: 3.1). Preparative column chromatography on silica gel of unbound lipid separated a series of subfractions, preliminary labelled as less polar lipid. glycolipid, phospholipid and highly polar lipid. Glycoplipid was the dominant component of spruce litter unbound lipid (45%) and further increased in the Aeh horizon (60%), while less polar lipid showed highest percentages in the L (41%) and Oh layers (42%). Phospholipid (9–15%) and highly polar lipid (2–7%) were generally minor lipid subfractions in the forest humus profile.