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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 <2.5 kPa in sand and <4.0 kPa in silt؉clay, equivalent to <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 [␦ 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.

Soil microbial community characterization is increasingly being used to determine the responses of soils to stress and disturbances and to assess ecosystem sustainability. However, there is little experimental evidence to indicate that predictable patterns in microbial community structure or composition occur during secondary succession or ecosystem restoration. This study utilized a chronosequence of developing jarrah (Eucalyptus marginata) forest ecosystems, rehabilitated after bauxite mining (up to 18 years old), to examine changes in soil bacterial and fungal community structures (by automated ribosomal intergenic spacer analysis [ARISA]) and changes in specific soil bacterial phyla by 16S rRNA gene microarray analysis. This study demonstrated that mining in these ecosystems significantly altered soil bacterial and fungal community structures. The hypothesis that the soil microbial community structures would become more similar to those of the surrounding nonmined forest with rehabilitation age was broadly supported by shifts in the bacterial but not the fungal community. Microarray analysis enabled the identification of clear successional trends in the bacterial community at the phylum level and supported the finding of an increase in similarity to nonmined forest soil with rehabilitation age. Changes in soil microbial community structure were significantly related to the size of the microbial biomass as well as numerous edaphic variables (including pH and C, N, and P nutrient concentrations). These findings suggest that soil bacterial community dynamics follow a pattern in developing ecosystems that may be predictable and can be conceptualized as providing an integrated assessment of numerous edaphic variables.

LettersFine endophytes (Glomus tenue) are related to Mucoromycotina, not GlomeromycotaFine endophytes are arbuscule-producing fungi of unclear phylogenetic placement Fine endophytes (FE), Glomus tenue, are traditionally considered to be arbuscular mycorrhizal fungi (AMF) with distinctive microscopic morphology when stained. FE have fine hyphae (c. 1.5 lm diameter) which branch intra-cellularly in a distinctive fan-like pattern (Gianinazzi-Pearson et al., 1981;Abbott, 1982). The hyphae contain small swellings along their length, sometimes referred to as vesicle-like swellings (Hall, 1977). FE form arbuscules (or arbuscule-like structures) with fine elements in a tapered, conical shape (Greenall, 1963;Merryweather & Fitter, 1998). Spores of FE are very small (< 20 lm) compared to the majority of Glomeromycota, and colourless (Hall, 1977). Morphological variations indicate that FE may consist of multiple species (Thippayarugs et al., 1999), hence we use the term FE to indicate a species group.Within the kingdom Fungi, both morphological and genetic characteristics are used to determine taxonomic classification (St€ urmer, 2012). In 2001, all AMF were placed within the phylum Glomeromycota (Sch€ ußler et al., 2001). In the listing of glomeromycotan species by Sch€ ußler & Walker (2010), some members of the genus Glomus were not revised due to insufficient taxonomic knowledge, and this included FE. A key reason for classifying FE within the Glomeromycota was the presence of arbuscules, considered apomorphic for the phylum (Morton, 1990). However, the morphological features of root colonization by FE are distinct from other, coarse, AMF so their placement within the genus Glomus and the Glomeromycota was questioned (Hall, 1977;Sch€ ußler & Walker, 2010), and their status as mycorrhizal fungi is ambivalent.Accurate determination of FE usually requires magnification ≥ 9100, hence, where assessments of AMF colonization use lower magnifications they may not be identified. Furthermore, FE may be undetected if samples are not processed within 2 d of harvesting (Orchard et al., 2016a). Nevertheless, FE are globally distributed and prolific within many ecosystems, examples include: pastures and native bushland of New Zealand (Crush, 1973) and Australia (Abbott & Robson, 1982;McGee, 1989), Venezuelan cloud forests (Rabatin et al., 1993), riverine and alpine regions of Europe (Read & Haselwandter, 1981;Turnau et al., 1999;Binet et al., 2011) and an old-field in the United States (Hilbig & Allen, 2015). However, the difficulty of isolating and, hence, genetically characterizing FE has hindered the determination of their phylogenetic placement. A novel method to enrich colonization by fine endophytesTo clarify the identity of FE, we targeted the SSU (18S) ribosomal RNA gene using roots from two independent glasshouse experiments where individual pots contained multiple plants. For each pot we used one root system for DNA extraction and one root system to visually assess the percentage of total root length colonized (%TRL; see Sup...

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