Non-native plants and soil microbes: potential contributors to the consistent reduction in soil aggregate stability caused by the disturbance of North American grasslands

Effects of riparian pioneer plants on soil aggregate stability: Roles of root traits and rhizosphere microorganisms

Pioneer plants are vital in stabilizing soil structure while restoring reservoir drawdown areas. However, uncertainties persist regarding the mechanism of pioneer plants to soil stability in these delicate ecosystems. This study aims to unravel the plant-soil feedback mechanisms from the roles of root traits and rhizosphere microorganisms. We conducted a mesocosm experiment focusing on four common pioneer plants from the drawdown area of Three Gorges Reservoir, China. Using the wet sieving methodology, trait-based approach and high-throughput sequencing technology, we explored soil aggregate stability parameters, plant root traits and rhizosphere microbial communities in experimental plant groups. The interacting effect of pioneer plant species richness, root traits, and rhizosphere microbial communities on soil aggregate stability was quantified by statistical and machine-learning models. Our results demonstrate that diverse pioneer plant communities significantly enhance soil aggregate stability. Notably, specific species, such as Cynodon dactylon (L.) Pers. and Xanthium strumarium L., exert a remarkably strong influence on soil stability due to their distinctive root traits. Root length density (RLD) and root specific surface area (RSA) were identified as crucial root traits mediating the impact of plant diversity on soil aggregate stability. Additionally, our study highlights the link between increased rhizosphere fungal richness, accompanied by plant species richness, and enhanced soil aggregate stability, likely attributable to elevated RLD and RSA. These insights deepen our understanding of the role of pioneer vegetation in soil structure and stability, providing valuable implications for ecological restoration and management practices in reservoir drawdown areas.

Soil characteristics and bare ground cover differ among jurisdictions and disturbance histories in Western US protected area-centered ecosystems

Introduction

Ecological conditions at a given site are driven by factors including resource availability, habitat connectivity, and disturbance history. Land managers can influence disturbance history at a site by harvesting resources, creating transportation pathways, introducing new species, and altering the frequency and severity of events such as fires and floods. As a result, locations with different land management histories have also likely experienced different disturbance trajectories that, over time, are likely to result in different ecological characteristics.

Methods

To understand how the presence of different management histories may shape ecological conditions across large landscapes, we examined plant and soil characteristics at matched sampling points across jurisdictional boundaries within four Protected Area-Centered Ecosystems (PACEs) in the western US. We employed Bayesian modeling to explore 1) the extent to which specific ecological variables are linked to disturbance and jurisdiction both among and within individual PACEs, and 2) whether disturbance evidence differs among jurisdictions within each PACE.

Results

Across all jurisdictions we found that disturbances were associated with ecologically meaningful shifts in percent cover of bare ground, forbs, grass, shrubs, and trees, as well as in tree species richness, soil stability, and total carbon. However, the magnitude of shifts varied by PACE. Within PACEs, there were also meaningful associations between some ecological variables and jurisdiction type; the most consistent of these were in soil stability and soil carbon:nitrogen ratios. Disturbance evidence within each PACE was relatively similar across jurisdictions, with strong differences detected between contrast jurisdictions only for the Lassen Volcanic National Park PACE (LAVO).

Discussion

These findings suggest an interaction between management history and geography, such that ecotones appear to manifest at jurisdictional boundaries within some, but not all, contexts of disturbance and location. Additionally, we detected numerous differences between PACEs in the size of disturbance effects on ecological variables, suggesting that while the interplay between disturbance and management explored here appears influential, there remains a large amount of unexplained variance in these landscapes. As continued global change elevates the importance of large landscape habitat connectivity, unaligned management activities among neighboring jurisdictions are likely to influence existing ecological conditions and connectivity, conservation planning, and desired outcomes.

The Shift of Soil Bacterial Community After Afforestation Influence Soil Organic Carbon and Aggregate Stability in Karst Region

Soil microbes regulate the carbon cycle and affect the formation and stabilization of soil aggregates. However, the interactions between the soil microbial community and soil organic carbon (SOC) fractions, organic carbon (OC) content in aggregates, and soil aggregate stability after afforestation are remain poorly understood. In our study, we investigated SOC fractions in bulk soil, aggregate-associated OC content, soil aggregate stability, and soil bacterial community with high-throughput 16S rRNA sequencing at sites representing natural secondary forest (NF) and managed forest (MF), with cropland (CL) as reference in a degraded karst region of Southwest China. Our results showed that afforestation remarkably increased the SOC fraction and OC content in aggregates, the mean weight diameter (MWD), and the mean geometric diameter (GMD). The most dominant bacterial phyla detected were Acidobacteriota, Actinobacteriota, Proteobacteria, and Chloroflexi across all soils. Afforestation remarkably altered the relative abundances of most of the dominant soil bacteria at the phylum, class, and order levels. Interestingly, such changes in the abundance of soil bacteria taxa had significantly effects on SOC fraction, aggregate-associated OC content, MWD, and MGD. The abundance of dominant bacterial taxa such as Methylomirabilota, Latescibacterota, Methylomirabilia, MB-A2-108, norank_Latescibacterota; Dehalococcoidia, Rokubacteriales, Gaiellales, Microtrichales, norank_c__MB-A2-108, norank_c__norank_p__Latescibacterota, Rhizobiales, and S085 not only remarkably increased but also had significant positive effects on SOC fractions and aggregate-associated OC content after afforestation. Moreover, MWD and MGD were positively correlated with the relative abundance of Methylomirabilota, Methylomirabilia, Rokubacteriales, Latescibacterota, and Rhizobiales. Results indicated the importance of certain soil bacteria for regulating SOC storage and soil aggregate stability. We concluded that afforestation on cropland could alter the abundance of soil bacteria, and these changes modulate the stability of soil aggregates and SOC fractions.

Factors controlling the spatial variability of soil aggregates and associated organic carbon across a semi-humid watershed

Soil aggregates (SA) play crucial roles in soil organic carbon (SOC) sequestration. Different SA fractions contribute differently to the sequestration of SOC. However, few studies have examined the factors controlling SA fractions and associated SOC contents across a watershed. Soil samples were collected at 0-10 cm (surface layer) and 10-20 cm (subsurface layer) from 88 sites across a semi-humid watershed (1.1 km²) on the Loess Plateau, China. These samples were separated into macroaggregates (MA), microaggregates (MI), and silt + clay fractions (SC) by wet-sieving, and SOC content of each fraction was determined. The objectives were to: 1) investigate the spatial variability of SA fractions and associated SOC contents as well as their main controls across an entire watershed, and 2) explore the linkages between soil aggregation and SOC sequestration. The bulk and aggregate SOC contents of all SA fractions showed moderate variability, with coefficient of variations of 23.3-31.9%. Geostatistical analysis indicated that the spatial patterns of SA fractions and SOC content varied with aggregate size. From combined Spearman's correlation analysis and structural equation modelling, we found that soil texture was an important control on the spatial variability of all SA fractions and associated SOC contents. Vegetation dynamics and management practices associated with land use were also important controls on MA and MI and their associated SOC contents, especially in the surface layer. However, SC and its associated SOC content were more sensitive to eco-hydrological processes related to topography. Among the land uses, grassland had the greatest SOC sequestration potential. The fine roots of herbs can wrap MI in MA and increase SOC content within MA, which is the primary mechanism responsible for SOC sequestration in grasslands. These results indicate that using vegetation with fine root systems for restoration is a good strategy to increase SOC sequestration in this region.

Symbionts as Filters of Plant Colonization of Islands: Tests of Expected Patterns and Environmental Consequences in the Galapagos

The establishments of new organisms that arrive naturally or with anthropogenic assistance depend primarily on local conditions, including biotic interactions. We hypothesized that plants that rely on fungal symbionts are less likely to successfully colonize remote environments such as oceanic islands, and this can shape subsequent island ecology. We analyzed the mycorrhizal status of Santa Cruz Island, Galapagos flora compared with the mainland Ecuador flora of origin. We experimentally determined plant responsiveness and plant-soil feedback of the island flora and assessed mycorrhizal density and soil aggregate stability of island sites. We found that a greater proportion of the native island flora species belongs to families that typically do not associate with mycorrhizal fungi than expected based upon the mainland flora of origin and the naturalized flora of the island. Native plants benefited significantly less from soil fungi and had weaker negative soil feedbacks than introduced species. This is consistent with the observation that field sites dominated by native plant species had lower arbuscular mycorrhizal (AM) fungal density and lower soil aggregate stability than invaded field sites at the island. We found support for a mycorrhizal filter to the initial colonization of the Galapagos.

The enhancing effect of afforestation over secondary succession on soil quality under semiarid climate conditions

Semiarid climate conditions hamper natural re-vegetation, leaving the soil vulnerable to erosion after the cessation of agriculture. Therefore, soil and landscape protective measures, especially afforestations, have been implemented in the Mediterranean region since the early 20th century. This study aims to determine the long term impact of afforestation on soil functioning, in comparison with natural re-vegetation (secondary succession) on abandoned fields and semi-natural vegetation. A comparison of secondary succession and afforestation with the present traditional rain fed cereal fields and semi-natural (open) forest, including natural resource islands, was made as well. Composite soil samples were taken to study the physical (i.e. texture, aggregate stability) and chemical (i.e. carbon content, nutrient availability) soil characteristics after 20 and 40 years of afforestation and secondary natural succession. To take into account the resource island effect, the spatial heterogeneity induced by differences in plant cover, samples were taken both below and in between the tree canopy of the semi-natural and afforested Pinus halepensis trees. Our results indicate that under secondary succession on abandoned fields, soil quality improves non-linearly and only marginally over a time of 40 years. The afforestation showed a much more pronounced linear increase for most soil quality indicators, resulting in soil conditions comparable to what can be found under the semi-natural forest vegetation. Site preparation might have been a crucial factor for the success of ecosystem restoration in the studied dry land area as it improved water availability for the afforestation.

Disturbance reduces the differentiation of mycorrhizal fungal communities in grasslands along a precipitation gradient

Given that mycorrhizal fungi play key roles in shaping plant communities, greater attention should be focused on factors that determine the composition of mycorrhizal fungal communities and their sensitivity to anthropogenic disturbance. We investigate changes in arbuscular mycorrhizal (AM) fungal community composition across a precipitation gradient in North American grasslands as well as changes occurring with varying degrees of site disturbance that have resulted in invasive plant establishment. We find strong differentiation of AM fungal communities in undisturbed remnant grasslands across the precipitation gradient, whereas communities in disturbed grasslands were more homogeneous. These changes in community differentiation with disturbance are consistent with more stringent environmental filtering of AM fungal communities in undisturbed sites that may also be promoted by more rigid functional constraints imposed on AM fungi by the native plant communities in these areas. The AM fungal communities in eastern grasslands were particularly sensitive to anthropogenic disturbance, with disturbed sites having low numbers of AM fungal operational taxonomic units (OTUs) commonly found in undisturbed sites, and also the proliferation of AM fungal OTUs in disturbed sites. This proliferation of AM fungi in eastern disturbed sites coincided with increased soil phosphorus availability and is consistent with evidence suggesting the fungi represented by these OTUs would provide reduced benefits to native plants. The differentiation of AM fungal communities along the precipitation gradient in undisturbed grasslands but not in disturbed sites is consistent with AM fungi aiding plant adaptation to climate, and suggests they may be especially important targets for conservation and restoration in order to help maintain or re-establish diverse grassland plant communities.

Complementarity among plant growth promoting traits in rhizospheric bacterial communities promotes plant growth

An assessment of roles of rhizospheric microbial diversity in plant growth is helpful in understanding plant-microbe interactions. Using random combinations of rhizospheric bacterial species at different richness levels, we analysed the contribution of species richness, compositions, interactions and identity on soil microbial respiration and plant biomass. We showed that bacterial inoculation in plant rhizosphere enhanced microbial respiration and plant biomass with complementary relationships among bacterial species. Plant growth was found to increase linearly with inoculation of rhizospheric bacterial communities with increasing levels of species or plant growth promoting trait diversity. However, inoculation of diverse bacterial communities having single plant growth promoting trait, i.e., nitrogen fixation could not enhance plant growth over inoculation of single bacteria. Our results indicate that bacterial diversity in rhizosphere affect ecosystem functioning through complementary relationship among plant growth promoting traits and may play significant roles in delivering microbial services to plants.