Changes in litter properties during decomposition: A study by differential thermogravimetry and scanning calorimetry

To verify the paradigm that organic matter (OM) quality (q) decreases with decomposition it is necessary to define q in strictly chemical, operational terms. We suggest defining q as the result of a balance between the energy stored in OM and the external supply of energy needed to release it. We apply this concept to the study of litter decomposition in four European pine forests: boreal, cool Atlantic, Mediterranean and warm Atlantic. Intact litter cores were taken and transported to the laboratory, where needles were sorted into six classes that summarize the main facts of the decomposition: melanisation, fragmentation and perforation by mesofauna. Each class was analyzed by both differential thermogravimetry and differential scanning calorimetry to obtain its spectra of weight loss and energy release.In the non-decomposed needles, two peaks of weight loss and energy release appear: a labile peak at about 350 °C, and a recalcitrant peak at about 450 °C. During decomposition, both peaks (but especially the recalcitrant one) move to lower temperatures, and their shapes change from well defined to flattened. In Mediterranean litters, a third peak appears at about 500 °C, due probably to refractory products of neoformation. There is a continuous increase in the energy stored in the remaining litter (in Joules per unit OM): this increase is concentrated in both the most thermolabile fractions (lost at temperatures <350 °C) and the most thermostable ones (>450 °C). With decomposition OM becomes more recalcitrant (i.e., it is lost at higher temperatures), but its stored energy becomes more available (i.e., it is released at lower temperatures). Overall, the energetic benefit/cost ratio increases. Thus, our results to date do not agree with the current paradigm that q decreases with decomposition; rather, they suggest that, at least in the first phases we studied, q is maintained or even increases.     

Effect of understory fern (Dicranopteris dichotoma) removal on substrate utilization patterns of culturable soil bacterial communities in subtropical Eucalyptus plantations

Although understory vegetation is known to play an important role in driving the processes and functions of forest ecosystems, little is known about how understory vegetation affects the composition and function of soil microbial communities in forest ecosystems, especially in subtropical and tropical forests. This study used the experimental removal of understory fern (Dicranopteris dichotoma) to investigate the effect of the fern on substrate utilization patterns of culturable soil bacterial communities in two subtropical Eucalyptus plantations. One year after treatment, the removal of understory fern significantly increased soil temperature by 2–3 °C and retarded litter decomposition by 5.6–23.1%. However, understory fern removal did not affect the substrate utilization pattern of soil bacterial communities. Our study provides evidence that, although understory fern removal significantly alters soil temperature and litter decomposition rate, the disturbance caused by understory removal one year after treatment is too weak to cause detectable changes in substrate utilization pattern of culturable soil bacterial communities in subtropical Eucalyptus plantations.     

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.     

Soil microbial biomass and activity: the effect of site characteristics in humid temperate forest ecosystems

Microbial biomass, respiratory activity, and in‐situ substrate decomposition were studied in soils from humid temperate forest ecosystems in SW Germany. The sites cover a wide range of abiotic soil and climatic properties. Microbial biomass and respiration were related to both soil dry mass in individual horizons and to the soil volume in the top 25 cm. Soil microbial properties covered the following ranges: soil microbial biomass: 20 µg C g^–1–8.3 mg C g^–1 and 14–249 g C m^–2, respectively; microbial C–to–total organic C ratio: 0.1%–3.6%; soil respiration: 109–963 mg CO₂‐C m^–2 h^–1; metabolic quotient (qCO₂): 1.4–14.7 mg C (g C_mic)^–1 h^–1; daily in‐situ substrate decomposition rate: 0.17%–2.3%. The main abiotic properties affecting concentrations of microbial biomass differed between forest‐floor/organic horizons and mineral horizons. Whereas microbial biomass decreased with increasing soil moisture and altitude in the forest‐floor/organic horizons, it increased with increasing N_tot content and pH value in the mineral horizons. Quantities of microbial biomass in forest soils appear to be mainly controlled by the quality of the soil organic matter (SOM), i.e., by its C : N ratio, the quantity of N_tot, the soil pH, and also showed an optimum relationship with increasing soil moisture conditions. The ratio of C_mic to C_org was a good indicator of SOM quality. The quality of the SOM (C : N ratio) and soil pH appear to be crucial for the incorporation of C into microbial tissue. The data and functional relations between microbial and abiotic variables from this study provide the basis for a valuation scheme for the function of soils to serve as a habitat for microorganisms.     

Effect of N addition,moisture, and temperature on soil microbial respiration and microbial biomass in forest soil at different stages of litter decomposition

Purpose

The objective of the present study was to investigate the interactive effects of nitrogen (N) addition, temperature, and moisture on soil microbial respiration, microbial biomass, and metabolic quotient (qCO₂) at different decomposition stages of different tree leaf litters.

Materials and methods

A laboratory incubation experiment with and without litter addition was conducted for 80 days at two temperatures (15 and 25 °C), two wetting intensities (35 and 50 % water-filled porosity space (WFPS)) and two doses of N addition (0 and 4.5 g N m^?2, as NH₄NO₃). The tree leaf litters included three types of broadleaf litters, a needle litter, and a mixed litter of them. Soil microbial respiration, microbial biomass, and qCO₂ along with other soil properties were measured at two decomposition stages of tree leaf litters.

Results and discussion

The increase in soil cumulative carbon dioxide (CO₂) flux and microbial biomass during the incubation depended on types of tree leaf litters, N addition, and hydrothermal conditions. Soil microbial biomass carbon (C) and N and qCO₂ were significantly greater in all litter-amended than in non-amended soils. However, the difference in the qCO₂ became smaller during the late period of incubation, especially at 25 °C. The interactive effect of temperature with soil moisture and N addition was significant for affecting the cumulative litter-derived CO₂-C flux at the early and late stages of litter decomposition. Furthermore, the interactive effect of soil moisture and N addition was significant for affecting the cumulative CO₂ flux at the late stage of litter decomposition but not early in the experiment.

Conclusions

This present study indicated that the effects of addition of N and hydrothermal conditions on soil microbial respiration, qCO₂, and concentrations of labile C and N depended on types of tree leaf litters and the development of litter decomposition. The results highlight the importance of N availability and hydrothermal conditions in interactively regulating soil microbial respiration and microbial C utilization during litter decomposition under forest ecosystems.

     

Assessing soil particle-size distribution on experimental plots with similar texture under different management systems using multifractal parameters

Soil particle-size distribution (PSD) is a fundamental soil physical attribute with dominant influence on many other soil properties. Laser diffraction combined with multifractal analyses have proven to be useful to obtain precise information from PSDs. The aim of this work was to assess similitude or difference of PSDs sampled on plots of an experimental field and belonging to the same textural class using multifractal parameters. The field experiment consisted of two tillage treatments and two cropping systems. It was conducted following a randomized complete split-block design with four replications on a Humic Dystrudept. Tillage treatments were conventional tillage (CT) and no tillage (NT) while crop rotations were ryegrass-sorghum (RS) and ryegrass-corn (RC). Particle-size distribution analysis by the sieve-pipette and by laser diffraction corroborate that all the samples were assigned to the same textural class. Singularity spectra f(α) and Rényi spectra, D_q, showed that multifractal distribution was a suitable model for PSDs obtained by laser diffraction. However, in the range of moments − 10 < q < 10, the r² values for the linear fits leading to a Rényi spectrum, D_q, were higher than those for the singularity spectrum, suggesting the former was better defined than the latter. No significant differences in multifractal parameters were found between plots with contrasted crop rotation, RS and RC. In contrast, Hölder exponent of order zero (α₀) and several parameters derived from the left branch of both, the f(α) and the D_q spectra, were significantly different between CT and NT treatments. No effects of mixing by cultivation were detected in our work, so that differences in PSDs between no-tilled and conventionally-tilled plots were simply attributed to patchiness and variation on the experimental field. Multifractal analysis of PSDs measured by laser diffraction provides further insight in verifying patterns of between plot soil texture variations (i.e. randomness or trends) in completely randomized block designs.     

Temperature and substrate controls on microbial phospholipid fatty acid composition during incubation of grassland soils contrasting in organic matter quality

Soil incubations are often used to investigate soil organic matter (SOM) decomposition and its response to increased temperature, but changes in the activity and community composition of the decomposers have rarely been included. As part of an integrated investigation into the responses of SOM components in laboratory incubations at elevated temperatures, fungal and bacterial phospholipid fatty acids (PLFAs) were measured in two grassland soils contrasting in SOM quality (i.e. SOM composition), and changes in the microbial biomass and community composition were monitored. Whilst easily-degradable SOM and necromass released from soil preparation may have fuelled microbial activity at the start of the incubation, the overall activity and biomass of soil microorganisms were relatively constant during the subsequent one-year soil incubation, as indicated by the abundance of soil PLFAs, microbial respiration rate (r), and metabolic quotient (qCO₂). PLFAs relating to fungi and Gram-negative bacteria declined relative to Gram-positive bacteria in soils incubated at higher temperatures, presumably due to their vulnerability to disturbance and substrate constraints induced by faster exhaustion of available nutrient sources at higher temperatures. A linear correlation was found between incubation temperatures and the microbial stress ratios of cyclopropane PLFA-to-monoenoic precursor (cy17:0/16:1ω7c and cy19:0/18:1ω7c) and monoenoic-to-saturated PLFAs (mono/sat), as a combined effect of temperature and temperature-induced substrate constraints. The microbial PLFA decay patterns and ratios suggest that SOM quality intimately controls microbial responses to global warming.     

Indications for soil organic matter quality in soils under different management

Changes in management practice are reflected by soil carbon and nitrogen status, in particular by the proportion of soil organic matter (SOM) being easily transformed (active SOM). We describe SOM quality for three management practices, Organic Farming system (OF), Integrated Crop Production (ICP) and pasture sites (G), which intend to achieve sustainable management practice. The experimental sites were conventionally farmed until 1992. SOM quality was examined by describing active SOM pools, such as the decomposed ‘young soil organic matter’ (YSOM), ratio of microbial biomass carbon (C_mic) to organic carbon (C_org), ecophysiological status of the microbial biomass (qCO₂), and the ratio of light particulate organic matter (POM-LF) to C_org. Ratios of soil microbial biomass (C_mic/C_org) and POM-LF (POM-LF/C_org) and the amount of decomposed YSOM were relatively similar to each other, despite differences in management practice and soil texture. Soil microbial parameters (C_mic, C_mic/C_org and qCO₂) were significantly (p<0.05) affected by the amount of decomposed YSOM and the silt content in the OF. In the ICP, soil microbial parameters depended only on the amount of decomposed YSOM, which was considered to be a consequence of the more heterogeneous texture at the OF-sites. Management effects were detectable for no-tillage in the ICP leading to an accumulation of active SOM in the surface soil (0–10 cm). The ratio POM-LF/C_org showed no difference between G and OF despite markedly higher C_org-contents at the G-sites. Conclusively, all methods used indicate comparable SOM qualities for the three management systems, despite differences in soil texture and soil management during 7 years. Management practices seem to be well adapted to the site conditions.     

Functional diversity of microbial communities in the mixed boreal plain forest of central Canada

This study was designed to examine whether or not specific tree species (Picea glauca, Picea mariana, Pinus banksiana, Populus tremuloides), their post-fire stand age, or their position in a successional pathway had any significant effect on the functional diversity of associated soil microbial communities in a typical mixed boreal forest ecosystem (Duck Mountain Provincial Forest, Manitoba, Canada). Multivariate analyses designed to identify significant biotic and/or abiotic variables associated with patterns of organic substrate utilization (assessed using the BIOLOG™ System) revealed the overall similarity in substrate utilization by the soil microbial communities. The five clusters identified differed mainly by their substrate-utilization value rather than by specific substrate utilization. Variability in community functional diversity was not strongly associated to tree species or post-fire stand age; however, redundancy analysis indicated a stronger association between substrate utilization and successional pathway and soil pH. For example, microbial communities associated with the relatively high pH soils of the P. tremuloides-P. glauca successional pathway, exhibited a greater degree of substrate utilization than those associated with the P. banksiana-P. mariana successional pathway and more acidic soils. Differences in functional diversity specific to tree species were not observed and this may have reflected the mixed nature of the forest stands and of their heterogeneous forest floor. In a densely treed, mixed boreal forest ecosystem, great overlap in tree and understory species occur making it difficult to assign a definitive microbial community to any particular tree species. The presence of P. tremuloides in all stand types and post fire stand ages has probably contributed to the large amount of overlap in utilization profiles among soil samples.     

Exotic plant influences soil nematode communities through litter input

The impact of exotic plant invasions on soil communities and nutrient cycling processes has received an increasing attention in recent years. To test whether the exotic plant invasions affect nematode communities through altering litter quality, we compared mass loss and nematode colonization during the stem litter decomposition of invasive Spartina alterniflora and native Phragmites australis in salt marshes of the Yangtze River estuary, China. Plastic drinking straws were synchronously used as controls. The addition of plant residues was found stimulating the growth of nematodes, particularly bacterial feeders on day 16 after burial. A top-down control of bacterivous nematodes by carnivores existed in nematode succession during the litter decomposition. With higher nitrogen content and lower C:N ratio, stem litter of the invasive S. alterniflora decayed faster and supported more abundant nematodes than the native P. australis. The greater nematode abundance in S. alterniflora was mainly due to two dominant genera of bacterial nematodes, namely Diplolaimelloides and Diplolaimella. Lower values of maturity index and structure index in S. alterniflora than in P. australis litter indicate that a more degraded food web condition resulted from the faster litter decay. A considerable difference in nematode community structures between two litter types only occurred in a certain period of the decomposition (from 8 to 32 days after burial), suggesting that the changes in faunal community structure are time dependent. In summary, this study confirmed the hypothesis that the invasion of S. alterniflora stimulates the growth of bacterial nematodes by producing higher quality of litter than the native P. australis. The results obtained here suggest that the invasion of exotic plant is likely to alter ecosystem functions indirectly through exerting its effect on soil decomposer communities such as nematodes.     

Catabolic profiles of soil fungal communities along a geographic climatic gradient in Israel

The great distances that typically separate different climatic zones have previously restricted the investigation of climatic controls over soil microbial function to only short-scale gradients, or alternatively, to the investigation of extreme instead of real gradients. In this study, we comparatively assessed sole carbon source utilization profiles and resultant catabolic diversity levels of soil fungal communities along a geographically continuous steep climatic gradient, stretching from the Upper Galilee in the north of Israel to the Negev Desert in the south. Four sites along the gradient were studied, representing humid-Mediterranean, Mediterranean, semi-arid and arid climate types, and they were characterized by a mean annual rainfall of 780, 537, 300, and 90 mm, respectively.The relationships between abiotic soil characteristics and fungal activity parameters (basal respiration, biomass, and the metabolic quotient qCO₂) as well as fungal substrate utilization profiles and functional diversity were explored in the context of climate and seasonality (rainy versus dry seasons).To make an ecologically relevant assessment of fungal activity in soil, a MicroResp? microrespiratory system capable of analyzing whole-soil samples was modified to allow, for the first time, the testing of fungal utilization of low-solubility carbon substrates, alongside other major plant-derived substrates, reflective of the soil environment.The accurate profiling of substrate utilization in the calcareous soils comprising this Mediterranean gradient could not have been accomplished without the correction of measured CO₂ evolution data, which were biased due to CaCO₃–CO₂–H₂O equilibria reactions. The correction of the data was conducted according to a protocol presented in the preceding article of this series.The utilization potential of all 14 substrates tested was higher during the dry than during the rainy season, particularly in the Mediterranean sites, with polymeric substrates being utilized preferentially in the arid sites in comparison with the Mediterranean sites.The distinctive approach exercised in this study enabled us to make a relevant ecological interpretation, particularly in face of the prospect of fungi being responsible for the bulk of recalcitrant organic matter decomposition in soil.     

Density loss and respiration rates in coarse woody debris of Pinus radiata, Eucalyptus regnans and Eucalyptus maculata

This study compared field and laboratory decomposition rates of coarse woody debris (CWD) (>10 cm diameter) from three tree species: Pinus radiata, Eucalyptus regnans, and Eucalyptus maculata. For this purpose, the density loss of logs on the ground sampled from chronosequences of sites following harvesting was determined using the water replacement technique. P. radiata logs were sampled 1, 2.5, 6, and 9 years following harvesting, and logs of E. regnans and E. maculata were collected from sites that were harvested 1, 3.5, 6.5, and 12 and 1.5, 6.5, and 11.5 years ago, respectively. In addition, the C/N ratio of wood was determined and current respiration rates of logs from these different age classes were measured through laboratory incubation. The times for loss of 95% of material (t_0.95) determined from density loss for these species were 24 years for P. radiata, 43 years for E. regnans, and 62 years for E. maculata. The decomposition rates of CWD derived from laboratory respiration were 6.1, 5.9 and 11.9 times higher than the decay rates from density loss in P. radiata, E. regnans, and E. maculata, respectively. This points to severe constraints of decomposition through adverse conditions in the field. The changes in respiration rates and C/N ratio with age of decaying logs indicated that the single component, negative exponential decay model could be applied satisfactorily only to P. radiata. In the case of the eucalypt species, substrate quality (expressed through respiration rates) declined in the oldest samples. This may be explained by the loss of rapidly decomposing sapwood and the retention of more decay-resistant heartwood. In these cases, a two-component model will be more suitable to describe the density loss of decaying wood.     

The adverse effects of soil salinization on the growth of Trifolium alexandrinum L. and associated microbial and biochemical properties in a soil from Iran

The aim of this study was to determine the effects of increasing concentrations of salt solutions (including 0.12, 2, 6, and 10 dS m⁻¹) on the growth of berseem clover (Trifolium alexandrinum L.) and related soil microbial activity, biomass and enzyme activities. Results showed that the dry weights of root and shoot decreased with an increase in the concentrations of salt solutions. Soil salinization depressed the microbiological activities including soil respiration and enzyme activities. Substrate-induced respiration was consistently lower in salinized soils, whereas microbial biomass C did not vary among salinity levels. Higher metabolic quotients (qCO₂) and unaffected microbial biomass C at high EC values may indicate that salinity is a stressful factor, inducing either a shift in the microbial community with less catabolic activity or reduced efficiency of substrate utilization. Acid phosphatase and alkaline phosphatase activities decreased with increasing soil salinity. We found significant, positive correlations between the activities of phosphatase enzymes and plant's root mass, suggesting that any decrease in the activities of the two enzymes could be attributed to the reduced root biomass under saline conditions.     

Gaseous diffusion equations for porous materials

Commonly used equations of gaseous transport by diffusion are examined and the two parameters required for each equation are tabulated from the literature. The advantages and limitations of the commonly used linear —D/D_o = a(S?b) — and curvilinear — D/D_o = KS^m— equations are considered and a new diffusion equation is proposed to combine the advantages of the previous equations. This new equation, like the others, requires only two parameters. It takes the form D/D_o = [(S?u)/(1?u)]^v and may serve as a basis for studying and more accurately modeling gas transport in porous media.     

Three decades of soil microbial biomass studies in Brazilian ecosystems: Lessons learned about soil quality and indications for improving sustainability

Soil microbial biomass plays important roles in nutrient cycling, plant-pathogen suppression, decomposition of residues and degradation of pollutants; therefore, it is often regarded as a good indicator of soil quality. We reviewed more than a hundred studies in which microbial biomass-C (MB-C), microbial quotient (MB-C/TSOC, total soil organic carbon) and metabolic quotient (qCO₂) were evaluated with the objective of understanding MB-C responses to various soil-management practices in Brazilian ecosystems. These practices included tillage systems, crop rotations, pastures, organic farming, inputs of industrial residues and urban sewage sludge, applications of agrochemicals and burning. With a meta-analysis of 233 data points, we confirmed the benefits of no-tillage in preserving MB-C and reducing qCO₂ in comparison to conventional tillage. A large number of studies described increases in MB-C and MB-C/TSOC due to permanent organic farming, also benefits from crop rotations particularly with several species involved, whereas application of agrochemicals and burning severely disturbed soil microbial communities. The MB-C decreased in overgrazed pastures, but increased in pastures rotated with well-managed crops. Responses of MB-C, MB-C/TSOC and qCO₂ to amendment with organic industrial residues varied with residue type, dose applied and soil texture. In conclusion, MB-C and related parameters were, indeed, useful indicators of soil quality in various Brazilian ecosystems. However, direct relationships between MB-C and nutrient-cycling dynamics, microbial diversity and functionality are still unclear. Further studies are needed to develop strategies to maximize beneficial effects of microbial communities on soil fertility and crop productivity.     

Prediction of hydraulic conductivity and sorptivity in soils at steady-state infiltration

The purpose of this study was (1) to find a matching factor (u) between infiltration rate and hydraulic conductivity during steady-state infiltration, and (2) to propose equations based on infiltration and soil moisture-retention functions for prediction of the hydraulic conductivity K(θ) within the rapidly (non-capillary) drainable pores (RDP) and capillary-matrix pores of soils. The K(θ) of capillary pores was divided into K(θ)_SDP, K(θ)_WHP and K(θ)_FCP within slowly drainable pores (SDP), water-holding pores (WHP) and fine capillary pores (FCP), respectively. Five soil profiles of calcareous sandy loam, alluvial saline and non-saline clay, located at the Nile Delta, were used to apply the proposed equations. The highest and the lowest values of K(θ)_RDP were observed in calcareous and saline clay soil profiles, respectively. Values of K(θ)_RDP remained higher than those for capillary pores in the studied soils. The predicted values of K(θ) in capillary and non-capillary pores classes were in the expected range for unsaturated hydraulic conductivity. Water sorptivity (S) was determined at initial unsaturated soil water conditions and calculated at steady-state infiltration (S _w) using a derived equation. There was a decrease in S with an increase in soil water content; i.e. at steady-state infiltration, S decreased by 35–40% in calcareous soils and by 45–60% in alluvial clay soils. The parameter values of u and S _w tended to be uniform in calcareous soils, but nonuniform in saline and non-saline clay soils.     

Effects of multiple heavy metal contamination and repeated phytoextraction by Sedum plumbizincicola on soil microbial properties

The threat of heavy metal contamination to food and human health in south and east China has become a public concern as industrial development continues. The aims of this study were to investigate the influence of repeated phytoextraction over a two-year period by successive crops of the Zn and Cd hyperaccumulator Sedum plumbizincicola on multiple metal contaminated soils and to assess recovery of soil quality. Total and NH₄OAc-extractable Zn and Cd concentrations were significantly reduced in planted soils compared to unplanted soils. Microbial biomass C (C_mic), basal respiration and microbial quotient (qM) were significantly and positively correlated and soil metabolic quotient (qCO₂) was negatively correlated with heavy metal concentrations in unplanted soils (P < 0.05). However, C_mic, basal respiration and qM values increased significantly after phytoremediation by five crops over two years compared to unplanted soil. Urease, β-glucosidase, neutral phosphatase and arylsulfatase activities also increased significantly with decreasing heavy metal contents and hydrolase activity was enhanced in planted soil (P < 0.05) compared to the unplanted control. The data indicate the capacity of S. plumbizincicola to extract Zn and Cd from contaminated soil and also that phytoremediation had beneficial effects on soil microbial and hydrolase activities, with the metal phytoextraction procedure restoring soil quality.     

Diversity of macro-detritivores in dead wood is influenced by tree species,decay stage and environment

Diplopoda (millipedes) and Isopoda (woodlice) are among the most abundant macro-detritivores in temperate forests. These key regulators of plant litter decomposition are influenced by habitat and substrate quality, including that of dead wood. Dead wood provides shelter and resources to macro-detritivores, but the relative effects of tree species, wood decay stage, forest environment and their interactions on macro-detritivore communities are poorly known. To unravel these effects, we combined a reciprocal field incubation experiment and direct field sampling to compare the Diplopoda and Isopoda communities in logs of silver birch (Betula pendula) and Norway spruce (Picea abies) in two contrasting sites in terms of soil texture, pH, fertility and microclimate. We found: (1) a curvilinear relationship between wood decay stage and abundance of Diplopoda and Isopoda, by using wood density as a measure for the decay stage; (2) the pH of dead wood was a good predictor of wood decay stage in a site with pH close to neutrality but not in an acidic site; (3) Diplopoda and Isopoda community composition on different tree species converged during the decay process, consequently tree species are more important in the substrate selection of macro-detritivores at the beginning of their dead wood decomposition; (4) tree species, the growing environment of the trees and the decomposition environment of the logs strongly determined Diplopoda and Isopoda community composition in dead wood, these drivers of macro-detritivore communities interacted with each other and with the wood decay stage. Thus, when trying to understand and predict future patterns of macro-detritivore diversity under regimes of changing land-use and climate, these interactions should be taken into account. An important next step will be to quantify the feedback of macro-detritivore community composition to dead wood decomposition itself. This feedback may be better understood from the combination of (1) the complex interactions of tree species, wood decay stage and forest environment on the macro-detritivore community and (2) the functional traits of these macro-detritivore species. A better knowledge about these feedbacks can help in predicting carbon storage and nutrient cycling functions of dead wood in forests differing or changing in tree species composition and abiotic environment.     

Metabolic profiling reveals a functional succession of active fungi during the decay of Mediterranean plant litter

We evaluated the relationship between the potential activity shown by pure fungal isolates (in vitro) and their ecological role in the field. Fungal species frequency and occurrence during trophic succession on a natural resource were studied and used as a tool to gain a better understanding of the metabolic profiles obtained in vitro, thus permitting clarification of the role played by some of the key fungal species involved in the decomposition process within a Mediterranean natural ecosystem. Litter bags containing pure (homogeneous) and mixed leaves of low maquis plant species were incubated for one year in a field experiment. Functional diversity of a representative pool of fungal communities was analysed and compared using a “phenotype microarray” technique. Taking as its basis the screening of metabolic profiles using culturable isolates, this investigation showed that there was a high potentiality of functional redundancy between low, intermediate and high frequency taxa, and potential niche overlap during the different phases that occur during leaf litter decomposition. The early stages of decomposition appeared to be characterized by the presence of rare taxa (“Low” species), with a significantly higher functional potential, especially for sugar compounds. Conversely, the later phases of decomposition appeared to be characterized by species of intermediate frequency, while rare taxa at these stages seem to lose their dominant role. The importance of the different groups of substrates found across the decomposition phases also represented a key factor. For example, redundancy in the utilization of N-related compounds indicated a high potential for overlapping between species, especially in the early phases of the decomposition process, suggesting that the capacity for interspecific competition on some substrates can be considerable, particularly at the start of substrate exploitation.     

Soil carbon content and relative abundance of high affinity H2-oxidizing bacteria predict atmospheric H2 soil uptake activity better than soil microbial community composition

Soil–atmosphere exchange of H₂ is controlled by gas diffusion and the microbial production and oxidation activities in soil. Among these parameters, the H₂ oxidation activity catalyzed by soil microorganisms harboring high affinity hydrogenase is the most difficult variable to parameterize because it is influenced by many unknown edaphic factors that shape microbial community structure and function. Here we seek to formulate a model combining microbiological and physicochemical variables to predict the H₂ oxidation rate (u) in soil. Soil sample replicates collected from a grassland and three forests exhibited different H₂ oxidation potentials. We examined the microbial community structure based on ribotyping analysis, the relative abundance of high affinity H₂-oxidizing bacteria (HOB) estimated by qPCR and soil physicochemical characteristics as predictors for u. A single linear regression parameterized by total carbon content and a multiple linear regression using total carbon content and HOB relative abundance in soil explained 66 and 92% of the variance in u, respectively. Microbial community composition based on 16S rRNA gene pyrosequencing profiles was not a reliable predictor for u. Indeed, we found that HOB are members of the rare biosphere, comprising less than 1% of total bacteria as estimated by qPCR. We confirmed this relationship of u with total carbon content and HOB by an independent soil survey of 14 samples collected from maize monocultures, grasslands, deciduous forests and larch plantations. Observations made from both soil surveys thus were combined to build a predictive model for u parameterized with total carbon content and HOB relative abundance. Our results show that molecular biogeochemistry is a potential approach to improve performance of classical H₂ surface flux models which estimate u empirically without considering variation in HOB distribution and activity in soil.