Background and aims
While large differences in microbial biomass and extracellular enzyme activities (EEAs) between rhizosphere and bulk soils have been demonstrated, the potentially different ...response of microbes and their EEAs in rhizosphere and bulk soils to nitrogen (N) deposition is still not elucidated.
Methods
We analyzed the microbial biomass and EEAs in the rhizosphere and bulk soils of
Sibiraea angustata
in an alpine shrubland on the eastern Qinghai-Tibet Plateau after chronic N application. We also analyzed the stoichiometric linkages between plants, microbes, enzymes and soils to clarify the coupled responses of aboveground plants and belowground ecological processes.
Results
Microbial nutrient concentrations and activities of EAAs responded differently to N addition in the rhizosphere and bulk soils. In the rhizosphere, N addition caused a significant increase in microbial biomass carbon (C), N and phosphorus (P) concentrations and greater P-degrading enzyme activity (relative to the activities of C- and N-degrading enzymes), which induced a significant reduction in enzyme C:P and N:P ratios. The rhizosphere enzyme N:P ratio was negatively correlated with the N:P ratios of plant, soil and microbe, implying that increased plant and microbial P uptake under N addition may gradually aggravate rhizosphere P limitation. However, for the bulk soil, N addition did not affect microbial biomass but significantly enhanced C-degrading enzyme activity and decreased the enzyme C:N ratio. Meanwhile, the bulk-soil enzyme C:N ratio was negatively correlated with the soil C:N ratio but independent of the plant C:N ratio, implying that N addition may enhance bulk-soil microbial C limitation.
Conclusions
Our study suggests that elevated N deposition may induce differential microbial nutrient limitation between the rhizosphere and bulk soils due to the plant-microbe-soil interactions in the rhizosphere. This study highlights the importance of incorporating rhizosphere microbial processes into biogeochemical models describing environmental changes.
Ecological understandings of soil bacterial community succession and assembly mechanism along elevational gradients in mountains remain not well understood. Here, by employing the high-throughput ...sequencing technique, we systematically examined soil bacterial diversity patterns, the driving factors, and community assembly mechanisms along the elevational gradients of 1800-4100 m on Gongga Mountain in China. Soil bacterial diversity showed an extraordinary stair-step pattern along the elevational gradients. There was an abrupt decrease of bacterial diversity between 2600 and 2800 m, while no significant change at either lower (1800-2600 m) or higher (2800-4100 m) elevations, which coincided with the variation in soil pH. In addition, the community structure differed significantly between the lower and higher elevations, which could be primarily attributed to shifts in soil pH and vegetation types. Although there was no direct effect of MAP and MAT on bacterial community structure, our partial least squares path modeling analysis indicated that bacterial communities were indirectly influenced by climate via the effect on vegetation and the derived effect on soil properties. As for bacterial community assembly mechanisms, the null model analysis suggested that environmental filtering played an overwhelming role in the assembly of bacterial communities in this region. In addition, variation partition analysis indicated that, at lower elevations, environmental attributes explained much larger fraction of the β-deviation than spatial attributes, while spatial attributes increased their contributions at higher elevations. Our results highlight the importance of environmental filtering, as well as elevation-related spatial attributes in structuring soil bacterial communities in mountain ecosystems.
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•Nematode community structure varies among forest successional stages.•Forest succession leads to higher nematode abundance and maturity index.•Increasing fungivores and decreasing ...herbivores occur with forest succession.•Forest succession exhibits layer-specific effects on nematode community structure.
Nematodes are among the most important faunal groups in soil food webs and serve as important bioindicators in evaluating subsurface ecological processes and ecosystem functioning. Although previous studies have reported the effects of vegetation types on soil nematode communities, the community patterns of soil nematodes at different soil depths during secondary forest succession remain unclear. Through a field survey, we investigated how soil nematode community characteristics changed at two soil depths (0–10 cm and 10–20 cm) across subalpine forest successional stages consisting of grasslands, shrublands and forests in Southwest China. We measured soil properties and microbial biomass to analyze significant environmental factors influencing variations in soil nematode communities. With forest succession, we observed increases in the total nematode abundance and maturity index and a decreased plant parasitic index, indicating more stable food webs and lower root herbivory hazards in forests than in grasslands and shrublands. Fungivore diversity was significantly increased and herbivore diversity was significantly decreased during forest succession, despite no significant difference existing in the total nematode diversity among the three successional stages. The abundances of the dominant nematode genera Helicotylenchus and Tylencholaimus were significantly correlated with forest succession. In addition, the variations in soil nematode communities differed between the two soil depths, presumably due to various soil variables. NO-3-N and pH were the dominant determinants of the variations in soil nematode communities at 0–10 cm soil depth, while SOC was the dominant determinant at 10–20 cm soil depth. Overall, our results suggested that forest succession exhibited layer-specific effects on soil nematode communities via different soil variables, and that specific nematode genera could serve as useful bioindicators during forest succession.
•Herb root exudates promote P. asperata, but inhibit B. albosinensis and B. platyphylla.•Root exudates increase soil bacterial richness and diversity.•Soil bacteria have a greater role on seedling ...growth than soil properties and fungi.•Root exudates affect seedling growth of subalpine trees by regulating soil bacteria.
Root exudates play an important role in belowground ecological processes, by which plants can regulate the soil ecosystem. However, studies on the effects of root exudates on aboveground plant emergence and growth and thus vegetation regeneration are still lacking, particularly in subalpine forests. In this study, we collected the root exudates of dominant plants at different successional stages (early-stage herbs, mid-stage shrubs and late-stage broadleaf and coniferous trees) in a subalpine region of southwestern China, and used them to condition the soil collected in the field. Using the conditioned soil, a pot experiment on the seed germination and seedling growth of subalpine dominant trees (Betula albosinensis, Betula platyphylla and Picea asperata) was conducted in an artificial climate chamber. The seed germination rate and seedling biomass of the subalpine trees, as well as the soil properties and microbial community structure in the potting soils, were measured. Our results showed that root exudates of different successional stages had significant and distinct effects on the seed germination and seedling growth of the subalpine trees, and these effects differed among the tree species. Specifically, the seed germination rate and seedling biomass of P. asperata were promoted, while those of B. albosinensis and B. platyphylla were inhibited under the addition treatments of the herb root exudates. Furthermore, our study suggested that the soil bacterial community contributed more than soil properties and the fungal community to the variations in the seedling biomass of subalpine trees, and the effects of root exudates on seedling biomass were significantly mediated by the soil bacterial community. Overall, these results implied that root exudates of different successional stages could affect the seedling growth of subalpine trees by regulating soil microbial communities, highlighting the critical role of root exudates during forest succession and providing important insights into the potential rhizosphere mechanisms of forest succession.
Ectomycorrhizal fungi (EcMF) are involved in soil nutrient cycling in forest ecosystems. These fungi can promote the uptake of nutrients (e.g., nitrogen (N) and phosphorus (P)) and water by host ...plants, as well as facilitate host plant growth and resistance to stresses and diseases, thereby maintaining the aboveground primary productivity of forest ecosystems. Moreover, EcMF can acquire the carbon (C) sources needed for their growth from the host plants. The nutrient regulation mechanisms of EcMF mainly include the decay of soil organic matter via enzymatic degradation, nonenzymatic mechanism (Fenton chemistry), and priming effects, which in turn promote C and N cycling. At the same time, EcMF can secrete organic acids and phosphatases to improve the availability of soil P, or increase mycelium inputs to facilitate plant acquisition of P. The spatiotemporal distribution of EcMF is influenced by a combination of historical factors and contemporary environmental factors. The community of EcMF is associated with various factors, such as climate change, soil conditions, and host distribution. Under global climate change, investigating the relationships between the nutrient cycling functions of EcMF communities and their distribution patterns under various spatiotemporal scales is conducive to more accurate assessments of the ecological effects of EcMF on the sustainable development of forest.
-type and
-type denitrifier communities mediate the conversion of nitrite to nitric oxide, which is the key step in denitrification. Results of previous studies have indicated that
-type and
-type ...denitrifiers may occupy different niches; however, the mechanisms and drivers of their responses to environmental changes within community assembly are poorly understood. In this study, we evaluated the distribution and assembly of
-type and
-type denitrifier communities along an elevation gradient from 1,800 to 4,100 m at Mount Gongga, China. Results showed that elevational patterns of alpha diversity in
-type and
-type denitrifier communities followed hump-backed patterns along the elevation gradient. However,
-type denitrifier communities formed two distinct clusters that were primarily separated by elevation, whereas
-type denitrifier communities formed three distinct clusters that were primarily separated by forest type along the elevation gradient. Moreover, deterministic processes were dominant in governing the assemblages of
-type and
-type denitrifiers. Soil pH was a key factor influencing the alpha and beta diversity of the
-type denitrifier communities, whereas plant richness was a primary variable influencing
-type denitrifiers. Additionally, our work revealed that soil denitrification potential was mainly explained by the variation in the beta diversity of denitrifier communities rather than the alpha diversity of denitrifier communities or denitrifier abundances over a large elevation gradient, and
-type denitrifiers played more important roles in soil denitrification. These results may contribute to predicting the consequences of global changes on denitrifier communities and their ecological services.
Mount Gongga is the highest peak in the Hengduan Mountain region and is located at the southeastern fringe of the Tibetan Plateau, Sichuan Province, southwest China. As a transitional zone between the Tibetan Plateau and Sichuan Basin, Gongga Mountain features particularly diverse topography, geology, climate, and biodiversity and is a globally significant hot spot of biodiversity. In this contribution, we comprehensively describe the diversity and assembly of denitrifier communities along an elevation gradient on Gongga Mountain. Our findings established for the first time that the distribution patterns of beta diversity and driving factors differed between
-type and
-type denitrifier communities, and deterministic processes were dominant in shaping communities of denitrifiers. Moreover, the beta diversity of denitrifier communities rather than alpha diversity or denitrifier abundance played an important role in explaining denitrification potential, and the beta diversity of
-type denitrifier communities was more important than
-type denitrifier communities in soil denitrification. This work provides crucial insights into the spatial distribution of denitrifier communities and their ecological function and increases our understanding of the mechanisms underlying spatial distribution of community assembly along large elevation gradients.
Knowledge of variations in abundant and rare soil microbial communities and interactions during secondary forest succession is lacking. Soil samples were gathered from different secondary ...successional stages (grassland, shrubland, and secondary forest) to study the responses of abundant and rare bacterial and fungal communities, interactions and driving factors to secondary forest succession by Illumina sequencing of the 16S and ITS rRNA genes. The results showed that the
-diversities (Shannon index) of abundant bacteria and fungi revealed no significant changes during secondary forest succession, but increased significantly for rare bacteria. The abundant and rare bacterial and fungal
-diversities changed significantly during secondary forest succession. Network analysis showed no obvious changes in the topological properties (nodes, links, and average degree) of abundant microbial networks during secondary forest succession. In contrast, these properties of the rare microbial networks in the secondary forest were higher than those in the grassland and shrubland, indicating that rare microbial networks are more responsive to secondary forest succession than abundant microorganisms. Additionally, rare microbial networks revealed more microbial interactions and greater network complexity than abundant microbial networks due to their higher numbers of nodes and links. The keystone species differed between the abundant and rare microbial networks and consisted of 1 and 48 keystone taxa in the abundant and rare microbial networks, respectively. Soil TP was the most important influencing factor of abundant and rare bacterial communities. Successional stages and plant richness had the most important influences on abundant and rare fungal communities, respectively. C:P, SM and N:P were mainly related to abundant and rare microbial network topological properties. Our study indicates that abundant and rare microbial communities, interactions and driving factors respond differently to secondary forest succession.
The aboveground carbon sequestration rate (ACSR) of forests serves as an indicator of their carbon sequestration capacity over time, providing insights into the potential carbon sequestration ...capacity of forest ecosystems. To explore the long-term Spatiotemporal variation of ACSR in the transitional ecotone of the eastern Tibetan Plateau under climate change scenarios, we utilized a forest landscape model that was parameterized with forest inventory data from the eastern Tibetan Plateau to simulate this ecological function changes. The study found that climate warming had significant effect on forests ACSR in different types of forests. ACSR was significantly reduced (p<0.05) in cold temperate coniferous and temperate coniferous forests, whereas it was significantly increased in deciduous broad-leaved forests. However, the impact of climate warming on evergreen broad-leaved forests was found to be negligible. At the species level, climate warming has mostly suppressed the ACSR of coniferous trees, except for Chinese hemlock. The main dominant species, spruce and fir, have been particularly affected. Conversely, the ACSR of most broad-leaved trees has increased due to climate warming. In addition, at the landscape scale, the ACSR within this region is expected to experience a steady decline after 2031s-2036s. Despite the effects of climate warming, this trend is projected to persist. In conclusion, the forests ACSR in this region will be significantly affected by future climate warming. Our research indicates that climate warming will have a noticeable suppressive effect on conifers. It is imperative that this factor be taken into account when devising forest management plans for the future in this region.
Fast-growing pioneer tree species play a crucial role in triggering late successional development in forests. Experimental evidence of the soil legacy effects of pre-existing plants on pioneer tree ...performance is lacking. We explored the legacy effects of soils conditioned by early successional herbs (Poa poophagorum Bor and Potentilla fragarioides L.) and mid-successional shrubs (Rhododendron fortunei Lindl. and Enkianthus quinqueflorus Lour.) on late-successional ectomycorrhizal (ECM) pioneer tree (Betula platyphylla Sukaczev) seedling growth. The soils were analyzed for soil nutrient status and fungal and bacterial compositions using ITS and 16S rRNA gene sequencing. B. platyphylla seedlings produced higher biomass in soils conditioned by shrubs. Soil organic carbon (SOC) and bacterial and fungal legacies most impacted pioneer tree seedling growth. Additionally, the partial least squares path model revealed that soil nutrients, especially SOC, indirectly affected seedling biomass by their direct effects on the bacterial and fungal communities. The changes in bacterial community composition had a stronger effect on seedling biomass than those of fungi because bacteria with shorter turnover times are generally considered to be more efficient than fungi in enhancing nutrient availability. Our study integrates soil microbial and nutrient legacies to explain the potential mechanisms of pioneer tree regeneration.
The performance of Rumex nepalensis, an important medicinal herb, varies significantly among subalpine grasslands, shrublands and forest ecosystems in southwestern China. Plant–soil feedback is ...receiving increasing interest as an important driver influencing plant growth and population dynamics. However, the feedback effects of soils from different ecosystems on R. nepalensis remain poorly understood. A greenhouse experiment was carried out to identify the effects of different soil sources on the photosynthesis and biomass of R. nepalensis. R. nepalensis was grown in soils collected from the rooting zones of R. nepalensis (a grassland soil, RS treatment), Hippophae rhamnoides (a shrub soil, HS treatment), and Picea asperata (a forest soil, PS treatment). The chlorophyll contents, net photosynthetic rates, and biomasses of R. nepalensis differed significantly among the three soils and followed the order of RS > HS > PS. After soil sterilization, these plant parameters followed the order of RS > PS > HS. The total biomass was 16.5 times higher in sterilized PS than in unsterilized PS, indicating that the existence of soil microbes in P. asperata forest ecosystems could strongly inhibit R. nepalensis growth. The root to shoot biomass ratio of R. nepalensis was the highest in the sterilized PS but the lowest in the unsterilized PS, which showed that soil microbes in PS could change the biomass allocation. Constrained redundancy analysis and path analysis suggested that soil microbes could impact the growth of R. nepalensis via the activities of soil extracellular enzymes (e.g., β-1,4-N-acetylglucosaminidase (NAG)) in live soils. The soil total soluble nitrogen concentration might be the main soil factor regulating R. nepalensis performance in sterilized soils. Our findings underline the importance of the soil microbes and nitrogen to R. nepalensis performance in natural ecosystems and will help to better predict plant population dynamics.
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