Local variation in conspecific plant density influences plant–soil feedback in a natural grassland

Density-dependent plant-soil feedbacks of two plant species affected by plant competition

Both plant-soil feedbacks (PSF) and plant competition drive plant community assembly, but their interactive effects have rarely been investigated, and the role of community composition in modulating these interactions is unknown. We conducted a fully reciprocal experiment with two plant species (Polygonum criopolitanum and Carex thunbergii) and their associated soil biota communities to untangle the relative importance of PSF and competition. The two species were grown either in monoculture or in mixed plantings, with various densities of each plant's neighbor, and either in the presence or absence of the soil biota associated with either species. When grown individually, each plant species showed a neutral PSF in the presence of its own soil biota, while feedbacks in competition were predominantly negative. P. criopolitanum produced more biomass in the presence of soil biota than in autoclaved soil, but the opposite was observed for C. thunbergii. In competition, both plant species were suppressed by its presence of their own soil biota, and neither had a competitive advantage, resulting in a significant negative pairwise PSF in most cases. C. thunbergii also showed stronger per capita effects than did P. criopolitanum. The predication on species coexistence largely depended on the presence of soil biota, the soil origin, and the plant species ratio. When planted in equal proportion, P. criopolitanum and C. thunbergii only coexisted in the presence of soil biota of P. criopolitanum, while C. thunbergii usually excluded P. criopolitanum under all other conditions. When the density of the neighboring plant increased, either species was excluded in the presence of their own soil biota. However, in the absence of a soil biota, there was a priority effect, giving an advantage to the species that was planted first. Our study revealed that the strength and direction of these interactions were altered by interspecific competition at the community level, and both the identity and the density of the competitors must be taken into account in interpreting plant community assembly processes.

Influence of Intraspecific Competition Stress on Soil Fungal Diversity and Composition in Relation to Tree Growth and Soil Fertility in Sub-Tropical Soils under Chinese Fir Monoculture

Soil microorganisms provide valuable ecosystem services, such as nutrient cycling, soil remediation, and biotic and abiotic stress resistance. There is increasing interest in exploring total belowground biodiversity across ecological scales to understand better how different ecological aspects, such as stand density, soil properties, soil depth, and plant growth parameters, influence belowground communities. In various environments, microbial components of belowground communities, such as soil fungi, respond differently to soil features; however, little is known about their response to standing density and vertical soil profiles in a Chinese fir monoculture plantation. This research examined the assemblage of soil fungal communities in different density stands (high, intermediate, and low) and soil depth profiles (0–20 cm and 20–40 cm). This research also looked into the relationship between soil fungi and tree canopy characteristics (mean tilt angle of the leaf (MTA), leaf area index (LAI), and canopy openness index (DIFN)), and general growth parameters, such as diameter, height, and biomass. The results showed that low-density stand soil had higher fungal alpha diversity than intermediate- and high-density stand soils. Ascomycota, Basidiomycota, Mucromycota, and Mortierellomycota were the most common phyla of the soil fungal communities, in that order. Saitozyma, Penicillium, Umbelopsis, and Talaromyces were the most abundant fungal genera. Stand density composition was the dominant factor in changing fungal community structure compared to soil properties and soil depth profiles. The most significant soil elements in soil fungal community alterations were macronutrients. In addition, the canopy openness index and fungal community structure have a positive association in the low-density stand. Soil biota is a nutrient cycling driver that can promote better plant growth in forest ecosystems by supporting nutrient cycling. Hence, this research will be critical in understanding soil fungal dynamics, improving stand growth and productivity, and improving soil quality in intensively managed Chinese fir plantations.

Fungal Pathogens in Grasslands

Grasslands are major primary producers and function as major components of important watersheds. Although a concise definition of grasslands cannot be given using a physiognomic or structural approach, grasslands can be described as vegetation communities experiencing periodical droughts and with canopies dominated by grasses and grass-like plants. Grasslands have a cosmopolitan distribution except for the Antarctic region. Fungal interactions with grasses can be pathogenic or symbiotic. Herbivorous mammals, insects, other grassland animals, and fungal pathogens are known to play important roles in maintaining the biomass and biodiversity of grasslands. Although most pathogenicity studies on the members of Poaceae have been focused on economically important crops, the plant-fungal pathogenic interactions involved can extend to the full range of ecological circumstances that exist in nature. Hence, it is important to delineate the fungal pathogen communities and their interactions in man-made monoculture systems and highly diverse natural ecosystems. A better understanding of the key fungal players can be achieved by combining modern techniques such as next-generation sequencing (NGS) together with studies involving classic phytopathology, taxonomy, and phylogeny. It is of utmost importance to develop experimental designs that account for the ecological complexity of the relationships between grasses and fungi, both above and below ground. In grasslands, loss in species diversity increases interactions such as herbivory, mutualism, predation or infectious disease transmission. Host species density and the presence of heterospecific host species, also affect the disease dynamics in grasslands. Many studies have shown that lower species diversity increases the severity as well as the transmission rate of fungal diseases. Moreover, communities that were once highly diverse but have experienced decreased species richness and dominancy have also shown higher pathogenicity load due to the relaxed competition, although this effect is lower in natural communities. This review addresses the taxonomy, phylogeny, and ecology of grassland fungal pathogens and their interactions in grassland ecosystems.

Unravelling the Functional Diversity of the Soil Microbial Community of Chinese Fir Plantations of Different Densities

The structure and function of forest ecosystems are directly or indirectly affected by their stand density. However, what effect the density of Chinese fir plantations has on the functional diversity of the soil microbial community remains unclear. The microbial metabolic functional diversity of soils sampled at the topsoil (0–20 cm) of 35-year-old Chinese fir plantations of five initial densities (D1: 1667 stems∙hm−2, D2: 3333 stems∙hm−2, D3: 5000 stems∙hm−2, D4: 6667 stems∙hm−2, and D5: 10,000 stems∙hm−2) was studied by using Biolog ECO technology. The results showed that the soil pH, oxidizable organic carbon (SOOC), available N (AN), available P (AP), and available K (AK) contents all showed a gradual increase from D1 to D4 and a decrease from D4 to D5, while the number of culturable bacteria and total microorganisms, the average well color development (AWCD) values for the single carbon substrate and six types of carbon sources used by the microbial community, as well as the Shannon-Wiener diversity index (H’), Pielou evenness index (J), and McIntosh Diversity Index (U), were the opposite, suggesting that low-densities favored C and N mineralization and the nutrient cycle. The density of Chinese fir plantations had a significant effect on the use of carbohydrates, amino acids, carboxylic acids, and phenolic acids by the soil microbial community, but it had no significant effect on the use of polymers (p < 0.05). Principal component analysis (PCA) revealed that carbohydrates, polymers, and phenolic acids were sensitive carbon sources that caused differences in the metabolic functions of soil microbial communities in Chinese fir plantations. Redundancy analysis (RDA) showed that physicochemical factors have a significant influence on the metabolic function of soil microbial communities (RDA1 and RDA2 explained >85% variance). The changes in density affected the soil physicochemical properties, the composition, and the metabolic functional diversity of microbial communities in Chinese fir plantations, which is certainly useful for the stand density regulation of Chinese fir plantations.

Density-dependency and plant-soil feedback: former plant abundance influences competitive interactions between two grassland plant species through plant-soil feedbacks

BACKGROUNDS AND AIMS

Negative plant-soil feedbacks (PSFs) are thought to promote species coexistence, but most evidence is derived from theoretical models and data from plant monoculture experiments.

METHODS

We grew Anthoxanthum odoratum and Centaurea jacea in field plots in monocultures and in mixtures with three ratios (3:1, 2:2 and 1:3) for three years. We then tested in a greenhouse experiment the performance of A. odoratum and C. jacea in pots planted with monocultures and 1:1 mixtures and filled with live and sterile soils collected from the field plots.

RESULTS

In the greenhouse experiment, C. jacea produced less aboveground biomass in soil conditioned by C. jacea monocultures than in soil conditioned by A. odoratum monocultures, while the aboveground biomass of A. odoratum in general did not differ between the two monospecific soils. The negative PSF effect was greater in the 1:1 plant mixture than in plant monocultures for A. odoratum but did not differ for C. jacea. In the greenhouse experiment, the performance of C. jacea relative to A. odoratum in the 1:1 plant mixture was negatively correlated to the abundance of C. jacea in the field plot where the soil was collected from. This relationship was significant both in live and sterile soils. However, there was no relationship between the performance of A. odoratum relative to C. jacea in the 1:1 plant mixture in the greenhouse experiment and the abundance of A. odoratum in the field plots.

CONCLUSIONS

The response of a plant to PSF depends on whether the focal species grows in monocultures or in mixtures and on the identity of the species. Interspecific competition can exacerbate the negative plant-soil feedbacks compared to intraspecific competition when a plant competes with a stronger interspecific competitor. Moreover, the abundance of a species in mixed plant communities, via plant-soil feedback, negatively influences the relative competitiveness of that species when it grows later in interspecific competition, but this effect varies between species. This phenomenon may contribute to the coexistence of competing plants under natural conditions through preventing the dominance of a particular plant species.

Ecology of Fungal Plant Pathogens

Fungal plant pathogens are ubiquitous and highly diverse. Key to their success is high host density, which notably is the case in agroecosystems. Several hypotheses related to the effects of plant pathogens on plant diversity (the Janzen-Connell hypothesis, the dilution effect hypothesis) and the phenomenon of higher biomass in plant mixtures (i.e., overyielding) can all be explained by the quantitative interplay between host and pathogen density. In many agroecosystems, fungal plant pathogens cause great losses, since in monocultures diseased plants cannot be replaced by healthy plants. On the other hand, in natural ecosystems fungal plant pathogens shape the succession of vegetation and enhance the biodiversity of forests and grasslands. When pathogens are introduced into areas outside their natural range, they may behave differently, causing severe damage. Once introduced, changes may occur such as hybridization with other closely related pathogens or host shifts, host jumps, or horizontal gene transfer. Such changes can be hazardous for both agricultural and natural ecosystems.