Effects of soil conditioning, root and shoot litter addition interact to determine the intensity of plant–soil feedback

Interactive effects of rhizospheric soil microbes and litter on the growth of the invasive hyperaccumulator Bidens pilosa in cadmium-contaminated soil

Both rhizospheric soil microbes and shoot litter input can have profound effects on plant performance; however, their interactive effects on plants in Cd-contaminated soils remain poorly understood. We grew an invasive hyperaccumulator, Bidens pilosa, in sterilized and unsterilized rhizosphere soil without litter or with a low (0.2%, dry weight ratio) or a high amount (1%) of litter from B. pilosa in soil with low (5 mg kg-1) or high (10 mg kg-1) concentrations of Cd. The total, shoot, and root biomass of B. pilosa increased significantly with litter addition, by an average of 27%, 28%, and 20%, respectively. The biomass of B. pilosa was significantly lower in unsterilized rhizosphere soil than in sterilized rhizosphere soil, decreasing by 19% for total, 18% for shoot, and 24% for root, respectively. Furthermore, the effects of different litter amounts (0.2% vs. 1%) on biomass did not vary in sterilized rhizosphere soils but significantly varied in unsterilized rhizosphere soils, showing that the biomass was significantly lower with 1% litter addition than with 0.2% litter addition in unsterilized rhizosphere soils, decreasing by 28% for total, 29% for shoot, and 21% for root, respectively. Tissue Cd concentrations were significantly higher in highly Cd-contaminated soils (+75% for shoot and +51% for root) than in low Cd-contaminated soils; however, higher tissue Cd concentrations did not cause a significant decrease in the biomass of B. pilosa. Soil fungal communities, particularly the dominant phyla, Ascomycota and Basidiomycota, play crucial roles in modulating the effects of rhizosphere soil microbes and litter on the growth of B. pilosa. Our results suggest that rhizosphere soil microbes and litter interact and affect the growth of B. pilosa: litter addition promoted growth by increasing the abundance of saprotrophs (especially Basidiomycota) and decreasing Cd accumulation in plant tissues, and rhizosphere soil inhibition was associated with a decreased abundance of Basidiomycota. Our findings highlight the importance of the interactive effects of rhizospheric soil microbes and litter on plant growth in Cd-contaminated soils.

Beyond the rootzone: Unveiling soil property and biota gradients around plants

Although the effects of plants on soil properties are well known, the effects of distance from plant roots to root-free soil on soil properties and associated soil organisms are much less studied. Previous research on the effects of distance from a plant explored specific soil organisms and properties, however, comparative studies across a wide range of plant-associated organisms and multiple model systems are lacking. We conducted a controlled greenhouse experiment using soil from two contrasting habitats. Within each soil type, we cultivated two plant species, individually and in combination, studying soil organisms and properties in the root centre, the root periphery, and the root-free zones. We showed that the distance from the cultivated plant (representing decreasing amount of plant roots) had a significant impact on the abiotic properties of the soil (pH and available P and N) and also on the composition of the fungal, bacterial, and nematode communities. The specific patterns, however, did not always match our expectations. For example, there was no significant relationship between the abundance of fungal pathogens and the distance from the cultivated plant compared to a strong decrease in the abundance of arbuscular mycorrhizal fungi. Changes in soil chemistry along the distance from the cultivated plant were probably one of the important drivers that affected bacterial communities. The abundance of nematodes also decreased with distance from the cultivated plant, and the rate of their responses reflected the distribution of their food sources. The patterns of soil changes along the gradient from plant to root-free soil were largely similar in two contrasting soil types and four plant species or their mixtures. This suggests that our results can be generalised to other systems and contribute to a better understanding of the mechanisms of soil legacy formation.

Temperate trees locally engineer decomposition and litter-bound microbiomes through differential litter deposits and species-specific soil conditioning

Leaf decomposition varies widely across temperate forests, shaped by factors like litter quality, climate, soil properties, and decomposers, but forest heterogeneity may mask local tree influences on decomposition and litter-associated microbiomes. We used a 24-yr-old common garden forest to quantify local soil conditioning impacts on decomposition and litter microbiology. We introduced leaf litter bags from 10 tree species (5 arbuscular mycorrhizal; 5 ectomycorrhizal) to soil plots conditioned by all 10 species in a full-factorial design. After 6 months, we assessed litter mass loss, C/N content, and bacterial and fungal composition. We hypothesized that (1) decomposition and litter-associated microbiome composition would be primarily shaped by the mycorrhizal type of litter-producing trees, but (2) modified significantly by underlying soil, based on mycorrhizal type of the conditioning trees. Decomposition and, to a lesser extent, litter-associated microbiome composition, were primarily influenced by the mycorrhizal type of litter-producing trees. Interestingly, however, underlying soils had a significant secondary influence, driven mainly by tree species, not mycorrhizal type. This secondary influence was strongest under trees from the Pinaceae. Temperate trees can locally influence underlying soil to alter decomposition and litter-associated microbiology. Understanding the strength of this effect will help predict biogeochemical responses to forest compositional change.

Invasive Cirsium arvense displays different resource-use strategies along local habitat heterogeneity in the trans-Himalayan region of Ladakh

Climate change and anthropogenic pressures have resulted in a significant shift in the invasion susceptibility and frequency of non-native species in mountain ecosystems. Cirsium arvense (L.) Scop. (Family: Asteraceae) is an invasive species that spreads quickly in mountains, especially in the trans-Himalayan region of Ladakh. The current study used a trait-based approach to evaluate the impact of local habitat heterogeneity (soil physico-chemical properties) on C. arvense. Thirteen plant functional traits (root, shoot, leaf, and reproductive traits) of C. arvense were studied in three different habitat types (agricultural, marshy, and roadside). Functional trait variability in C. arvense was higher between, than within habitats (between different populations). All the functional traits interacted with habitat change, except for leaf count and seed mass. Soil properties strongly affect C. arvense's resource-use strategies across habitats. The plant adapted to a resource-poor environment (roadside habitat) by conserving resources and to a resource-rich environment (agricultural and marshy land habitat) by acquiring them. The ability of C. arvense to use resources differently reflects its persistence in introduced habitats. In summary, our study shows that C. arvense invades different habitats in introduced regions through trait adaptations and resource-use strategies in the trans-Himalayan region.

Plant-soil interactions in the native range of two congeneric species with contrasting invasive success

The aim of this study was to compare plant-soil interactions in the native range of two congeneric European species differing in their invasive success in the world: a globally invasive Cirsium vulgare and non-invasive C. oleraceum. We assessed changes in soil nutrients and soil biota following soil conditioning by each species and compared performance of plants grown in self-conditioned and unconditioned soil, from which all, some or no biota was excluded. The invasive species depleted more nutrients than the non-invasive species and coped better with altered nutrient levels. The invasive species had higher seedling establishment which benefited from the presence of unconditioned biota transferred by soil filtrate. Biomass of both species increased in soil with self-conditioned soil filtrate and decreased in soil with self-conditioned whole-soil inoculum compared to unconditioned filtrate and inoculum. However, the increase was smaller and the decrease greater for the invasive species. The invasive species allocated less biomass to roots when associated with harmful biota, reducing negative effects of the biota on its performance. The results show that in the native range the invasive species is more limited by self-conditioned pathogens and benefits more from unconditioned mutualists and thus may benefit more from loss of effectively specialized soil biota in a secondary range. Our study highlights the utility of detailed plant-soil feedback research in species native range for understanding factors regulating species performance in their native range and pinpointing the types of biota involved in their regulation.