Disentangling the role of oomycete soil pathogens as drivers of plant-soil feedbacks

Quantifying soil microbial effects on plant species coexistence: A conceptual synthesis

Soil microorganisms play a critical role in shaping the biodiversity dynamics of plant communities. These microbial effects can arise through direct mediation of plant fitness by pathogens and mutualists, and over the past two decades, numerous studies have shined a spotlight on the role of dynamic feedbacks between plants and soil microorganisms as key determinants of plant species coexistence. Such feedbacks occur when plants modify the composition of the soil community, which in turn affects plant performance. Stimulated by a theoretical model developed in the 1990s, a bulk of the empirical evidence for microbial controls over plant coexistence comes from experiments that quantify plant growth in soil communities that were previously conditioned by conspecific or heterospecific plants. These studies have revealed that soil microbes can generate strong negative to positive frequency-dependent dynamics among plants. Even as soil microbes have become recognized as a key player in determining plant coexistence outcomes, the past few years have seen a renewed interest in expanding the conceptual foundations of this field. New results include re-interpretations of key metrics from classic two-species models, extensions of plant-soil feedback theory to multispecies communities, and frameworks to integrate plant-soil feedbacks with processes like intra- and interspecific competition. Here, I review the implications of theoretical developments for interpreting existing empirical results and highlight proposed analyses and designs for future experiments that can enable a more complete understanding of microbial regulation of plant community dynamics.

Contrasting roles of fungal and oomycete pathogens in mediating nitrogen addition and winter grazing effects on biomass

Both bottom-up and top-down processes modulate plant communities. Fungal and oomycete pathogens are most common in global grasslands, and due to differences in their physiology, function, host range, and life cycles, they may differentially affect plants (in both intensity and direction). However, how fungal and oomycete pathogens regulate bottom-up and top-down effects on plant community biomass remains unclear. To this end, we conducted a 3-year field experiment in an alpine meadow incorporating mammalian herbivore exclosure, fungicide/oomyceticide application, and nitrogen addition treatments. We arranged 12 blocks with half randomly assigned to be mammalian herbivore exclosures (fenced to exclude grazing sheep), and the other half were fenced most of the year but not in winter (winter grazing control). Six 2.5 × 2.5 m square plots were established in each block, with each of the six plots assigned as control, nitrogen addition, fungicide application, oomyceticide application, nitrogen addition + fungicide application, and nitrogen addition + oomyceticide application. We found that fungicide application significantly increased plant community biomass (mainly Poaceae species) under nitrogen addition and promoted the bottom-up effect of nitrogen addition on plant community biomass by altering the community-weighted mean of plant height (via species turnover). Meanwhile, oomyceticide application significantly increased plant community biomass (mainly Poaceae species) when mammalian herbivores were excluded and weakened the top-down effect of winter grazing on plant community biomass by driving intraspecific variation in plant height. Our results highlight that fungal and oomycete pathogens play important (but differing) roles in mediating the effects of nutrient availability and higher trophic levels on plant community biomass. Mechanistically, we demonstrated that plant pathogen-related modulation of plant community biomass is achieved by alterations to plant height. Overall, this study combines both community and disease ecology to reveal complex interactions among higher trophic levels and their potential impacts on terrestrial ecosystem functioning under human disturbance.

Fungal pathogens increase community temporal stability through species asynchrony regardless of nutrient fertilization

Natural enemies and their interaction with host nutrient availability influence plant population dynamics, community structure, and ecosystem functions. However, the way in which these factors influence patterns of community stability, as well as the direct and indirect processes underlying that stability, remains unclear. Here, we investigated the separate and interactive roles of fungal/oomycete pathogens and nutrient fertilization on the temporal stability of community biomass and the potential mechanisms using a factorial experiment in an alpine meadow. We found that fungal pathogen exclusion reduced community temporal stability mainly through decreasing species asynchrony, while fertilization tended to reduce community temporal stability by decreasing species stability. However, there was no interaction between pathogen exclusion and nutrient fertilization. These effects were largely due to the direct effects of the treatments on plant biomass and not due to indirect effects mediated through plant diversity. Our findings highlight the need for a multitrophic perspective in field studies examining ecosystem stability.

Light availability and plant shade tolerance modify plant-microbial interactions and feedbacks in subtropical trees

Plant-soil feedbacks (PSFs) are an important mechanism of species coexistence in forest communities. However, evidence remains limited for how light availability regulates PSFs in species with different shade tolerance via changes in plant-microbial interactions. Here we tested in a glasshouse experiment how PSFs changed as a function of light availability and tree shade tolerance. Soil bacterial and fungal communities were profiled using the 16S rRNA and ITS2 gene sequencing, respectively. Under low light, individual PSFs were positively related to shade tolerance, while the least shade-tolerant species produced the most positive PSFs under high light. Pairwise PSFs between species with contrasting shade tolerance were strongly positive under high light but negative under low light, thereby promoting the dominance of less shade-tolerant species in forest gaps and species coexistence under closed canopy, respectively. Under high light, PSFs were related to soil microbial composition and diversity, with the relative abundance of arbuscular mycorrhizal fungi being the primary driver of PSFs. Under low light, none of soil microbial properties were significantly related to PSFs. These findings indicate PSFs and plant shade tolerance interact to promote species coexistence and improve our understanding of how soil microbes contribute to variation in PSFs.