Elevational decline in soil bacterial diversity and its positive role in driving soil multifunctionality: a global meta-analysis

Plants and soil bacteria exert a vital function in mediating soil ecosystem multifunctionality (EMF). Nevertheless, plant and soil bacteria interaction during forest secondary succession is poorly understood, and their roles in soil EMF remain largely unexplored. The dynamics of soil physicochemical properties and bacterial diversity was studied in southwest China during forest succession from coniferous to monsoon broadleaf evergreen. Interdomain ecological networks (IDEN) were adopted for investigating plant–bacteria associations. With the purpose of assessing how soil factors, bacterial community and plant diversity influenced soil EMF, the structural equation model (SEM) was used. It was discovered that both soil bacterial diversity and soil EMF gradually increased with the succession. IDEN analysis revealed that plant–bacteria ecological networks differed significantly across successional stages. Keystone plant species richness (KSR) increased with succession, which benefited soil bacteria diversity (path coefficient = 0.802, p < 0.001) while having a direct negative impact on pH (path coefficient = −0.602, p < 0.05) and C:N (path coefficient = −0.759, p < 0.001). Soil pH and C:N ratio declines were observed during forest secondary succession and were found to be inversely related to soil EMF. Furthermore, soil pH was found to be inversely related to bacterial diversity. The SEM analysis explained 88.2% of the variation in soil multifunctionality. The findings suggested that keystone plant species played a key function in regulating the variations of soil bacterial community and EMF (total effect = 0.813). Our study clarified the critical functions of keystone species in driving soil bacterial diversity and multifunctionality during forest succession and provided a new perspective on the relationship between above‐ and below‐ground.

To understand effects of soil microbes on soil biochemistry in alpine grassland ecosystems under environmental changes, we explored relationships between soil microbial diversity and soil total nitrogen, organic carbon, available nitrogen and phosphorus, soil microbial biomass and soil enzyme activities in alpine meadow, alpine steppe and cultivated grassland on the Qinghai-Tibetan plateau under three-year warming, enhanced precipitation and yak overgrazing. Soil total nitrogen, organic carbon and NH4-N were little affected by overgrazing, warming or enhanced precipitation in three types of alpine grasslands. Soil microbial biomass carbon and phosphorus along with the sucrase and phosphatase activities were generally stable under different treatments. Soil NO3-N, available phosphorus, urease activity and microbial biomass nitrogen were increased by overgrazing in the cultivated grassland. Soil bacterial diversity was positively correlated with, while soil fungal diversity negatively with soil microbial biomass and enzyme activities. Soil bacterial diversity was negatively correlated with, while soil fungal diversity positively with soil available nutrients. Our findings indicated soil bacteria and fungi played different roles in affecting soil nutrients and microbiological activities that might provide an important implication to understand why soil biochemistry was generally stable under environmental changes in alpine grassland ecosystems.

Afforestation plays a pivotal role in ecosystem restoration, exemplified by the Saihanba Mechanized Forest Farm, the world's largest planted forest; however, the assembly mechanisms and interactions of soil microbial communities in such forests remain inadequately understood. This study aimed to elucidate the impact of different afforestation tree species, namely Larix gmelinii var. principis-rupprechtii, Picea asperata, and Pinus sylvestris var. mongolica, on soil bacterial diversity and community structure in comparison to grassland. Sixty soil samples were collected at a 20 cm depth, and high-throughput sequencing was employed to identify bacterial communities and assess their interactions with environmental factors. A total of 6528 operational taxonomic units (OTUs) were identified, with Solirubrobacter, Conexibacter, Bacillus, Massilia, Gaiella, Acidibacter, and Vicinamibacter being the dominant genera. Afforestation significantly impacted soil bacterial alpha diversity, with notable influence from key soil chemical properties, including available phosphorus (AP), cation exchange capacity (CEC), and the carbon-to-nitrogen ratio of soil organic matter (SOM-C/N). The Mantel test highlighted pH, the Normalized Difference Vegetation Index (NDVI), and spatial variable (dbMEM) as primary environmental factors influencing dominant bacterial genera. The bacterial community structure demonstrated deterministic homogeneous selection, wherein SOM-C/N emerged as a significant factor influencing the dissimilarity of soil bacterial communities. Furthermore, plantation soils exhibited a more complex network structure than grassland soil, highlighting the crucial role of bacterial communities in vegetation changes and providing valuable insights into their response to environmental factors during the reforestation process.

Anthropogenic Dark Earth (ADE) is one of the most fertile soils in the world. ADE soils have received this nomination due to the pre-historical origin of these archaeological sites, established by pre-colombian populations. ADEs are small areas of soil which present high nutrient and organic matter contents and are randomly distributed throughout the Amazonian region. The true origin of these soils is not known yet. Due to the lack of information concerning the bacterial diversity, this work studied the bacterial diversity in ADE soils collected from two regions: Lagoa Balbina -site Terra Preta (Central Amazonia-Amazonas state) and National Forest of Caxiuan -archaeological site Mina I (Oriental Amazonia -Par state), using culture-independent molecular techniques. The total genomic DNAs extracted from the soil samples were used as templates in the PCR reactions using the universal primers for the 16S rRNA bacterial gene. The PCR-products were cloned into the pGEM-T vector and 980 clones were selected and searched using the GenBank (NCBI-USA) and the RDP II program. Data analyses indicated predominance of unknown microorganisms, representing 41.6 % among the sequences from ADE-Balbina, 68.3 % from Adjacent-Balbina, 84.8% from ADE-Mina and 47.7 % from Adjacent-Mina. The predominant phylum in ADE-Balbina was Firmicutes, representing 37.1% of the total sequences from that site, followed by Proteobacteria (9.6%), Verrucomicrobia (5.6%) Acidobacteria (2,5%), Gemmatimonadetes (2,5%), Actinobacteria (0,5%) and Nitrospira (0.5%). On the other hand, in the adjacent soil ADJ-Balbina, the predominant phylla were Proteobacteria (15.1%), Acidobacteria (12.5%), Firmicutes (2.3%), Nitrospira (1.1%) and Verrucomicrobia (0.8%). In the Oriental Amazon, the prevalent phylla from the ADE-Mina soil were Proteobacteria (6.5%), Acidobacteria (4.7%), Firmicutes (1.4%), Nitrospira (1.1%), Planctomycetes (1.1%) and Verrucomicrobia (0.4%). In the ADJ-Mina verificou a presena dos filos Acidobacteria 27.2%, Proteobacteria 14.2%, Firmicutes 3.8%, Verrucomicrobia 3.8%, Nitrospira 1.3%, Planctomycetes 1.3%, Actinobacteria 0.4% e Gemmatimonadetes 0.4%. The soil pH may be of the soil attributes which may have directly influenced the bacterial diversity in those soils, as well as the above-ground vegetation from the natural forest in Caxiuan-Par. The estimates of the Operational Taxonomic Units (OTUs) richness using Bootstrap directly corroborated the diversity values obtained from the Simpson and Shannon indexes. Unique UTOs using Jackknife estimator were correlated with a higher percentage of the low frequencies of phylla in all the four clone libraries. The nonparametric ACE and Chao1 methods to estimate the OTUs richness also corroborated the Jackknife values.

PDF HTML阅读 XML下载 导出引用 引用提醒 基于16S rDNA测序对茶园土壤细菌群落多样性的研究 DOI: 10.5846/stxb201808141733 作者: 作者单位: 云南农业大学龙润普洱茶学院,云南农业大学龙润普洱茶学院,云南农业大学龙润普洱茶学院,云南农业大学龙润普洱茶学院,云南农业大学龙润普洱茶学院,云南农业大学龙润普洱茶学院,云南农业大学资源与环境学院 作者简介: 通讯作者: 中图分类号: 基金项目: 国家自然科学基金项目（31660225，41461059）；国家现代农业茶叶产业技术体系建设专项（CARS-19） Analysis of the bacterial community and diversity in tea plantation soil via 16S rDNA sequencing Author: Affiliation: College of Longrun Pu-erh Tea,Yunnan Agricultural University,College of Longrun Pu-erh Tea,Yunnan Agricultural University,College of Longrun Pu-erh Tea,Yunnan Agricultural University,College of Longrun Pu-erh Tea,Yunnan Agricultural University,College of Longrun Pu-erh Tea,Yunnan Agricultural University,College of Longrun Pu-erh Tea,Yunnan Agricultural University,College of Resources and Environment,Yunnan Agricultural University Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:土壤细菌群落组成和多样性，对茶园土壤生态系统健康和肥力可持续性具有的重要理论意义。利用Illumina高通量测序技术测定分析16S rDNA，研究云南景迈山、布朗山和南糯山的现代茶园、古茶园（林）和森林土壤的细菌群落结构与多样性。结果表明：古茶园土壤细菌的丰度和多样性高于现代茶园及森林；研究土壤样本细菌共分属47个菌门、89个目，其中变形菌门、酸杆菌门、放线菌门、厚壁菌门与拟杆菌门是优势类群，它们在森林、现代茶园和古茶园土壤中的相对丰度累计分别达91.86%、82.48%和77.08%；伯克霍尔德氏菌目、根瘤菌目是优势菌群，其平均丰度分别达13.91%和8.17%，黄单胞菌目、红螺菌目、芽孢杆菌目、放线菌目和拟杆菌目等12个目的丰度较高，达2%以上；PCA分析表明：森林、现代茶园和古茶园土壤的细菌群落结构差异明显，除景迈山外，主要优势细菌丰度依次为：古茶园 > 现代茶园 > 森林，古茶园土壤细菌多样性有增强趋势。 Abstract:The structure and diversity of soil bacterial communities in tea plantation soil is important for maintaining a healthy, fertile, and sustainable soil ecosystem. In the present study, soil bacterial communities and their diversity in ancient and modern tea plantations and forests in Jingmai, Bulang, and Nannuo mountains in the Yunnan Province were analyzed via Illumina sequencing of 16S rDNA. Soil bacterial abundance and diversity in ancient tea plantations were higher than those in modern tea plantations and forests. Operational taxonomic units (OTUs) were assigned to 47 phyla and 89 orders. Furthermore, Proteobacteria, Acidobacteria, Actinobacteria, Firmicutes, and Bacteroidetes were the dominant phyla with relative abundances of 91.86%, 82.48% and 77.08% in forest, modern, and ancient tea plantations, respectively. At the order level, both Burkholderiales and Rhizobiales were dominant with average abundances of 13.91% and 8.17%, respectively. Moreover, the relative abundance of twelve orders including Xanthomonadales, Rhodospirillales, Bacillales, Actinomycetales, and Bacteroidales, etc., were greater than 2%. Principal component analysis revealed that soil bacterial community structures among forest, modern, and ancient tea plantations were significantly different. With the exception of the Jingmai mountain, the abundances of the primarily dominant soil bacteria were in order of ancient tea plantation > modern tea plantation > forest. Thus, the soil bacterial diversity in ancient tea plantations showed an increasing trend. 参考文献 相似文献 引证文献

Carbon nanotubes (CNTs) have huge industrial potential, and their environmental impacts need to be evaluated. Knowledge of CNT impacts on soil microbial communities is still limited. To address this knowledge gap, we systematically examined dynamic effects of one type of single-walled carbon nanotubes (SWs) and three multiwalled carbon nanotubes (MWs) with different outer diameters on the soil bacterial community in an agricultural soil over 56 days. The results showed that SWs differently affected soil bacterial abundance, diversity, and composition as compared to MWs. The differences could have resulted from the materials' distinct physical structure and surface composition, which in turn affected their bioavailability in soil. For certain treatments, soil bacterial diversity and the relative abundance of certain predominant phyla were correlated with their exposure duration. However, many phyla recovered to their initial relative abundance within 56 days, reflecting resilience of the soil bacterial community in response to CNT-induced disturbance. Further analysis at the genus level showed differential tolerance to MWs, as well as size- and dose-dependent tolerance among bacterial genera. Predictive functional profiling showed that while CNTs initially caused fluctuations in microbial community function, community function largely converged across all treatments by the end of the 56 day exposure.

The introduction of rhizoma peanut (RP Arachis glabrata Benth) into bahiagrass (Paspalum notatum Flüggé) may require time to develop stable plant–soil microbe interactions as the microbial legacy of the previous plant community may be long-lasting. A previous study showed that <2 years of introducing rhizoma peanut into bahiagrass pastures minimally affected soil bacterial diversity and community composition. In this study, we compared the effects of the long-term inclusion of rhizoma peanut (>8 years) into bahiagrass on soil bacterial diversity and community composition against their monocultures at 0 to 15 and 15 to 30 cm soil depths using next-generation sequencing to target bacterial 16S V3–V4 regions. We observed that a well-established RP–bahiagrass mixed stand led to a 36% increase in bacterial alpha diversity compared to the bahiagrass monoculture. There was a shift from a soil bacterial community dominated by Proteobacteria (~26%) reported in other bahiagrass and rhizoma peanut studies to a soil bacterial community dominated by Firmicutes (39%) in our study. The relative abundance of the bacterial genus Crossiella, known for its antimicrobial traits, was enhanced in the presence of RP. Differences in soil bacterial diversity and community composition were substantial between 0 to 15 and 15 to 30 cm soil layers, with N2-fixing bacteria belonging to the phylum Proteobacteria concentrated in 0 to 15 cm. Introducing RP into bahiagrass pastures is a highly sustainable alternative to mineral N fertilizer inputs. Our results provide evidence that this system also promotes greater soil microbial diversity and is associated with unique taxa that require further study to better understand their contributions to healthy pastures.

Soil bacterial communities are a crucial biological indicator of soil health and crop performance; however, their response to climate change remains poorly understood. Wild blueberry farms are experiencing unprecedented temperature changes, which may exacerbate microbial responses and potentially harm the crop. Here, we aimed to elucidate the response of bacterial communities in wild blueberry fields to warming. We employed passive and active open-top chambers to simulate climate warming scenarios, which elevated atmospheric temperatures by 1.2 and 3.3°C, respectively. Soils in the active warming treatment exhibited significantly lower water content than in ambient conditions. Overall soil bacterial diversity and richness under the warming (passive and active) treatments and ambient controls did not demonstrate significant differences after 2 years of warming. However, we found significantly higher bacterial evenness and diversity under warming treatments in the early growing season (June). Our study found pronounced seasonal shifts in the evenness and diversity of bacteria in wild blueberry soils, suggesting that the variation in bacterial community structure may be more influenced by seasonal changes in temperature and plant activity than by warming treatments. Our study reveals the limited impact of warming on overall soil bacterial diversity in a temperate crop, but the seasonal dynamics of bacterial diversity and composition are sensitive to warming. The increased bacterial evenness and diversity under warming treatments in June could be attributed to advanced plant phenology, indicating a potential future shift in seasonal dynamics of bacterial activity under global warming.

Current Australian legislation permits the beneficial application of grease trap waste (GTW) to agricultural soil, viewing it as a beneficial source of organic matter and soil conditioner containing no/low amounts of metals or pathogenic organisms. However, little is known about the influence of GTW on soil bacterial community. A field experiment was established at Menangle in south western Sydney in Australia to quantitatively assess the impacts of different types (GTW CO and GTW CL) and amounts of GTW application on the soil bacterial community and diversity. Furthermore, a municipal solid waste (MSW) compost was simultaneously examined to compare against the other organic wastes. Knowledge about the shifts in microbial community structure and diversity following the applications of organic wastes could help to evaluate the ecological consequences on the soil and thus to develop sound regulatory guidelines for the beneficial reuse of organic wastes in agricultural lands. Soil samples were collected from recycled organics plots treated with different types and quantity of organic wastes. The field experimental treatments included control (CK, without application of any organic wastes), low amount of GTW CO (COL), GTW CL (CLL), and MSW (ML), and high amounts of GTW CO (COH), GTW CL (CLH), and MSW compost (MH). Microbial DNA was extracted from soil samples and the 16S rRNA genes were polymerase chain reaction (PCR)-amplified. The PCR products were analyzed by denaturing gradient gel electrophoresis (DGGE), cloning, and sequencing. The bacterial community structures and diversity were assessed using the DGGE profiles and clone libraries constructed from the excised DGGE bands. DGGE-based analyses showed that application of the GTW CO, regardless of the amount applied, had significant negative effects on soil bacterial genotypic diversity and community structure compared with the control, while the applications of other organic wastes including the GTW CL and MSW had no clear effects. The effects of the rate of organic waste application on soil bacterial community characteristics varied with the types of organic wastes applied. Sequence-based analyses of 126 clones indicated that Proteobacteria (53.2%) was the dominant taxa at the experimental site, followed by Actinobacteria (9.5%), Bacteroidetes (7.9%), Firmicutes (7.9%), Gemmatimonadetes (5.6%), Chloroflexi (2.4%), Acidobacteria (1.6%) and the unclassified group (11.9%). In the COH treatment, Acidobacteria, Bacteroidetes, and Gemmatimonadetes were not detected; the percentages of Firmicutes, Proteobacteria, and Actinobacteria in the COH treatment were significantly different from those in CK. There is a significant positive correlation (r = 0.71, p = 0.002) between the C/N ratio of organic wastes and the bacterial genotypic communities. Both the type and the amount of GTW applied affected soil bacterial genotypic diversity and community structure. The different effects of various types of organic wastes on soil bacterial characteristics may be predicted by the differences in specific properties of organic wastes such as C/N ratio, as evidenced by the strong and significant positive relationship between the bacterial community distance and the environmental distance of C/N ratio. This also indicates that the C/N ratio of GTW applied can be a major driver for the shift in the soil bacterial community. Our results revealed that the effects of organic wastes on soil bacterial communities varied with the types of organic wastes, and depending on the rate of application. Application of the GTW CO led to significant shifts in soil bacterial community diversity and structure. The effects of different types of organic wastes on the soil bacterial characteristics can be predicted by the differences of specific properties of organic wastes, such as the C/N ratio. Sequence-based analyses of 126 clones indicated that Proteobacteria was the dominant taxa at the experimental site. Our results have important implications for developing sound regulatory guidelines for the beneficial reuse of organic wastes, indicating that GTW CO and similar organic waste treatments may not be suitable for application in agricultural soils due to its significant negative effect on soil bacterial community.

Application of organic fertilizers or their combination with chemical fertilizers is a feasible practice for improving soil fertility and reducing soil degradation in agroecosystems, and these regulations are mainly mediated though soil microbial communities. Despite bacteria ranking among the most abundant and diverse groups of soil microorganisms, the effects of long-term organic fertilization (OF) and chemical–organic fertilization (COF) on soil bacterial diversity and community composition remain unclear. In this study, we conducted a meta-analysis and demonstrated that OF had no significant effect on bacterial alpha diversity. Application of chemical fertilizer and crop residue significantly decreased bacterial Richness index. Both OF and COF significantly altered bacterial community structure, with these changes being predominately attributed to shifts in soil pH. For bacterial phyla, both OF and COF significantly increased the relative abundance of Proteobacteria and Bacteroidetes, suggesting that OF and COF may cause the enrichment of copiotrophic taxa. In addition, COF significantly increased the relative abundance of Gammaproteobacteria but decreased the relative abundance of Acidobacteria. Overall, our results suggest that organic and chemical–organic fertilization can effectively maintain bacterial diversity and enhance soil fertility in agroecosystems, and the alteration of soil bacterial community structure is closely intertwined with soil pH.

Understanding soil bacterial diversity under nitrogen reduction is necessary for the crucial role in soil nitrogen cycling. However, the effects of combined fertilization on soil chemical properties, microbial community structure, and yield are unknown. This study was conducted to investigate the effect of nitrogen fertilizer reduction with bio-organic fertilizer on soil bacterial community diversity of red raspberry orchard. Six treatments were set in this study: NF-100%, NF-75%, NF-50%, NF-25% and CF, no nitrogen fertilizer and bio-organic fertilizer for CK. The bacterial community structures of soil were analyzed by 16S rRNA gene amplification high-throughput sequencing technology. Nitrogen fertilizer reduction with bio-organic fertilizer increased soil organic matter (SOM), total nitrogen (TN), alkali-hydrolyzable nitrogen (AN), available phosphorus (AP), available potassium (AK), and reduced soil pH. NF-50% and NF-25% treatments increased the yield of red raspberry. Nitrogen reduction combined with bio-organic fertilizer increased the relative abundance of copiotrophic bacteria and decreased the relative abundance of oligotrophic bacteria. The increase in copiotrophic bacteria in the soil of red raspberry orchard could indicate an increase in soil nutrient availability, which have positive implications for soil fertility and production. However, nitrogen fertilizer reduction with bio-organic fertilizer altered the abundance and diversity of soil bacteria, which was reduced compared to CF treatments. The PCoA analysis of the soil bacterial community showed that the community structure of NF-25% treatment was more different from other treatments, indicating that the fertilization method changed the community structure of soil bacteria. The results of a redundancy analysis showed that SOM, pH, AN, TN, and AP were the main factors affecting the microbial community structure. Overall, the reduction of nitrogen fertilizer with bio-organic fertilizer significantly increased the soil nutrient content, reduced the relative abundance and diversity of soil bacteria, increased the relative abundance of beneficial bacteria in the soil, changed the bacterial community structure of soil, increased production and created suitable soil conditions for the red raspberry growth.

Change in soil bacterial community during secondary succession depend on plant and soil characteristics

Altitudinal changes in soil bacterial diversity, composition, biotic interactions, and function are prevalent. However, the overall patterns and associations among these dimensions remain unclear, particularly in vulnerable alpine mountain ecosystems. Here, we investigated soil bacterial communities along a high-altitude gradient to elucidate patterns and associations in taxonomic and phylogenetic diversity, co-occurrence networks, and functional potentials. We observed increasing altitudinal trends in bacterial richness and phylogenetic diversity, along with significant differences in taxonomic and phylogenetic composition across altitudes. The connectivity component of the co-occurrence network properties showed a negative association with altitude. We also observed high redundancy in functional potentials, resulting in insignificant variation in functional diversity along the altitudinal gradient. However, the strength of functional diversity varied based on the interaction between network connectivity and phylogenetic diversity. Additionally, functional dissimilarity was more closely associated with phylogenetic rather than taxonomic dissimilarity or differences in network properties, highlighting the role of phylogenetic lineages in functional redundancy. This study characterizes the altitudinal distribution of soil bacteria and explores their covariations, enhancing our understanding of soil bacterial diversity and functional potentials along altitudinal gradients and providing valuable insights for predicting community changes and improving alpine ecosystem conservation.

Understanding ecosystem respiration (Reco) response to various management practices is important for the sustainable development of solar greenhouse systems, which represent a large and profitable industry in Northwest China. A tomato (Lycopersicon esculentum Mill.)–cucumber (Cucumis sativus L.)–tomato rotation experiment was conducted with four treatments including two irrigation levels (regular irrigation [RI] and low irrigation [LI]) and two N fertilizer sources (organic‐N fertilizer [N1] and inorganic‐N fertilizer [N2]). Ecosystem respiration and soil variables were measured multiple times. Mean Reco flux was 511, 319, and 437 mg m–2 h–1 for tomato‐2016, cucumber‐2016, and tomato‐2017, respectively. Soil temperature was the most important factor affecting Reco magnitude. Tomato seasons with higher temperature had significantly greater Reco flux than the cucumber season. Organic‐N fertilizer application can stimulate Reco flux, cumulative Reco, and Reco intensity mainly by providing favorable soil N and C condition for soil bacterial growth and activity, whereas the difference between irrigation levels was inconsistent. The fitted exponential function describing the relationship between Reco and soil temperature showed that the temperature sensitivity decreased from tomato‐2016 to tomato‐2017 for all treatments, corresponding to the continuously reduced soil bacterial alpha diversity. Soil bacterial Shannon index was an effective prediction factor for Reco in the relatively warmer and drier season, with an R2 &gt;.8 during tomato‐2017. The RIN1 treatment consistently had the greatest cumulative Reco and the highest soil bacterial alpha diversity throughout the rotation. Further studies are needed on the mechanisms of how the change of soil properties would regulate the response of Reco to soil abiotic and biotic factors.

One of soil microbiology's most intriguing puzzles is how so many different bacterial species can coexist in small volumes of soil when competition theory predicts that less competitive species should decline and eventually disappear. We provide evidence supporting the theory that low pore connectivity caused by low water potential (and therefore low water content) increases the diversity of a complex bacterial community in soil. We altered the pore connectivity of a soil by decreasing water potential and increasing the content of silt- and clay-sized particles. Two textures were created, without altering the chemical properties or mineral composition of the soil, by adding silt- and clay-sized particles of quartz to a quartz-based sandy soil at rates of 0% (sand) or 10% (silt+clay). Both textures were incubated at several water potentials, and the effect on the active bacterial communities was measured using terminal restriction fragment length polymorphism (TRFLP) of bacterial 16S rRNA. Bacterial richness and diversity increased as water potential decreased and soil became drier (P < 0.012), but they were not affected by texture (P > 0.553). Bacterial diversity increased at water potentials of <or=2.5 kPa in sand and <or=4.0 kPa in silt+clay, equivalent to <or=56% water-filled pore space (WFPS) in both textures. The bacterial community structure in soil was affected by both water potential and texture (P < 0.001) and was correlated with WFPS (sum of squared correlations [delta(2)] = 0.88, P < 0.001). These findings suggest that low pore connectivity is commonly experienced by soil bacteria under field conditions and that the theory of pore connectivity may provide a fundamental principle to explain the high diversity of bacteria in soil.