Impacts of extreme winter warming events on litter decomposition in a sub-Arctic heathland

Arctic climate change is expected to lead to a greater frequency of extreme winter warming events. During these events, temperatures rapidly increase to well above 0 °C for a number of days, which can lead to snow melt at the landscape scale, loss of insulating snow cover and warming of soils. However, upon return of cold ambient temperatures, soils can freeze deeper and may experience more freeze-thaw cycles due to the absence of a buffering snow layer. Such loss of snow cover and changes in soil temperatures may be critical for litter decomposition since a stable soil microclimate during winter (facilitated by snow cover) allows activity of soil organisms. Indeed, a substantial part of fresh litter decomposition may occur in winter. However, the impacts of extreme winter warming events on soil processes such as decomposition have never before been investigated. With this study we quantify the impacts of winter warming events on fresh litter decomposition using field simulations and lab studies.Winter warming events were simulated in sub-Arctic heathland using infrared heating lamps and soil warming cables during March (typically the period of maximum snow depth) in three consecutive years of 2007, 2008, and 2009. During the winters of 2008 and 2009, simulations were also run in January (typically a period of shallow snow cover) on separate plots. The lab study included soil cores with and without fresh litter subjected to winter-warming simulations in climate chambers.Litter decomposition of common plant species was unaffected by winter warming events simulated either in the lab (litter of Betula pubescens ssp. czerepanovii), or field (litter of Vaccinium vitis-idaea, and B. pubescens ssp. czerepanovii) with the exception of Vaccinium myrtillus (a common deciduous dwarf shrub) that showed less mass loss in response to winter warming events. Soil CO₂ efflux measured in the lab study was (as expected) highly responsive to winter warming events but surprisingly fresh litter decomposition was not. Most fresh litter mass loss in the lab occurred during the first 3-4 weeks (simulating the period after litter fall).In contrast to past understanding, this suggests that winter decomposition of fresh litter is almost non-existent and observations of substantial mass loss across the cold season seen here and in other studies may result from leaching in autumn, prior to the onset of “true” winter. Further, our findings surprisingly suggest that extreme winter warming events do not affect fresh litter decomposition.     

Litter- and ecosystem-driven decomposition under elevated CO2 and enhanced N deposition in a Sphagnum peatland

Peatlands represent massive global C pools and sinks. Carbon accumulation depends on the ratio between net primary production and decomposition, both of which can change under projected increases of atmospheric CO₂ and N deposition. The decomposition of litter is influenced by 1) the quality of the litter, and 2) the microenvironmental conditions in which the litter decomposes. This study aims at experimentally testing the effects of these two drivers in the context of global change. We studied the in situ litter decomposition from three common peatland species (Eriophorum vaginatum, Polytrichum strictum and Sphagnum fallax) collected after one year of litter production under pre-treatment conditions (elevated CO₂: 560 ppm or enhanced N: 3 g m⁻² y⁻¹ NH₄NO₃) and decomposed the following year under treatment conditions (same as pre-treatment). By considering the cross-effects between pre-treatments and treatments, we distinguished between the effects on mass loss of 1) the pre-treatment-induced litter quality and 2) the treatment conditions under which the litters were decomposing. The combination between CO₂ pre-treatment and CO₂ treatment reduced Polytrichum decomposition by −24% and this can be explained by litter quality-driven decomposition changes brought by the pre-treatment. CO₂ pre-treatment reduced Eriophorum litter quality, although this was not sufficient to predict decomposition. The N addition pre-treatment reduced the decomposition of Eriophorum, due to enhanced lignin and soluble phenols concentrations in the initial litter, and reduced litter-driven losses of starch and enhanced litter-driven losses of soluble phenols. While decomposition indices based on initial litter quality provide a broad explanation of quantitative and qualitative decomposition, they can only be taken as first approximations. Indeed, the microbial ATP activity, the litter N loss and resulting litter quality, were strongly altered irrespective of the compounds' initial concentration and by means of processes that occurred independently of the initial litter-qualitative changes. The experimental design was valuable to assess litter- and ecosystem-driven decomposition pathways simultaneously or independently. The ability to separate these two drivers makes it possible to attest the presence of litter-qualitative changes even without any litter biochemical determinations, and shows the screening potential of this approach for future experiments dealing with multiple plant species.     

Lignin and cellulose degradation and nitrogen dynamics during decomposition of three leaf litter species in a Mediterranean ecosystem

Cellulose and lignin degradation dynamics was monitored during the leaf litter decomposition of three typical species of the Mediterranean area, Cistus incanus L., Myrtus communis L. and Quercus ilex L., using the litter bag method. Total N and its distribution among lignin, cellulose and acid-detergent-soluble fractions were measured and related to the overall decay process. The litter organic substance of Cistus and Myrtus decomposed more rapidly than that of Quercus. The decay constants were 0.47 year⁻¹, 0.75 year⁻¹ and 0.30 year⁻¹ for Cistus, Myrtus and Quercus, respectively. Lignin and cellulose contents were different as were their relative amounts (34 and 18%, 15 and 37%, 37 and 39% of the overall litter organic matter before exposure, for Cistus, Myrtus and Quercus, respectively). Lignin began to decrease after 6 and 8 months of exposure in Cistus and Myrtus, respectively, while it did not change significantly during the entire study period in Quercus. The holocellulose, in contrast, began to decompose in Cistus after 1 year, while in Quercus and Myrtus immediately. Nitrogen was strongly immobilized in all the litters in the early period of decay. Its release began after the first year in Cistus and Myrtus and after 2 years of decomposition in Quercus. These litters still contained about 60, 20 and 90% of the initial nitrogen at the end of the experiment (3 years). Prior to litter exposure nitrogen associated with the lignin fraction was 65, 54 and 37% in Cistus, Myrtus and Quercus, while that associated with the cellulose fraction was 30, 24 and 28%. Although most of the nitrogen was not lost from litters, its distribution among the litter components changed significantly during decomposition. In Cistus and Myrtus the nitrogen associated with lignin began to decrease just 4 months after exposure. In Quercus this process was slowed and after 3 years of decomposition 8% of the nitrogen remained associated with lignin or lignin-like substances. The nitrogen associated with cellulose or cellulose-like substances, in contrast, began to decrease from the beginning of cellulose decomposition in all three species. At the end of the study period most of the nitrogen was not associated to the lignocellulose fraction but to the acid-detergent-soluble substance (87, 88 and 84% of the remaining litter nitrogen).     

Effects of seasonal and diurnal temperature fluctuations on population dynamics of two epigeic earthworm species in forest soil

Temperature fluctuations are a fundamental entity of the soil environment in the temperate zone and show fast (diurnal) and slow (seasonal) dynamics. However, responses of soil ecosystem engineers, such as earthworms, to annual temperature dynamics are virtually unknown. We studied growth, mortality and cocoon production of epigeic earthworm species (Lumbricus rubellus and Dendrobaena octaedra) exposed to temperature fluctuations in root-free soil of a mid-European beech-oak forest. Both earthworm species (3 + 3 individuals of each species) were kept in microcosms containing soil stratified into L, F + H and A_h horizons. In the field, earthworm responses to smoothing of diurnal temperature fluctuations were studied, simulating possible global change. In the laboratory, earthworm responses to seasonal (±5 °C of the annual mean) and diurnal temperature fluctuations (±5 °C of the seasonal levels) were analyzed in a two-factorial design. Both experiments lasted 12 months to differentiate between seasonal and diurnal responses. In the third experiment overwintering success of both earthworm species was investigated by comparing effects of constant temperature regime (+2 °C), and daily or weekly temperature fluctuations (2 °C ± 5 °C).Temperature regime strongly affected population performance of the earthworms studied. In the field, smoothed temperature fluctuations beneficially affected population development of both earthworm species (higher biomass, faster maturity and reproduction, lower mortality). Consequently, density of both species increased faster at smoothed than at ambient temperature conditions. In the laboratory, responses of L. rubellus and D. octaedra to temperature treatments differed; however, in general, earthworms benefited from the absence of diurnal fluctuations. Total earthworm numbers were at a maximum at constant temperature and lowest in the treatment with both diurnal and seasonal temperature fluctuations. However, after one year L. rubellus tended to dominate irrespective of the temperature regime. In the overwintering experiment L. rubellus sensitively responded to even short-term winter frost and went extinct after one week of frost whereas D. octaedra much better tolerated frost conditions. Earthworms of both species which survived frosts were characterized by a significant body weight decrease during the period of frosts and fast recovery in spring suggesting a different pattern of individual resource expenditure as compared with constant +2 °C winter regime. Contrasting trends in the population dynamics of L. rubellus and D. octaedra during the frost-free period and during winter suggest that in the long-term temperature fluctuations contribute to the coexistence of decomposer species of similar trophic position in the forest litter. The results are discussed in context of consequences of climate change for the functioning of soil systems.     

Palatability trials on hardwood leaf litter grown under elevated CO2: a stable carbon isotope study

The palatability to isopods and microbes of a broad range of hardwood leaf litter, derived from three field CO₂-enrichment experiments in the USA, was investigated, using δ¹³C, to trace the C flow from litter to isopods and to CO₂ respired by microbial decomposition. Leaf litter grown under elevated CO₂ had δ¹³C values ranging from −39 to −45‰, which were significantly different from ambient litter δ¹³C values of around −30‰. Litter palatability to isopods of the Porcellio sp. was tested by incubating ambient- and elevated-CO₂ litter, and a mixture of the two, in the presence of isopods for 14 days, under environmentally controlled conditions; δ¹³C was measured on litter and isopods' body before and after incubation. In an additional experiment, litter was incubated in the absence of fauna for 30 days, and on five occasions the δ¹³C of the CO₂ respired from litter was measured. The ¹³C label was clearly carried from the litter source to the isopods' bodies, and their faeces. For microbial-respired CO₂, δ¹³C was significantly higher than that of the litter source, suggesting preferential degradation of substrates enriched in ¹³C as compared to those in the overall litter. With the exception of Quercus myrtifolia leaf litter, elevated CO₂ did not affect the palatability to isopods nor the microbial degradation of any of the litters, possibly as a result of unaltered litter N concentration. However, significant differences in litter palatability and decay rates were observed among the different species. With this study, the use of isotopically labelled litter material was confirmed as a key methodology that can significantly contribute to the advancement of the understanding of litter decomposition and of the quantification of C fluxes in the process.     

Decomposition and substrate quality of leaf litters and fine roots from three exotic plantations and a native forest in the southwestern highlands of Ethiopia

Substrate quality and decomposition (measured as CO₂ release in laboratory microcosms) of fresh leaf litter and fine roots of Cupressus lusitanica, Pinus patula, Eucalyptus grandis and native forest trees were studied. Changes in litter chemistry in each forest stand were analysed by comparing fresh leaf litter (collected from trees) and decomposed litter from the forest floor. Elemental concentrations, proximate fractions including monomeric sugars, and cross polarisation magic-angle spinning (CPMAS) ¹³C NMR spectra were analysed in leaf litters, decomposed litter and fine roots. Leaf litters and fine roots varied in their initial substrate chemistry with Ca concentration in leaf litters being higher than that in fine roots. In each stand, fine roots had a higher acid unhydrolysable residue (AUR) (except for the Pinus stand), higher holocellulose concentration and lower concentration of water-soluble extractives (WSE) and dichloromethane extractives (NPE) than fresh leaf litter. Likewise, ¹³C NMR spectra of fine roots showed lower alkyl and carboxyl C, and higher phenolic (except P. patula), aromatic and O-alkyl C proportions than leaf litters. Compared with fresh leaf litter, decomposed litter had lower concentrations of potassium, holocellulose, WSE, NPE, arabinose and galactose, similar or higher concentrations of Mg, Ca, S and P, and higher concentrations of N and AUR. CPMAS ¹³C NMR spectra of decomposed litter showed a higher relative increase in signal intensity due to methoxyl C, aromatic C, phenolic C and carboxylic C compared with alkyl C. In a microcosm decomposition study, the proportion of initial C remaining in leaf litter and fine roots significantly fitted an exponential regression model. The decomposition constants (k) ranged between 0.0013 and 0.0030 d⁻¹ for leaf litters and 0.0010-0.0017 d⁻¹ for fine roots. In leaf litters there was a positive correlation between the k value and the initial Ca concentration, and in fine roots there was an analogous positive correlation with initial WSE. Leaf litters decomposed in the order Cupressus>native forest>Eucalyptus∼Pinus, and fine roots in the order Pinus>native forest>Cupressus∼Eucalyptus. In each stand the fine root decomposition was significantly lower than the leaf litter decomposition, except for the P. patula stand where the order was reversed.     

UVB exposure does not accelerate rates of litter decomposition in a semi-arid riparian ecosystem

Aboveground litter decomposition is controlled mainly by substrate quality and climate factors across terrestrial ecosystems, but photodegradation from exposure to high-intensity ultraviolet-B (UVB) radiation may also be important in arid and semi-arid environments. We investigated the interactive effects of UVB exposure and litter quality on decomposition in a Tamarix-invaded riparian ecosystem during the establishment of an insect biological control agent in northern Nevada. Feeding by the northern tamarisk beetle (Diorhabda carinulata) on Tamarix spp. trees leads to altered leaf litter quality and increased exposure to solar UVB radiation from canopy opening. In addition, we examined the dynamics of litter decomposition of the invasive exotic Lepidium latifolium, because it is well-situated to invade beetle-infested Tamarix sites. Three leaf litter types (natural Tamarix, beetle-affected Tamarix, and L. latifolium) differing in substrate quality were decomposed in litterbags for one year in the field. Litterbags were subjected to one of three treatments: (1) Ambient UVB or (2) Reduced UVB (where UVB was manipulated by using clear plastic films that transmit or block UVB), and (3) No Cover (a control used to test for the effect of using the plastic films, i.e. a cover effect). Results showed a large cover effect on rates of decomposition and nutrient release, and our findings suggested that frequent cycles of freeze-thaw, and possibly rainfall intensity, influenced decomposition at this site. Contrary to our expectations, greater UVB exposure did not result in faster rates of decomposition. Greater UVB exposure resulted in decreased rates of decomposition and P release for the lower quality litter and no change in rates of decomposition and nutrient release for the two higher quality litter types, possibly due to a negative effect of UVB on soil microbes. Among litter types, rates of decomposition and net release of N and P followed this ranking: L. latifolium > beetle-affected Tamarix > natural Tamarix. Altered nutrient dynamics with beetle introduction as well as the rapid decomposition rates exhibited by L. latifolium are consistent with vulnerability to secondary invasion. In this desert ecosystem, decomposition and nutrient release were strongly affected by litter type and much less so by UVB exposure.     

Interactions between residue placement and earthworm ecological strategy affect aggregate turnover and N2O dynamics in agricultural soil

Previous laboratory studies using epigeic and anecic earthworms have shown that earthworm activity can considerably increase nitrous oxide (N₂O) emissions from crop residues in soils. However, the universality of this effect across earthworm functional groups and its underlying mechanisms remain unclear. The aims of this study were (i) to determine whether earthworms with an endogeic strategy also affect N₂O emissions; (ii) to quantify possible interactions with epigeic earthworms; and (iii) to link these effects to earthworm-induced differences in selected soil properties. We initiated a 90-day ¹⁵N-tracer mesocosm study with the endogeic earthworm species Aporrectodea caliginosa (Savigny) and the epigeic species Lumbricus rubellus (Hoffmeister). ¹⁵N-labeled radish (Raphanus sativus cv. Adagio L.) residue was placed on top or incorporated into the loamy (Fluvaquent) soil. When residue was incorporated, only A. caliginosa significantly (p < 0.01) increased cumulative N₂O emissions from 1350 to 2223 μg N₂O-N kg⁻¹ soil, with a corresponding increase in the turnover rate of macroaggregates. When residue was applied on top, L. rubellus significantly (p < 0.001) increased emissions from 524 to 929 μg N₂O-N kg⁻¹, and a significant (p < 0.05) interaction between the two earthworm species increased emissions to 1397 μg N₂O-N kg⁻¹. These effects coincided with an 84% increase in incorporation of residue ¹⁵N into the microaggregate fraction by A. caliginosa (p = 0.003) and an 85% increase in incorporation into the macroaggregate fraction by L. rubellus (p = 0.018). Cumulative CO₂ fluxes were only significantly increased by earthworm activity (from 473.9 to 593.6 mg CO₂-C kg⁻¹ soil; p = 0.037) in the presence of L. rubellus when residue was applied on top. We conclude that earthworm-induced N₂O emissions reflect earthworm feeding strategies: epigeic earthworms can increase N₂O emissions when residue is applied on top; endogeic earthworms when residue is incorporated into the soil by humans (tillage) or by other earthworm species. The effects of residue placement and earthworm addition are accompanied by changes in aggregate and SOM turnover, possibly controlling carbon, nitrogen and oxygen availability and therefore denitrification. Our results contribute to understanding the important but intricate relations between (functional) soil biodiversity and the soil greenhouse gas balance. Further research should focus on elucidating the links between the observed changes in soil aggregation and controls on denitrification, including the microbial community.     

The fate of condensed tannins during litter consumption by soil animals

Condensed tannins (CT) can strongly affect litter decomposition, but their fate during the decomposition process, in particular as influenced by detritivore consumption, is not well understood. We tested the hypothesis that litter CT are reduced by the gut passage of two functionally distinct detritivores of Mediterranean forests, the millipede Glomeris marginata, and the land snail Pomatias elegans, as a fixed proportion of initial litter CT, but more so in Pomatias since snails are known to have a more efficient enzymatic capacity. Contrary to our hypothesis, both detritivore species reduced litter CT to near zero in their faecal pellets irrespective of the wide range in initial leaf litter CT concentrations of 9-188 mg g⁻¹ d m among three Mediterranean tree species (Pistacia terebinthus, Quercus ilex, Alnus glutinosa) and different decomposition stages of their litter. The almost complete disappearance of CT even from some litter types highly concentrated in CT, due to either degradation by gut microorganism or complexation of CT into insoluble high molecular weight structures, suggests a high “de-tanning” efficiency across functionally distinct detritivore species. The transformation of CT-rich litter into virtually CT-free faecal pellets by detritivores might be highly relevant for the subsequent decomposition process in ecosystems with a high macrofauna abundance and CT-rich plant species such as Mediterranean forests.     

FT-IR study of the changes in carbohydrate chemistry of three New Jersey pine barrens leaf litters during simulated control burning

Low intensity control burns are a standard fuel reduction management tool used in pine barrens ecosystems. Periodic disturbances through fire can be an important influence on the cycling of nutrients within the ecosystem. Previous studies have shown that the inorganic chemistry of leaf litter residues differs with increasing temperature. Our study compared chemical changes in white oak (Quercus alba), pitch pine (Pinus rigida) and black huckleberry (Gaylussacia baccata), characteristic of the New Jersey pine barrens, during thermal decomposition using FT-IR spectroscopy. Three replicates of senescent leaf material were ground and separately heated for 2 h at: 100, 200, 300, 400 and 550 °C. These temperatures are representative of the range seen in fuel reducing prescribed burns in the pine barrens. Unburned litter of each species was used as a control. An optimization process using varying amounts of KBr and oak litter was performed to develop favorable FT-IR spectral conditions for a sample to KBr ratio of 0.75%. Chemometric analysis of the FT-IR spectra using principal component analysis (PCA) was used to analyze the changes in carbohydrate chemistry of each litter plant species (leaf litter species) at each temperature. In general, it appears that there is clear separation of leaf litter species at the different combustion temperatures. Infrared spectroscopy illustrated that all three species shared wavenumbers characteristic of the primary components of leaves such as cellulose, lignin and hemicellulose. Results from the PCA indicated separation of litter species and species by combustion temperature. PC axis 1 corresponds to the effects of temperature on leaf litter species and PC axis 2 separates the leaf litter species. At the low temperatures (control-200 °C), oak, pine and huckleberry litter species separated from each other. Wavenumbers that contributed to the separation of species at low temperatures belonged to functional group stretching frequencies of outer surface waxes, basic sugars, fatty acids and aldehydes. It appears that oak had more IR bands specific to suberin content. Convergence of these species occurs at 300 °C. Complexity of chemical composition decreases at this particular temperature as is shown by the decrease in wavenumber richness when compared to litters at low and high temperatures. Oak, pine and huckleberry had similar IR spectra showing bands belonging to outer surface wax content, pectin, lignin and hemicellulose. With increasing temperatures (400-550 °C), differences between litter species increased slightly. Plant material was reduced to similar composition due to thermal decomposition, which consisted of inorganic materials such as carbonate, phosphate and sulfate ions and possible fused aromatics.     

Mineralization of forest litter nutrients by heat and combustion

Temperature dependant mineralization dynamics during fire of litter species characteristic of the New Jersey pine barrens was determined. Senescent leaf material of pitch pine (Pinus rigida), white oak (Quercus alba) and black huckleberry (Gaylusssacia baccata) were collected at the time of abscission; sorted, ground and oven-dried at 70 °C. Replicate samples were then heated for 2 h at: 70, 100, 200, 300, 400, and 550 °C. Mass loss and total nitrogen and total phosphorus concentration of the heated material were determined. Additional samples of the residual material were extracted with deionized water, and the filtrate was assayed for the anions: , , ; and cations: , K⁺, Mg⁺⁺, and Ca⁺⁺.By heating leaf litter over a range of temperatures, to simulate the heterogeneous nature of forest litter burning, we identified patterns of nutrient mineralization characteristic of specific temperatures, some of which were common to all three litter species and others unique to individual species. In general, it appears that black huckleberry leaf litter was the most nutrient rich and the most labile. In huckleberry litter, there was a large reserve of soluble nitrogen, sulfur, phosphate, calcium and magnesium that became available upon heating to 200 °C. Pitch pine litter was the most nutrient poor, and the rates of nutrient mineralization were also generally the lowest of the three species studied. White oak litter nutrient concentration and rates of mineralization along the temperature gradient were intermediate. For all three litter species examined organic and inorganic nitrogen losses due to volatilization were >99% upon heating to 550 °C, and soluble magnesium concentrations declined significantly at temperatures of 300 °C, despite having a volatilization temperature greater than 1100 °C. Under the temperature range employed, heating of leaf litter resulted in little volatilization loss of phosphorus; however, the amount of soluble phosphate phosphorus was much lower in all three litter types at temperatures of 300 °C and above. With increasing temperatures, inorganic phosphate ions presumably became bound to cations in the ash, forming insoluble metal phosphates. The dramatic increase of the ratio of total phosphorus to soluble inorganic phosphate at higher temperatures, the loss of soluble magnesium above 300 °C, and the near complete loss of nitrogen at 550 °C suggests that after intense fires availability of these minerals may be dramatically reduced.     

Field investigations of Lumbricus terrestris spatial distribution and dispersal through monitoring of manipulated, enclosed plots

A field experiment in managed woodland was set up to examine the effects of manipulated population density and resource availability on spatial distribution and dispersal of the anecic earthworm Lumbricus terrestris. Experiments over 2 years, made use of 1 m² field enclosures with associated trapping units to assess emigration rates at control and enhanced L. terrestris densities and different levels of leaf litter availability. Densities were manipulated twice; at the outset and again after 1 year when visually tagged animals obtained from 2 origins were introduced. Population density had a significant effect on dispersal (p < 0.01, p < 0.05 in Year 1 and Year 2 respectively) with more captures (pro rata) at the higher density compared with controls over the experimental period. Food availability only had a significant effect during the initial week of the experiment. L. terrestris midden arrangement was found to be regular across 1 m² plots and regularity increased with an increase in midden number. Mean (±S.E.) midden number was 30.34 ± 0.77 m⁻² and 28.06 ± 0.5 m⁻², during the first and second year of the experiment respectively and this was unaffected by additions. Inter-midden distance was recorded at 0.13 ± 0.0014 m. Results suggest that L. terrestris dispersal can be affected by population density and resource availability.     

Carbon dioxide emissions of soils under pure and mixed stands of beech and spruce, affected by decomposing foliage litter mixtures

Soil respiration is the largest terrestrial source of CO₂ to the atmosphere. In forests, roughly half of the soil respiration is autotrophic (mainly root respiration) while the remainder is heterotrophic, originating from decomposition of soil organic matter. Decomposition is an important process for cycling of nutrients in forest ecosystems. Hence, tree species induced changes may have a great impact on atmospheric CO₂ concentrations. Since studies on the combined effects of beech-spruce mixtures are very rare, we firstly measured CO₂ emission rates in three adjacent stands of pure spruce (Picea abies), mixed spruce-beech and pure beech (Fagus sylvatica) on three base-rich sites (Flysch) and three base-poor sites (Molasse; yielding a total of 18 stands) during two summer periods using the closed chamber method. CO₂ emissions were higher on the well-aerated sandy soils on Molasse than on the clayey soils on Flysch, characterized by frequent water logging. Mean CO₂ effluxes increased from spruce (41) over the mixed (55) to the beech (59) stands on Molasse, while tree species effects were lower on Flysch (30-35, mixed > beech = spruce; all data in mg CO₂-C m⁻² h⁻¹). Secondly, we studied decomposition after fourfold litter manipulations at the 6 mixed species stands: the O_i - and O_e horizons were removed and replaced by additions of beech -, spruce - and mixed litter of the adjacent pure stands of known chemical quality and one zero addition (blank) in open rings (20 cm inner diameter), which were covered with meshes to exclude fresh litter fall. Mass loss within two years amounted to 61-68% on Flysch and 36-44% on Molasse, indicating non-additive mixed species effects (mixed litter showed highest mass loss). However, base cation release showed a linear response, increasing from the spruce - over the mixed - to the beech litter. The differences in N release (immobilization) resulted in a characteristic converging trend in C/N ratios for all litter compositions on both bedrocks during decomposition. In the summers 2006 and 2007 we measured CO₂ efflux from these manipulated areas (a closed chamber fits exactly over such a ring) as field indicator of the microbial activity. Net fluxes (subtracting the so-called blank values) are considered an indicator of litter induced changes only and increased on both bedrocks from the spruce - over the mixed - to the beech litter. According to these measurements, decomposing litter contributed between 22-32% (Flysch) and 11-28% (Molasse) to total soil respiration, strengthening its role within the global carbon cycle.     

Fungal growth on a common wood substrate across a tropical elevation gradient: Temperature sensitivity, community composition, and potential for above-ground decomposition

Fungal breakdown of plant material rich in lignin and cellulose (i.e. lignocellulose) is of central importance to terrestrial carbon (C) cycling due to the abundance of lignocellulose above and below-ground. Fungal growth on lignocellulose is particularly influential in tropical forests, as woody debris and plant litter contain between 50% and 75% lignocellulose by weight, and can account for 20% of the C stored in these ecosystems. In this study, we evaluated factors affecting fungal growth on a common wood substrate along a wet tropical elevation gradient in the Peruvian Andes. We had three objectives: 1) to determine the temperature sensitivity of fungal growth - i.e. Q₁₀, the factor by which fungal biomass increases given a 10 °C temperature increase; 2) to assess the potential for above-ground fungal colonization and growth on lignocellulose in a wet tropical forest; and 3) to characterize the community composition of fungal wood decomposers across the elevation gradient. We found that fungal growth had a Q₁₀ of 3.93 (95% CI of 2.76-5.61), indicating that fungal biomass accumulation on the wood substrate nearly quadrupled with a 10 °C increase in temperature. The Q₁₀ for fungal growth on wood at our site is higher than Q₁₀ values reported for litter decomposition in other tropical forests. Moreover, we found that above-ground fungal growth on the wood substrate ranged between 37% and 50% of that measured in the soil, suggesting above-ground breakdown of lignocellulose represents an unexplored component of the C cycle in wet tropical forests. Fungal community composition also changed significantly along the elevation gradient, and Ascomycota were the dominant wood decomposers at all elevations. Fungal richness did not change significantly with elevation, directly contrasting with diversity patterns observed for plant and animal taxa across this gradient. Significant variation in fungal community composition across the gradient suggests that the characteristics of fungal decomposer communities are, directly or indirectly, influenced by temperature.     

Wetland macrophyte decomposition under different nutrient conditions: Relationships between decomposition rate, enzyme activities and microbial biomass

We used oligotrophic, P-limited herbaceous wetlands of northern Belize as a model system, on which to document and explain how changes in nutrient content along a salinity gradient affect activities of extracellular enzymes involved in macrophyte decomposition. To determine what is more important for decomposition, the initial litter quality, or site differences, we used reciprocal litter placement in a combined “site quality” and “litter quality” experiment running from August 2003 to April 2004. The experiment was set up in long-term control and nutrient addition plots (P, N, and NP) established in 2001 in 15 limestone-based inland marshes with a wide range of water conductivities (200-6000 μS) and a uniform pH (7.0-7.7) dominated by emergent macrophytes, Eleocharis spp. There were no differences among the plots in total sediment N and water NH₄-N, but total and KCl-extractable sediment P and water PO₄-P were significantly higher in P and NP plots throughout the duration of the experiment. The initial litter N content was slightly but significantly different between control and N plots versus P and NP plots (5.7 and 7.1 mg g⁻¹, respectively). The difference was much bigger for litter P content, 0.1 and 0.7 mg g⁻¹, respectively. Enzyme activities of alkaline phosphatase, leucine-aminopeptidase, arylsulfatase, and β-glucosidase were measured fluorometrically in Eleocharis litter in both the litterbag experiment and the naturally decomposing material. Total phospholipid fatty acid (PLFA) content in litter samples was used as a measure of microbial biomass present. Phosphatase always exhibited the highest activity of the enzymes studied, followed by leucine-aminopeptidase, arylsulfatase and β-glucosidase. There were no significant differences between enzyme activities from litterbags and the unconfined litter. Phosphatase activity was significantly suppressed in P-addition plots under all salinity levels while the activities of the remaining enzymes were significantly higher in P-enriched plots. There was a strong correlation between decomposition coefficient k-values and most of the enzymes as well as between the amount of PLFA and enzyme activities. PLFA, arylsulfatase, and litter C/P were the best predictors of k-values.     

Impact of litter quality on mineralization processes in managed and abandoned pasture soils in Southern Ecuador

Tropical regions are currently undergoing remarkable rates of land use change accompanied by altered litter inputs to soil. In vast areas of Southern Ecuador forests are clear cut and converted for use as cattle pastures. Frequently these pasture sites are invaded by bracken fern, when bracken becomes dominant pasture productivity decreases and the sites are abandoned. In the present study implications of invasive bracken on soil biogeochemical properties were investigated. Soil samples (0-5 cm) were taken from an active pasture with Setaria sphacelata as predominant grass and from an abandoned pasture overgrown by bracken. Grass (C₄ plant) and bracken (C₃ plant) litter, differing in C:N ratio (33 and 77, respectively) and lignin content (Klason-lignin: 18% and 45%, respectively), were incubated in soils of their corresponding sites and vice versa for 28 days at 22 °C. Unamended microcosms containing only the respective soil or litter were taken as controls. During incubation the amount of CO₂ and its δ¹³C-signature were determined at different time intervals. Additionally, the soil microbial community structure (PLFA-analysis) as well as the concentrations of KCl-extractable C and N were monitored. The comparison between the control soils of active and abandoned pasture sites showed that the massive displacement of Setaria-grass by bracken after pasture abandonment was characterized by decreased pH values accompanied by decreased amounts of readily available organic carbon and nitrogen, a lower microbial biomass and decreased activity as well as a higher relative abundance of actinomycetes. The δ¹³C-signature of CO₂ indicated a preferential mineralization of grass-derived organic carbon in pasture control soils. In soils amended with grass litter the mineralization of soil organic matter was retarded (negative priming effect) and also a preferential utilization of easily available organic substances derived from the grass litter was evident. Compared to the other treatments, the pasture soil amended with grass litter showed an opposite shift in the microbial community structure towards a lower relative abundance of fungi. After addition of bracken litter to the abandoned pasture soil a positive priming effect seemed to be supported by an N limitation at the end of incubation. This was accompanied by an increase in the ratio of Gram-positive to Gram-negative bacterial PLFA marker. The differences in litter quality between grass and bracken are important triggers of changes in soil biogeochemical and soil microbial properties after land use conversion.     

Performance of para-nitrophenyl phosphate and 4-methylumbelliferyl phosphate as substrate analogues for phosphomonoesterase in soils with different organic matter content

Phosphomonoesterase (PMEase) activity plays a key role in nutrient cycling and is a potential indicator of soil condition and ecosystem stress. We compared para-nitrophenyl phosphate (pNPP) and 4-methylumbelliferyl phosphate (MUP) as substrate analogues for PMEase in 7 natural ecosystem soils and 8 agricultural top soils with contrasting C contents (8.0-414 g kg⁻¹ C) and pH (3.0-7.5). PMEase activities obtained with pNPP (0.05-5 μmol g⁻¹ h⁻¹) were significantly less than activities obtained with MUP (0.9-13 μmol g⁻¹ h⁻¹), especially in soils with a high organic matter content (>130 g kg⁻¹). Only PMEase activities assayed with MUP correlated significantly with total C and total N (r=0.7, P<0.01 all), and pH (r=−0.71, P<0.01). PMEase activities obtained with the two substrate analogues were correlated when expressed on a C-content basis (r=0.8, P<0.001), but not when expressed on an oven-dry soil weight basis. This indicated that interference by organic matter is related to the quantity rather than to the quality of organic matter. Overall, assaying with MUP was more sensitive compared to assaying with pNPP, particularly in the case of high organic and acid soils.     

Quantitative PCR assays to enumerate Rhizobium leguminosarum strains in soil also target non viable cells and overestimate those detected by the plant infection method

The plant infection method is commonly used to estimate the Most Probable Number (MPN) of soil rhizobia. Here, a qPCR method was set-up and validated with newly developed ANU (strain specific) and RHIZ (more general) primers to quantify the specific Rhizobium leguminosarum bv. trifolii ANU843 strain or general R. leguminosarum strains. Detection limits of qPCR protocols in soil were 1.2 × 10⁴ (ANU) and 4.2 × 10³ (RHIZ) cells per g soil. The qPCR assay appears robust and accurate in freshly inoculated soils but overestimated MPN for indigenous soil rhizobia. An incubation experiment showed that qPCR detected added DNA or non viable cells in soils up to 5 months after addition and incubation at 20 °C in moist conditions.     

Cross-fostering in coyotes: Evaluation of a potential conservation and research tool for canids

Cross-fostering has been attempted opportunistically with endangered canids as a means of increasing populations. Due to the usefulness of cross-fostering for conservation, an understanding of factors influencing success rates is essential. Using captive coyotes (Canis latrans) as a model, we assessed the willingness of adult pairs to foster young born to other parents. We assessed the efficacy of fostering pups into existing litters (augmentation) and completely switching litters (replacement). We augmented four litters with two pups of similar age when pups were <7 days old. In addition, we replaced four entire litters when pups were <10 days old. We also augmented litters with pups 3-4 and 6-7 weeks of age. Survival, weight gain, and dominance status of pups were monitored for six weeks and compared to four control litters to determine success. All complete litter replacements were successful with survival rates among replaced pups (89.5%) similar to those of control litters (90%). For augmented litters, pup survival was dependent on the age at which fostering occurred. All pups fostered into 4 litters at <1 week of age survived beyond 6 weeks of age, two of three fostering attempts with 3-4-week-old pups succeeded, while neither of two attempts to foster 6-week-old pups succeeded. Surviving fostered pups appeared to be at no disadvantage. Weight gains were similar for pups in all treatments, and there was no evidence of reduced dominance status among fostered pups as compared to natal pups in the same litters. These results illustrate that genetic relatedness is not essential for successful fostering and does not appear to alter dominance patterns; however the age at which pups are fostered may affect the success of fostering attempts.     

Competition between invasive earthworms (Amynthas corticis, Megascolecidae) and native North American millipedes (Pseudopolydesmus erasus, Polydesmidae): Effects on carbon cycling and soil structure

Invasive earthworms can have significant impacts on C dynamics through their feeding, burrowing, and casting activities, including the protection of C in microaggregates and alteration of soil respiration. European earthworm invasion is known to affect soil micro- and mesofauna, but little is known about impacts of invasive earthworms on other soil macrofauna. Asian earthworms (Amynthas spp.) are increasingly being reported in the southern Appalachian Mountains in southeastern North America. This region is home to a diverse assemblage of native millipedes, many of which share niches with earthworm species. This situation indicates potential for earthworm-millipede competition in areas subject to Amynthas invasion.In a laboratory microcosm experiment, we used two ¹³C enriched food sources (red oak, Quercus rubra, and eastern hemlock, Tsuga canadensis) to assess food preferences of millipedes (Pseudopolydesmus erasus), to determine the effects of millipedes and earthworms (Amynthas corticis) on soil structure, and to ascertain the nature and extent of the interactions between earthworms and millipedes. Millipedes consumed both litter species and preferred red oak litter over eastern hemlock litter. Mortality and growth of millipedes were not affected by earthworm presence during the course of the experiment, but millipedes assimilated much less litter-derived C when earthworms were present.Fauna and litter treatments had significant effects on soil respiration. Millipedes alone reduced CO₂ efflux from microcosms relative to no fauna controls, whereas earthworms alone and together with millipedes increased respiration, relative to the no fauna treatment. CO₂ derived from fresh litter was repressed by the presence of macrofauna. The presence of red oak litter increased CO₂ efflux considerably, compared to hemlock litter treatments.Millipedes, earthworms, and both together reduced particulate organic matter. Additionally, earthworms created significant shifts in soil aggregates from the 2000-250 and 250-53 μm fractions to the >2000 μm size class. Earthworm-induced soil aggregation was lessened in the 0-2 cm layer in the presence of millipedes. Earthworms translocated litter-derived C to soil throughout the microcosm.Our results suggest that invasion of ecosystems by A. corticis in the southern Appalachian Mountains is unlikely to be limited by litter species and these earthworms are likely to compete directly for food resources with native millipedes. Widespread invasion could cause a net loss of C due to increased respiration rates, but this may be offset by C protected in water-stable soil aggregates.