Plant and soil communities are associated with the response of soil water repellency to environmental stress

Changes in soil water repellency and soil erosion resistance as affected by land uses in karst environments

Soil water repellency (SWR) exists in various soil ecosystems and can interrupt water infiltration and cause soil erosion. Anthropogenic land-use change can cause shifts in SWR and soil erosion resistance (SER) in sensitive soils. However, the direction and magnitude of these shifts in SWR and SER and their interrelations remain largely unclear. This study examined the changes in SWR, soil erodibility (K), and aggregate fractions in different land uses of a karst trough valley in southwest China. Soil samples were gathered from five land uses: cropland (CL), orchard (OP), secondary forest (SF), grassland (GL), and riverbank (RB), using a 1000 m × 1000 m grid, resulting in 210 sampling sites. Results showed that the water drop penetration time was significantly higher by 86.43%, 68.85%, and 71.47% in the SF, OP, and RB than in the CL, respectively (p < 0.05). The soil organic carbon (SOC) was the greatest in the SF and the lowest in the GL and CL (p < 0.05). The 1000-250 μm aggregate fraction, mean weight diameter (MWD), and geometric mean diameter (GMD) varied markedly among the different land uses (p < 0.05). The silt content, fractal dimension (D), and K were markedly lower in the RB than in the CL, OP, SF and GL (p < 0.05). The SWR had pronouncedly positive correlations with the SOC and 1000-250 μm aggregate fraction (p < 0.05). The K displayed a negative correlation with the GMD but a positive correlation with the D (p < 0.05). The structural equation modeling analysis revealed that the SOC was affected positively by the silt content and the 1000-250 μm aggregate fraction and negatively by the K. These factors directly regulated the SWR. Our results shed light on the mechanisms of land use changes impacting the SWR, SER and other soil properties in karst environments.

Improved description of terrestrial habitat types by including microbial communities as indicators

Soils host diverse communities of microorganisms essential for ecosystem functions and soil health. Despite their importance, microorganisms are not covered by legislation protecting biodiversity or habitats, such as the Habitats Directive. Advances in molecular methods have caused breakthroughs in microbial community analysis, and recent studies have shown that parts of the communities are habitat-specific. If distinct microbial communities are present in the habitat types defined in the Habitats Directive, the Directive may be improved by including these communities. Thus, monitoring and reporting of biodiversity and conservation status of habitat types could be based not only on plant communities but also on microbial communities. In the present study, bacterial and plant communities were examined in six habitat types defined in the Habitats Directive by conducting botanical surveys and collecting soil samples for amplicon sequencing across 19 sites in Denmark. Furthermore, selected physico-chemical properties expected to differ between habitat types and explain variations in community composition of bacteria and vegetation were analysed (pH, electrical conductivity (EC), soil texture, soil water repellency, soil organic carbon content (OC), inorganic nitrogen, and in-situ water content (SWC)). Despite some variations within the same habitat type and overlaps between habitat types, habitat-specific communities were observed for both bacterial and plant communities, but no correlation was observed between the alpha diversity of vegetation and bacteria. PERMANOVA analysis was used to evaluate the variables best able to explain variation in the community composition of vegetation and bacteria. Habitat type alone could explain 46% and 47% of the variation in bacterial and plant communities, respectively. Excluding habitat type as a variable, the best model (pH, SWC, OC, fine silt, and Shannon's diversity index for vegetation) could explain 37% of the variation for bacteria. For vegetation, the best model (pH, EC, ammonium content and Shannon's diversity index for bacteria) could explain 25% of the variation. Based on these results, bacterial communities could be included in the Habitats Directive to improve the monitoring, as microorganisms are more sensitive to changes in the environment compared to vegetation, which the current monitoring is based on.

Changes in air dose rates due to soil moisture content in the Fukushima prefecture forests

Radionuclides released and deposited because of the 2011 Fukushima Dai-ichi Nuclear Power Plant accident caused an increase in air dose rates in Fukushima Prefecture forests. Although an increase in air dose rates during rainfall was previously reported, the air dose rates in the Fukushima forests decreased during rainfall. This study aimed to develop a method to estimate rainfall-related changes in air dose rates, even in the absence of soil moisture data, in Namie-Town and Kawauchi-Village, Futaba-gun, Fukushima Prefecture. Moreover, we examined the relationship between preceding rainfall (Rw) and soil moisture content. The air dose rate was estimated by calculating the Rw in Namie-Town from May to July 2020. We found that the air dose rates decreased with increasing soil moisture content. The soil moisture content was estimated from Rw by combining short-term and long-term effective rainfall using half-live values of 2 h and 7 d and considering the hysteresis of water absorption and drainage processes. Furthermore, the soil moisture content and air dose rate estimations showed a good agreement with coefficient of determination (R2) scores >0.70 and >0.65, respectively. The same method was tested to estimate the air dose rates in Kawauchi-Village from May to July 2019. At the Kawauchi site, variation in estimated value is relatively large due to the presence of water repellency in dry conditions, and the amount of 137Cs inventory was low, so estimating air dose from rainfall remained a challenge. In conclusion, rainfall data were successfully used to estimate soil moisture and air dose rates in areas with high 137Cs inventories. This leads to the possibility of removing the influence of rainfall on measured air dose rate data and could contribute to the improvement of methods currently used to estimate the external air dose rates for humans, animals, and terrestrial forest plants.

Discrepant Effects of Flooding on Assembly Processes of Abundant and Rare Communities in Riparian Soils

Numerous rare species coexist with a few abundant species in microbial communities and together play an essential role in riparian ecosystems. Relatively little is understood, however, about the nature of assembly processes of these communities and how they respond to a fluctuating environment. In this study, drivers controlling the assembly of abundant and rare subcommunities for bacteria and archaea in a riparian zone were determined, and their resulting patterns on these processes were analyzed. Abundant and rare bacteria and archaea showed a consistent variation in the community structure along the riparian elevation gradient, which was closely associated with flooding frequency. The community assembly of abundant bacteria was not affected by any measured environmental variables, while soil moisture and ratio of submerged time to exposed time were the two most decisive factors determining rare bacterial community. Assembly of abundant archaeal community was also determined by these two factors, whereas rare archaea was significantly associated with soil carbon-nitrogen ratio and total carbon content. The assembly process of abundant and rare bacterial subcommunities was driven respectively by dispersal limitation and variable selection. Undominated processes and dispersal limitation dominated the assembly of abundant archaea, whereas homogeneous selection primarily driven rare archaea. Flooding may therefore play a crucial role in determining the community assembly processes by imposing disturbances and shaping soil niches. Overall, this study reveals the assembly patterns of abundant and rare communities in the riparian zone and provides further insight into the importance of their respective roles in maintaining a stable ecosystem during times of environmental perturbations.

Physicochemical and biological factors determining the patchy distribution of soil water repellency among species of dominant vegetation in loess hilly region of China

Soil water repellency (SWR) is a physical phenomenon whereby water cannot penetrate or has difficulty penetrating the soil surface. There are many factors involved in its occurrence, but the main factors controlling its emergence in loess remain unclear. In this work, we have studied numerous physicochemical and biological factors functioning in different dominant vegetations (Pinus tabulaeformis Carr., Robinia pseudoacacia L., and Hippophae rhamnoides L.) in a loess hilly region by gas chromatography-mass spectrometry (GC-MS) and high-throughput sequencing techniques. We observed that more than 75% of the soils under Robinia and Hippophae are categorized as slightly or strongly water repellent, while nearly 50% of the soils under Pinus are categorized as severely to extremely water repellent. The relative concentrations of total free lipids in the soil in the same water-repellency class were Pinus > Robinia > Hippophae, where fatty acids, alkanols, and sterols were positively correlated with SWR, whereas alkanes were not. For the abundance and diversity index of bacterial and fungal communities, the three species ranked in the following order: Robinia ≈ Hippophae > Pinus. Thus, solvent-extractable polar waxes were indicated to be better preserved in water-repellent soils under Pinus due to lower microbial diversity than Robinia and Hippophae. Here, we demonstrate polar waxes to be the principal factor controlling SWR. Moreover, the dominant phyla of fungi varied greatly than those of bacteria under three vegetation types. Correlation analysis showed that the abundance of Actinobacteria in dominant bacteria increased with SWR. Nonmetric multidimensional scaling suggested the fungal community in different water-repellent soils under Pinus to vary more than those under Robinia and Hippophae. The indicator species mainly belonged to Actinobacteria in bacteria and Basidiomycota in fungi at the phylum level; this finding was further supported by the linear discriminant analysis (LDA) effect size (LEfSe). Additionally, GC-MS identified a small amount of ergosterol, a specific biomarker of fungi under Pinus. These pieces of evidence collectively reveal that severe to extreme SWR occurs under Pinus and appears to be the most influenced by fungi and actinomycetes when the topsoil is close to air drying. However, there is a need for further testing on different plant species or land use.

Harnessing microbial multitrophic interactions for rhizosphere microbiome engineering

The rhizosphere is a narrow and dynamic region of plant root-soil interfaces, and it's considered one of the most intricate and functionally active ecosystems on the Earth, which boosts plant health and alleviates the impact of biotic and abiotic stresses. Improving the key functions of the microbiome via engineering the rhizosphere microbiome is an emerging tool for improving plant growth, resilience, and soil-borne diseases. Recently, the advent of omics tools, gene-editing techniques, and sequencing technology has allowed us to unravel the entangled webs of plant-microbes interactions, enhancing plant fitness and tolerance to biotic and abiotic challenges. Plants secrete signaling compounds with low molecular weight into the rhizosphere, that engage various species to generate a massive deep complex array. The underlying principle governing the multitrophic interactions of the rhizosphere microbiome is yet unknown, however, some efforts have been made for disease management and agricultural sustainability. This review discussed the intra- and inter- microbe-microbe and microbe-animal interactions and their multifunctional roles in rhizosphere microbiome engineering for plant health and soil-borne disease management. Simultaneously, it investigates the significant impact of immunity utilizing PGPR and cover crop strategy in increasing rhizosphere microbiome functions for plant development and protection using omics techniques. The ecological engineering of rhizosphere plant interactions could be used as a potential alternative technology for plant growth improvement, sustainable disease control management, and increased production of economically significant crops.

Fire as a driver of fungal diversity - A synthesis of current knowledge

Fires occur in most terrestrial ecosystems where they drive changes in the traits, composition, and diversity of fungal communities. Fires range from rare, stand-replacing wildfires to frequent, prescribed fires used to mimic natural fire regimes. Fire regime factors, including burn severity, fire intensity, and timing, vary widely and likely determine how fungi respond to fires. Despite the importance of fungi to post-fire plant communities and ecosystem functioning, attempts to identify common fungal responses and their major drivers are lacking. This synthesis addresses this knowledge gap and ranges from fire adaptations of specific fungi to succession and assembly fungal communities as they respond to spatially heterogenous burning within the landscape. Fires impact fungi directly and indirectly through their effects on fungal survival, substrate and habitat modifications, changes in environmental conditions, and/or physiological responses of the hosts with which fungi interact. Some specific pyrophilous, or "fire-loving," fungi often appear after fire. Our synthesis explores whether such taxa can be considered cosmopolitan, and whether they are truly fire-adapted or simply opportunists adapted to rapidly occupy substrates and habitats made available by fires. We also discuss the possible inoculum sources of post-fire fungi and explore existing conceptual models and ecological frameworks that may be useful in generalizing fungal fire responses. We conclude with identifying research gaps and areas that may best transform the current knowledge and understanding of fungal responses to fire.

The role of plant species on runoff and soil erosion in a Mediterranean shrubland

Shrubland is a Mediterranean biome characterized by densely growing evergreen shrubs adapted to fire events. To date, scientific research has focused on the impact of vegetation on soil erosion mainly through the control that plant biomass or plant cover exerts on sediment delivery and runoff discharge, being the individual plant species influence on hydrological and erosional processes not achieved in detail. The objective of this research is to determine: i) runoff and soil losses in a shrubland-covered rangeland at Sierra de Enguera, Spain; and ii) how four plant species affect soil and water losses. We measured soil cover, soil properties, runoff discharge and sediment yield under natural rainfall for five years (2010-2014) in a typical shrubland burnt in 1999. Four plant species were selected with 4 plots each: Ulex parviflorus Pourr., Pistacia lentiscus L., Quercus coccifera L. and Rosmarinus officinalis L. Despite that the soil properties and plant cover did not exhibit statistically significant differences among plant species, the runoff discharge was lower on Q. coccifera (4.87%, SE 0.24) and P. lentiscus (6.24%, SE 0.51) than on U. parviflorus (13.41%, SE 0.58) and R. officinalis (13.84%, SE 1.23). Sediment concentrations were, respectively, 3.91, 4.33, 4.31 and 4.88 g l^-1, and the differences between R. officinalis and the other species were statistically significant. The runoff discharge determined differences in soil erosion rates among the plant species with lower rates on P. lentiscus (1.36 Mg ha^-1 y^-1) and Q. coccifera (1.53 Mg ha^-1 y^-1), than on U. parviflorus (3.17 Mg ha^-1 y^-1) and R. officinalis (3.85 Mg ha^-1 y^-1). This long term in situ study indicated that Q. coccifera and P. lentiscus are more efficient in controlling runoff discharge and soil losses than U. parviflorus and R. officinalis one decade after a fire. We discuss these results in light of the recent findings by the scientific community of the role of the canopy cover (rainfall interception), soil macropore and root system, and the water repellency that control the hydrological response of the soil (e.g. runoff generation, infiltration). The information supplied by 5 years of research is relevant for restoration and rehabilitation programs and advise that Q. coccifera and P. lentiscus are the most efficient plant species to control soil and water losses within the Mediterranean shrubland. This is an applied science approach for a better management of rangelands.

Soil water repellency and the five spheres of influence: A review of mechanisms, measurement and ecological implications

Soil water repellency (SWR) is a widespread phenomenon that influences patterns of soil wetting, runoff, evapotranspiration and availability of water for plants. In natural ecosystems there is emerging evidence that some plants can take advantage of non-uniform wetting patterns, leading to the emergence of co-evolutionary behaviour. In this review, SWR is considered in terms of five spheres of influence. Given the presence of hydrophobic organic material in the biosphere, the strength, severity and persistence of SWR is influenced by properties at the surface of the lithosphere and prevailing conditions in the atmosphere and hydrosphere. These in turn, can be modified by activities in the anthroposphere. This review thus examines the strength, severity and persistence of non-wetting behaviour with reference to these five spheres of influence and also the interactions between the spheres. It is focused on (i) how SWR is characterised to provide insight into how different measurement techniques have specific operational ranges, (ii) how SWR has developed as an indirect consequence of evolution in natural ecosystems and (iii) how feedbacks across the different spheres have emerged. It demonstrates that management and restoration of natural ecosystems with water repellent soils is very different from management of productive crops in monocultural agricultural systems, controlled in the anthroposphere.

Artificial Plantation Responses to Periodic Submergence in Massive Dam and Reservoir Riparian Zones: Changes in Soil Properties and Bacterial Community Characteristics

Simple Summary

This study focuses on plants in riparian zones that are very vulnerable due to water stress and anthropogenic disturbances, which are particularly important regarding their ecological and environmental role. Although plants and microbiome interactions are necessary for plant nutrient acquisition, relatively little is known about the responses of roots, bulk, and rhizosphere soil microbial communities of different artificial vegetation types in riparian areas of massive dams and reservoirs. Therefore, this study aims to assess the responses of woody and herbaceous plants in the riparian zones of the Three Gorges Dam Reservoir, China. Results revealed that the weight of dominant soil bacteria in different periods, including Proteobacteria, Acidobacteria, Actinobacteria, Chloroflexi, and Cyanobacteria, was higher, and their composition was different in the rhizosphere, bulk soil, and endophyte. In the soil co-occurrence networks, the weight of soil physical properties was higher than chemical properties in the early emergence stage. The current study provides knowledge about bacteria in bulk, rhizosphere soils, and within roots in different emergence phases. Additionally, these results provide valuable information to inoculate the soil with key microbiota members by applying fertilizers, potentially improving plant and soil production and health.

Abstract

Plant and microbiome interactions are necessary for plant nutrient acquisition. However, relatively little is known about the responses of roots, bulk, and rhizosphere soil microbial communities in different artificial vegetation types (woody and herbaceous) in riparian areas of massive dams and reservoirs. Therefore, this study aims to assess such responses at elevations of 165–170 m a.s.l. in the riparian zones of the Three Gorges Dam Reservoir, China. The samples were collected containing the rhizosphere soil, bulk soil, and roots of herbaceous and woody vegetation at different emergence stages in 2018. Then, all the samples were analyzed to quantify the soil properties, bacterial community characteristics, and their interaction in the early and late emergence phases. In different periods, the weight of dominant soil bacteria, including Proteobacteria, Acidobacteria, Actinobacteria, Chloroflexi, and Cyanobacteria, was higher, and their composition was different in the rhizosphere, bulk soil, and endophytes. Moreover, the soil co-occurrence networks indicated that the weight of soil physical properties was higher than chemical properties in the early emergence stage. In contrast, the weight of chemical properties was relatively higher in the late emergence stage. Furthermore, the richness and diversity of the bacterial community were mainly affected by soil organic matter. This study suggests that these herbaceous and woody vegetation are suitable for planting in reservoir areas affected by hydrology and human disturbance in light of soil nutrients and soil microbial communities, respectively. Additionally, these results provide valuable information to inoculate the soil with key microbiota members by applying fertilizers, potentially improving plant health and soil production.

The Impacts of Vineyard Afforestation on Soil Properties, Water Repellency and Near-Saturated Infiltration in the Little Carpathians Mountains

Vineyards are a 7000-year-old land-use tradition and both management and abandonment have result in altered soil properties. These have a great effect on water resources and soil services, and this inspired our investigation into the effects of land-use and land-use change on soils in the Modra wine-growing region in South-western Slovakia. Ten topsoil samples were taken at each of the seven research sites (n = 70) on granite parent material in cultivated and afforested vineyards and original forest soils. Laboratory analyses included determination of soil texture, organic carbon content, soil pH, and water repellency. This was supplemented by infiltration measurements under near-saturated conditions at the vineyard and afforested study sites. Studied soils have a low clay content and a high proportion of sand. The vineyard soils have significantly higher pH than the forest and afforested soils because the naturally acidic soils have been limed. The forest and afforested soils have similar properties, with higher organic carbon content. This makes them strongly to extremely water repellent and contrasts sharply with the wettability of cultivated vineyard soils. One afforested site, however, was less acidic and therefore was considered transitional between forest and vineyard soils. Our infiltration measurements established the influence of soil water repellency on the infiltration process, and our results highlighted that the infiltration rate in the vineyard soils was significantly higher than in afforested soils. The infiltration rate also gradually increased over time in afforested soils due to decreasing water repellency. Physically impossible negative sorptivity values in afforested soils were noted because of changes in water repellency during the infiltration process. Finally, we conclude that soil afforestation results in increased soil water repellency and a subsequent reduction in the infiltration rate at the matrix scale.