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Promoting seed decay is an ecological approach to reducing weed persistence in the soil seedbank. Previous work demonstrated that Fusarium avenaceum F.a.1 decays dormant Avena fatua (wild oat) caryopses and induces several defense enzyme activities in vitro. The objectives of this study were to obtain a global perspective of proteins expressed after F.a.1-caryopsis colonization by conducting proteomic evaluations on (i) leachates, soluble extrinsic (seed-surface) proteins released upon washing caryopses in buffer and (ii) proteins extracted from whole caryopses; interactions with aluminum (Al) were also evaluated in the latter study because soil acidification and associated metal toxicity are growing problems. Of the 119 leachate proteins classified as defense/stress, 80 were induced or repressed. Defense/stress proteins were far more abundant in A. fatua (35%) than in F.a.1 (12%). Avena defense/stress proteins were also the most highly regulated category, with 30% induced and 35% repressed by F.a.1. Antifungal proteins represented 36% of Avena defense proteins and were the most highly regulated, with 36% induced and 37% repressed by F.a.1. These results implicate selective regulation of Avena defense proteins by F.a.1. Fusarium proteins were also highly abundant in the leachates, with 10% related to pathogenicity, 45% of which were associated with host cell wall degradation. In whole caryopsis extracts, fungal colonization generally resulted in induction of a similar set of Avena proteins in the presence and absence of Al. Results advance the hypothesis that seed decay pathogens elicit intricate and dynamic biochemical responses in dormant seeds.

Land application of biosolids is a primary means of recycling human waste products globally; however, because biosolids are also heavily enriched with nutrients and pollutants, it is necessary to consider the environmental impacts of land applications in various agro-ecosystems. To reduce costs and divert waste, biosolids from Joint Base Lewis-McChord, a military installation south of Tacoma, WA, USA, are composted with organic materials derived from the base itself. The potential for turfgrass management using the composted biosolids was tested in a field study at the military recreation facility, Eagles Pride golf course. The composted biosolids were surface-amended to golf course fairways (n = 3) at a rate of 46 Mg ha−1, in split applications, fall and spring, over two years, 2015 and 2016. Soil bacterial and fungal community responses were evaluated in amended and unamended soils from each fairway, along with soil chemical properties, including soil pH, total carbon (C), total nitrogen (N), and the C:N ratio. Soil microbial community analysis on soils sampled pre- and post-amendment did not demonstrate changes in community structure as a result of the composted biosolids application. Differences observed in soil microbial community structure appears to reflect edaphic and vegetative differences between fairways. Generally, correlations between soil properties and bacterial community OTUs were stronger than those with fungal OTUs, and predicted bacterial community functional analysis revealed several KO term groups were significantly correlated with soil TC and TN. Turfgrass biomass was unaffected, while leaf tissue N was increased by 4.5 and 10.6% in 2015 and 2016, respectively. These results suggest that amendment with composted biosolids does not strongly impact microbial communities across two years; however, long-term amendment could have affects that are not detected during this short-term study.

In wheat (Triticum aestivum L.)-based cropping systems, rotation crops often provide a yield benefit to the following wheat crop. This has been documented for legume crops and canola (Brassica napus L.). In the present study, we examined bacterial and fungal communities in the rhizosphere of wheat and canola at six paired locations on fall and spring sampling dates using high-throughput sequencing of marker genes (bacterial 16S rRNA gene and fungal internal transcribed spacers). Soil type was the primary driver of both fungal and bacterial communities, with the communities of locations geographically closest, and with the same soil type, clustering together. Season (fall vs spring sampling) had the next strongest effect, followed by the crop species. Differences between the wheat and canola were more evident in the spring than in the fall. However, in contrast, bacterial and fungal species richness and diversity were not related to location or season, and instead were consistently higher in the rhizosphere of canola compared to wheat. Many bacterial groups were shared across both crop species, especially Janthinobacterium and Kaistobacter. Bacterial communities of wheat in the fall were dominated by Actinomycetes and Acidobacteria. But in the spring, these communities shifted to fast growing bacterial copiotrophs, including Pseudomonas, Janthinobacterium, Flavobacterium, Oxalobacteraceae, and Sphingobacteriaceae. Saprotrophic fungi such as Ulocladium, Mortierella, Cryptoccocus, Chaetomium, Penicillium and Trichoderma were also common to, and equally abundant in, the rhizosphere of both crops. Potential wheat root pathogens such as Thanatephorus (Rhizoctonia) Ceratobasidium, Oculimacula, Typhula, and Microdochium were more abundant in the wheat rhizosphere compared to canola. In conclusion, most of the communities of fungi and bacteria are not “host specific” but colonize the roots of both wheat and canola with differing relative abundance dependent on the season. A much smaller group of taxa were unique to the specific crop. This work provides novel insight into changes in crop rhizosphere communities when canola is introduced into dryland monoculture wheat cropping systems.

Soils under continuous no-till (NT) often produce stratified soil acidification due to fertilizer placement and subsequent lack of mixing. Because aluminum (Al) becomes more bioavailable with lower soil pH, Al toxicity is an emerging problem in many soils throughout the Palouse region in the dryland cropping areas of the inland Pacific Northwest of the United States. When various lime materials are surface applied to NT soils, the depth to which these materials may impact soil pH, available Al, and microbial community response has not been explored in the fine-silty soils of this arid region. The objectives of the current study were to: (1) determine if soil sampling at small, discrete depth increments captures stratification of chemical and microbial properties in NT soils and (2) evaluate the impacts of two types of surface-broadcast lime on Al availability and microbial community composition at each depth interval. In April 2014 and 2015 (6 mo and 18 mo after liming), soil samples were collected at 2-cm depth increments and analyzed for soil pH and metals bioavailability, as well as microbial community composition using terminal restriction fragment length polymorphism analysis. In 2015, surface soil (0–2 cm) pH increased from 4.6 in untreated control to 6.6 in limed plots, and KCl-extractable Al decreased from approximately 50 mg kg−1 to below 10 mg kg−1 in the 2–4 cm depth of soil. The surface soil microbial community composition changed significantly in response to lime application, and was highly correlated with soil pH and Al concentrations. No significant effects of treatment were detected when data were pooled across the 10 cm depth. The 2-cm sampling increments provided statistically significant resolution in soil chemical stratification and related shifts in microbial community composition, revealing different effects at different depths. We are able to conclude from this study that (1) A finely resolved, spatially explicit sampling design is necessary to capture soil chemical stratification complexity within a variety of variables, and microbial community response over time; and (2) Surface application of lime in Palouse NT systems will take >18 mo to be effective at depths below 6 cm.

Plant physical and chemical characteristics are known to alter rhizosphere microbial communities, but the effect of introducing canola (Brassica napus L.) into monoculture wheat (Triticum aestivum L.) rotations is not clear. Results from a field study in eastern Washington showed that winter canola (WC) influenced the bulk soil microbial community and differentiated it from the community associated with winter wheat (WW). Abundance of soil fungi, including mycorrhizae, was reduced with the introduction of WC. The objective of this research was to determine the differences and similarities in the rhizosphere microbial communities of WC and WW. Canola and wheat rhizosphere soil was collected from six dryland farms in Adams and Douglas Counties, WA. Each farm was a paired site with WC and WW grown in adjacent fields of the same soil type, landscape orientation, and crop history. Canola, or any non-cereal crop, had never been grown previously at the experimental sites. Rhizosphere microbial biomass and community composition, determined using phospholipid fatty acid analysis (PLFA), revealed differences associated with landscape position at the initial fall sampling when WC and WW were in the rosette and tiller development stages, respectively. However, data from spring samples, when WC and WW were in the early bolting and stem elongation growth stages, respectively, showed significant differences in microbial communities between WC and WW rhizosphere soils. Data suggest that initial (fall) microbial community composition were an artifact of previous histories of monocrop wheat production and varied with expected differences in landscape position. As the crops developed, microbial communities became more dissimilar and were differentiated by crop species. Our results show that WC can have significant effects on rhizosphere microbial biomass of specific microbial groups and community structure in wheat-based cropping systems, such as reductions in fungi and gram+ bacteria in some locations. Crop-related changes in the abundance of specific rhizosphere microbial groups may play a key role in the yield of subsequent crops, and overall soil health and nutrient turnover.

Leaf chlorosis in vineyards is associated with reduced crop yields and quality. While iron (Fe) is understood to play a crucial role in chlorosis, total plant and soil Fe are not always indicative of chlorosis in grapevines. Physiology of chlorosis in vineyards has been well-studied, but the soil microbial consequences of and contributions to chlorosis have received little attention. We used next-generation sequencing (NGS) to examine the bacterial and fungal communities associated with grapevines demonstrating varying degrees of visual chlorosis symptoms. Additionally, chemical analyses of soils and grape leaves were used to explore the influence of plant nutritional status and soil chemistry on microbial community composition. Finally, factors influencing bacterial community composition were correlated with predicted bacterial community function. Leaf tissue magnesium (leaf Mg) concentrations and chlorosis rank were correlated with bacterial community composition as determined via dbRDA (distance-based Redundancy Analysis) using Bray-Curtis dissimilarities. Non-metric multidimensional scaling (NMDS) revealed a significant correlation between fungal community composition and soil Fe and pH, along with leaf N, Mg, and Ca (mg.kg-1). Chlorosis rank was moderately correlated with KEGG Orthology (KO) terms associated with nitrogen (N) and carbon (C) metabolism in soils, while leaf Mg was associated with a spectrum of KO terms including glycosphingolipid biosynthesis, glycan degradation, transporters, and porphyrin and chlorophyll metabolism. Additionally, abundance of many bacterial operational taxonomic units was significantly correlated with leaf Mg, including those from the following orders: Rhodobacterales, Acidobacteriales, Opitutales, Sphingomonadales, Burkholderiales, Saprospirales, and Flavobacteriales. Our findings suggest grapevine chlorosis is interrelated with soil microbial community structure and function, plant nutrition, and soil chemistry.

Fusarium avenaceum F.a.1 is a novel strain of a fungal plant pathogen capable of preferentially decaying wild oat (Avena fatua) caryopses compared with those of wheat (Triticum aestivum). Understanding the molecular mechanisms governing weed seed-pathogen interactions is crucial to developing novel weed seed suppression technologies. Additionally, wild oat often competes with wheat in regions undergoing soil acidification, which leads to increases in soluble concentrations of many metals, including aluminum (Al). There is a dearth of information regarding the gene expression responses of Fusarium species to Al toxicity, or how metal toxicity might influence caryopsis colonization. To address this, a transcriptomic approach was used to investigate molecular responses of F.a.1 during wild oat caryopsis colonization in the presence and absence of chronic, sublethal concentrations of Al (400 μM). Caryopsis colonization was associated with induction of genes related to virulence, development, iron metabolism, oxidoreduction, stress, and detoxification, along with repression of genes associated with development, transport, cell-wall turnover, and virulence. Caryopsis colonization during Al exposure resulted in the induction of genes associated with virulence, detoxification, stress, iron metabolism, oxidoreduction, and cell wall turnover, along with repression of genes associated with cell wall metabolism, virulence, development, detoxification, stress, and transcriptional regulation. Aluminum exposure in the absence of caryopses was associated with induction of genes involved in siderophore biosynthesis, secretion, uptake, and utilization, along with several other iron metabolism-related and organic acid metabolism-related genes. The siderophore-related responses associated with Al toxicity occurred concurrently with differential regulation of genes indicating disruption of iron homeostasis. These findings suggest Al toxicity is attenuated by siderophore metabolism in F.a.1. In summary, both caryopsis colonization and Al toxicity uniquely influence transcriptomic responses of F.a.1.

With increasing canola (Brassica napus L.) acreage in the Inland Pacific Northwest of the USA, we investigated the effect of this relatively new rotational crop on soil microbial communities and the performance of subsequent wheat (Triticum aestivum L.) crops. In a 6-year on-farm canola-wheat rotation study conducted near Davenport, WA, grain yields of spring wheat (SW) following winter canola (WC) were reduced an average of 17% compared to SW yields following winter wheat (WW). Using soil samples collected and analyzed every year from that study, the objective of this research was to determine the differences and similarities in the soil microbial communities associated with WC and WW, and if those differences were associated with SW yield response. Microbial biomass and community composition were determined using phospholipid fatty acid analysis (PLFA). The WC-associated microbial community contained significantly less fungi, mycorrhizae, and total microbial biomass than WW. Additionally, reduced fungal and mycorrhizal abundance in SW following WC suggests that the canola rotation effect can persist. A biocidal secondary metabolite of canola, isothiocyanate, may be a potential mechanism mediating the decline in soil microbial biomass. These results demonstrate the relationship between soil microbial community composition and crop productivity. Our data suggest that WC can have significant effects on soil microbial communities that ultimately drive microbially mediated soil processes.

One of the primary goals of organic agriculture is increasing soil quality through the enhancement of soil biological diversity and activity. Greater soil microbial activity and diversity increase soil organic matter turnover and contribute to soil fertility, one of the main challenges associated with organic management. The objectives of this study were to 1) compare soil microbial abundance and activity between organic and conventional cropping systems, and 2) explore connections between soil microbial community indicators and crop productivity in organic and conventional winter wheat (Triticum aestivum)/spring wheat/winter pea (Pisum sativum) rotations. Soil and plant tissue was sampled following six years of organic and conventional management, and soil was analyzed for microbial abundance and activity. Fungal and bacterial abundance, soil enzyme activity, and soil organic carbon (C) were greater in the organic system than in the conventional system, and all four measures were positively correlated. Community-level physiological profiling (CLPP) indicated that C substrate utilization was greater in the organic than in the conventional system, though bacterial T-RFLP data did not demonstrate different community structure between systems, suggesting that management type affected bacterial community function, but not structure. Fungal T-RFLP results indicated that fungal community structure was different between the organic and conventional systems. Hay yield and tissue nitrogen (N) were greater in the organic system, and were positively correlated with fungal and bacterial abundance, but grain yield and protein were greater in the conventional system. The results of this study indicate that management type (organic vs. conventional) has implications for microbial abundance and microbial community function, and that differences in soil microbial abundance and activity likely impact crop yields and N uptake.

Siderophores (low-molecular weight metal chelating compounds) are important in various ecological phenomenon ranging from iron (Fe) biogeochemical cycling in soils, to pathogen competition, plant growth promotion, and cross-kingdom signaling. Furthermore, siderophores are also of commercial interest in bioleaching and bioweathering of metal-bearing minerals and ores. A rapid, cost effective, and robust means of quantitatively assessing siderophore production in complex samples is key to identifying important aspects of the ecological ramifications of siderophore activity, including, novel siderophore producing microbes. The method presented here was developed to assess siderophore activity of in-tact microbiome communities, in environmental samples, such as soil or plant tissues. The samples were homogenized and diluted in a modified M9 medium (without Fe), and enrichment cultures were incubated for 3 days. Siderophore production was assessed in samples at 24, 48, and 72 hours (h) using a novel 96-well microplate CAS (Chrome azurol sulphonate)-Fe agar assay, an adaptation of the traditionally tedious and time-consuming colorimetric method of assessing siderophore activity, performed on individual cultivated microbial isolates. We applied our method to 4 different genotypes/Lines of wheat (Triticum aestivum L.), including Lewjain, Madsen, and PI561725, and PI561727 commonly grown in the inland Pacific Northwest. Siderophore production was clearly impacted by the genotype of wheat, and in the specific types of plant tissues observed. We successfully used our method to rapidly screen for the influence of plant genotype on siderophore production, a key function in terrestrial and aquatic ecosystems. We produced many technical replicates, yielding very reliable statistical differences in soils and within plant tissues. Importantly, the results show the proposed method can be used to rapidly examine siderophore production in complex samples with a high degree of reliability, in a manner that allows communities to be preserved for later work to identify taxa and functional genes.

For over three centuries, soil scientists and microbiologists have been studying the most diverse habitat on the planet, characterizing the structure, function, and composition of the soil microbial community through a diverse array of techniques. Yet our understanding of the soil microbiome and the reciprocal dynamics with metal speciation and bioavailability remain primarily limited to model systems and isolated organisms. Metals, however, are ubiquitous in the environment, pervasive in agriculture, and essential to life functions. The purpose of this chapter is to review our existing knowledge on the soil factors that control metal bioavailability and bring that knowledge together with our understanding of dynamics of metals and metalloids with microbial communities in soil to highlight the current gaps in metal-microbiome research. Ultimately, restoration of ecosystem function, enhanced soil health and quality, and any type of management for microbial metal transformations will only be possible with a thorough understanding of how soil microbiomes interact with each other, the soil, their associated plant communities, and the impacts these interactions have on the molecular details underlying their biogeochemical function.

Soil acidification is a global issue that often results in increased aluminum (Al) toxicity. While no-till (NT) management has many benefits regarding sustainability, a discrete zone of acidification often occurs when ammoniacal fertilizers are banded below the seed. The full agroecological consequences of NT stratification and impacts on bacterial communities are largely unknown. Using next-generation sequencing (NGS) and Phylogenetic Investigation of Communities by Reconstruction of Unobserved States (PICRUSt), we characterized the influence of liming amendment and soil stratification on bacterial community composition and predicted function in 2-cm depth increments. Soil depth, pH, DTPA extractable aluminum (DTPA-Al), and KCl extractable Al (KCl-Al) were all significantly correlated with bacterial community structure and function. In soils with the lowest pH and greatest extractable Al, bacterial community was distinct, with highest relative abundance of the Koribacteraceae family, an indicator of soil degradation. Additionally, aspects of bacterial metabolism and nutrient turnover were impacted in the lowest pH zones, including secondary metabolite, carbohydrate, and energy metabolism. These results suggest that soil stratification (Al and pH) in NT systems has direct impacts on microbial community structure and function, potentially influencing ecosystem services at a highly resolved spatial scale within surface depths relevant to seed germination and emergence.

Because Zn, Cd, and Cu are chalcophilic (sulfur-loving) heavy-metals, understanding the spatial variability in soil chemical variables and relationships with soil microbial community composition will lead to a better understanding of their cycling and availability. In this study, we evaluate field-scale spatial variability in soil chemical parameters and relationships with the soil microbial community composition in the Elba muck soils of Western NY which are drained annually. In May, 2005, soil samples were collected along a series of transects in order to capture spatial heterogeneity across the site (approximate field dimensions: 36 × 90 m). Soils were collected at two depths, 0–15 and 15–30 cm, and analyzed for % H 2 0, pH, total, as well as 0.01 M CaCl 2 -extractable S, Zn, Cd, and Cu. Soil Bacteria and Archaea community composition were characterized by terminal restriction fragment length polymorphism (T-RFLP) analysis. Field hedgerow willow ( Salix purpurea L.) tissues were also analyzed for total S, Zn, Cd, and Cu. Spatial variability across the field was vast within all measured parameters. Soil pH, total soil S, extractable soil Cu and Zn exerted the greatest influence on the composition of the soil microbial communities, and willow tissue concentrations reflected bioavailable fractions of soil heavy metals. Our results demonstrate the intricate link between soil chemistry and soil microbial community composition over a field scale in organic soils with high soil S and heavy metals content.

Summary Soil aggregate stability is an important ecosystem property which deteriorates overtime due to agricultural practices. The cessation of cultivation allows the potential recovery of soil aggregate binding agents such as soil micro-organisms and biochemical properties. Consequently, an increase in soil aggregate stability is expected. However, this outcome is difficult to predict because the response of each individual soil component and its contribution to soil stability varies. This study utilized a chronosequence of 12 ex-arable fields in the Bolivian Altiplano, representing six soil ages of abandonment after cessation of potato cultivation, to examine whether soil aggregate stability increases after abandonment and the extent to which changes in soil bacterial and fungal community composition and soil chemical properties are involved in stability recovery. Fields with the longest time since disturbance (15 and 20 years) have a greater proportion of water-stable aggregates than more recently abandoned fields (1 and 3 years) and exhibit larger differences in bacterial and fungal composition. Total N, , C and organic matter also increased with time since the last intensive agricultural practice. Water-stable aggregates were strongly correlated with soil fungal community composition. Analysis of covariance is also consistent with the soil fungal community being an important mediator of the recovery of aggregate stability. Synthesis and applications. Soil aggregate stability increased by 50% over the 20 years following disturbance. This recovery was associated with shifts in soil fungal community composition, as is consistent with fungal mediation of this recovery. Land management strategies focusing on restoration of the soil fungal community may enhance soil aggregate stability, a key aspect for soil conservation, restoration, sustainability of agroecosystems and erosion prevention.

On numerous occasions, rhizosphere microbial activities have been identified as a key factor in metal phytoavailability to various plant species and in phytoremediation of metal-contaminated sites. For soil bioremediation efforts in heavy metal contaminated areas, microbes adapted to higher concentrations of heavy metals are required. This study was a field survey undertaken to examine rhizosphere microbial communities and biogeochemistry of soils associated with Zn accumulation by indigenous willows (Salix purpurea L.) in the naturally metalliferous peat soils located near Elba, NY. Soil and willow leaf samples were collected from seven points, at intervals 18 m apart along a willow hedgerow, on four different dates during the growing season. Soil bacterial community composition was characterized by terminal restriction fragment length polymorphism (T-RFLP) analysis and a 16S clone library was created from the rhizosphere of willows and soils containing the highest concentrations of Zn. Bacterial community composition was correlated with soil sulfate, but not with soil pH. The clone library revealed comparable phylogenetic associations to those found in other heavy metal-contaminated soils, and was dominated by affiliations within the phyla Acidobacteria (32%), and Proteobacteria (37%), and the remaining clones were associated with a wide array of phyla including Actinobacteria, Gemmatimonadetes, Planctomycetes, Verrucomicrobia, Bacteriodetes, and Cyanobacteria. Diverse microbial populations were present in both rhizosphere and bulk soils of these naturally metalliferous peat soils with community composition highly correlated to the soil sulfate cycle throughout the growing season indicative of a sulfur-oxidizing rhizosphere microbial community. Results confirm the importance of soil characterization for informing bioremediation efforts in heavy metal contaminated areas and the reciprocity that microbial communities uniquely adapted to specific conditions and heavy metals may have on an ecosystem.

Chlorine is the most widely used carcass sanitizer in poultry processing in the USA. The objective of this study was to determine the effects of varying concentrations of organic matter on the susceptibility of twelve most prevalent poultry-associated Salmonella serotypes (MPPSTs) to chlorine. To mimic the microenvironment of the water used for immersion chilling, we manipulated organic matter contamination levels in pre-chilled (pH∼6, T∼4 °C) chlorinated (50 ppm) water using varying concentrations (0, 1, 2, 3, 4, and 5%) of chicken-meat-extract (CME) produced from frozen chicken carcasses. This CME-based in vitro model was challenged with ∼1 × 105 CFUs of each MPPST isolate and the bacterial survival was tested at 5, 30, 60 and 90 min post-challenge. In this model, the decimal reduction time (D90-values) of each MPPST was linearly correlated with the concentration of CME. Significant inter-serotype differences in the D90-values were observed. The results show that the pH, concentration of total- and free-chlorine were also linearly correlated with the presence of CME in a concentration-dependent manner. The findings of this study indicate that the serotype and the levels of organic matter contamination significantly influence Salmonella survival and that both variables should be included in models that predict effectiveness of chlorine treatment in immersion chilling.

Anthropogenic sources of lead contamination in soils include mining and smelting activities, effluents and wastes, agricultural pesticides, domestic garbage dumps, and shooting ranges. While Pb is typically considered relatively insoluble in the soil environment, some fungi may potentially contribute to mobilization of heavy metal cations by means of secretion of low-molecular-weight organic acids (LMWOAs). We sought to better understand the potential for metal mobilization within an indigenous fungal community at an abandoned shooting range in Oak Ridge, TN, where soil Pb contamination levels ranged from 24 to >2,700 mg Pb kg dry soil −1 . We utilized culture-based assays to determine organic acid secretion and Pb-carbonate dissolution of a diverse collection of soil fungal isolates derived from the site and verified isolate distribution patterns within the community by 28S rRNA gene analysis of whole soils. The fungal isolates examined included both ascomycetes and basidiomycetes that excreted high levels (up to 27 mM) of a mixture of LMWOAs, including oxalic and citric acids, and several isolates demonstrated a marked ability to dissolve Pb-carbonate at high concentrations up to 10.5 g liter −1 (18.5 mM) in laboratory assays. Fungi within the indigenous community of these highly Pb-contaminated soils are capable of LMWOA secretion at levels greater than those of well-studied model organisms, such as Aspergillus niger . Additionally, these organisms were found in high relative abundance (>1%) in some of the most heavily contaminated soils. Our data highlight the need to understand more about autochthonous fungal communities at Pb-contaminated sites and how they may impact Pb biogeochemistry, solubility, and bioavailability, thus consequently potentially impacting human and ecosystem health.

Iron (Fe) deposition from African dust has been implicated in a variety of environmental impacts on downwind terrestrial and marine ecosystems throughout the Caribbean. The most abundant form of Fe in African dust is FeIII, which is often not bioavailable. The objective of this study was to determine to what degree microorganisms isolated from African dust collected in Trinidad and Tobago are capable of producing siderophores that mobilize bioavailable Fe into the environment. Aerosol samples were collected for microbial analyses during African dust conditions in the source region (Mali) and downwind sites (Trinidad and Tobago). Microbial community fingerprints, obtained by means of terminal restriction fragment length polymorphism analysis, were compared among aerosol samples and possible Trinidadian sources of locally aerosolized microorganisms (sea water and soils). Ordination of the fingerprint data revealed similarities between aerosols from the source region and the aerosols and soils of downwind regions. Aerosol isolates from the downwind sites were screened for siderophore production using a modified chrome azurol-S (CAS) assay. Twenty-five percent of isolates tested that were sampled under non-dust conditions and 65% of African dust isolates produced at least one type of siderophore; among African dust isolates, all known classes of siderophores were produced. These data support African dust microorganism siderophore production as a viable mechanism by which Fe bioavailability may be increased in downwind locations, given appropriate conditions for microbial proliferation.

Approximately 70,150 dry Mg of biosolids from over 450 wastewater treatment facilities are applied to the semi-arid rangelands of Colorado every year. Research on semi-arid grassland responses to biosolids has become vital to better understand ecosystem dynamics and develop effective biosolids management strategies. The objectives of this study were to determine the long-term (w12 years) effects of a single biosolids application, and the short-term (w2 years) effects of a repeated application, on plant and microbial community structure in a semi-arid grassland soil. Specific attention was paid to arbuscular mycorrhizal fungi (AMF) and linkages between shifts in plant and soil microbial community structures. Biosolids were surface applied to experimental plots once in 1991 (long-term plots) and again to short-term plots in 2002 at rates of 0, 2.5, 5, 10, 21, or 30 Mg ha K1 . Vegetation (species richness and above-ground biomass), soil chemistry (pH, EC, total C, total N, and extractable P, NO3‐N, and NH4‐N), and soil microbial community structure [ester-linked fatty acid methyl esters (EL-FAMEs)], were characterized to assess impacts of biosolids on the ecosystem. Soil chemistry was significantly affected and shifts in both soil microbial and plant community structure were observed with treatment. In both years, the EL-FAME biomarker for AMF decreased with increasing application rate of biosolids; principal components analysis of EL-FAME data yielded shifts in the structure of the microbial communities with treatment primarily related to the relative abundance of the AMF specific biomarker. Significant (p%0.05) correlations existed among biomarkers for Gram-negative and Gram-positive bacteria, AMF and specific soil chemical parameters and individual plant species’ biomass. The AMF biomarker was positively correlated with biomass of the dominant native grass species blue grama (Bouteloua gracilis [Willd. ex Kunth] Lagasca ex Griffiths) and was negatively correlated with western wheatgrass (Agropyron smithii Rydb.) biomass. This study demonstrated that applications of biosolids at relatively low rates can have significant long-term effects on soil chemistry, soil microbial community structure, and plant community species richness and structure in the semi-arid grasslands of northern Colorado. Reduced AMF and parallel shifts in the soil microbial community structure and the plant community structure require further investigation to determine precisely the sequence of influence and resulting ecosystem dynamics. q 2005 Elsevier Ltd. All rights reserved.

A plot study was conducted to quantify long-term (>12 years) impacts of a single biosolids application, and short-term impacts (