Sounds of Soil: A New World of Interactions under Our Feet?

Soils are biodiversity-dense and constantly carry chemical flows of information, with our mental image of soil being dark and quiet. But what if soil biota tap sound, or more generally, vibrations as a source of information? Vibrations are produced by soil biota, and there is accumulating evidence that such vibrations, including sound, may also be perceived. We here argue for potential advantages of sound/vibration detection, which likely revolve around detection of potential danger, e.g., predators. Substantial methodological retooling will be necessary to capture this form of information, since sound-related equipment is not standard in soils labs, and in fact this topic is very much at the fringes of the classical soil research at present. Sound, if firmly established as a mode of information exchange in soil, could be useful in an ‘acoustics-based’ precision agriculture as a means of assessing aspects of soil biodiversity, and the topic of sound pollution could move into focus for soil biota and processes

A general stochastic model shows that plant-soil feedbacks can buffer plant species from extinction risks in unpredictable environments

Theory and experiments have demonstrated that negative plant-soil feedback (PSF) promotes coexistence between plant species. Plants and soils, however, face the challenge of an increasingly unpredictable environment due to multiple global change factors. Environmental stochasticity induces fluctuations that increase the variability and unpredictability of population dynamics, plant associations in the community and thus properties such as overall productivity. In this paper, we formulate a stochastic version of a classic PSF deterministic model, which describes the outcome of plant species competition in the presence of soil feedback. Especially when the soil feedback is negative, the deterministic expectation is that pulse perturbations to the system (e.g. a drought episode) cause plants and soil to move away from their equilibrium and then return to it. Environmental stochasticity alters this expectation: the system can either settle into a fluctuation regime around the deterministic expectation, or plant species may go extinct. Probability of extinction predictably increases with environmental stochasticity but the more negative the PSF, the more it can counteract the increase in extinction probability caused by increased environmental stochasticity. We stress that in nature the actual impact of PSF will depend on the interactions that link different types of soil organisms to plant species. We conclude that theory shows that plant communities with strong negative PSF are best placed to withstand the risk posed by increased environmental stochasticity but also that we still need more experimental evidence to validate theory and develop applications

Spatial characterization of arbuscular mycorrhizal fungal molecular diversity at the submetre scale in a temperate grassland

Although arbuscular mycorrhizal fungi (AMF) form spatially complex communities in terrestrial ecosystems, the scales at which this diversity manifests itself is poorly understood. This information is critical to the understanding of the role of AMF in plant community composition. We examined small-scale (submetre) variability of AMF community composition (terminal restriction fragment length polymorphism fingerprinting) and abundance (extraradical hyphal lengths) in two 1 m2 plots situated in a native grassland ecosystem of western Montana. Extraradical AMF hyphal lengths varied greatly between samples (14–89 m g soil−1) and exhibited spatial structure at scales <30 cm. The composition of AMF communities was also found to exhibit significant spatial autocorrelation, with correlogram analyses suggesting patchiness at scales <50 cm. Supportive of overall AMF community composition analyses, individual AMF ribotypes corresponding to specific phylogenetic groups exhibited distinct spatial autocorrelation. Our results demonstrate that AMF diversity and abundance can be spatially structured at scales of <1 m. Such small-scale heterogeneity in the soil suggests that establishing seedlings may be exposed to very different, location dependent AMF communities. Our results also have direct implications for representative sampling of AMF communities in the field

Legacy effect of microplastics on plant–soil feedbacks

Microplastics affect plants and soil biota and the processes they drive. However, the legacy effect of microplastics on plant–soil feedbacks is still unknown. To address this, we used soil conditioned from a previous experiment, where Daucus carota grew with 12 different microplastic types (conditioning phase). Here, we extracted soil inoculum from those 12 soils and grew during 4 weeks a native D. carota and a range-expanding plant species Calamagrostis epigejos in soils amended with this inoculum (feedback phase). At harvest, plant biomass and root morphological traits were measured. Films led to positive feedback on shoot mass (higher mass with inoculum from soil conditioned with microplastics than with inoculum from control soil). Films may decrease soil water content in the conditioning phase, potentially reducing the abundance of harmful soil biota, which, with films also promoting mutualist abundance, microbial activity and carbon mineralization, would positively affect plant growth in the feedback phase. Foams and fragments caused positive feedback on shoot mass likely via positive effects on soil aeration in the conditioning phase, which could have increased mutualistic biota and soil enzymatic activity, promoting plant growth. By contrast, fibers caused negative feedback on root mass as this microplastic may have increased soil water content in the conditioning phase, promoting the abundance of soil pathogens with negative consequences for root mass. Microplastics had a legacy effect on root traits: D. carota had thicker roots probably for promoting mycorrhizal associations, while C. epigejos had reduced root diameter probably for diminishing pathogenic infection. Microplastic legacy on soil can be positive or negative depending on the plant species identity and may affect plant biomass primarily via root traits. This legacy may contribute to the competitive success of range-expanding species via positive effects on root mass (foams) and on shoot mass (PET films). Overall, microplastics depending on their shape and polymer type, affect plant–soil feedbacks

Research trends of microplastics in the soil environment: Comprehensive screening of effects

We collated and synthesized previous studies that reported the impacts of microplastics on soil parameters. The data were classified and integrated to screen for the proportion of significant effects, then we suggest several directions to alleviate the current data limitation in future experiments. We compiled 106 datasets capturing significant effects, which were analyzed in detail. We found that polyethylene and pellets (or powders) were the most frequently used microplastic composition and shape for soil experiments. The significant effects mainly occurred in broad size ranges (0.1–1 mm) at test concentrations of 0.1%–10% based on soil dry weight. Polyvinyl chloride and film induced significant effects at lower concentrations compared to other compositions and shapes, respectively. We adopted a species sensitivity distribution (SSD) and soil property effect distribution (SPED) method using available data from soil biota, and for soil properties and enzymes deemed relevant for microplastic management. The predicted-no-effect-concentration (PNEC)-like values needed to protect 95% of soil biota and soil properties was estimated to be between 520 and 655 mg kg−1. This study was the first to screen microplastic levels with a view toward protecting the soil system. Our results should be regularly updated (e.g., quarterly) with additional data as they become available

Controls on the production, incorporation and decomposition of glomalin - a novel fungal soil protein important to soil carbon

OAK B263 Glomalin is an operationally defined soil protein, produced by arbuscular mycorrhizal fungi (AMF), with importance in soil carbon sequestration through its relationship with soil aggregation. The goal of the project was to further explore the natural history of glomalin and to address several questions regarding basic behavior of this compound in soil (production, incorporation, decomposition). We have obtained a significant amount of novel information on the arbuscular mycorrhizal fungal soil protein, concerning factors controlling its production to mechanisms of incorporation and decomposition. These findings have resulted in 10 publications in peer-reviewed journals, with several more submitted or in preparation, and 16 contributed presentations at meetings. I have sought collaborative opportunities whenever they fit within the research proposed to enhance our productivity. Additionally, although not part of the original proposed work, we have made a significant effort to elucidate the molecular biology of glomalin (in response to Program Officer suggestions). In addition to peer-reviewed publications there have also been a number of invited presentations, including a keynote address delivered by the PI at the International Conference on Mycorrhizae (ICOM4) in Montreal, summer 2003. Two Master's students have been trained (and graduated), and a postdoctoral associate has been mentored, as well as numerous undergraduate researchers at UM. In this report I summarize the major findings of the project in the areas of glomalin production control (host factors, elevated CO2), incorporation, and decomposition. Section D is newly added and describes recent progress in molecular biology. Briefly, we found that glomalin production is influenced by the host, as shown by host species effects and responses to elevated CO2. We have recently made a significant breakthrough in understanding how glomalin may become deposited into soil; apparently the dominant pathway is via hyphal turnover rather than by secretion (as previously assumed). In terms of decomposition, we have learned that glomalin is surprisingly stable (data from soil incubation experiments and from carbon dating) and has a residence time far greater than the AMF hyphae (on the order of decades, putting at least some glomalin fractions in the slow soil C pool). Finally, our exploratory work on molecular biology of glomalin has yielded some promising preliminary data (including an immunoreactive band that was used to obtain N-terminal amino acid sequence). While the gene has not yet been identified, this strongly suggests that glomalin is a unique compound; - a significant step from an operational definition (based on soil extraction conditions) to biochemical characterization

Microplastic transport in soil by earthworms

Despite great general benefits derived from plastic use, accumulation of plastic material in ecosystems, and especially microplastic, is becoming an increasing environmental concern. Microplastic has been extensively studied in aquatic environments, with very few studies focusing on soils. We here tested the idea that microplastic particles (polyethylene beads) could be transported from the soil surface down the soil profile via earthworms. We used Lumbricus terrestris L., an anecic earthworm species, in a factorial greenhouse experiment with four different microplastic sizes. Presence of earthworms greatly increased the presence of microplastic particles at depth (we examined 3 soil layers, each 3.5 cm deep), with smaller PE microbeads having been transported downward to a greater extent. Our study clearly shows that earthworms can be significant transport agents of microplastics in soils, incorporating this material into soil, likely via casts, burrows (affecting soil hydraulics), egestion and adherence to the earthworm exterior. This movement has potential consequences for exposure of other soil biota to microplastics, for the residence times of microplastic at greater depth, and for the possible eventual arrival of microplastics in the groundwater

Effect of different root endophytic fungi on plant community structure in experimental microcosms

Understanding the effects of root-associated microbes in explaining plant community patterns represents a challenge in community ecology. Although typically overlooked, several lines of evidence point out that nonmycorrhizal, root endophytic fungi in the Ascomycota may have the potential to drive changes in plant community ecology given their ubiquitous presence, wide host ranges, and plant species-specific fitness effects. Thus, we experimentally manipulated the presence of root endophytic fungal species in microcosms and measured its effects on plant communities. Specifically, we tested whether (1) three different root endophyte species can modify plant community structure; (2) those changes can also modified the way plant respond to different soil types; and (3) the effects are modified when all the fungi are present. As a model system, we used plant and fungal species that naturally co-occur in a temperate grassland. Further, the soil types used in our experiment reflected a strong gradient in soil texture that has been shown to drive changes in plant and fungal community structure in the field. Results showed that each plant species responded differently to infection, resulting in distinct patterns of plant community structure depending on the identity of the fungus present. Those effects depended on the soil type. For example, large positive effects due to presence of the fungi were able to compensate for less nutrients levels in one soil type. Further, host responses when all three fungi were present were different from the ones observed in single fungal inoculations, suggesting that endophyte–endophyte interactions may be important in structuring plant communities. Overall, these results indicate that plant responses to changes in the species identity of nonmycorrhizal fungal community species and their interactions can modify plant community structure