Soil-litter Interface Research Articles

Representing vapour and capillary rise from the soil improves a leaf litter moisture model

• Soil water affects litter moisture predictions. • Vapour and capillary rise from the soil were represented to estimate litter moisture. • Models ignoring soil water effects underestimated litter moisture. • Hydraulic coupling between the litter and soil is important. Litter moisture content plays a critical role in fire danger rating systems and forest ecosystems. Soil moisture content has been proposed to affect litter moisture due to vapour flux and capillary rise from the soil. However, few models consider soil water content when predicting litter moisture, and to our knowledge, no model includes capillary fluxes. In this study, we represented soil moisture coupling in a physics-based litter moisture prediction model by describing the vapour and capillary fluxes from the soil to litter. We aimed to evaluate if litter moisture predictions can be improved by incorporating the hydrological process at the soil-litter interface and explored the possible role of soil moisture in litter moisture simulations. Three model versions were compared against observations at a dry and wet experimental site in Australia: the original physics-based model, a model version extended with soil vapour flux only, and a version that incorporates both soil vapour flux and capillary rise. The simulation results suggest that soil moisture considerably influences litter moisture through soil vapour flux and capillary rise, which can lead to more than 10% of oven-dried weight, particularly under wet soil conditions. The corresponding model showed the best performance in comparison with field observations. The contribution of upward soil moisture fluxes was small after long dry and warm periods but noticeable during more moist periods. Further research is needed to evaluate the revised model for a broader range of weather, soil and litter conditions.

Controlled field experiment clarifies the influence of soil moisture on litter moisture content

Fuel moisture content (FMC) of fine surface litter fuel plays a decisive role in determining fire ignition and spread. Understanding physical processes that determine litter moisture dynamics is necessary to accurately predict litter FMC. It is known that vapour exchange, precipitation and latent heat transfer are dominant factors governing changes in litter FMC. However, limited research has been undertaken on physical processes at the soil-litter interface, despite the presumed importance of soil moisture in hydrating litter. In this study, we investigated the influence of soil moisture on litter FMC in factorial controlled field experiments at a dry and a wet site in Canberra, Australia. Each site had two treatments: with and without contact between soil and litter. Litter FMC, temperature and relative humidity were automatically and continuously measured in both surface and subsurface litter with a combination of fuel moisture, temperature and relative humidity sensors. The two treatments showed a different influence of soil moisture on litter FMC at the dry and wet site. There was limited moisture transport from soil to litter at the dry site, almost certainly dominated by vapour flux. Here, the influence of soil moisture on litter FMC is through its influence on local humidity in the litter layer. However, at the wet site capillary flow occurred in addition to vapour flux from the soil. The capillary flow responded to the matric potential gradient between soil and litter. The measurements confirmed that subsurface litter was more strongly coupled to the soil than the surface litter, which was more strongly coupled with the atmosphere. The improved understanding of physical processes governing water transport from soil to litter should help predict litter FMC more accurately, especially under wet soil conditions. Further research is needed to incorporate soil-litter interactions in litter FMC simulations.

Overwintering behavior reduces mortality for a terrestrial turtle in forests managed with prescribed fire

Prescribed fire is an essential management practice in pyrogenic ecosystems, but fire can also be a significant disturbance and source of mortality for both target and non-target species. Seasonal periods of animal inactivity may provide opportunities to design burn plans that minimize negative impacts to species of conservation concern, but few studies have rigorously examined this possibility. Using radiotelemetry, we studied overwintering behavior and interactions with fire in a forest-dwelling terrestrial turtle, the Eastern Box Turtle (Terrapene carolina carolina), over an eight-year period at two sites that use prescribed fire in forest management. Turtles at both sites selected predominantly hardwood forests and mesic habitats and avoided upland pine forests. Turtles buried deepest (2.9 – 3.2 cm) below the soil-litter interface in late February and then moved gradually shallower until emergence in early April. Emergence timing varied over a 58-day period, but was consistent within individuals from year to year. Turtles also maintained fidelity to refuge locations, but those overwintering in burned areas selected sites over twice as far from refuges used in previous years compared to those in unburned areas. The areas and habitats selected by turtles during winter served as refugia from fire, and those whose refuges did burn remained buffered from lethal temperatures even at shallow burial depths. The only fire-related injury or mortality occurred during seasons of surface activity. Timing burning and other forest management practices during periods of winter dormancy may thus minimize threats to turtle populations, but modifications to prescribed fire regimes must also be balanced with other management objectives.

Twigs occupied by Pheidole Westwood, 1839: Is there a difference between species?

Abstract: Pheidole is a genus with wide geographical distribution and diversity, especially in the leaf litter of neotropical forests, where nests are found at the soil-litter interface, in the soil and vegetation, among leaves, seeds, and twigs. Despite the availability of twigs and Pheidole species diversity in the leaf litter, most of this resource is not occupied, which suggests the existence of filters. This study analyzes whether twigs occupied by Pheidole species differ for the outer structure and anatomy of the wood. Twigs were collected from preserved Atlantic Forest fragments in southeastern Brazil. Twigs with Pheidole colonies were measured and the wood anatomy analyzed. We collected 224 twigs with Pheidole colonies, but the analysis was done at 41% due to wood decomposition. Five species were recorded in these twigs, which differ for the outer structure and anatomy of the wood. These results suggest the existence of preference in the occupation of twigs determined by wood structure.

The Neotropical mite genus Neopilizetes (Oribatida: Galumnidae): redescription of N. neotropicus, rediagnosis of the genus, and descriptions of four new species.

Neopilizetes is a genus of the oribatid mite family Galumnidae that was first characterized by having well-developed setae, a dorsosejugal suture, and raised striae on the prodorsum and notogaster. It was proposed to accommodate Pilizetes neotropicus Balogh Mahunka 1978 from Brazil-the only supposed Pilizetes species known outside the Ethiopian Region-and it has remained monotypic. We found adults of N. neotropicus and four related species in soil-litter interface samples of a single cacao plantation in Ilhéus, Bahia, Brazil. Herein we: redescribe N. neotropicus; describe Neopilizetes thoracicus n. sp., Neopilizetes tigris n. sp., Neopilizetes triumnasus n. sp. and Neopilizetes unumnasus n. sp., and use this information to develop a new concept and expanded diagnosis of Neopilizetes. A key to the five species is provided, and we offer reasons for rejecting the classification of Neopilizetes as a subgenus of Pilizetes.

Carbon flow from litter through soil microorganisms: From incorporation rates to mean residence times in bacteria and fungi

Resource quality and availability modify the microbial contribution to soil organic matter turnover and formation. We created a microbial hotspot at the soil-litter interface in a microcosm experiment to better understand and integrate specific microbial habitats into C turnover models. Reciprocal transplantation of 13C and 12C litter on top of soil cores allowed us to follow C flow into specific members of the microbial food web (bacteria and fungi) and to calculate the turnover times of litter-derived C in these microorganisms at three different stages of maize litter decomposition; early stage (0–4 days), intermediate stage (5–12 days) and later stage (29–36 days).Litter age influenced the incorporation rate of C into bacteria and fungi and subsequent turnover in phospholipid fatty acid (PLFA) biomarkers. When fresh litter was applied, both fungi and bacteria were able to assimilate labile litter C in the early stage of decomposition, while lower substrate quality in the intermediate stage of decomposition promoted fungal utilization. Utilization of complex litter C sources was minor in both fungi and bacteria in the later stage of decomposition. Different bacterial substrate utilization strategies were reflected by either a decline of the isotopic signal after exchange of 13C by 12C litter or by storage and/or reuse of previously released microbial 13C. The mean residence time of C in the fungal PLFA 18:2ω6,9 was estimated from 46 to 32 days, which is the same or shorter time than that of bacterial PLFAs. This highlights the role of fungi in rapid turnover processes of plant residues, with implications for implementation of bacterial and fungal processes into C turnover models.

Relationships between soil–litter interface enzyme activities and decomposition in Pinus massoniana plantations in China

Enzyme activities in decomposing litter are directly related to the rate of litter mass loss and have been widely accepted as indicators of changes in belowground processes. Studies of variation in enzyme activities of soil–litter interface and its effects on decomposition are lacking. Evaluating enzyme activities in this layer is important to better understand energy flow and nutrient cycling in forest ecosystems. Litter decomposition and the seasonal dynamics of soil–litter enzyme activities were investigated in situ in 20- (younger) and 46-year-old (older) Pinus massoniana stands for 540 days from August 2010 to March 2012 by litterbag method. We measured potential activities of invertase, cellulase, urease, polyphenol oxidase, and peroxidase in litter and the upper mineral soils, and evaluated their relationships with the main environment factors. Remaining litter mass was 57.6 % of the initial weights in the younger stands and 61.3 % in the older stands after 540-day decomposition. Levels of enzyme activity were higher in the litter layer than in the soil layer. Soil temperature, litter moisture, and litter nitrogen (N) concentration were the most important factors affecting the enzyme activities. The enzyme activity showed significantly seasonal dynamics in association with the seasonal variations in temperature, water, and decomposition stages. Remaining litter dry mass was found to be significantly linearly correlated with enzyme activities (except for litter peroxidase), which indicates an important role of enzyme activity in the litter decomposition process. Our results indicated the important effects of biotic (litter N) and abiotic factors (soil temperature and litter moisture) on soil–litter interface enzyme activities. Overall significant linear relationship between remaining dry mass and enzyme activities highlighted the important role of enzyme activity in affecting litter decomposition processes, which will further influence nutrient cycling in forest ecosystems. Our results contributed to the better understanding of the mechanistic link between upper soil–litter extracellular enzyme production and litter decomposition in forest ecosystems.

Understanding earthworm – Collembola interactions and their importance for ecosystem processes needs consideration of species identity

Soil animals and their interactions exert strong effects on ecosystem processes, such as leaf litter decomposition and nitrogen (N) cycling, thereby contributing to ecosystem functioning and stability. The understanding of how and why certain species interact is important to predict the effect of soil animal communities on ecosystem processes. Species interactions are discussed as being facilitative, antagonistic or neutral. We investigated interactions between two earthworm (Lumbricus terrestris, Aporrectodea caliginosa) and two Collembola species (Heteromurus nitidus and Protaphorura armata), representing major soil decomposer taxa. The two earthworm species are representatives of the soil macrofauna, with L. terrestris living in permanent vertical burrows and feeding on leaf litter, and A. caliginosa living in non-permanent horizontal burrows and feeding predominantly on resources in organo-mineral soil. The Collembola species are representatives of the soil mesofauna, with H. nitidus predominantly colonizing the soil litter interface and feeding on litter associated resources, whereas P. armata colonizes deeper soil layers and mainly feeds on resources in organo-mineral soil. Therefore, the species were assumed to differ in two major traits, i.e. body size (macrofauna vs. mesofauna) and food microhabitat association (“litter-associated” vs. “soil-associated”). Mesocosms with natural forest floor containing one beech (Fagus sylvatica) sapling were set up and incubated in the laboratory for three months. 15N labeled beech litter was added to follow the effect of detritivore animals on N cycling and N uptake by beech saplings. We hypothesized antagonistic interactions to dominate in species with similar body size or food microhabitat association via hampering the performance (biomass, abundance) of each other thereby reducing effects on leaf litter decomposition and N cycling. On the contrary, we expected species of different body size or food microhabitat association to facilitate each other's effects on ecosystem processes. In contrast to our expectation there was no clear relationship between similarity of body size or food microhabitat association and soil fauna effects on each other. Interactions between detritivores were not consensual with L. terrestris facilitating biomass gain of A. caliginosa, while its own biomass was reduced in presence of A. caliginosa. Additionally, 15N incorporation into A. caliginosa and the two Collembola species decreased in presence of L. terrestris, irrespective of similarity of body size or food microhabitat association. Leaf litter decomposition was increased by L. terrestris, while none of the other species affected litter decomposition. Generally, 15N incorporation into beech saplings was significantly increased in presence of L. terrestris or H. nitidus, but reduced in two species treatments due to antagonistic interactions. Interestingly, it was increased if L. terrestris, A. caliginosa and P. armata were present together, indicating facilitative interactions between these species. The results suggest that soil fauna interactions mainly vary with the identity of species and community composition rather than with similarity of traits. This highlights the complexity of soil fauna interactions and the difficulty to predict their effects on ecosystem processes, such as litter decomposition and N cycling, in species rich communities.

Production and effects of volatile organic compounds during interspecific interactions

Competition between mycelia of saprotrophic cord-forming basidiomycetes occurs both within dead woody resources and in the soil-litter interface, and involves a variety of antagonistic mechanisms including the production of volatile organic compounds (VOCs). The antagonistic potential of VOC profiles from interactions in wood blocks and in soil microcosms was assessed using shared headspace experiments, and the profile of VOCs emitted over the course of interactions elucidated using solid phase microextraction (SPME) with gas chromatography-mass spectrometry (GC–MS). Quantitative and qualitative changes in VOC production occurred in interactions compared to self-pairing controls, with different VOC profiles from fungi growing in wood blocks compared to soil trays. There were both stimulatory and inhibitory effects of VOCs on target mycelial extension rate, hyphal coverage and fractal dimension. VOC-mediated effects were greater in self-pairing controls compared to interactions, and differed depending on the substratum in which the VOC-producing fungi were growing.

Metabolic profiling of Minimedusa polyspora (Hotson) Weresub & P.M. LeClair, a cellulolytic fungus isolated from Mediterranean maquis, in southern Italy

The fungus Minimedusa polyspora (Hotson) Weresub & P.M. Le Clair was isolated during a survey aimed at collecting cellulolytic fungi from the soil–litter interface of the Mediterranean maquis environment in Italy. The metabolic profiling of the species and its ability to translocate nutrients between decaying leaf litter and soil have been investigated by means of Phenotype MicroArray™, Scanning Electron Microscopy and microanalysis techniques. The Phenotype MicroArray™ showed that the fungus possesses a preference for polysaccharides at the initial phases of its growth, and that it prefers hexoses and then oligosaccharides in the later phases of its development. M. polyspora proved to be capable of concentrating several important biogenic microelements (N, P, S, K and Ca), which are absent in the cellulosic substrate before fungal colonization. This capacity for nutrient uptake and translocation from other sources than cellulose makes this fungus a very efficient pioneer colonizer that requires little nitrogen, is fast growing, changes its own metabolism according to the early modifications of the substrate and uses inhibitory substances to make the habitat unfavourable for other species.

The influence of litter composition across the litter–soil interface on mass loss, nitrogen dynamics and the decomposer community

Many studies have investigated the influence of plant litter species composition on decomposition, but results have been context-dependent. Litter and soil are considered to constitute a decomposition continuum, but whether litter and soil ecosystems respond to litter identity and mixing in the same manner is unsure. In a field experiment utilizing 5 litter species and their mixture, we investigated whether the effects of litter identity and mixing on mass loss, nutrient dynamics, and decomposer communities are consistent across the litter–soil interface. In monoculture, mass loss and nitrogen (N) dynamics in the litter layer corresponded to the underlying soil N availability, demonstrating the continuum of resources from litter to soil. Litter microbial biomass and mesofauna abundance tended to be greater on litter with a faster decay rate and greater N release. However, soil decomposer abundance and diversity were not greater with higher soil N, causing litter and soil communities to respond differently to litter identity. Non-additive mass loss and N dynamics were observed after 6 months, and were correlated with non-additive litter microbial community composition and litter mesofauna communities, but all other aspects of the litter community and all measures of the soil community were additive. Decomposer communities and N dynamics did not respond similarly to the litter mixture across the litter–soil interface. This study is one of the few to comprehensively examine how a litter mixture influences decomposition dynamics and communities across the soil–litter interface, including multiple taxa and trophic levels. Our results demonstrate that processes associated with decomposition are decoupled for litter and soil, particularly in that litter showed non-additivity in mass loss, N release and decomposer community, but soil responses were largely additive.

Succession of bacterial and fungal 4-chloro-2-methylphenoxyacetic acid degraders at the soil-litter interface

Phenoxyacetic acids can be degraded by diverse soil microorganisms. Nevertheless, we miss information about the succession of 4-chloro-2-methylphenoxyacetic acid (MCPA) degraders in micro-environments of soils as well as specific functions of different microbial groups during MCPA degradation. We studied MCPA degradation at the soil-litter interface in a microcosm experiment and followed the succession of different degrader populations by quantifying the abundance of 16S rRNA genes as well as, the fungal ITS fragment and the functional genes tfdA (in total and divided into three classes) and cadA. Adjacent to the litter layer, a dynamic depletion zone of MCPA indicated that the litter effect on MCPA degradation depends on substrate availability and the affected soil volume. The increase of the tfdA class III and cadA genes was linked to MCPA mineralisation. Total abundance of tfdA genes was dominated by class I MCPA degraders and did not reflect MCPA degradation potential of the soil. Litter addition induced the development of pioneer and late-stage fungal communities, which were probably both involved in MCPA degradation. The results underline the importance of the ecological behaviour of different degrader populations for the understanding of herbicide degradation in soils.

Alkane-degrading bacteria at the soil-litter interface: comparing isolates with T-RFLP-based community profiles

Alkane-degrading bacteria were isolated from uncontaminated soil microcosms, which had been incubated with maize litter as natural alkane source. The isolates served to understand spatio-temporal community changes at the soil-litter interface, which had been detected using alkB as a functional marker gene for bacterial alkane degraders. To obtain a large spectrum of isolates, liquid subcultivation was combined with a matrix-assisted enrichment (Teflon membranes, litter). Elevated cell numbers of alkane degraders were detected by most probable number counting indicating enhanced alkane degradation potential in soil in response to litter treatment. Partial 16S rRNA gene sequencing of 395isolates revealed forty different phylogenetic groups [operational taxonomic units (OTUs)] and spatio-temporal shifts in community composition. TenOTUs comprised so far unknown alkane degraders, and five OTUs represented putative new bacterial genera. The combination of enrichment methods yielded a higher diversity of isolates than liquid subcultivation alone. Comparison of 16S rRNA gene T-RFLP profiles indicated that many alkane degraders present in the enrichments were not detectable in the DNA extracts from soil microcosms. These possibly rare specialists might represent a seed bank for the alkane degradation capacity in uncontaminated soil. This relevant ecosystem function can be fostered by the formation of the soil-litter interface.

Functional and ecological consequences of saprotrophic fungus–grazer interactions

Saprotrophic fungi are key regulators of nutrient cycling in terrestrial ecosystems. They are the primary agents of plant litter decomposition and their hyphal networks, which grow throughout the soil-litter interface, represent highly dynamic channels through which nutrients are readily distributed. By ingesting hyphae and dispersing spores, soil invertebrates, including Arthropoda, Oligochaetae and Nematoda, influence fungal-mediated nutrient distribution within soil. Fungal physiological responses to grazing include changes to hydrolytic enzyme production and respiration rates. These directly affect nutrient mineralisation and the flux of CO(2) between terrestrial and atmospheric pools. Preferential grazing may also exert selective pressures on saprotrophic communities, driving shifts in fungal succession and community composition. These functional and ecological consequences of grazing are intrinsically linked, and influenced by invertebrate grazing intensity. High-intensity grazing often reduces fungal growth and activity, whereas low-intensity grazing can have stimulatory effects. Grazing intensity is directly related to invertebrate abundance, and varies dramatically between species and functional groups. Invertebrate diversity and community composition, therefore, represent key factors determining the functioning of saprotrophic fungal communities and the services they provide.

Resource, biological community and soil functional stability dynamics at the soil–litter interface

The interface between decaying plant residues and soil is a focus for soil ecological processes because of resources from the residues diffusing into the soil, and microfauna that proliferate in the adjacent soil. Given that the recovery of soil function following disturbance depends on immigration, colonization and establishment of exotic organisms from adjacent un-disturbed habitats, and the availability of bio-available resources, we hypothesized that the soil–litter interface could contribute to soil functional stability. In laboratory pot trials, soil was separated into two parts by a mesh bag with the inner section amended, or not amended, with rice straw; an outer layer of unamended soil, adjacent to the litter (1.5 cm thick, either heated or not), provided a soil–litter interface. This enabled us to examine the dynamics of dissolved organic carbon (DOC), mineral nitrogen, microbial biomass carbon (MBC), nematode assemblages and functional stability during 35 days incubation. Either 1 mm or 5 μm meshes were used, which allowed nematodes to migrate (SR1) or not (SR5) through the mesh to the soil–litter interface; thus also enabling us to evaluate the role of nematodes in soil functional stability. Higher DOC and MBC but lower mineral nitrogen concentrations were found at the soil–litter interface. Heating increased the availability of soil resources such as mineral nitrogen and DOC, but decreased the MBC and total nematode abundance in the soil. The soil–litter interface was characterized by a higher abundance of nematodes, particularly microbivores, regardless of mesh aperture or disturbance. The difference in nematode abundance between SR1 and SR5 indicated that nematode propagation, due to resource diffusion and nematode migration through the mesh, contributed to the changing numbers of microbivorous nematodes depending on incubation time. The soil functional stability was calculated as a relative change in the functioning of short-term barley decomposition. Soil functional resistance, defined as the instantaneous effect of disturbance on decomposition measured on the first day, was highest in the SR5 treatment. However, soil functional resilience, defined as the recovery of soil function over the whole incubation period (35d), was highest in the SR1 treatment, which is most probably attributed to the functioning of microbivorous nematodes. Our results suggest that small-scale spatial heterogeneity, due to organic residue decomposition, can help maintain soil functions following disturbance.

The importance of mycelial connection at the soil–litter interface for nutrient translocation, enzyme activity and litter decomposition

The translocation of nutrients by networks of fungal hyphae is an important mechanism controlling decomposition rates of plant residues. We performed a 15-month long field experiment aiming to assess the effects of the C and N availability and the role of the mycelial connection between litter and underlying mineral soil on the decomposition rates and levels of enzyme activities in plant litter. Litterbags containing either N-rich litter of alder ( Alnus glutinosa) or N-poor litter of aspen ( Populus tremula) were exposed on top of A-horizon of forest soil. The experiment was set up in a full-factorial design with litter species, C and N availability, and presence of mycelial connection as factors. Labile C and N were added as sucrose and ammonium nitrate solutions in four combinations: −C−N, +C−N, −C+N and +C+N. To disrupt the mycelial connection between litter and soil half of the litterbags were slightly moved, whereas others were left untouched. All experimental manipulations were performed every ten days during snow-free period. At the natural levels of C and N availability, N-poor aspen litter, but not N-rich alder litter, accumulated significant amounts of N. The addition of easily available C decreased the activity of xylanase and phenoloxidase and reduced litter decomposition rates of aspen litter; in alder litter similar but less pronounced trends were also observed. The severance of the mycelial connection between plant litter and underlying soil generally decreased xylanase activity and decomposition rates of the N-limited litter, likely due to the reduced presence of basidiomycete fungi in the litterbags. This effect was not observed, if the N-limitation was artificially relieved by the addition of easily available N, likely due to the increased activity of bacteria and microfungi.

Regulation of bacterial and fungal MCPA degradation at the soil–litter interface

Much is known about mechanisms and regulation of phenoxy acid herbicide degradation at the organism level, whereas the effects of environmental factors on the performance of the phenoxy acid degrading communities in soils are much less clear. In a microcosm experiment we investigated the small-scale effect of litter addition on the functioning of the MCPA degrading communities. 14C labelled MCPA was applied and the functional genes tfdA and tfdAα were quantified to characterise bacterial MCPA degradation. We identify the transport of litter compounds as an important process that probably regulates the activity of the MCPA degrading community at the soil–litter interface. Two possible mechanisms can explain the increased tfdA abundance and MCPA degradation below the litter layer: 1) transport of α-ketoglutarate or its metabolic precursors reduces the costs for regenerating this co-substrate and thereby improves growth conditions for the MCPA degrading community; 2) external supply of energy and nutrients changes the internal resource allocation towards enzyme production and/or improves the activity of bacterial consortia involved in MCPA degradation. In addition, the presence of litter compounds might have induced fungal production of litter-decaying enzymes that are able to degrade MCPA as well.

Microbial consumption and production of volatile organic compounds at the soil-litter interface

Substantial amounts of volatile organic compounds (VOCs) can be released during decom- position and these compounds can affect atmospheric chemistry, belowground processes, and the structure of microbial communities in litter and soil. However, we have a limited understanding of the types, quantities and ecological impacts of VOCs emitted from litter. Here we used a closed flow-through system and proton transfer reaction mass spectrom- etry (PTR-MS) to characterize VOC emissions from soil and two litter types (Pinus taeda and Acer rubrum) over a 72-day incubation period. Microbial respiration rates were measured throughout the incubation, and the soils were harvested at the end of the incubation to determine how litter VOCs influenced soil C dynamics, N mineralization rates, and bacterial communities. Using the PTR-MS we identified over 100 VOCs, with 10 VOCs making up the majority of emissions. VOCs accounted for up to 2.5% of the C flux from litter. Soil was a net sink of litter VOCs, absorbing up to 80% of VOCs released by litter, and exposure of soil to litter VOCs increased microbial respiration rates in soil by up to 15%. However, we observed negligible impacts of litter VOCs on soil nutrient levels and bacterial community structure, suggesting that soils must be exposed to higher concentrations of VOCs than observed in our study, to cause effects on these soil characteristics. Overall, VOCs appear to have an important influence on C dynamics at the soil-litter interface and VOC emissions from decomposing litter may represent an understudied component of biosphere-atmosphere interactions.

Saprotrophic cord systems: dispersal mechanisms in space and time

In natural terrestrial environments, nutrients are often patchily and sparsely distributed, and the microclimate is constantly changing both temporally and spatially. To survive, fungi must be able to transfer to a new resource before the nutrient supplies in their current food base are exhausted. While the majority of fungi propagate as spores, some basidiomycetes can grow out of a resource as mycelium in search of new resources. The mycelium of these fungi typically aggregates to form linear organs, termed cords or rhizomorphs, that ramify at the soil–litter interface in forests, interconnecting disparate litter components to form extensive (many square meters or even hectares), long-lived (many years) systems. These mycelial systems form effective dispersal mechanisms in space and time. This article reviews the two main, but not mutually exclusive, mycelial dispersal (resource capture) strategies: (1) a “sit and wait” strategy, whereby a large mycelial network waits for resources to land on it and then actively colonises those resources; and (2) growing and searching actively for new resources. The way in which mycelia balance exploration and nutrient transport, and robustness to damage, against “cost” of production and speed with which an area can be colonised, is explored using techniques borrowed from graph theory and statistical mechanics.

Endogeic earthworms alter carbon translocation by fungi at the soil–litter interface

The effect of endogeic earthworms ( Octolasion tyrtaeum (Savigny)) on the translocation of litter-derived carbon into the upper layer of a mineral soil by fungi was investigated in a microcosm experiment. Arable soil with and without O. tyrtaeum was incubated with 13C/ 15N-labelled rye leaves placed on plastic rings with gaze (64 μm mesh size) to avoid incorporation of leaves by earthworms. The plastic rings were positioned either on or 3 cm above the soil surface, to distinguish between biotic and chemical/physical translocation of nutrients by fungi and leaching. Contact of leaves to the soil increased 13C translocation, whereas presence of O. tyrtaeum reduced the incorporation of 13C into the mineral soil in all treatments. Although biomass of O. tyrtaeum decreased during the experiment, more 13C and 15N was incorporated into earthworm tissue in treatments with contact of leaves to the soil. Contact of leaves to the soil and the presence of O. tyrtaeum increased cumulative 13CO 2-C production by 18.2% and 14.1%, respectively. The concentration of the fungal bio-indicator ergosterol in the soil tended to be increased and that of the fungal-specific phospholipid fatty acid 18:2 ω6 was significantly increased in treatments with contact of leaves to the soil. Earthworms reduced the concentration of ergosterol and 18:2 ω6 in the soil by 14.0% and 43.2%, respectively. Total bacterial PLFAs in soil were also reduced in presence of O. tyrtaeum, but did not respond to the addition of the rye leaves. In addition, the bacterial community in treatments with O. tyrtaeum differed from that without earthworms and shifted towards an increased dominance of Gram-negative bacteria. The results indicate that litter-decomposing fungi translocate litter-derived carbon via their mycelial network in to the upper mineral soil. Endogeic earthworms decrease fungal biomass by grazing and disruption of fungal hyphae thereby counteracting the fungal-mediated translocation of carbon in soils.