Composition of the microbial communities in the mineral soil under different types of natural forest

Phospholipid fatty acid (PLFA) patterns were used to describe the composition of the soil microbial communities under 12 natural forest stands including oak and beech, spruce-fir-beech, floodplain and pine forests. In addition to the quantification of total PLFAs, soil microbial biomass was measured by substrate-induced respiration and chloroform fumigation-extraction. The forest stands possess natural vegetation, representing an expression of the natural site factors, and we hypothesised that each forest type would support a specific soil microbial community. Principal component analysis (PCA) of PLFA patterns revealed that the microbial communities were compositionally distinct in the floodplain and pine forests, comprising azonal forest types, and were more similar in the oak, beech and spruce-fir-beech forests, which represent the zonal vegetation types of the region. In the nutrient-rich floodplain forests, the fatty acids 16:1ω5, 17:0cy, a15:0 and a17:0 were the most prevalent and soil pH seemed to be responsible for the discrimination of the soil microbial communities against those of the zonal forest types. The pine forest soils were set apart from the other forest soils by a higher abundance of PLFA 18:2ω6,9, which is typical of fungi and may also indicate ectomycorrhizal fungi associated with pine trees, and high amounts of PLFA 10Me18:0, which is common in actinomycetes. These findings suggest that the occurrence of azonal forest types at sites with specific soil conditions is accompanied by the development of specific soil microbial communities. The study provides information on the microbial communities in undisturbed forest soils which may facilitate interpretation of data derived from managed or even damaged or degraded forests.     

Plant canopy effects on litter accumulation and soil microbial biomass in two temperate forests

The objective of this study was to determine whether differences in canopy structure and litter composition affect soil characteristics and microbial activity in oak versus mixed fir-beech stands. Mean litter biomass was greater in mixed fir-beech stands (51.9t ha⁻¹) compared to oak stands (15.7t ha⁻¹). Canopy leaf area was also significantly larger in mixed stands (1.96m² m⁻²) than in oak stands (1.73m² m⁻²). Soil organic carbon (C _org) and moisture were greater in mixed fir-beech stands, probably as a result of increased cover. Soil microbial biomass carbon (C _mic), nitrogen (N _mic), and total soil nitrogen (N _tot) increased slightly in the mixed stand, although this difference was not significant. Overall, mixed stands showed a higher mean C _org/N _tot ratio (22.73) compared to oak stands (16.39), indicating relatively low rate of carbon mineralization. In addition, the percentage of organic C present as C _mic in the surface soil decreased from 3.17% in the oak stand to 2.26% in the mixed stand, suggesting that fir-beech litter may be less suitable as a microbial substrate than oak litter.     

Variation in quality and decomposability of red oak leaf litter along an urban-rural gradient

 This study tested whether urban land use can affect the chemistry and decomposability of Quercus rubra L. (red oak) leaf litter in forests within and near a large metropolitan area. Cities may affect the quality of leaf litter directly through foliar uptake of atmospheric pollutants, and indirectly through alterations in local climate and changes in soil fertility caused by pollutant loads and altered nutrient cycling regimes. Using a microbial bioassay, we tested whether red oak leaf litter collected from urban and suburban forests in and near New York City differed in decomposability from litter of the same species collected from rural forests 130 km from the city. We found that oak litter from the urban forests decayed 25% more slowly and supported 50% less cumulative microbial biomass in a laboratory bioassay than rural litter. Rural litter contained less lignin and more labile material than urban litter, and the amounts of these chemical constituents were highly correlated with the decay rate coefficients and integrated microbial growth achieved on the litter. The specific causes of the variation in litter chemistry are not known. The results of this study suggest that decomposer activity and nutrient cycling in forests near large cities may be affected both by altered litter quality and by altered biotic, chemical and physical environments. The sensitivity of the microbial bioassay makes it useful for distinguishing differences in within-species litter quality that result from natural or anthropogenic variation in the environment. Received: 7 January 1999     

Structural and functional diversity of soil bacterial and fungal communities following woody plant encroachment in the southern Great Plains

In the southern Great Plains (USA), encroachment of grassland ecosystems by Prosopis glandulosa (honey mesquite) is widespread. Mesquite encroachment alters net primary productivity, enhances stores of C and N in plants and soil, and leads to increased levels of soil microbial biomass and activity. While mesquite’s impact on the biogeochemistry of the region is well established, it effects on soil microbial diversity and function are unknown. In this study, soils associated with four plant types (C₃ perennial grasses, C₄ midgrasses, C₄ shortgrasses, and mesquite) from a mesquite-encroached mixed grass prairie were surveyed to in an attempt to characterize the structure, diversity, and functional capacity of their soil microbial communities. rRNA gene cloning and sequencing were used in conjunction with the GeoChip functional gene array to evaluate these potential differences. Mesquite soil supported increased bacterial and fungal diversity and harbored a distinct fungal community relative to other plant types. Despite differences in composition and diversity, few significant differences were detected with respect to the potential functional capacity of the soil microbial communities. These results may suggest that a high level of functional redundancy exists within the bacterial portion of the soil communities; however, given the bias of the GeoChip toward bacterial functional genes, potential functional differences among soil fungi could not be addressed. The results of this study illustrate the linkages shared between above- and belowground communities and demonstrate that soil microbial communities, and in particular soil fungi, may be altered by the process of woody plant encroachment.     

Effects of tree species and N additions on forest floor microbial communities and extracellular enzyme activities

Forest nitrogen (N) retention and soil carbon (C) storage are influenced by tree species and their associated soil microbial communities. As global change factors alter forest composition, predicting long-term C and N dynamics will require understanding microbial community structure and function at the tree species level. Because atmospheric N deposition is increasing N inputs to forested ecosystems across the globe, including the northeastern US, it is also important to understand how microbial communities respond to added N. While prior studies have examined these topics in mixed-species stands, we focused on the responses of different tree species and their associated microbial communities within a single forest type - a northern hardwood forest in the Catskills Mountains, NY. Based on prior studies, we hypothesized that N additions would stimulate extracellular enzyme activities in relatively labile litters, but suppress oxidative enzyme activities in recalcitrant litters, and tested for independent tree species effects within this context. During the 2007 growing season (May-June), we measured enzyme activities and microbial community composition (using phospholipid fatty acid analysis - PLFA) of the forest floor in single-species plots dominated by sugar maple (Acer saccharum), yellow birch (Betula alleghaniensis), red oak (Quercus rubra), American beech (Fagus grandifolia) and eastern hemlock (Tsuga canadensis), species whose litters range from relatively labile to recalcitrant. Half the plots were fertilized with N by adding NH₄NO₃ (50 kg ha⁻¹ y⁻¹) from 1997 to 2009. Non-metric multidimensional scaling (NMS) and multi-response permutation procedures (MRPP) were used to examine microbial community structure and relationship to enzyme activities.We found that in response to N additions, both microbial community composition and enzyme activities changed; however the strength of the changes were tree species-specific and the direction of these changes was and not readily predictable from prior studies conducted in mixed-species stands. For example, in contrast to other studies, we found that N additions caused a significant overall increase in fungal biomass that was strongest for yellow birch (24% increase) and weakest for sugar maple (1% increase). Contrary to our initial hypotheses and current conceptual models, N additions reduced hydrolytic enzyme activities in hemlock plots and reduced oxidative enzyme activity in birch plots, a species with relatively labile litter. These responses suggest that our understanding of the interactions between microbial community composition, enzyme activity, substrate chemistry, and nutrient availability as influenced by tree species composition is incomplete. NMS ordination showed that patterns in microbial community structure (PLFA) and function (enzyme activity) were more strongly influenced by tree species than by fertilization, and only partially agreed with the structure-function relationships found in other studies. This finding suggests that tree species-specific responses are likely to be important in determining the structure and function of northeastern hardwood forests in the future. Enhanced understanding of microbial responses to added N in single and mixed-species substrates with varying amounts of lignin and phenols may be needed for accurate predictions of future soil C and N dynamics.     

Acquisition of microbial communities and enhanced availability of soil nutrients by the isopod Porcellio scaber (Latr.) (Isopoda: Oniscidea)

 In microcosm experiments Porcellio scaber increased litter disappearance of oak and alder litter. Alder litter disappeared at more than twice the rate of oak litter. Soil texture did not influence the disappearance of oak litter; however, disappearance of alder litter was enhanced on silt rather than on sand. P. scaber enhanced microbial communities (i.e. microbial respiration and microbial biomass) on both silt and sand when feeding on either alder or oak. Overall, microbial respiration increased 10-fold when isopods fed on oak litter on sand and 20-fold when isopods fed on alder litter on sand. On silt, the initially high microbial respiration remained constant when isopods fed on oak and doubled when they fed on alder litter. In all treatments without P. scaber there was a decrease in microbial respiration over the 12-week experimental period. The availability of macronutrients (C_org, N_tot, P, K, Mg, Ca) in the topsoil was increased when P. scaber fed on alder litter but less pronounced when P. scaber fed on oak litter. Using sand as a substrate, there was an apparent increase only for C_org, Mg and Ca; on silt, increases in C_org, N_tot and P were measured. Under field conditions the contribution of P. scaber to nutrient fluxes will be higher on sand than on silt. Received: 1 July 1999     

Rhizosphere soil enzymatic and microbial activities in bamboo forests in southeastern China

Bamboo has been introduced to coastal sandy areas in southeastern China to protect and restore coastal ecosystems. An understanding of the chemistries and enzymatic and microbial activities in the soils of these bamboo forests will aid our understanding of how bamboo plantations can improve soil fertility and will provide scientific evidence for policy makers for encouraging the planting of bamboo in other coastal areas. We investigated the physical and chemical properties of the rhizosphere soil [soil moisture content (SMC), pH, and contents of soil organic matter (SOM), total nitrogen (TN), available nitrogen (AN), total phosphorus (TP), available phosphorus (AP), total potassium (TK) and available potassium (AK)], enzymatic activities (sucrase, protease, urease and catalase) and microbial properties (counts of bacteria, fungi and actinomycetes) in five bamboo forests. The bamboo forests had significantly higher levels of SOM, TN, AN, TP, AP, TK, and AK and lower pH relative to a control soil sample from an area devoid of plants. Soil enzymatic activities and microbial communities were considerably higher in the bamboo forests than in the soil from the barren land. The chemical contents, enzymatic activities and microbial counts of the soil and the litter and root biomasses were higher in forests with the bamboo species Dendrocalamopsis oldhami and Pseudosasa amabilis than in forests with the other three species (Acidosasa edulis, Dendrocalamopsis vario-striata, and Dendrocalamopsis beecheyana var. pubescens), which suggests that these two species could adapt to sandy soil and grow well in a hostile environment. These results indicate that planting bamboo may help to both enrich soil fertility and increase the diversity of tree species in coastal ecosystems. The difference between aboveground and belowground biomass may have been responsible for these changes in soil properties.     

Microbial community structure mediates response of soil C decomposition to litter addition and warming

Microbial activity has been highlighted as one of the main unknowns controlling the fate and turnover of soil organic matter (SOM) in response to climate change. How microbial community structure and function may (or may not) interact with increasing temperature to impact the fate and turnover of SOM, in particular when combined with changes in litter chemistry, is not well understood. The primary aim of this study was to determine if litter chemistry impacted the decomposition of soil and litter-derived carbon (C), and its interaction with temperature, and whether this response was controlled by microbial community structure and function. Fresh or pre-incubated eucalyptus leaf litter (¹³C enriched) was added to a woodland soil and incubated at 12, 22, or 32 °C. We tracked the movement of litter and soil-derived C into CO₂, water-extractable organic carbon (WEOC), and microbial phospholipids (PLFA). The litter additions produced significant changes in every parameter measured, while temperature, interacting with litter chemistry, predominately affected soil C respiration (priming and temperature sensitivity), microbial community structure, and the metabolic quotient (a proxy for microbial carbon use efficiency [CUE]). The direction of priming varied with the litter additions (negative with fresh litter, positive with pre-incubated litter) and was related to differences in the composition of microbial communities degrading soil-C, particularly gram-positive and gram-negative bacteria, resulting from litter addition. Soil-C decomposition in both litter treatments was more temperature sensitive (higher Q₁₀) than in the soil-only control, and soil-C priming became increasingly positive with temperature. However, microbes utilizing soil-C in the litter treatments had higher CUE, suggesting the longer-term stability of soil-C may be increased at higher temperature with litter addition. Our results show that in the same soil, the growth of distinct microbial communities can alter the turnover and fate of SOM and, in the context of global change, its response to temperature.     

Elevated CO<Subscript>2</Subscript> concentration affected pine and oak litter chemistry and the respiration and microbial biomass of soils amended with these litters

Elevated atmospheric CO₂ concentration ([CO₂]) may change litter chemistry which affects litter decomposability. This study investigated respiration and microbial biomass of soils amended with litter of Pinus densiflora (a coniferous species; pine) and Quercus variabilis (a deciduous species; oak) that were grown under different atmospheric [CO₂] and thus had different chemistry. Elevated [CO₂] increased lignin/N through increased lignin concentration and decreased N concentration. The CO₂ emission from the soils amended with litter produced under the same [CO₂] regime was greater for oak than pine litter, confirming that broadleaf litter with lower lignin decomposes faster than needle leaf litter. Within each species, however, soils amended with high lignin/N litter grown under elevated [CO₂] emitted more CO₂ than those with low lignin/N litter grown under ambient [CO₂]. Such contrasting effects of lignin/N on inter- and intra-species variations in litter decomposition should be ascribed to the effects of other litter chemistry variables including nonstructural carbohydrate, calcium and manganese as well as inhibitory effect of N on lignin decomposition. The microbial biomass was also higher in the soils amended with high lignin/N litter than those with low lignin/N litter probably due to low substrate use efficiency of lignin by microbes. Our study suggests that elevated [CO₂] increases lignin/N for both species, but increased lignin/N does not always reduce soil respiration and microbial biomass. Further study investigating a variety of tree species is required for more comprehensive understanding of inter- and intra-species variations of litter decomposition under elevated [CO₂].     

Long-term litter decay in Canadian forests and the influence of soil microbial community and soil chemistry

Long-term rates of litter decay have been shown to be primarily influenced by temperature, moisture and litter quality. However, while decomposition is a biological process, the relative importance of microbial communities and other soil chemistry factors is not well understood. Our analysis examined long-term litter decay parameters, microbial community composition via phospholipid fatty acid (PLFA) analysis, and soil organic horizon chemistry at 14 upland forested sites. Data were collected as part of the Canadian Intersite Decomposition Experiment (CIDET), a 12-year national litter decomposition experiment. Residual errors from a two-pool exponential decay model with decay rates modified by mean annual air temperature and moisture stress were compared to PLFA marker groups and chemistry variables. Residual errors were not well explained by soil PLFA marker group abundance or concentration, soil pH, nor soil C:N ratios. The best predictor of residual error was soil carbon percent (%C), with higher %C associated with slower than predicted decomposition.     

Relationship between soil enzyme activities, nutrient cycling and soil fungal communities in a northern hardwood forest

Soil fungi are highly diverse and act as the primary agents of nutrient cycling in forests. These fungal communities are often dominated by mycorrhizal fungi that form mutually beneficial relationships with plant roots and some mycorrhizal fungi produce extracellular and cell-bound enzymes that catalyze the hydrolysis of nitrogen (N)- and phosphorus (P)- containing compounds in soil organic matter. Here we investigated whether the community structure of different types of mycorrhizal fungi (arbuscular and ectomycorrhizal fungi) is correlated with soil chemistry and enzyme activity in a northern hardwood forest and whether these correlations change over the growing season. We quantified these relationships in an experimental paired plot study where white-tailed deer (access or excluded 4.5 yrs) treatment was crossed with garlic mustard (presence or removal 1 yr). We collected soil samples early and late in the growing season and analyzed them for soil chemistry, extracellular enzyme activity and molecular analysis of both arbuscular mycorrhizal (AM) and ectomycorrhizal/saprotrophic fungal communities using terminal restriction fragment length polymorphism (TRFLP). AM fungal communities did not change seasonally but were positively correlated with the activities of urease and leucine aminopeptidase (LAP), enzymes involved in N cycling. The density of garlic mustard was correlated with the presence of specific AM fungal species, while deer exclusion or access had no effect on either fungal community after 4.5 yrs. Ectomycorrhizal/saprotrophic fungal communities changed seasonally and were positively correlated with most soil enzymes, including enzymes involved in carbon (C), N and P cycling, but only during late summer sampling. Our results suggest that fine scale temporal and spatial changes in soil fungal communities may affect soil nutrient and carbon cycling. Although AM fungi are not generally considered capable of producing extracellular enzymes, the correlation between some AM taxa and the activity of N acquisition enzymes suggests that these fungi may play a role in forest understory N cycling.     

Tree influence on soil biological activity: What can be inferred from the optical examination of humus profiles?

Humus forms may vary in different forest stands, but the local influence of trees upon soil microbial and faunal activities is still imperfectly known. Optical methods could help to discern processes of litter transformation and formation of organo-mineral assemblages, allowing a better diagnostic of tree influences upon humus-soil development. The microstratification of humus was studied under a beech (Fagus crenata), a mixed oak forest (Quercus crispula and Quercus serrata), and a cedar (Cryptomeria japonica) plantation. The three sites are located in Kyoto (Japan), and share similar environmental conditions. Litter decomposition rates and soil fauna were also investigated. At the beech site, which had the thickest O horizon, the main process was the gradual fragmentation of litter. This process, together with shallow root and weak fungal development, gave rise to a stable sandwich-like structure in the O horizon. In contrast, the oak site showed a two-step transformation of litter. Initially, litter decomposition was triggered by the activity of white rot fungi, and the discarded litter decayed much more slowly thereafter. The cedar site exhibited a sharp vertical delineation between upper thick Oe horizon developed since plantation time and a relict A horizon. The optical method thus demonstrated differences in soil biological activities and litter transformation patterns under the three sites.     

Annual dynamics of phosphatase activities in an evergreen oak litter: influence of biotic and abiotic factors

As part of a study of the processes involved in litter biodegradation, we considered the variations over 1 year of the phosphatase activities in sclerophyllous evergreen oak litter (Quercus ilex L.). Evergreen oak is representative of tree species in the forests of the French Mediterranean area. Acid (E.C. 3.1.3.2.) and alkaline (E.C. 3.1.3.1.) phosphatases, were measured over 13 months in the forest litter, along with several biotic and abiotic variables, potentially involved in the regulation of these enzymes. These comprised moisture, temperature, pH, water-extractable inorganic P (P_I), fungi, culturable heterotrophic bacteria and protein concentrations. Moisture considerably affected the production of proteins and acid phosphatases, probably formed by litter microorganisms. This result corroborated the study of Criquet et al. [Soil Biology and Biochemistry 34 (2002) 1111] which indicated that rainfall was the most important factor regulating the production and the activity of numerous enzymes in sclerophyllous forest litter. However, it appeared that moisture cannot alone predict all of the variations in phosphatase activities and the mineralisation rate of organic P (P_O). Indeed, principal component analyses (PCA) and multiple regressions showed that temperature and bacterial communities were also implicated in phosphatase dynamics and P_O mineralisation. Acid phosphatases were negatively correlated with the temperature, whilst alkaline phosphatases were positively correlated with this variable. The significant correlation obtained between bacteria and P_I concentrations, and the lack of correlation between bacteria and both acid and alkaline phosphomonoesterases, suggest that other important phosphatase types, such as phosphodiesterases, must be strongly implicated in P_O mineralisation of the litter and in the regulation of P microbial metabolism.     

Suppression of other soil microorganisms by mycelium of arbuscular mycorrhizal fungi in root-free soil

The influence of mycelium of two arbuscular mycorrhizal (AM) fungi, Glomus intraradices and Glomus mosseae, on other soil microorganisms, was examined in root-free soil with and without organic substrate amendment in terms of cellulose. The AM fungi were grown in symbiosis with cucumber in a compartmented growth system, which allowed AM fungal external mycelium to grow into root-free compartments. The fungicide Benomyl was applied to the root-free compartments to create an alternative non-mycorrhizal control treatment. Whole cell biomarker fatty acids were employed to quantify different groups of soil microorganisms including the two AM fungi. Abundance of most microbial groups were reduced by external mycelium of both AM fungi, though differential effects on the microbial community composition were observed between the two AM fungi as revealed from principal component analysis. Inhibition of other soil microorganisms was more pronounced in root-free soil with mycelium of G. mosseae than with mycelium of G. intraradices. In general, cellulose increased the amount of biomarker fatty acids of most groups of soil microorganisms, but cellulose did not affect the influence of AM fungi on other soil microorganisms. Benomyl suppressed growth of the external mycelium of the two AM fungi and had limited non-target effects on other microbial groups. In conclusion, our results show differential effects of external mycelium of AM fungi on other soil microbial communities, though both AM fungi included in the study overall inhibited most microbial groups as examined using whole cell biomarker fatty acids.     

Water and Element Fluxes of Red Oak Ecosystems during Stand Development on Post-Mining Sites (Lusatian Lignite District)

Most spoils in the Lusatian lignite district have been afforested successfully. However, since soil chemical status remains very different compared to unmined locations even after decades it is still unclear if forest ecosystems develop to a self-sustainable status or if restoration will fail in the long term. To clarify this question water and element fluxes of different-aged red oak(Q. rubra L.) stands on both Quaternary pure sands and Tertiary, pyritic loamy sands were investigated and compared to a nearby, unmined location. Due to increasing interception and evapotranspiration, the deep percolation rates of red oak forests decline drastically within30 yr. Annual groundwater recharge is about 300 mm yr^-1(41–46% of precipitation) initially. Deep percolation of the older stands decrease to less than 15% of precipitation, independent of substrate type. Tertiary substrates are characterized by intensive leaching of H, Fe, Al, Mg, Ca, NH₄-N and SO₄-S due to pyrite oxidation and therefore enhanced silicate weathering. Despite of drastically declining output rates with progressing ecosystem development, they remain remarkably higher as compared to Quaternary sands. However, despite differences in soil chemistry, the aged stands act as an almost complete sink for N, P and K. Compared to adjacent oak forests on undisturbed soils the stands on reclamation sites show very similar patterns with regard to N, P, K, Ca and Mg turnover.     

Controls on soil carbon accumulation during woody plant encroachment: Evidence from physical fractionation, soil respiration, and δC of respired CO2

Woody plant encroachment into grasslands and savannas is a globally extensive land-cover change that alters biogeochemical processes and frequently results in soil organic carbon (SOC) accrual. We used soil physical fractionation, soil respiration kinetics, and the isotopic composition of soil respiration to investigate microbial degradation of accrued SOC in sandy loam soils along a chronosequence of C₃woody plant encroachment into a C₄-dominated grassland in southern Texas. Our previous work in this system demonstrated significant changes in the chemistry and abundance of lignin and aliphatic biopolymers within particulate soil fractions during the first 40 yrs of woody plant encroachment, indicating selective accrual of purportedly more recalcitrant plant chemicals. However, during the long-term soil laboratory incubation presented herein, a greater proportion of SOC was mineralized in soils from older woody stands (34-86 yrs) than in soils from younger woody stands (14-23 yrs) and grasslands, providing no evidence for greater biochemical recalcitrance as a controlling mechanism for SOC accrual. In addition, δ¹³C values of respired CO₂ indicate that the mineralized SOC was predominately of C₃ origin from all woody stands along the chronosequence, and that respired CO₂ was primarily derived from the free light fraction (density <1.0 g/cm³) and macroaggregate-sized soil fraction. Our data suggested that the location of SOC among soil fractions was more important than plant polymer chemistry in determining SOC turnover rates during incubation. Surprisingly, estimates of the size and turnover rate of the active SOC pool based on respiratory kinetics did not increase with woody encroachment, and the turnover rate of the slower SOC pool decreased, again supporting the notion that increases in biochemically recalcitrant biopolymers did not hinder decomposition in the lab. These data indicate environmental conditions that may allow for C accrual in the field were alleviated during the controlled incubation. Therefore, C accrual in these sandy loam soils following woody encroachment should not be assumed stable, and this factor should be taken into account when considering responses of SOC to climate change or when making management decisions regarding land cover impacts on SOC.     

Modeling of the thermal influence of fires on the physicochemical properties and microbial activity of litter in cryogenic soils

Periodic surface fires in the cryolithozone (the northern taiga subzone) are the main factor determining the qualitative and quantitative characteristics of the soil organic matter. The specific features of the changes in the physicochemical parameters and microbial activity of the organic horizons in the cryogenic soils under larch forests of the northern taiga after the impact of high temperatures were revealed. The temperatures of fires of different intensity were simulated in laboratory conditions. The thermal impact on the litter organic matter during the surface fires may increase the CO₂ emission from the surface of the soil in the postfire communities due to the destruction of organic compounds only for a short time. After fires of high intensity with strong mineralization of the litters, during a period of more than 1 month, the pyrogenic effect on the organic horizons of the soils under the larch forests of the cryolithozone determined the reduction of the CO₂ emissions in the freshly burned areas as compared to the intact stands.     

Humus structure during a spruce forest rotation: quantitative changes and relationship to soil biota

Temporal dynamics of edaphic communities affect numerous processes in forests and also strongly influence the soil's organic matter status. We have linked long-term changes in the formation of organic matter (using humus micromorphological analyses) to changes in the soil's community structure during a spruce forest cycle on acid soil. The study was carried out at four sites of different age-classes in the Tharandter forest, Germany. The composition of the deeper humus layers (OH, A) was stable. Herbaceous litter, recent spruce litter, fragmented spruce litter, decomposed litter and faeces and fungi, which contributed to the organic layer (OL and OH horizon), significantly changed during the forestry cycle, especially with the shift from the early stage to intermediate stages. Parallel changes of the faunal assemblage of the soil showed quantitative relations between major stages of the forest development, humus dynamics and soil community composition. The herbaceous litter was correlated with surface-dwelling Collembola and microbial properties with faeces and fungi. Our results suggest that the long-term stability of deep organic layers provides a refuge for decomposers and detritivores that allows a rapid response to both adverse and favourable conditions, taking place in OL and OF layers. Furthermore, the opening of the canopy in mature stands allows the decomposers to adapt to changes in resource input long before the collapse of the forest.     

Changes in soil microbial substrate utilization in response to altered litter diversity and precipitation in a Mediterranean shrubland

This study aimed at quantifying the consequences of reduced precipitation and plant diversity on soil microbial community functioning in a Mediterranean shrubland of southern France. Across a natural gradient of shrub species diversity, we established a total of 92 plots (4 × 4 m) with and without a moderate rain exclusion treatment of about 12 % of total precipitation. Shrub diversity included all possible combinations of the four dominant species (Cistus albidus, Quercus coccifera, Rosmarinus officinalis, and Ulex parviflorus). Respective leaf litter mixtures of these species combinations were exposed in all plots over 2 years. We quantified how litter species richness and the reduction in precipitation affected the soil microbial substrate utilization (measured by CO₂ evolution using the MicroResp method) on soil samples collected underneath each individual litter mixture after 1 and 2 years of decomposition. Moderate precipitation reduction had a minor impact, but litter species richness and the dissimilarity in phenolic concentrations (estimated using Rao’s quadratic entropy) showed a positive effect on the diversity of substrates metabolized by the microbial communities. Moreover, litter species richness increased soil microbial activity by increasing the catabolic diversity of the soil microbial community. These effects were mostly driven by the presence of Quercus and Ulex leaf litter, which at the same time reduced microbial metabolic dominance, while the presence of Rosmarinus had opposite effects. Our data suggest that plant species loss can have stronger effects on the functioning of soil microbial communities than moderate drought, with potentially important feedbacks on biogeochemical cycling in Mediterranean shrubland ecosystems.     

Colonisation by arbuscular mycorrhizal and fine endophytic fungi in four woodland grasses – variation in relation to pH and aluminium

Acidic soils are harsh environments for plants. One of the major problems is the potential toxicity of aluminium (Al) and hydrogen ions at a pH below 5; another is the shortage of nutrients usually accompanying soil acidity. The aim of this study was to elucidate the relationship between arbuscular mycorrhizal (AM) colonisation and soil acidity (measured by pH and Al concentration) in order to evaluate the possibility that AM fungi facilitate the existence of plants on acidic soils. We sampled tussocks of four grass species, Elymus caninus, Poa nemoralis, Deschampsia cespitosa and Deschampsia flexuosa, together with samples of the surrounding soil, in oak forests of varying soil pH in southern Sweden. We determined pH, easily reacting Al (Al_r), extractable Al (Al_BaCl2) and phosphate in the soil samples, analysed the shoots for Al and phosphorous and quantified the degree of AM and fine endophyte (FE) colonisation in the roots. E. caninus was found on the least acidic soils and had the highest AM colonisation of all the species studied, while D. flexuosa, which was found on the most acidic soils, had the lowest AM colonisation. P. nemoralis and D. cespitosa were intermediate with respect to pH and AM colonisation. The colonisation of AM fungi exceeded FE colonisation for E. caninus and P. nemoralis, while the opposite was true for the two Deschampsia species. Our results indicated a negative relationship between Al_r and the degree of AM colonisation at the within-species level. The low colonisation of AM fungi on acidic soils may to some extent be explained by a sensitivity of AM fungi to Al_r: this parameter showed a stronger negative association with AM colonisation than did pH and Al_BaCl2. We hypothesize that Al toxicity is a critical factor for plant nutrition in forest soils through the impact on symbiotic fungi.