Forest Floor Solution Research Articles

Evidence of dissolved trivalent manganese in acidic forest soils

AbstractBackgroundEvidence of trivalent manganese (Mn3+) in the aqueous phase of soils is unknown so far although this strong oxidant has large environmental implications.AimsWe aimed to modify a spectrophotometric protocol (porphyrin method) and to discriminate between Mn2+ and Mn3+ in the aqueous phase of forest soils based on kinetic modeling.MethodsWe investigated manganese speciation in 12 forest floor solutions and 41 soil solutions from an acidic forest site by adjusting pH and correcting for absorbance.ResultsThe solutions showed broad ranges in pH (3.4−6.3), dissolved organic carbon (DOC, 1.78−77.1 mg C L−1), and total Mn (MnT, 23.9−908 µg L−1). For acidic solutions, a pH‐buffer was added to increase the pH of the solutions to 7.5−8.0, and background absorption was corrected for colored solutions, that is, solutions high in DOC. This was done to accelerate the reaction kinetics and avoid overestimation of MnT concentrations. After the pH and color adjustments, the comparison of MnT concentrations between the porphyrin method and optical emission spectrometry showed good agreement. Trivalent Mn, which is stabilized by organic ligands, constitutes significant proportions in both forest floor solutions (10−87%) and soil solutions (0.5−74%).ConclusionsThe dissolved Mn3+ is present in acidic forest soils. Thus, we revise the paradigm that this species is not stable and encourage to apply the revised method to other soils.

Dissolved Organic Matter Dynamics in Reference and Calcium Silicate‐Treated Watersheds at Hubbard Brook Experimental Forest, NH, USA

AbstractDissolved organic matter (DOM) can serve as an indicator of watershed carbon cycling, and links terrestrial to aquatic ecosystems. The extent DOM is microbially processed before it enters aquatic ecosystems is expected to change with soil depth and landscape position, and as some research suggests, in response to recovery from chronic acidification. Here we examined DOM within freely flowing soil pore water and stream water collected from reference and calcium silicate‐treated watersheds at the Hubbard Brook Experimental Forest, New Hampshire, USA. Dissolved organic carbon (DOC) and nitrogen (DON) concentrations as well as, absorption and fluorescence properties were analyzed to assess patterns in DOM quantity and quality. Parallel factor analysis was used to identify three predominant fluorescing DOM components (C1‐3). All three components were terrestrial in origin, however C3 exhibited spectral properties previously linked to the microbial processing of humic material. Distinct horizonal patterns emerged between genetic soil horizons. DOC quantity decreased with soil depth, so too did, the plant derived lignin‐like fraction (C2), while the C3 fraction increased. High‐elevation coniferous zones with shallow bedrock contributed to higher DOC concentrations in soil and stream water, while high and low‐elevation mineral soils exhibited greater C3 fractions and fluorescence indicators of microbially processed DOM. This pattern suggests these mineral soils are important organic carbon sinks, due to greater rates of microbial DOM processing. DOC in forest floor solutions, C3 fractions, and indicators of smaller molecular size (E2:E3) were all higher in the calcium silicate‐treated watershed, suggesting greater microbial processing of DOM.

Response of Dissolved Organic Matter in the Forest Floor of a Temperate Spruce Stand to Increasing Throughfall

In temperate spruce forests, dissolved organic matter (DOM) from forest floors is the major source of organic matter entering the mineral soil and thus it determines important soil properties and element cycling through the ecosystem. We examined effects of doubling locally collected throughfall for 6 yr on the concentrations of dissolved organic C (DOC) and properties of DOM (aromaticity, degree of molecule complexity) in the forest floor. Forest floor solutions below the Oi, Oe, and Oa horizons were sampled every 2 to 4 wk using tension lysimeters. For the controls, the average DOC concentrations in 2002 to 2007 were 43.8 ± 2.6 mg L−1 below the Oi, 49.6 ± 2.7 mg L−1 below the Oe, and 61.0 ± 2.0 mg L−1 below the Oa horizon. Doubling throughfall resulted in average DOC concentrations of 37.4 ± 1.8 mg L−1 (Oi), 49.3 ± 1.6 mg L−1 (Oe), and 50.1 ± 8.0 mg L−1 (Oa). The decreases in concentrations due to throughfall addition as well as the effects on DOM properties were, however, not statistically significant. It is commonly assumed that throughfall inputs are linearly related to water fluxes within and from the forest floor. Under that assumption, the results suggest that DOM fluxes are controlled by water fluxes rather than by the quantity of C that can be mobilized from the soil organic matter. Hence, increasing precipitation due to future climate changes presumably will result in enhanced DOM fluxes into the mineral horizons.

Effects of forest canopy cover and floor on chemical quality of water in broad leaved and coniferous forests of Istanbul, Turkey

The objective of this study was to determine how the canopy cover and forest floor of sessile oak, oriental beech, and Anatolian black pine stands affect the rainwater chemical parameters. The chemical parameters investigated were pH, electrical conductivity, total nitrogen (N), total phosphorus (P), sulfate (SO4)2-, nitrate (NO3)-, ammonia (NH3)+, permanganate index (PI), chloride (Cl−), sodium (Na+), potassium (K+), calcium (Ca2+), magnesium (Mg2+), iron (Fe), aluminum (Al), and lead (Pb). Forest canopy covers significantly affected pH, total N, total P, SO42-, Na+, and K+ contents in the throughfall, while forest floors influenced all chemical properties, except for NH3+ and Pb in the forest floor solution. While rainwater pH changed from medium acid (5.79) to strongly acid for both the throughfall (5.48) and forest floor solution (5.50) in the pine stand, it increased to neutral (6.57) in the oak and slightly acid (6.15) in the beech forest floor solution for the throughfall. Regardless of forest stand types, mean total P values of rainwater significantly decreased after passing through the canopy, but a change was not detected in the forest floor solution. A significant amount of SO42- was retained in the forest floor. Average SO42- values decreased from 4.90mgL−1 in the rainwater to 1.14–1.04mgL−1 in the forest floor solution of oak, beech and pine stands. Forest floor under the beech stand had significantly higher nitrate leaching than that under other two stands. Canopy covers of oak, beech and pine stands did not significantly influence the concentrations of Ca2+, Mg2+, Fe, and Al in the throughfalls. However, the same chemical parameters significantly increased in the forest floor. Given that the forest canopy cover and floor of all three stands had significantly different effect on rainwater chemistry, various tree species appear to differently influence the stream water chemistry. Therefore, selection of tree species for afforestation projects in fresh water producing watersheds becomes important to maintain water quality in streams for aquatic life and fresh water consumers.

Insect herbivory, organic matter deposition and effects on belowground organic matter fluxes in a central European oak forest

Apart from the forest floor, the canopy of forested ecosystems functions as the second most important source for dissolved and particulate fractions of organic and inorganic C and N compounds. However, under mass outbreak situations of insect herbivores this flux path of organic matter is considerably intensified clearly exceeding C and N fluxes from the forest floor. In this paper we report on herbivore-altered C and N fluxes from the canopy to the forest floor and effects on forest floor nutrient fluxes during severe defoliating herbivory of the winter moth (Operophtera brumata) and the mottled umber moth (Eranis defoliaria) in an oak forest in Germany. Over the course of 6.5 months we followed the C and N fluxes with bulk deposition, throughfall solution, insect frass deposits (green-fall together with insect faeces) and with forest floor solution in an 117-yr-old oak (Quercus petraea) forest. Compared to the control, herbivore defoliation significantly enhanced throughfall inputs of total and dissolved organic carbon and nitrogen by a factor of 3 and 2.5 (for TOC and DOC), and by 1.4 and 1.3 times (for TNb and DNb), respectively. Frass plus green-fall C and N fluxes peaked in May with 592 kg C ha−1 and 33.5 kg N ha−1 representing 79.6% (for C) and 78.3% (for N) of the total C and N input over 2.5 months. The quantitative and qualitative C and N input via faeces and litter deposition significantly differ between the insect affected and non-affected site. However, the C and N fluxes with throughfall did not significantly correlate with forest floor leachates. In this context, forest floor fluxes of TOC, DOC and NO3-N were significantly lower at the infested site compared to the control, whereas fluxes of NH4-N together with DON were significantly higher. The study demonstrates the importance of linking the population and associated frass dynamics of herbivorous insects with the cycling of nutrients and organic matter in forest ecosystems, highlighting the remarkable alterations in the timing, amounts and nature of organic matter dynamics on the ecosystem level. Consequently, the ecology of phytophagous insects allows partly to explain temporal-spatial alterations in nutrient cycling and thus ecosystem functioning.

Soil solution chemistry at one mountain beech (Fagus sylvatica L.) CONECOFOR plot, 1999 to 2005

Soil solution monitoring aims to understand various temporal scales of soil processes. The first eight years of observation in ABR1 Level II site have brought significant elements of understanding about the shorter temporal scales. It is suggested that certain solutes, regularly produced by forest floor microbial processes, are transferred to the highly mobile portion of the soil solution by a non linear process, producing irregular pulses and creating a strong high frequency component. Seasonal processes remain nonetheless detectable after simple and rough filtering. A multi-year trend of diminished nitrate mineralization and increased pH of forest floor solutions is possible. It is estimated that much more accurate analysis will be possible in a relatively short time span of further monitoring.

Linking Foliar Chemistry to Forest Floor Solid and Solution Phase Organic C and N in Picea abies [L.] Karst Stands in Northern Bohemia

Dissolved organic carbon and nitrogen (DOC and DON) produced in the forest floor are important for ecosystem functions such as microbial metabolism, pedogenesis and pollutant transport. Past work has shown that both DOC and DON production are related to litterfall and standing stocks of C and N in the forest floor. This study, conducted in spring, 2003, investigated variation in forest floor water extractable DOC (WEDOC) and DON (WEDON) and forest floor C and N as a function of lignin, cellulose and N contained in live canopy foliage across eight Picea abies [L.] Karst stands in northern Bohemia. Based on Near Infrared Spectroscopy (NIR) analysis of foliar materials, lignin:N and cellulose:N content of the youngest needles (those produced in 2002) were positively and significantly related to WEDOC (R 2 = 0.82–0.97; P<0.01) and to forest floor C:N ratio (R = 0.72–0.78; P<0.01). Foliar N was strongly and negatively related to WEDOC and C:N ratio (R = −0.91 and 0.72; P<0.05) among our study sites. WEDON was positively correlated to foliar lignin:N (R = 0.48; P<0.05; n=40). Forest floor C pools were not positively correlated with foliar lignin and cellulose and forest floor N pools were not positively correlated with foliar N. Instead, a significant negative correlation was found between forest floor N pools and foliar cellulose (R=−0.41; P<0.05), and between forest floor C pools and foliar N (R = −0.44; P<0.05). From a remote sensing standpoint, our results are important because canopy reflectance properties are primarily influenced by the most recent foliage, and it was the chemistry of the most recently produced needles that showed a stronger relationship with forest floor WEDOC and C:N ratio suggesting forest floor production of WEDOC can be calculated regionally with remote sensing.

Potential acidifying capacity of deposition: Experiences from regions with high NH4+ and dry deposition in China

Acid rain may cause soil acidification possibly leading to indirect forest damage. Assessment of acidification potential of atmospheric deposition is problematic where dry and occult deposition is significant. Furthermore, uncertainty is enhanced where a substantial part of the potential acidity is represented by deposition of ammonium (NH4+) since the degree of assimilation and nitrification is not readily available.Estimates of dry deposition based on deposition velocity are highly uncertain and the models need to be verified or calibrated by field measurements of total deposition. Total deposition may be monitored under the forest canopy. The main problem with this approach is the unknown influence of internal bio-cycling. Moreover, bio-cycling may neutralize much of the acidity by leaching of mainly K+. When the water percolates down into the rooting zone this K+ is assimilated again and acidity is regenerated. Most monitoring stations only measure deposition. Lacking measurements of output flux of both NH4+ and NO3− from the soil one cannot assess current net N transformation rates. Assumptions regarding the fate of ammonium in the soil have strong influence on the estimated acid load. Assuming that all the NH4+ is nitrified may lead to an overestimation of the acidifying potential.In parts of the world where dry deposition and ammonium are important special consideration of these factors must be made when assessing the acidification potential of total atmospheric loading. In China dry and occult deposition is considerable and often greater than wet deposition. Furthermore, the main part of the deposited N is in its reduced state (NH4+). The IMPACTS project has monitored the water chemistry as it moves through watersheds at 5 sites in China. This paper dwells at two important findings in this study. 1) Potassium leached from the canopy by acid rain is assimilated again upon entering the mineral soil. 2) Nitrification apparently mainly takes place in forest floor (H- and O-) horizon as NH4+ that escapes this horizon is efficiently assimilated in the A-horizon. This suggests that the potential acidification capacity of the deposition may be found in the throughfall and forest floor solution by treating K+ and NH4+, respectively, as acid cations in a base neutralization capacity (BNC) calculation.

Importance of canopy herbivores to dissolved and particulate organic matter fluxes to the forest floor

Few studies deal with the origin and contribution of organic matter (OM) fluxes from forest canopies to the variability of DOM and nutrient dynamics in the forest floor. Recent studies indicate a microbial and phytophages origin of OM in the throughfall of temperate forest ecosystems and reported inputs of dissolved (<0.45 μm) organic carbon (DOC) with throughfall range between 40 and 160 kg C ha −1 year −1. Data on particulate organic matter (0.45 μm<POM<2 mm) associated with throughfall fluxes are rare and not often considered in terrestrial ecosystem element budgets. Using field and laboratory experiments, we investigated the direct effects of canopy infestation by two functional groups of herbivores (sap and leaf feeders) on dissolved and particulate organic matter fluxes in throughfall and their impact on forest floor processes. Throughfall fluxes of particulate amino-nitrogen beneath infested spruce were about 60% and those of hexose-carbon beneath infested hardwood trees up to 70% higher compared to uninfested control trees. Corresponding fluxes to filtered samples showed no statistically significant differences. In column irrigation experiments, different levels of aphid infestation (uninfested, moderately, heavy infested) were simulated by adding honeydew to the irrigation solution. Forest floor solution chemistry from forest floor showed a significant reduction in NH 4–N and NO 3–N fluxes, slightly reduced DON fluxes and significantly increased CO 2 effluxes following honeydew application. We suggest that the amount of POM compounds transported with throughfall solution and its subsequent effects on forest floor processes depend on the herbivore functional group and on the level of infestation. From both a quantitative and qualitative point of view, our data indicate that herbivory-mediated organic matter in particulate and dissolved forms contribute considerably to the overall throughfall input of organic substances into the forest floor.

Lactovum miscens gen. nov., sp. nov., an aerotolerant, psychrotolerant, mixed-fermentative anaerobe from acidic forest soil

An aerotolerant, psychrotolerant anaerobe, anNAG3, was isolated from an acidic forest floor solution (in situ pH of 4.5). Cells of anNAG3 stained Gram-positive did not form spores, and were not motile. Cells were ovoid, approximately 1 μm long and 0.7 μm wide, mostly in pairs, and contained a multi-layered cell wall and intracytoplasmic membranes. Growth was observed at pH 3.5–7.5 and 0–35 °C. Glucose, galactose, fructose, mannitol, glucosamine, N-acetylglucosamine, cellobiose, and maltose supported growth. Lactate, ethanol, formate, and acetate were end products. H 2 and CH 4 were not detected, and only very minor amounts of CO 2 were produced. The relative amount of a particular product was dependent on the substrate utilized, and product profiles indicated that (i) sugars were initially metabolized to pyruvate via glycolysis, and (ii) lactate dehydrogenase and pyruvate-formate lyase were responsible for the subsequent metabolism of pyruvate. O 2 was not significantly utilized and was not toxic to growth. anNAG3 did not contain detectable membranous or cytoplasmic cytochromes. Nitrate, sulfate, and Fe(III) were not dissimilated. Thus, anNAG3 was characterized as an aerotolerant, non-acetogenic chemoorganotroph with a mixed-fermentative metabolism. The G + C content of the DNA was 37.6 mol%. The similarity of the 16S rRNA gene sequence of anNAG3 to that of its closest phylogenetic relatives (which were in the genera Lactococcus and Streptococcus) approximated 88–89%, indicating that anNAG3 constitutes the type species of a new genus. Based on the collective properties of anNAG3, it is proposed that anNAG3 be termed Lactovum miscens.

Effects of chronic nitrogen amendment on dissolved organic matter and inorganic nitrogen in soil solution

Increased atmospheric deposition of N to forests is an issue of global concern, with largely undocumented long-term effects on soil solution chemistry. In contrast to bulk soil properties, which are typically slow to respond to a chronic stress, soil solution chemistry may provide an early indication of the long-term changes in soils associated with a chronic stress. At the Harvard Forest, soil solution was collected beneath the forest floor in zero tension lysimeters for 10 years (1993–2002) as part of an N saturation experiment. The experiment was begun in 1988 with 5 or 15 g N m −2 per year added to hardwood and pine forest plots, and our samples thus characterize the long-term response to N fertilization. Samples were routinely analyzed for inorganic nitrogen, dissolved organic nitrogen (DON), and dissolved organic carbon (DOC); selected samples were also analyzed to determine qualitative changes in the composition of dissolved organic matter. Fluxes of DOC, DON, and inorganic N were calculated based on modeled water loss from the forest floor and observed concentrations in lysimeter samples. The concentration and flux of inorganic N lost from the forest floor in percolating soil solution are strongly affected by N fertilization and have not shown any consistent trends over time. On average, inorganic N fluxes have reached or exceeded the level of fertilizer application in most plots. Concentrations of DOC were unchanged by N fertilization in both the hardwood and pine stands, with long-term seasonal averages ranging from 31–57 mg l −1 (hardwood) and 36–93 mg l −1 (pine). Annual fluxes of DOC ranged from 30–50 g m −2 per year. DON concentrations more than doubled, resulting in a shift toward N-rich organic matter in soil solution percolating from the plots, and DON fluxes of 1–3 g m −2 per year. The DOC:DON ratio of soil solution under high N application (10–20) was about half that of controls. The organic chemistry of soil solution undergoes large qualitative changes in response to N addition. With N saturation, there is proportionally more hydrophilic material in the total DON pool, and a lower C:N ratio in the hydrophobic fraction of the total DOM pool. Overall, our data show that fundamental changes in the chemistry of forest floor solution have occurred in response to N fertilization prior to initiation of our sampling. During the decade of this study (years 5–14 of N application) both inorganic N and dissolved organic matter concentrations have changed little despite the significant biotic changes that have accompanied N saturation.

The composition of dissolved organic matter in forest soil solutions: changes induced by seasons and passage through the mineral soil

Dissolved organic matter in forest soils is a mixture of specific low-molecular-weight compounds and high-molecular-weight polyelectrolytes. The distribution of molecules of different molecular charge, weight, and size controls the reactivity of dissolved organic matter in the soil and, once exported from the soil, in the aqueous environment. The objective of this study was to track changes in the molecular composition of dissolved organic matter in soil as induced by seasons and the passage through the soil. In a 27-month field experiment, we collected seepage water from the organic forest floor layers and porewater in the mineral subsoil under a Scots pine ( Pinus sylvestris L.) and a European beech ( Fagus sylvatica L.) stand. In the solutions, we measured the concentrations of organic carbon (C) and the composition of organic matter in combined 3-month samples using liquid-state 1H-NMR spectroscopy. Parallel to seasonal variations in the concentration of dissolved organic C, the 1H-NMR spectra of dissolved organic matter in forest floor leachates showed remarkable changes. At the pine site, dissolved organic matter released during winter and spring contained larger proportions of H associated with O-containing structures and of low-molecular-weight compounds. During summer and autumn, the contribution of O-containing structures and low-molecular-weight compounds declined and broad signals in the aromatic and aliphatic region indicated the release of high-molecular-weight aromatic and aliphatic macromolecules. Forest floor leachates under beech showed a similar trend but a larger number of low-molecular-weight compounds was present in winter and spring samples. In spring, summer, and autumn samples acetate and sometimes succinate were most prominent. During the passage of dissolved organic matter through the upper mineral soil at the pine site, the number and intensity of signals due to low-molecular-weight compounds increased. These substances were thus more mobile than the macromolecules and/or released due to microbial transformation of organic matter. At the beech site subsoil porewater contained less low-molecular-weight compounds than the forest floor solutions but the present ones had increased signal intensities. We suggest that the differences of dissolved organic matter transformations during the passage through the upper mineral soil at the two sites were due to different microbial activity in the soils.

The effect of organic acids on base cation leaching from the forest floor under six North American tree species

SummaryOrganic acidity and its degree of neutralization in the forest floor can have large consequences for base cation leaching under different tree species. We investigated the effect of organic acids on base cation leaching from the forest floor under six common North American tree species. Forest floor samples were analysed for exchangeable cations and forest floor solutions for cations, anions, simple organic acids and acidic properties. Citric and lactic acid were the most common of the acids under all species. Malonic acid was found mainly under Tsuga canadensis (hemlock) and Fagus grandifolia (beech). The organic acids were positively correlated with dissolved organic carbon and contributed significantly to the organic acidity of the solution (up to 26%). Forest floor solutions under Tsuga canadensis contained the most dissolved C and the most weak acidity among the six tree species. Under Tsuga canadensis we also found significant amounts of strong acidity caused by deposition of sulphuric acid from the atmosphere and by strong organic acids. Base cation exchange was the most important mechanism by which acidity was neutralized. Organic acids in solution from Tsuga canadensis, Fagus grandifolia, Acer rubrum (red maple) and Quercus rubra (red oak) were hardly neutralized while much more organic acidity was neutralized for Acer saccharum (sugar maple) and Fraxinus americana (white ash). We conclude that quantity, nature and degree of neutralization of organic acids differ among the different tree species. While the potential for base cation leaching with organic acids from the forest floor is greatest under Tsuga canadensis, actual leaching with organic anions is greatest under Acer saccharum and Fraxinus americana under which the forest floor contains more exchangeable cations than does the strongly acidified forest floor under Tsuga canadensis.

Biodegradable dissolved organic carbon in forest soil solution and effects of chronic nitrogen deposition

Using a flow-through bioreactor, biodegradable dissolved organic carbon (BDOC) in forest floor solution was determined for a red pine ( Pinus resinosa) plantation and a naturally regenerated mixed hardwood forest (dominant species: Quercus velutina, Q. rubra, Acer rubrum, Fagus grandifolia, Prunus serotina). The forests have received chronic-N fertilization at three different rates (0, 50, 150 kg N ha −1 year −1) since 1988. Both BDOC concentration and %BDOC [% of dissolved organic carbon (DOC)] were significantly higher in the summer (18–20 mg C l −1, ≈30%) than spring or fall for both forest types. The BDOC depletion that was hypothesized to occur with chronic-N application and N saturation was not found in either stand. Instead, for the hardwood stand, BDOC concentration increased with N application, probably due to increased BDOC production in the forest floor with increases in available N. The strong negative relationship between litterfall mass and both BDOC concentration ( r 2=0.36) and %BDOC ( r 2=0.60) for the hardwood stand suggests that the primary source of BDOC in the hardwood forest floor is recent photosynthate allocated to fine roots for their growth, and organic compounds released from them (e.g., exudates and mucilage) rather than the leachate of freshly fallen autumn litter. A significant but weaker positive relationship between BDOC and ambient temperature for both stands suggests that the decomposition of forest floor organic matter is an additional source of BDOC in the forest floor. The production of non-biodegradable DOC (NBDOC) in the forest floor was not sensitive to ambient temperature or the chronic-N treatment. This suggests that an abiotic process, such as chemical equilibration between forest floor and forest floor solution, is responsible for the concentration of non-labile DOC. High %BDOC in throughfall (50–75%) for both stands suggests that throughfall is possibly an additional source of BDOC in the forest floor.

Effects of phytophagous insects on soil solution chemistry: herbivores as switches for the nutrient dynamics in the soil

Abstract In a column irrigation experiment the effects of different amounts of honeydew on the dynamics of soil respiration, dissolved organic carbon and nitrogen (DOC and DON, respectively) and inorganic nitrogen species in the organic layer of a forest soil were investigated. On the base of hourly and daily sampling, the application of different field-relevant amounts of honeydew-DOC caused short-time effects on the chemical composition of the gas and solution phase in the forest floor. Within one hour, the micro-organisms of the forest floor responded to the input of easily decomposable carbon compounds producing significantly higher rates of CO 2 than the control. In the forest floor solution the concentrations of inorganic nitrogen were reduced in the presence of high amounts of honeydew-DOC. The input of honeydew-DOC also slightly affected the DON dynamics causing reduced concentrations compared to the control. In case of DOC concentrations, differences between the treatments disappeared three days after the application of honeydew DOC has stopped. The column experiment showed that high temporal resolution measurements are necessary to achieve a better understanding of processes concerning the effects of phytophagous insects in the canopy on the solution chemistry of the forest floor.

Aphids on Norway spruce and their effects on forest-floor solution chemistry

Many species of aphids on spruce excrete large quantities of honeydew. As providers of carbohydrates, it is likely that they affect several ecosystem functions by fueling biological processes which are energy-limited. In a rainfall simulation experiment, we manipulated the level of aphid infestation on spruce and studied the effects of honeydew on forest-floor solution chemistry collected underneath infested and uninfested trees. A non-destructive sampling method was used to allow repeated measurements using the same forest floor during the experimental period.Although the input of dissolved organic carbon (DOC) underneath infested trees to the forest floor was considerably higher compared to uninfested trees, carbon concentrations of forest-floor leachates did not differ between treatments. Concentrations of dissolved organic nitrogen (DON), NO3–N and Ntotal of forest-floor leachates, underneath infested and uninfested trees followed a marked seasonal trend with low concentrations recorded in June and after-frost treatment of the forest floor, while the highest concentrations were recorded in July/August. Statistically significant differences in soil solution properties underneath infested and uninfested trees were recorded in July when honeydew-affected forest-floor leachates had lower concentrations of NH4–N, Ntotal, a lower conductivity and a higher pH. Despite a large input of honeydew, no pronounced seasonal trend was found in the carbon leachate concentrations of the forest floor (e.g. DOC, hexose-C). Discriminant function analysis showed that forest-floor leachates can be classified according to the experience of frost and nitrogen concentrations, which were affected by the honeydew of aphids. At the end of the experiment, the number of micro-organisms present in the forest floor was not significantly different between treatments but was higher in the Oh compared to the Ol+f horizon. Microbial communities did not appear to be severely influenced by deep temperatures. It seems likely that, in coniferous forests, aphids can considerably reduce local fluxes of nitrogen from the forest floor.

Canopy leaching, dry deposition, and cycling of calcium in Austrian oak stands as a function of calcium availability and distance from a lime quarry

Three Austrian oak stands were chosen along a 4-km distance gradient from a lime quarry to study effects of Ca availability both on dry deposition rates and on Ca cycling in these ecosystems. A fourth stand was used as a more regional reference site, some 30 km west of the lime quarry. Calcium bulk precipitation fluxes decreased with increasing distance from the lime quarry, contributing to major differences in available Ca along the transect over the last decades. Higher supply of Ca changed biogeochemical cycling by increasing pool sizes and fluxes of Ca in foliage, litter, throughfall, forest floor, soil, herbaceous vegetation, and soil solution. Regression analyses of net throughfall was a useful tool for separating between dry deposition and leaching of Ca. Dry deposition rates of particulate Ca declined rapidly with increasing distance from the Ca source. Leaching of Ca from the canopy declined along the gradient according to Ca content of the green foliage during the growing season. Leaching rates as a percentage of the stand's annual requirement indicated a relative shift from solid (litter) toward more solute Ca fluxes reaching the forest floor with increasing Ca availability of the stand.

Mobilization and immobilization of dissolved organic matter in forest soils

AbstractField and laboratory studies combined with destructive and nondestructive analytical methods were used to characterize dissolved organic matter (DOM) in acid forest soils. DOM is produced in significant amounts in the forest canopy and in the forest floor. A major part of the organic solutes are lignocellulose‐degradation products being strongly microbially altered in the course of ligninolysis. The release of lignin‐derived moieties into the soil solution is controlled by their degree of biooxidation. Microorganisms contribute also directly to the organic solutes through the release of microbial metabolites. DOM released from the forest floor passes the upper mineral soil almost conservatively, whereas in the subsoil most DOM is removed from solution. Immobilization of DOM is mainly due to sorption on Fe and Al oxides. The highly oxidized lignin‐derived moieties are preferentially removed from the soil solution whereas the saccharides are relatively enriched. We conclude that DOM in the forest soil output to the hydrosphere is a result of (1) the release of microbially degraded lignocellulose compounds and of microbial metabolites into the forest floor solution and (2) selective sorptive removal of the lignin‐derived constituents in the subsoil.

Linking aphid honeydew, throughfall, and forest floor solution chemistry of Norway spruce

In a seminatural manipulation experiment with artificial irrigation we followed throughfall and forest floor solution chemistry collected underneath aphid infested and uninfested Norway spruce. Solutions underneath infested trees showed significantly higher concentrations of dissolved organic carbon (DOC) but lower concentrations of dissolved organic nitrogen (DON), NO3‐N, and NH4‐N in throughfall solutions and of NH4‐N in forest floor solutions. Average concentrations were 40.5% (DON), 27.5% (NO3‐N), and 46.2% lower (NH4‐N) underneath infested trees in throughfall solutions, and 19.5% (DON), 9.4% (NO3‐N), and 42.0% (NH4‐N) lower in forest floor solutions. Differences in throughfall were more pronounced than in forest floor leachates. It is likely that honeydew is fuelling the metabolism of micro‐organisms and thus critically affects above and below ground nutrient cycles. We emphasize the importance of linking the biology of herbivores and micro‐organisms with geochemical processes.

Significance of DOM in the translocation of cations and acidity in acid forest soils

AbstractWe estimated the contribution of dissolved organic matter (DOM) to cation leaching and the translocation of acidity in three acid forest soils. The analysis was based on monitored (2 years) concentrations of dissolved organic carbon (DOC) in the field, measured total acidities of DOM, and measured as well as predicted weighted mean dissociation constants of the organic acids. Although the forest floor solutions were strongly acidic (pH 3.47–4.10), a considerable proportion of the organic acids was dissociated and organic anions represented 22–40% of the total anions in the mineral soil input. The flux of DOM‐associated exchangeable protons from the forest floor to the mineral soil ranged from 0.35 (Wülfersreuth) to 3.72 (Hohe Matzen) kmol ha−1 yr−1. In the subsoil, this organic acidity may be neutralized by microbial decomposition of the organic acids, but a part of the hydrogen ions may dissociate and contribute to acidification of the soil solution and to weathering processes. Due to the pronounced retention of DOM in the mineral subsoil horizons, the contribution of DOM to the output of cations and acidity from the soil is much lower than in the surface horizons but still significant.?