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Microorganisms residing on root surfaces play a central role in plant development and performance and may promote growth in agricultural settings. Studies have started to uncover the environmental parameters and host interactions governing their assembly. However, soil microbial communities are extremely diverse and heterogeneous, showing strong variations over short spatial scales. Here, we quantify the relative effect of meter-scale variation in soil bacterial community composition among adjacent field microsites, to better understand how microbial communities vary by host plant genotype as well as soil microsite heterogeneity. We used bacterial 16S rDNA amplicon sequencing to compare rhizosphere communities from four Brassica rapa cultivars grown in three contiguous field plots (blocks) and evaluated the relative contribution of resident soil communities and host genotypes in determining rhizosphere community structure. We characterize concomitant meter-scale variation in bacterial community structure among soils and rhizospheres and show that this block-scale variability surpasses the influence of host genotype in shaping rhizosphere communities. We identified biomarker amplicon sequence variants (ASVs) associated with bulk soil and rhizosphere habitats, each block, and three of four cultivars. Numbers and percent abundances of block-specific biomarkers in rhizosphere communities far surpassed those from bulk soils. These results highlight the importance of fine-scale variation in the pool of colonizing microorganisms during rhizosphere assembly and demonstrate that microsite variation may constitute a confounding effect while testing biotic and abiotic factors governing rhizosphere community structure.

Microorganisms residing on root surfaces play a central role in plant development and performance and may promote growth in agricultural settings. Studies have started to uncover the environmental parameters and host interactions governing their assembly. However, soil microbial communities are extremely diverse and heterogeneous, showing strong variations over short spatial scales. Here, we quantify the relative effect of meter-scale variation in soil bacterial community composition among adjacent field microsites, to better understand how microbial communities vary by host plant genotype as well as soil microsite heterogeneity. We used bacterial 16S rDNA amplicon sequencing to compare rhizosphere communities from four Brassica rapa cultivars grown in three contiguous field plots (blocks) and evaluated the relative contribution of resident soil communities and host genotypes in determining rhizosphere community structure. We characterize concomitant meter-scale variation in bacterial community structure among soils and rhizospheres and show that this block-scale variability surpasses the influence of host genotype in shaping rhizosphere communities. We identified biomarker amplicon sequence variants (ASVs) associated with bulk soil and rhizosphere habitats, each block, and three of four cultivars. Numbers and percent abundances of block-specific biomarkers in rhizosphere communities far surpassed those from bulk soils. These results highlight the importance of fine-scale variation in the pool of colonizing microorganisms during rhizosphere assembly and demonstrate that microsite variation may constitute a confounding effect while testing biotic and abiotic factors governing rhizosphere community structure. [ABSTRACT FROM AUTHOR]

Abstract: In forest soils, the availability of phosphate is largely dependent on phosphatase activity. We used soil imprinting to compare in situ activity and fine-scale distribution of phosphatase on soil profiles located across forest chronosequences of four age classes young (5–6 yrs), canopy closure (24–30 yrs), stem exclusion (61–71 yrs), and older (90–103 yrs) of mixed Douglas-fir/paper birch stands regenerated after fire or clearcutting in southern interior British Columbia, Canada. Chromatography paper treated with a mixture of substrate and colorimetric reagent was applied directly to vertical soil surfaces, accessed through root windows. Stands older than 61 years had both the highest level of in situ phosphatase activity and larger, more intense regions of activity. Bray-extractable phosphorus was negatively related to imprintable phosphatase activity. We compared the changes in phosphatase activity with differences in the ectomycorrhizal fungal (EMF) community that had been documented previously in the same stands. Of 84 ectomycorrhizal fungi found on roots in at least two of the stand-age classes, eight taxa were positively correlated and one taxon (Rhizopogon vinicolor/vesiculosus) negatively correlated with high phosphatase activity. The frequency of three taxa appeared to be positively correlated with larger areas of activity on the soil profiles. By using an imprinting approach, this study was able to demonstrate, for the first time, that in situ phosphatase activity and physical attributes of that activity (i.e., number, size, and relative rates of each area of activity) were related to concentrations of soil nutrients and with the frequency of individual ectomycorrhizal fungi. [Copyright &y& Elsevier]

Field-scale observations of two upland soils derived from contrasting granite and basalt bedrocks are presented to hypothesize that redox activity of rhizospheres exerts substantial effects on mineral dissolution and colloidal translocation in many upland soils. Rhizospheres are redox-active microsites and in the absence of O2, oxidation of rhizodeposits can be coupled by reduction of redox-active species such as Fe, a biogenic reduction that leads to Fe translocation and oxidation, accompanied by substantial proton flux. Not only do rhizogenic Fe–C redox cycles demonstrate a process by which the rhizosphere affects an environment well outside the near-root zone, but these redox processes are also hypothesized to be potent weathering systems, such that rhizogenic redox-reactions complement acid- and ligand-promoted reactions as major biogeochemical processes that control crustal weathering. The potential significance of Fe–C redox cycling is underscored by the deep and extensive rooting and mottling of upland subsoils across a wide range of plant communities, lithologies, and soil-moisture and temperature regimes. [ABSTRACT FROM AUTHOR]