The effect of AMF combined with biochar on plant growth and soil quality under saline-alkali stress: Insights from microbial community analysis

Arbuscular mycorrhizal fungi (AMF) and biochar application individually can enhance plant tolerance to saline-alkali stress and promote plant growth efficiency. However, little is known about the potential synergistic effects of their combination on improving plant growth and soil quality under saline-alkali stress. This experiment adopted the potted method to explore the effects of four treatments on switchgrass growth and soil quality: biochar (BC), Rhizophagus irregularis (Ri), biochar + Ri (BR) and a control without biochar or Ri (CK). Compared to the CK treatment, the switchgrass biomass increased by 92.4 %, 148.6 %, and 177.3 % in the BC, Ri, and BR treatment groups, respectively. Similarly, the rhizosphere soil quality index increased by 29.33 %, 22.7 %, and 49.1 % in the respective treatment groups. The BR treatment significantly altered the rhizosphere soil microbial composition and diversity. Notably, compared to the other treatments, the archaeal α-diversity in the BR group showed a significant decrease. BR treatment significantly increased the relative abundance of bacteria, fungi and archaea at the genus level (e.g., Bacillus, Trichome and candidatus_methanopenens). Network analysis showed that the complexity and closeness of interactions between different microbial taxa were stronger in the BC, Ri and BR treatments than in the CK treatment, with BR being the more prominent. In summary, biochar combined with Ri has a better effect on promoting the growth of switchgrass under saline-alkali stress, improving the quality of saline-alkali soil, and increasing soil microbial diversity. This study provides a new approach for the efficient development and utilization of saline-alkali land.

Grazing weakens the linkages between plants and soil biotic communities in the alpine grassland

Livestock grazing alters the diversity and composition of plants and soil biota in grassland ecosystems. However, whether and how grazing affects plant-soil biota interactions are limited. Here, we performed a field investigation on the Tibetan grasslands to determine the relationships between plant community properties (biomass, diversity and richness) and soil biota (abundance, diversity and composition of bacteria, fungi and nematodes) in the long-term yak grazing and ungrazed plots, and responses of plant-soil biota linkages to grazing in alpine meadows and alpine swampy meadows were compared. The results found that grazing did not cause significant changes in plant community properties but increased the soil water content. Further, grazing weakened plant-soil microbes/nematode relationships in alpine meadows. The bacterial and fungal abundances were correlated with plant belowground biomass and Simpson index in the ungrazed plots of alpine meadows, while the correlation was not significant under grazing. Bacterial composition was correlated with plant richness only in the ungrazed meadows. Plant-soil nematode linkages were more sensitive to grazing than plant-microbes linkages. Grazing decoupled the relationships between the abundances of nematode trophic groups and plant aboveground biomass, richness and Simpson index in alpine meadows, while the decoupling phenomenon is less evident in alpine swampy meadows. The SEM results indicate that grazing altered the plant above- and belowground biomass to affect the soil nematode community, while influenced soil microbes only through alterations of plant belowground biomass. The findings highlight the importance of grazing in influencing the interactions between aboveground plant communities and soil biological communities in Tibetan grasslands.

Long-term livestock exclusion increases plant richness and reproductive capacity in arid woodlands

Herbivore exclusion is implemented globally to recover ecosystems from grazing by introduced and native herbivores, but evidence for large-scale biodiversity benefits is inconsistent in arid ecosystems. We examined the effects of livestock exclusion on dryland plant richness and reproductive capacity. We collected data on plant species richness and seeding (reproductive capacity), rainfall, vegetation productivity and cover, soil strength and herbivore grazing intensity from 68 sites across 6500 km2 of arid Georgina gidgee (Acacia georginae) woodlands in central Australia between 2018 and 2020. Sites were on an actively grazed cattle station and two destocked conservation reserves. We used structural equation modeling to examine indirect (via soil or vegetation modification) versus direct (herbivory) effects of grazing intensity by two introduced herbivores (cattle, camels) and a native herbivore (red kangaroo), on seasonal plant species richness and seeding of all plants, and the richness and seeding of four plant groups (native grasses, forbs, annual chenopod shrubs, and palatable perennial shrubs). Non-native herbivores had a strong indirect effect on plant richness and seeding by reducing vegetative ground cover, resulting in decreased richness and seeding of native grasses and forbs. Herbivores also had small but negative direct impacts on plant richness and seeding. This direct effect was explained by reductions in annual chenopod and palatable perennial shrub richness under grazing activity. Responses to grazing were herbivore-dependent; introduced herbivore grazing reduced native plant richness and seeding, while native herbivore grazing had no significant effect on richness or seeding of different plant functional groups. Soil strength decreased under grazing by cattle but not camels or kangaroos. Cattle had direct effects on palatable perennial shrub richness and seeding, whereas camels had indirect effects, reducing richness and seeding by reducing the abundance of shrubs. We show that considering indirect pathways improves evaluations of the effects of disturbances on biodiversity, as focusing only on direct effects can mask critical mechanisms of change. Our results indicate substantial biodiversity benefits from excluding livestock and controlling camels in drylands. Reducing introduced herbivore impacts will improve soil and vegetation condition, ensure reproduction and seasonal persistence of species, and protect native plant diversity.

Contrasting influences of two dominant plants, Dasiphora fruticosa and Ligularia virguarea, on aboveground and belowground communities in an alpine meadow

Soil organisms are abundant, phylogenetically and functionally diverse, and interact to catalyse and regulate critical soil processes. Understanding what structures belowground communities is therefore fundamental to gaining insight into ecosystem functioning. Dominant plants have been shown to influence belowground communities both directly and indirectly through changes in abiotic and biotic factors. In a field study, we used piecewise structural equation modelling to disentangle and compare the effects of a dominant allelopathic plant, Ligularia virgaurea, and a dominant facilitative plant, Dasiphora fruticosa, on understory plant, soil microbial and nematode community composition in an alpine meadow on the Tibetan plateau. Dasiphora fruticosa was associated with changes in edaphic variables (total nitrogen, soil organic carbon, pH and ammonium), understory plant and soil bacterial communities, whereas Ligularia virguarea was associated with increased soil ammonium content and soil fungal richness relative to dominant plant-free control plots. Moreover, nematode richness was significantly greater under D. fruticosa, with no change in nematode community composition. By contrast, nematode richness under Ligularia virgaurea was similar to that of dominant plant-free control plots, but nematode community composition differed from the control. The effects of both plants were predominantly direct rather than mediated by indirect pathways despite the observed effects on understory plant communities, soil properties and microbial assemblages. Our results highlight the importance of plants in determining soil communities and provide new insight to disentangle the complex above- and belowground linkages.

Dominant plant species play an important role in regulating bacterial antagonism in terrestrial Antarctica

In Antarctic terrestrial ecosystems, dominant plant species (grasses and mosses) and soil physicochemical properties have a significant influence on soil microbial communities. However, the effects of dominant plants on bacterial antagonistic interactions in Antarctica remain unclear. We hypothesized that dominant plant species can affect bacterial antagonistic interactions directly and indirectly by inducing alterations in soil physicochemical properties and bacterial abundance. We collected soil samples from two typical dominant plant species; the Antarctic grass Deschampsia antarctica and the Antarctic moss Sanionia uncinata, as well as bulk soil sample, devoid of vegetation. We evaluated bacterial antagonistic interactions, focusing on species from the genera Actinomyces, Bacillus, and Pseudomonas. We also measured soil physicochemical properties and evaluated bacterial abundance and diversity using high-throughput sequencing. Our results suggested that Antarctic dominant plants significantly influenced bacterial antagonistic interactions compared to bulk soils. Using structural equation modelling (SEM), we compared and analyzed the direct effect of grasses and mosses on bacterial antagonistic interactions and the indirect effects through changes in edaphic properties and bacterial abundance. SEMs showed that (1) grasses and mosses had a significant direct influence on bacterial antagonistic interactions; (2) grasses had a strong influence on soil water content, pH, and abundances of Actinomyces and Pseudomonas and (3) mosses influenced bacterial antagonistic interactions by impacting abundances of Actinomyces, Bacillus, and Pseudomonas. This study highlights the role of dominant plants in modulating bacterial antagonistic interactions in Antarctic terrestrial ecosystems.

Microbial interactions play an important role in regulating the effects of plant species on soil bacterial diversity

Plant species and microbial interactions have significant impacts on the diversity of bacterial communities. However, few studies have explored interactions among these factors, such the role of microbial interactions in regulating the effects of plant species on soil bacterial diversity. We assumed that plant species not only affect bacterial community diversity directly, but also influence bacterial community diversity indirectly through changing microbial interactions. Specifically, we collected soil samples associated with three different plant species, one evergreen shrub (Rhododendron simsii) and the other two deciduous shrubs (Dasiphora fruticosa and Salix oritrepha). Soil bacterial community composition and diversity were examined by high-throughput sequencing. Moreover, soil bacterial antagonistic interactions and soil edaphic characteristics were evaluated. We used structural equation modeling (SEM) to disentangle and compare the direct effect of different plant species on soil bacterial community diversity, and their indirect effects through influence on soil edaphic characteristics and microbial antagonistic interactions. The results showed that (1) Plant species effects on soil bacterial diversity were significant; (2) Plant species effects on soil microbial antagonistic interactions were significant; and (3) there was not only a significant direct plant species effect on bacterial diversity, but also a significant indirect effect on bacterial diversity through influence on microbial antagonistic interactions. Our study reveals the difference among plant species in their effects on soil microbial antagonistic interactions and highlights the vital role of microbial interactions on shaping soil microbial community diversity.

Fallow Land Enhances Carbon Sequestration in Glomalin and Soil Aggregates Through Regulating Diversity and Network Complexity of Arbuscular Mycorrhizal Fungi Under Climate Change in Relatively High-Latitude Regions

Soil aggregation and aggregate-associated carbon (C) play an essential function in soil health and C sequestration. Arbuscular mycorrhizal fungi (AMF) are considered to be primary soil aggregators due to the combined effect of extraradical hyphae and glomalin-related soil proteins (GRSPs). However, the effects of diversity and network complexity of AMF community on stability of soil aggregates and their associated C under long-term climate change (CC) and land-use conversion (LUC) in relatively high-latitude regions are largely unexplored. Therefore, an 8-year soil plot (with a 30-year cropping history) transplantation experiment was conducted to simulate CC and LUC from cropland to fallow land. The results showed that Glomus, Paraglomus, and Archaeospora were the most abundant genera. The diversity of AMF community in fallow land was higher than cropland and increased with increasing of mean annual temperature (MAT) and mean annual precipitation (MAP). Fallow land enhanced the network complexity of AMF community. The abundance families (Glomeraceae and Paraglomeraceae) exhibited higher values of topological features and were more often located in central ecological positions. Long-term fallow land had a significantly higher hyphal length density, GRSP, mean weight diameter (MWD), geometric mean diameter (GMD), and C concentration of GRSP (C-GRSP) than the cropland. The soil aggregate associated soil organic carbon (SOC) was 16.8, 18.6, and 13.8% higher under fallow land compared to that under cropland at HLJ, JL, and LN study sites, respectively. The structural equation model and random forest regression revealed that AMF diversity, network complexity, and their secreted GRSP mediate the effects of CC and LUC on C-GRSP and aggregate-associated SOC. This study elucidates the climate sensitivity of C within GRSP and soil aggregates which response symmetry to LUC and highlights the potential importance of AMF in C sequestration and climate change mitigation.

Oil degradation and variation of microbial communities in contaminated soils induced by different bacterivorous nematodes species

Oil pollution poses a great threat to environments and makes the remediation of oil-contaminated soils an urgent task. Microorganisms are the main biological factor for oil removal in the environment but microbial remediation is greatly affected by environmental factors. For our research, we inoculated three species of bacterivorous nematodes into oil-contaminated soil to explore how bacterivorous nematodes affect soil microbial activities and community structure in contaminated soil, as well as how efficiently different nematodes remove oil pollution from the soil. Six treatments were set in this experiment: sterilized oil-contaminated soil (SOC); nematode-free soil (S); oil-contaminated soil (OC); oil-contaminated soil + Caenorhabditis elegans (OCN1); oil-contaminated soil + Cephalobus persegnis (OCN2); oil-contaminated soil + Rhabditis marina (OCN3) for a 168-day incubation experiment. After the experiment was done, the oil contents in SOC, OC, OCN1, OCN2, and OCN3 were reduced by 6.5%, 32.3%, 38.2%, 42.8%, and 40.2%, respectively, compared with the beginning of the experiment. The amount of phospholipid fatty acids (PLFAs) of Gram-negative bacteria in OC, OCN1, OCN2, and OCN3 was increased by 50.9%, 43.4%, 37.7%, and 47.9%, respectively, compared with that of S. During the 168-day incubation period, the maximum growth of the number of nematodes in OCN1, OCN2, and OCN3 compared with the initial number of the nematodes were 2.25-, 1.52-, and 1.65-fold, respectively. The amount of oil residue in the contaminated soil negatively correlated with the populations of nematodes, total microorganisms, Gram-negative bacteria, actinomycetes, and eukaryotes. Thus, oil pollution increased the number of Gram-negative bacteria, decreased the ratio of Gram-positive bacteria/Gram-negative bacteria and Fungi/Bacteria significantly, and altered the community structure of soil microorganisms. Each species of bacterivorous nematodes has got its unique effect on the microbial activity and community structure in oil contaminated soils, but those tested can promote oil degradation and thus improve the environment of oil contaminated soils.

Site conditions interact with litter quality to affect home-field advantage and rhizosphere effect of litter decomposition in a subtropical wetland ecosystem

The home-field advantage (HFA) hypothesis predicts that plant litter would decompose more quickly beneath its own plant species in the soil than beneath other plant species. Theoretically, HFA can be induced by the rhizosphere of growing plants, due to so-called rhizosphere effect (RE). Despite growing evidence for the site condition-dependence of both effects, few work has be conducted to explore how site climate, vegetation type and soil properties interact to affect RE and HFA, and especially limited in situ representation from subtropical wetland systems. In a field experiment, we reciprocally incubated three root litter species (Rumex dentatus L., Carex thunbergii Steud., and Polygonum cripolitanum Hance) along a hydroperiod gradient in a subtropical wetland, which differed mainly with respect to vegetation and soil microclimate, with and without growing plants. The occurrence and magnitude of HFA and RE were mainly determined by litter quality and were stage-specific. Collectively, we detected significant HFA with chemically-recalcitrant litter from C. thunbergii and P. cripolitanum, but only at the first stage of decomposition. The presence of growing plants generally reduced litter decomposition, but the magnitude of the response was species-specific, with the positive effects detected only for root litters from C. thunbergii at the first stage of decomposition. In addition, we did not find a significant relationship between HFA and RE, indicating that plant species that produce litters exhibiting HFA may not accelerate litter decomposition via RE at same time. Structural equation models (SEM) revealed that site microclimate factors were conducive with soil properties in regulating C dynamics. Overall, soil microclimate in this wetland ecosystem was likely important in driving C cycling, either directly by changing environmental conditions, litter quality, and plant trait spectra, or indirectly by interrupting the interactions between litter and decomposers.

Clonal integration in Phragmites australis alters soil microbial communities in an oil-contaminated wetland

Clonal plants can share information and resources among connected ramets (asexual individuals). Such clonal integration can promote ramet growth, which may further influence soil microbial communities in the rooting zone. Crude oil contamination can negatively affect plant growth and alter soil microbial community composition. However, we still know little about how clonal integration affects soil microbial communities, especially under crude oil contamination. In a coastal wetland, ramets of the rhizomatous plant Phragmites australis in circular plots (60 cm in diameter) were subjected to 0, 5 and 10 mm depth of crude oil, and the rhizomes at the edge of the plots were either severed (preventing clonal integration) or left intact (allowing clonal integration). After three years of treatment, we analysed in each plot soil physiochemical properties and soil microbial community composition. The alpha-diversity of the soil microbial communities did not differ between intact and severed plots, but was overall lower in 10-mm than in 0-mm and 5-mm oil plots. Considering all three oil treatments together, soil microbial community dissimilarity (beta-diversity) was positively correlated with soil property distance in both severed and intact plots. Considering the three oil treatments separately, this pattern was also observed in 10-mm oil plots, but not in 0-mm or 5-mm oil plots. The soil microbial community composition was more sensitive to the oil addition than to the clonal integration. Moreover, the relative abundance of the nitrogen-cycling bacterial taxa was lower in intact than in severed plots, and that of the oil-degrading bacterial taxa increased with increasing oil-addition levels. Our results indicate that clonal integration and oil contamination can influence soil microbial communities independently through changing the relative abundance of the component bacteria taxa, which has important implications for ecosystem functions of the soil food web mediated by clonal plants.

Temporal dynamics of antibiotic resistant genes and their association with the bacterial community in a water-sediment mesocosm under selection by 14 antibiotics

Antibiotics in aquatic environments at high concentrations and sub-inhibitory concentrations potentially select for the evolution of antibiotic resistant genes (ARGs), posing a potential risk to aquatic ecological safety. Our knowledge of the temporal and successive dynamics of ARGs and bacterial community under the selective pressure of antibiotics in natural water-sediment system was limited. This study used a 120-d operating hydrodynamic mesocosm to explore the temporal dynamics of ARGs in water-sediment systems, and the main selective mechanisms following the attenuation and transport of 14 commonly used antibiotics. Under the selective pressures by antibiotics, ARGs propagated transiently, and persisted after antibiotic removal; the bacterial community structures likewise changed. Mantel test and network analysis indicated that ARGs significantly correlated with the bacterial community in the water and surface sediments. Structural equation model (SEM) further revealed that the evolution of ARGs was mainly due to the direct effect of the change in bacterial community and horizontal gene transfer (HGT) via the class 1 integron-integrase gene (intI1), but antibiotics indirectly influenced ARG profiles. The migration of ARGs in deep layer sediments was not related to the bacterial community and intI1, but may be explained by antibiotic selective effects and ARG transformation.

Grazing simplifies soil micro-food webs and decouples their relationships with ecosystem functions in grasslands

Livestock grazing often alters aboveground and belowground communities of grasslands and their mediated carbon (C) and nitrogen (N) cycling processes at the local scale. Yet, few have examined whether grazing-induced changes in soil food webs and their ecosystem functions can be extrapolated to a regional scale. We investigated how large herbivore grazing affects soil micro-food webs (microbes and nematodes) and ecosystem functions (soil C and N mineralization), using paired grazed and ungrazed plots at 10 locations across the Mongolian Plateau. Our results showed that grazing not only affected plant variables (e.g., biomass and C and N concentrations), but also altered soil substrates (e.g., C and N contents) and soil environment (e.g., soil pH and bulk density). Grazing had strong bottom-up effects on soil micro-food webs, leading to more pronounced decreases at higher trophic levels (nematodes) than at lower trophic levels (microbes). Structural equation modeling showed that changes in plant biomass and soil environment dominated grazing effects on microbes, while nematodes were mainly influenced by changes in plant biomass and soil C and N contents; the grazing effects, however, differed greatly among functional groups in the soil micro-food webs. Grazing reduced soil C and N mineralization rates via changes in plant biomass, soil C and N contents, and soil environment across grasslands on the Mongolian Plateau. Spearman's rank correlation analysis also showed that grazing reduced the correlations between functional groups in soil micro-food webs and then weakened the correlation between soil micro-food webs and soil C and N mineralization. These results suggest that changes in soil micro-food webs resulting from livestock grazing are poor predictors of soil C and N processes at regional scale, and that the relationships between soil food webs and ecosystem functions depend on spatial scales and land-use changes.

Effects of Vegetation Restoration on the Distribution of Nutrients, Glomalin-Related Soil Protein, and Enzyme Activity in Soil Aggregates on the Loess Plateau, China

Research Highlights: Soil enzymes have a significant impact on the production of glomalin-related soil protein (GRSP), directly and indirectly affecting the nutrient metabolism balance, but there is little available information on ecological stoichiometry in soil aggregates. Background and Objectives: Vegetation restoration changes community structure and species composition in ecosystems, thus changing the physicochemical properties of soil. Soil aggregate is the most basic physical structure of the soil. Therefore, in order to understand dynamic changes in soil aggregate nutrients as vegetation restoration progresses, we set out to investigate the nutrient distribution and utilization in aggregates, and how enzymes respond to the nutrient changes in achieving a nutritional balance along successive stages of vegetation restoration. Materials and Methods: We collected and analyzed soil from plots representing six different stages of a vegetation restoration chronosequence (0, 30, 60, 100, 130, and 160 years) after farmland abandonment on the Loess Plateau, China. We investigated soil nutrient stoichiometry, GRSP, and enzyme stoichiometry in the different successional stages. Results: The results revealed that soil organic carbon, total nitrogen, enzyme activity, and GRSP increased with vegetation recovery age, but not total phosphorus, and not all enzymes reached their maximum in the climax forest community. The easily extractable GRSP/total GRSP ratio was the largest at the shrub community stage, indicating that glomalin degradation was the lowest at this stage. Ecological stoichiometry revealed N-limitation decreased and P-limitation increased with increasing vegetation restoration age. Soil enzymes had a significant impact on the GRSP production, directly and indirectly affecting nutrient metabolism balance. Conclusions: Further study of arbuscular mycorrhizal fungi to identify changes in their category and composition is needed for a better understanding of how soil enzymes affect their release of GRSP, in order to maintain a nutrient balance along successive stages of vegetation restoration.

Sensitivity of plant species to warming and altered precipitation dominates the community productivity in a semiarid grassland on the Loess Plateau

Global warming and changes in precipitation patterns can critically influence the structure and productivity of terrestrial ecosystems. However, the underlying mechanisms are not fully understood. We conducted two independent but complementary experiments (one with warming and precipitation manipulation (+ or - 30%) and another with selective plant removal) in a semiarid grassland on the Loess Plateau, northwestern China, to assess how warming and altered precipitation affect plant community. Our results showed that warming and altered precipitation affected community aboveground net primary productivity (ANPP) through impacting soil moisture. Results of the removal experiment showed competitive relationships among dominant grasses, the dominant subshrub and nondominant species, which played a more important role than soil moisture in the response of plant community to warming and altered precipitation. Precipitation addition intensified the competition but primarily benefited the dominant subshrub. Warming and precipitation reduction enhanced water stresses but increased ANPP of the dominant subshrub and grasses, indicating that plant tolerance to drought critically meditated the community responses. These findings suggest that specie competitivity for water resources as well as tolerance to environmental stresses may dominate the responses of plant communities on the Loess Plateaus to future climate change factors.

Soil biological responses to, and feedbacks on, trophic rewilding

Trophic rewilding-the (re)introduction of missing large herbivores and/or their predators-is increasingly proposed to restore biodiversity and biotic interactions, but its effects on soils have been largely neglected. The high diversity of soil organisms and the ecological functions they perform mean that the full impact of rewilding on ecosystems cannot be assessed considering only above-ground food webs. Here we outline current understanding on how animal species of rewilding interest affect soil structure, processes and communities, and how in turn soil biota may affect species above ground. We highlight considerable uncertainty in soil responses to and feedbacks on above-ground consumers, with potentially large implications for rewilding interactions with global change. For example, the impact of large herbivores on soil decomposers and plant-soil interactions could lead to reduced carbon sequestration, whereas herbivore interactions with keystone biota such as mycorrhizal fungi, dung beetles and bioturbators could promote native plants and ecosystem heterogeneity. Moreover, (re)inoculation of keystone soil biota could be considered as a strategy to meet some of the objectives of trophic rewilding. Overall, we call for the rewilding research community to engage more with soil ecology experts and consider above-ground-below-ground linkages as integral to assess potential benefits as well as pitfalls.This article is part of the theme issue 'Trophic rewilding: consequences for ecosystems under global change'.

Network Analyses Can Advance Above-Belowground Ecology

An understanding of above-belowground (AG-BG) ecology is important for evaluating how plant interactions with enemies, symbionts, and decomposers affect species diversity and will respond to global changes. However, research questions and experiments often focus on only a limited number of interactions, creating an incomplete picture of how entire communities may be involved in AG-BG community ecology. Therefore, a pressing challenge is to formulate hypotheses of AG-BG interactions when considering communities in their full complexity. Here we discuss how network analyses can be a powerful tool to progress AG-BG research, link across scales from individual to community and ecosystem, visualize community interactions between the two (AG and BG) subsystems, and develop testable hypotheses.

Long-term effects of grazing and topography on extra-radical hyphae of arbuscular mycorrhizal fungi in semi-arid grasslands

Grazing and topography have drastic effects on plant communities and soil properties. These effects are thought to influence arbuscular mycorrhizal (AM) fungi. However, the simultaneous impacts of grazing pressure (sheep ha^-1) and topography on plant and soil factors and their relationship to the production of extra-radical AM hyphae are not well understood. Our 10-year study assessed relationships between grazing, plant species richness, aboveground plant productivity, soil nutrients, edaphic properties, and AM hyphal length density (HLD) in different topographic areas (flat or sloped). We found HLD linearly declined with increasing grazing pressure (1.5-9.0 sheep ha^-1) in sloped areas, but HLD was greatest at moderate grazing pressure (4.5 sheep ha^-1) in flat areas. Structural equation modeling indicates grazing reduces HLD by altering soil nutrient dynamics in sloped areas, but non-linearly influences HLD through plant community and edaphic changes in flat areas. Our findings highlight how topography influences key plant and soil factors, thus regulating the effects of grazing pressure on extra-radical hyphal production of AM fungi in grasslands. Understanding how grazing and topography influence AM fungi in semi-arid grasslands is vital, as globally, severe human population pressure and increasing demand for food aggravate the grazing intensity in grasslands.

Year-round breeding equatorial Larks from three climatically-distinct populations do not use rainfall, temperature or invertebrate biomass to time reproduction

Timing of reproduction in birds is important for reproductive success and is known to depend on environmental cues such as day length and food availability. However, in equatorial regions, where day length is nearly constant, other factors such as rainfall and temperature are thought to determine timing of reproduction. Rainfall can vary at small spatial and temporal scales, providing a highly fluctuating and unpredictable environmental cue. In this study we investigated the extent to which spatio-temporal variation in environmental conditions can explain the timing of breeding of Red-capped Lark, Calandrella cinerea, a species that is capable of reproducing during every month of the year in our equatorial east African study locations. For 39 months in three climatically-distinct locations, we monitored nesting activities, sampled ground and flying invertebrates, and quantified rainfall, maximum (T_max) and minimum (T_min) temperatures. Among locations we found that lower rainfall and higher temperatures did not coincide with lower invertebrate biomasses and decreased nesting activities, as predicted. Within locations, we found that rainfall, T_max, and T_min varied unpredictably among months and years. The only consistent annually recurring observations in all locations were that January and February had low rainfall, high T_max, and low T_min. Ground and flying invertebrate biomasses varied unpredictably among months and years, but invertebrates were captured in all months in all locations. Red-capped Larks bred in all calendar months overall but not in every month in every year in every location. Using model selection, we found no clear support for any relationship between the environmental variables and breeding in any of the three locations. Contrary to popular understanding, this study suggests that rainfall and invertebrate biomass as proxy for food do not influence breeding in equatorial Larks. Instead, we propose that factors such as nest predation, female protein reserves, and competition are more important in environments where weather and food meet minimum requirements for breeding during most of the year.

Effects of Short-Term Warming and Altered Precipitation on Soil Microbial Communities in Alpine Grassland of the Tibetan Plateau

Soil microbial communities are influenced by climate change drivers such as warming and altered precipitation. These changes create abiotic stresses, including desiccation and nutrient limitation, which act on microbes. However, our understanding of the responses of microbial communities to co-occurring climate change drivers is limited. We surveyed soil bacterial and fungal diversity and composition after a 1-year warming and altered precipitation manipulation in the Tibetan plateau alpine grassland. In isolation, warming and decreased precipitation treatments each had no significant effects on soil bacterial community structure; however, in combination of both treatments altered bacterial community structure (p = 0.03). The main effect of altered precipitation specifically impacted the relative abundances of Bacteroidetes and Gammaproteobacteria compared to the control, while the main effect of warming impacted the relative abundance of Betaproteobacteria. In contrast, the fungal community had no significant response to the treatments after 1-year. Using structural equation modeling (SEM), we found bacterial community composition was positively related to soil moisture. Our results indicate that short-term climate change could cause changes in soil bacterial community through taxonomic shifts. Our work provides new insights into immediate soil microbial responses to short-term stressors acting on an ecosystem that is particularly sensitive to global climate change.

The trophic responses of two different rodent-vector-plague systems to climate change

Plague, the causative agent of three devastating pandemics in history, is currently a re-emerging disease, probably due to climate change and other anthropogenic changes. Without understanding the response of plague systems to anthropogenic or climate changes in their trophic web, it is unfeasible to effectively predict years with high risks of plague outbreak, hampering our ability for effective prevention and control of the disease. Here, by using surveillance data, we apply structural equation modelling to reveal the drivers of plague prevalence in two very different rodent systems: those of the solitary Daurian ground squirrel and the social Mongolian gerbil. We show that plague prevalence in the Daurian ground squirrel is not detectably related to its trophic web, and that therefore surveillance efforts should focus on detecting plague directly in this ecosystem. On the other hand, plague in the Mongolian gerbil is strongly embedded in a complex, yet understandable trophic web of climate, vegetation, and rodent and flea densities, making the ecosystem suitable for more sophisticated low-cost surveillance practices, such as remote sensing. As for the trophic webs of the two rodent species, we find that increased vegetation is positively associated with higher temperatures and precipitation for both ecosystems. We furthermore find a positive association between vegetation and ground squirrel density, yet a negative association between vegetation and gerbil density. Our study thus shows how past surveillance records can be used to design and improve existing plague prevention and control measures, by tailoring them to individual plague foci. Such measures are indeed highly needed under present conditions with prevailing climate change.

Bacterial Respiration and Growth Rates Affect the Feeding Preferences, Brood Size and Lifespan of Caenorhabditis elegans

Bacteria serve as live food and nutrients for bacterial-feeding nematodes (BFNs) in soils, and influence nematodes behavior and physiology through their metabolism. Five bacterial taxa (Bacillus amyloliquefaciens JX1, Variovorax sp. JX14, Bacillus megaterium JX15, Pseudomonas fluorescens Y1 and Escherichia coli OP50) and the typical BFN Caenorhabditis elegans were selected to study the effects of bacterial respiration and growth rates on the feeding preferences, brood size and lifespan of nematodes. P. fluorescens Y1 and E. coli OP50 were found to be more active, with high respiration and rapid growth, whereas B. amyloliquefaciens JX1 and B. megaterium JX15 were inactive. The nematode C. elegans preferred active P. fluorescens Y1 and E. coli OP50 obviously. Furthermore, worms that fed on these two active bacteria produced more offspring but had shorter lifespan, while inactive and less preferred bacteria had increased nematodes lifespan and decreased the brood size. Based on these results, we propose that the bacterial activity may influence the behavior and life traits of C. elegans in the following ways: (1) active bacteria reproduce rapidly and emit high levels of CO₂ attracting C. elegans; (2) these active bacteria use more resources in the nematodes’ gut to sustain their survival and reproduction, thereby reducing the worm's lifespan; (3) inactive bacteria may provide less food for worms than active bacteria, thus increasing nematodes lifespan but decreasing their fertility. Nematodes generally require a balance between their preferred foods and beneficial foods, only preferred food may not be beneficial for nematodes.

Nitrogen deposition weakens plant-microbe interactions in grassland ecosystems

Soil carbon (C) and nitrogen (N) stoichiometry is a main driver of ecosystem functioning. Global N enrichment has greatly changed soil C : N ratios, but how altered resource stoichiometry influences the complexity of direct and indirect interactions among plants, soils, and microbial communities has rarely been explored. Here, we investigated the responses of the plant-soil-microbe system to multi-level N additions and the role of dissolved organic carbon (DOC) and inorganic N stoichiometry in regulating microbial biomass in semiarid grassland in northern China. We documented a significant positive correlation between DOC and inorganic N across the N addition gradient, which contradicts the negative nonlinear correlation between nitrate accrual and DOC availability commonly observed in natural ecosystems. Using hierarchical structural equation modeling, we found that soil acidification resulting from N addition, rather than changes in the plant community, was most closely related to shifts in soil microbial community composition and decline of microbial respiration. These findings indicate a down-regulating effect of high N availability on plant-microbe interactions. That is, with the limiting factor for microbial biomass shifting from resource stoichiometry to soil acidity, N enrichment weakens the bottom-up control of soil microorganisms by plant-derived C sources. These results highlight the importance of integratively studying the plant-soil-microbe system in improving our understanding of ecosystem functioning under conditions of global N enrichment.

Contrasting effects of elevated temperature and invertebrate grazing regulate multispecies interactions between decomposer fungi

Predicting the influence of biotic and abiotic factors on species interactions and ecosystem processes is among the primary aims of community ecologists. The composition of saprotrophic fungal communities is a consequence of competitive mycelial interactions, and a major determinant of woodland decomposition and nutrient cycling rates. Elevation of atmospheric temperature is predicted to drive changes in fungal community development. Top-down regulation of mycelial growth is an important determinant of, and moderator of temperature-driven changes to, two-species interaction outcomes. This study explores the interactive effects of a 4 °C temperature increase and soil invertebrate (collembola or woodlice) grazing on multispecies interactions between cord-forming basidiomycete fungi emerging from colonised beech (Fagus sylvatica) wood blocks. The fungal dominance hierarchy at ambient temperature (16 °C; Phanerochaete velutina > Resinicium bicolor > Hypholoma fasciculare) was altered by elevated temperature (20 °C; R. bicolor > P. velutina > H. fasciculare) in ungrazed systems. Warming promoted the competitive ability of the fungal species (R. bicolor) that was preferentially grazed by all invertebrate species. As a consequence, grazing prevented the effect of temperature on fungal community development and maintained a multispecies assemblage. Decomposition of fungal-colonised wood was stimulated by warming, with implications for increased CO₂ efflux from woodland soil. Analogous to aboveground plant communities, increasing complexity of biotic and abiotic interactions appears to be important in buffering climate change effects on soil decomposers.

Nitrogen addition and warming independently influence the belowground micro-food web in a temperate steppe

Climate warming and atmospheric nitrogen (N) deposition are known to influence ecosystem structure and functioning. However, our understanding of the interactive effect of these global changes on ecosystem functioning is relatively limited, especially when it concerns the responses of soils and soil organisms. We conducted a field experiment to study the interactive effects of warming and N addition on soil food web. The experiment was established in 2006 in a temperate steppe in northern China. After three to four years (2009–2010), we found that N addition positively affected microbial biomass and negatively influenced trophic group and ecological indices of soil nematodes. However, the warming effects were less obvious, only fungal PLFA showed a decreasing trend under warming. Interestingly, the influence of N addition did not depend on warming.

Structural equation modeling analysis suggested that the direct pathway between N addition and soil food web components were more important than the indirect connections through alterations in soil abiotic characters or plant growth. Nitrogen enrichment also affected the soil nematode community indirectly through changes in soil pH and PLFA. We conclude that experimental warming influenced soil food web components of the temperate steppe less than N addition, and there was little influence of warming on N addition effects under these experimental conditions.

Nitrogen addition regulates soil nematode community composition through ammonium suppression

Nitrogen (N) enrichment resulting from anthropogenic activities has greatly changed the composition and functioning of soil communities. Nematodes are one of the most abundant and diverse groups of soil organisms, and they occupy key trophic positions in the soil detritus food web. Nematodes have therefore been proposed as useful indicators for shifts in soil ecosystem functioning under N enrichment. Here, we monitored temporal dynamics of the soil nematode community using a multi-level N addition experiment in an Inner Mongolia grassland. Measurements were made three years after the start of the experiment. We used structural equation modeling (SEM) to explore the mechanisms regulating nematode responses to N enrichment. Across the N enrichment gradient, significant reductions in total nematode abundance, diversity (H' and taxonomic richness), maturity index (MI), and the abundance of root herbivores, fungivores and omnivores-predators were found in August. Root herbivores recovered in September, contributing to the temporal variation of total nematode abundance across the N gradient. Bacterivores showed a hump-shaped relationship with N addition rate, both in August and September. Ammonium concentration was negatively correlated with the abundance of total and herbivorous nematodes in August, but not in September. Ammonium suppression explained 61% of the variation in nematode richness and 43% of the variation in nematode trophic group composition. Ammonium toxicity may occur when herbivorous nematodes feed on root fluid, providing a possible explanation for the negative relationship between herbivorous nematodes and ammonium concentration in August. We found a significantly positive relationship between fungivores and fungal phospholipid fatty acids (PLFA), suggesting bottom-up control of fungivores. No such relationship was found between bacterivorous nematodes and bacterial PLFA. Our findings contribute to the understanding of effects of N enrichment in semiarid grassland on soil nematode trophic groups, and the cascading effects in the detrital soil food web.

Outcomes of fungal interactions are determined by soil invertebrate grazers

Saprotrophic fungal community composition, determined by the outcome of competitive mycelial interactions, is one of the many key factors affecting soil nutrient mineralisation and decomposition rates. Fungal communities are not generally predicted to be regulated by top-down factors, such as predation, but rather by bottom-up factors, including resource availability. We show that invertebrate grazers can exert selective pressures on fungal decomposer communities in soil, reversing the outcomes of competitive interactions. By feeding selectively on the cord-forming fungus Resinicium bicolor, isopods prevented the competitive exclusion of Hypholoma fasciculare and Phanerochaete velutina in soil and wood. Nematode populations also reversed the outcomes of competitive interactions by stimulating growth of less competitive fungi. These represent two opposing mechanisms by which soil fauna may influence fungal community composition and diversity. Factors affecting soil invertebrate communities will have direct consequences for fungal-mediated nutrient cycling in woodland soils.