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Self-succession of winter wheat (WW) in crop rotations results in substantial yield decline. This decline has been mostly attributed to the soil-borne fungus Gaeumannomyces graminis var. tritici (Ggt; take-all) causing earlier root senescence. A broad shift in the soil microbial community has also recently been proposed to confound this effect even in years without significant Ggt infestation in the field. We aimed to establish a mechanistic basis for the relationship between rotational position of WW and yield decline at an early wheat growth stage. To this end, an outdoor experiment with 1 m deep rhizotrons was set up using a sandy loam soil. WW was grown in soil after oilseed rape (KW1), soil after one season of WW (KW2) and soil after three successive seasons of WW (KW4). The plants were grown until the beginning of stem elongation (BBCH 30). At harvest, both shoot and root dry weight were markedly affected by the preceding crop, with a pronounced reduction of plant biomass of KW2 (-43%) and KW4 (-45%) compared to KW1. At BBCH 30, KW1 soil had much lower mineral N compared to KW2 (-49%) and KW4 (-39%). Non-purgeable organic C, a readily available energy source for soil microorganisms, was further reduced in successive WW rotations compared to KW1. Increased NH4+ and NPOC concentrations were found in root-affected soil compared to root-free bulk soil, indicating a strong hotspot for organic N mineralization in the rhizosphere. At the same time, the markedly higher shoot N concentration led to a lower C:N ratio of 31 for KW1 compared to KW2 and KW4, which had a C:N ratio of 46 and 44, respectively, suggesting a better exploitation of soil mineral N sources by KW1. In contrast, microbial biomass C and N were higher in KW2 and KW4 compared to KW1, pointing to enhanced microbial N immobilization in KW2 and KW4. The higher C:N ratio of WW straw compared to oilseed rape residues that are returned to the soil following harvest, obviously stimulated immobilization of soil N in microbial biomass, thereby limiting the availability of N for WW growth in KW2 and KW4. Root growth traits exhibited a strong response to WW rotational position, with higher root tissue density, root mean diameter and lower specific root length for KW1 compared to KW2 and KW4. Root length density (RLD) was overall higher in KW1 compared to KW2 (-29%) and KW4 (-31%), especially at 0-30 cm soil depth. Interestingly, higher RLD values for KW1 were also observed at the lowest depth of 60-100 cm compared to KW4, suggesting a strong effect of rotational position on nutrient accessibility in the subsoil. Successive WW invested more in acquisitive root traits that did not compensate for the reduction of biomass production. Our results highlight the effect of rotational position of WW on soil and plant properties and provide guidance for management-based adaptations at field level to improve WW productivity.

Since the resources for mineral fertilizers are limited and the energy costs have recently risen considerably, there is an increasing interest in closing nutrient cycles and the recycling of nitrogen and phosphorus in secondary raw materials. Nutrient-rich waste materials can be used directly as a fertilizer or after processing. If other valuable ingredients such as organic carbon are in the product after processing, pollutants can be present as well. In the present work, contamination by antibiotics will be discussed. Antibiotics are used in large quantities to treat bacterial infections in humans but also in animals for food production. Resistant or even multi-resistant strains can occur in feces if antibiotics are regularly used in higher quantities. It was the aim of the study to show which recycled organic fertilizers contain a particularly high amount of antibiotics and how this might affect the relative abundance of resistance genes and mobile genetic elements. Sewage sludge and composted sewage sludge, digestate with animal excrements, residues from food production and processed fertilizers such as ash products or struvite were studied. Exemplary, 14 antibiotics out of three classes were analyzed. Especially fluoroquinolones and tetracyclines are present at higher concentrations in fertilizers of animal origin. In processed fertilizers, the concentrations depend not only on the origin but also on the carbon content, since antibiotics can bind to organic matter.

We investigated the effects of substrate (cellulose or starch) and different clay contents on the production of microbial extracellular polymeric substances (EPS) and concomitant development of stable soil aggregates. Soils were incubated with different amounts of montmorillonite (+ 0.1%, + 1%, + 10%) both with and without two substrates of contrasting quality (starch and cellulose). Microbial respiration (CO2), biomass carbon (C), EPS-protein, and EPS-polysaccharide were determined over the experimental period. The diversity and compositional shifts of microbial communities (bacteria/archaea) were analysed by sequencing 16S rRNA gene fragments amplified from soil DNA. Soil aggregate size distribution was determined and geometric mean diameter calculated for aggregate formation. Aggregate stabilities were compared among 1–2-mm size fraction. Starch amendment supported a faster increase than cellulose in both respiration and microbial biomass. Microbial community structure and composition differed depending on the C substrate added. However, clay addition had a more pronounced effect on alpha diversity compared to the addition of starch or cellulose. Substrate addition resulted in an increased EPS concentration only if combined with clay addition. At high clay addition, starch resulted in higher EPS concentrations than cellulose. Where additional substrate was not provided, EPS-protein was only weakly correlated with aggregate formation and stability. The relationship became stronger with addition of substrate. Labile organic C thus clearly plays a role in aggregate formation, but increasing clay content was found to enhance aggregate stability and additionally resulted in the development of distinct microbial communities and increased EPS production.

The profound intensification of agricultural practices by increased application of agro-chemicals, short crop rotations and ploughing resulted in loss of soil fertility, erosion and accumulation of soil-borne plant pathogens. Soil microbial communities are key players in ecosystem processes and are intimately linked to crop productivity and health. Thus a better understanding of how farming practices affect soil microbiota is needed in order to promote sustainable agriculture. The long-term field trial in Bernburg (Germany) established in 1992 provides a unique opportunity to assess the effects of i) the crop (maize vs. rapeseed) preceding the actual winter wheat culture, ii) tillage practice (mouldboard plough vs. cultivator tillage) and iii) standard nitrogen (N)-fertilization intensity with application of growth regulators and fungicides (intensive) compared to reduced N-fertilization without growth regulators and fungicides (extensive). We hypothesized that these different farming practices affect the soil prokaryotic community structures with consequences for their functional potential. Total community-DNA was extracted directly from soils sampled at wheat harvest. Illumina sequencing of 16S rRNA genes amplified from total community-DNA revealed a significant effect of tillage practice and the preceding crop on prokaryotic community structures, whereas the influence of N-fertilization intensity was marginal. A number of differentially abundant prokaryotic genera and their predicted functions between mouldboard plough vs. cultivator tillage as well as between different preceding crops were identified. Compared to extensive N-fertilization, intensive N-fertilization resulted in higher abundances of bacterial but not of archaeal amoA genes, that are involved in ammonia oxidation. Our data suggest that long-term farming strategies differently shape the soil prokaryotic community structure and functions, which should be considered when evaluating agricultural management strategies regarding their sustainability, soil health and crop performance.

The effects of tillage on soil structure, physiology and microbiota structure were studied in a long-term field experiment, with side-to-side plots, established to compare effects of conventional tillage (CT) vs no-till (NT) agriculture. After 27 years, part of the field under CT was switched to NT and vice versa. Soil texture, soil enzymatic profiles and the prokaryotic community structure (16S rRNA genes amplicon sequencing) were analyzed at two soil depths (0–5 and 5–10 cm) in samples taken 6, 18 and 30 months after switching tillage practices. Soil enzymatic activities were higher in NT than CT, and enzymatic profiles responded to the changes much earlier than the overall prokaryotic community structure. Beta diversity measurements of the prokaryotic community indicated that the levels of stratification observed in long-term NT soils were already recovered in the new NT soils 30 months after switching from CT to NT. Bacteria and Archaea OTUs that responded to NT were associated with coarse soil fraction, soil organic carbon and C cycle enzymes, while CT responders were related to fine soil fractions and S cycle enzymes. This study showed the potential of managing the soil prokaryotic community and soil health through changes in agricultural management practices.

Conventional tillage and mineral fertilization (CTMF) jeopardize soil health in conventional vegetable production systems. Using a field experiment established in Uruguay in 2012, we aimed to compare the soil restoration potential of organic fertilization (compost and poultry manure) combined with conventional tillage and cover crop incorporated into the soil (CTOF) or with reduced tillage and the use of cover crop as mulch (RTOF). In 2017, table beet was cultivated under CTMF, CTOF and RTOF, and yields, soil aggregate composition and nutrients, as well as soil and table beet rhizosphere microbiota (here: bacteria and archaea) were evaluated. Microbiota was studied by high-throughput sequencing of 16S rRNA gene fragments amplified from total community DNA. RTOF exhibited higher soil aggregation, soil organic C, nutrient availability and microbial alpha-diversity than CTMF, and became more similar to an adjacent natural undisturbed site. The soil microbiota was strongly shaped by the fertilization source which was conveyed to the rhizosphere and resulted in differentially abundant taxa. However, 229 amplicon sequencing variants were found to form the core table beet rhizosphere microbiota shared among managements. In conclusion, our study shows that after only 5 years of implementation, RTOF improves soil health under intensive vegetable farming systems.

Long-term agricultural practices are assumed to shape the rhizosphere microbiome of crops with implications for plant health. In a long-term field experiment, we investigated the effect of different tillage and fertilization practices on soil and barley rhizosphere microbial communities by means of amplicon sequencing of 16S rRNA gene fragments from total community DNA. Differences in the microbial community composition depending on the tillage practice, but not the fertilization intensity were revealed. To examine whether these soil and rhizosphere microbiome differences influence the plant defense response, barley (cultivar Golden Promise) was grown in field or standard potting soil under greenhouse conditions and challenged with Blumeria graminis f. sp. hordei (Bgh). Amplicon sequence analysis showed that preceding tillage practice, but also aboveground Bgh challenge significantly influenced the microbial community composition. Expression of plant defense-related genes PR1b and PR17b was higher in challenged compared to unchallenged plants. The Bgh infection rates were strikingly lower for barley grown in field soil compared to potting soil. Although previous agricultural management shaped the rhizosphere microbiome, no differences in plant health were observed. We propose therefore that the management-independent higher microbial diversity of field soils compared to potting soils contributed to the low infection rates of barley.

Fertilization management affects not only plant performance but also soil microbiota, with specific rhizosphere interactions influenced by root activity, still poorly understood. We assumed that long-term fertilization exerts selective effects on rhizodeposition with consequences for recruitment of rhizosphere microbiota and plant performance. To address this hypothesis, we conducted a minirhizotron-experiment with lettuce as model plant, using field soils with contrasting properties from two long-term field experiments (HUB-LTE: loamy sand, DOK-LTE: silty loam) with organic and mineral fertilization history. Although no consistent fertilization effects were detectable on alpha-diversity of fungal and prokaryotic/archaeal rhizosphere communities, increased abundance of plant-beneficial arbuscular-mycorrhizal fungi was characteristic for the rhizospheres of the organically managed soils (HU-org; BIODYN2). However, their rhizosphere microbiota also comprised more fungal pathotrophs, and the plants systemically expressed defense-related genes in shoot tissues. However, there was no consistent effect of fertilization history on shoot biomass and root development, which remained unaffected in HUB-LTE. In the BIODYN2 soil, shoot and root growth was severely suppressed and affected by Olpidium infection, coinciding with the highest rhizosphere abundance of the pathogen (89 %). This was associated with a reduction of potentially plant growth-promoting microorganisms, such as Pseudomonaceae and Mortierella elongata by 87 and 97 %. These profound alterations of rhizosphere microbial communities were linked with characteristic changes in the composition of the rhizosphere solution. Olpidium-induced growth suppression and low rhizosphere abundance of Pseudomonadaceae were associated with reduced rhizosphere concentrations of the antifungal root exudate benzoic acid. This metabolite was described in the literature to be increased by Pseudomonas inoculation. By contrast, high abundance of Pseudomonadaceae (Gammaproteobacteria) in the rhizosphere of plants with long-term mineral fertilization (61-74 %) coincided with high rhizosphere concentrations of chemotactic dicarboxylates (succinate, malate) and a high C (sugar)/N (amino acid) ratio, known to support the growth of Gammaproteobacteria. Under these conditions no plant growth depressions were recorded even in presence of a high Olpidium abundance (76 %) in the rhizosphere. Our results suggest a complex network of belowground interactions between crop roots, site-specific factors and rhizosphere microbiota, modulating the impact of fertilization management with consequences for plant health and crop performance.

A better understanding of factors shaping the rhizosphere microbiota is important for sustainable crop production. We hypothesized that the effect of agricultural management on the soil microbiota is reflected in the assemblage of the rhizosphere microbiota with implications for plant performance. We designed a growth chamber experiment growing the model plant lettuce under controlled conditions in soils of a long-term field experiment with contrasting histories of tillage (mouldboard plough vs cultivator tillage), fertilization intensity (intensive standard nitrogen (N) + pesticides/growth regulators vs extensive reduced N without fungicides/growth regulators), and last standing field crop (rapeseed vs winter wheat). High-throughput sequencing of bacterial and archaeal 16S rRNA genes and fungal ITS2 regions amplified from total community DNA showed that these factors shaped the soil and rhizosphere microbiota of lettuce, however, to different extents among the microbial domains. Pseudomonas and Olpidium were identified as major indicators for agricultural management in the rhizosphere of lettuce. Long-term extensive fertilization history of soils resulted in higher lettuce growth and increased expression of genes involved in plant stress responses compared to intensive fertilization. Our work adds to the increasing knowledge on how soil microbiota can be manipulated by agricultural management practices which could be harnessed for sustainable crop production.

Soil microbial communities are key players of ecosystem processes and important for crop and soil health. The Humid Pampas region in Argentina concentrates 75% of the national soybean production, which is based on intensive use of agrochemicals, monocropping and no-till. A long-term field experiment under no-till management in the southeast of the Argentinean Pampas provides a unique opportunity to compare soybean under monocropping with cultivation including alternating cover crops or in a three-phase rotation. We hypothesized that cropping regimes and season affect soil microbial community composition and diversity. Amplicon sequencing of 16S rRNA genes and internal transcribed spacer fragments showed a stronger microbial seasonal dynamic in conservation regimes compared to monocropping. In addition, several bacterial (e.g. Catenulispora, Streptomyces and Bacillus) and fungal genera (e.g. Exophiala) with cropping regime-dependent differential relative abundances were identified. Despite a temporal shift in microbial and chemical parameters, this study shows that long-term cropping regimes shaped the soil microbiota. This might have important implications for soil quality and soybean performance and should therefore be considered in the development of sustainable agricultural managements.

Individual plant species form so-called resource islands in the barren semiarid landscape, whereby many soil properties are enhanced including soil structure. Within the soil structure, mostly studied as soil aggregates, microaggregates (During two sampling campaigns in spring 2017 and 2018, soil samples were taken over a distance gradient from two major semiarid grassland plant species in Southern Spain, the legume shrub Anthyllis cytisoides and the grass tussock Macrochloa tenacissima, to the surrounding bare soil. While topsoil was sampled in five distances to the plant during the first sampling campaign, the second one focused stronger on the root influence, hence rhizoplane and rhizosphere were sampled. Additionally, two sites with different parent materials were chosen to scale the effect of EPS on soil aggregation in the presence of inorganic binding agents (here carbonates). Total community DNA and EPS were extracted, followed by quantification of EPS-saccharides and bacterial abundance, as well as examination of the prokaryotic community composition by Illumina amplicon sequencing of the 16S rRNA genes. Further, the particle size distribution of (micro)aggregates in water was determined, with and without ultrasound treatment, as a measure of soil aggregate size distribution and stability.Based on the first sampling campaign, we found that the overall prokaryotic community composition differed between the two sites, but not between plant species. Interdependencies between the community composition and EPS content were revealed, whereby soil organic matter (SOM) seems to be a regulating factor as increasing SOM contents resulted in more EPS-saccharides. Soil microaggregation in the topsoil was enhanced by the plant canopy, especially at the edge of Macrochloa tussocks. Contrary to the expectation that increasing inorganic C contents would diminish the importance of EPS, the parent material richest in inorganic C results in a significant effect of EPS-saccharides on microaggregation.First results of the second sampling campaign indicate that even in the rhizoplane and rhizosphere, parent material had a dominating influence on the prokaryotic community composition. As EPS-saccharide contents and soil aggregation followed a similar decreasing trend with distance to the roots and canopy cover, interdependencies are expected.From the outcomes until now, we can conclude that the availability of decomposable OM influences the prokaryotic community composition and thereby triggers EPS production, whereas large contents of polyvalent cations from carbonates promote the stabilizing effect of EPS on microaggregates.

International audience; Long-term agricultural fertilization strategies gradually change soil properties including the associated microbial communities. Cultivated crops recruit beneficial microbes from the surrounding soil environment via root exudates. In this study, we aimed to investigate the effects of long-term fertilization strategies across field sites on the rhizosphere prokaryotic (Bacteria and Archaea) community composition and plant performance. We conducted growth chamber experiments with lettuce (Lactuca sativa L.) cultivated in soils from two long-term field experiments, each of which compared organic versus mineral fertilization strategies. 16S rRNA gene amplicon sequencing revealed the assemblage of a rhizosphere core microbiota shared in all lettuce plants across soils, going beyond differences in community composition depending on field site and fertilization strategies. The enhanced expression of several plant genes with roles in oxidative and biotic stress signalling pathways in lettuce grown in soils with organic indicates an induced physiological status in plants. Lettuce plants grown in soils with different fertilization histories were visibly free of stress symptoms and achieved comparable biomass. This suggests a positive aboveground plant response to belowground plant-microbe interactions in the rhizosphere. Besides effects of fertilization strategy and field site, our results demonstrate the crucial role of the plant in driving rhizosphere microbiota assemblage.

Our understanding of the interactions between minerals, organic matter, and microorganisms at so-called biogeochemical interfaces in soil is still hampered by the inherent complexity of these systems. Artificial soil maturation experiments can help to bridge a gap in complexity between simple abiotic sorption experiments and larger-scale field experiments. By controlling other soil-forming factors, the effect of a particular variable can be identified in a simplified system. Here, we review the findings of a series of artificial soil incubation experiments with the aim of revealing general trends and conclusions. The artificial soils were designed to determine the effect of mineral composition and charcoal presence on the development of abiotic and biotic soil properties during maturation. In particular, the development of soil aggregates, organic matter (OM) composition and turnover, sorption properties, and the establishment of microbial community composition and function were considered. The main objectives of the research were to determine (1) how surface properties and sorption of chemicals modify biogeochemical interfaces ; (2) how much time is required to form aggregates from mixtures of pure minerals, OM, and a microbial inoculum ; and (3) how the presence of different mineral and charcoal surfaces affects aggregation, OM turnover, and the development of microbial community composition.

Biocontrol inoculants often show inconsistency in their efficacy at field scale and the reason for this remains often unclear. A high rhizosphere competence of inoculant strains is assumed to be a key factor for successful biocontrol effects as the biocontrol strain has to compete with the indigenous microbial community in the rhizosphere. It is known that many factors, among them plant species and soil type shape the rhizosphere microbial community composition. However, microbial community composition in the rhizosphere can also be influenced by the presence of a pathogen. We hypothesized that plant species, soil type, and a pathogen affect the rhizosphere competence of a biocontrol strain and its biocontrol effect against a soil-borne pathogen. To test the hypothesis, we used an experimental plot system with three soil types (diluvial sand, alluvial loam, loess loam) kept under similar agricultural management at the same field site for 12 years. We investigate the rhizosphere competence of Pseudomonas sp. RU47 in two plant species (potato and lettuce) and its biocontrol effect against Rhizoctonia diseases. The colonization density of a rifampicin resistant mutant of RU47 in the rhizosphere of both crops was evaluated by plate counts. Bacterial community compositions were analyzed by denaturing gradient gel electrophoresis (DGGE) of 16S rRNA gene fragments amplified from total community DNA. The inoculant RU47 was able to colonize the rhizosphere of both model crops in a sufficient density and to reduce disease severity of black scurf on potato and bottom rot on lettuce in all three soils. DGGE indicated that RU47 affected the bacterial community composition stronger in the rhizosphere of lettuce than in the potato rhizosphere. In contrast, the effect of the pathogen Rhizoctonia solani on the bacterial community was much stronger in the rhizosphere of potato than in the lettuce rhizosphere. A significant effect of RU47 on the Pseudomonas-specific gacA fingerprints of the rhizosphere was only observed in lettuce in alluvial soil. The soil type and plant species independent biocontrol effects of RU47 and its minor influence on the indigenous bacterial community composition might be important criteria for the registration and use of RU47 as biocontrol strain.

The study of interactions between minerals, organic matter (OM) and microorganisms is essential for the understanding of soil functions such as OM turnover. Here, we present an interdisciplinary approach using artificial soils to study the establishment of the microbial community and the formation of macro-aggregates as a function of the mineral composition by using artificial soils. The defined composition of a model system enables to directly relate the development of microbial communities and soil structure to the presence of specific constituents. Five different artificial soil compositions were produced with two types of clay minerals (illite, montmorillonite), metal oxides (ferrihydrite, boehmite) and charcoal incubated with sterile manure and a microbial community derived from a natural soil. We used the artificial soils to analyse the response of these model soil systems to additional sterile manure supply (after 562 days). The artificial soils were subjected to a prolonged incubation period of more than two years (842 days) in order to take temporally dynamic processes into account. In our model systems with varying mineralogy, we expected a changing microbial community composition and an effect on macro-aggregation after OM addition, as the input of fresh substrate will re-activate the artificial soils. The abundance and structure of 16S rRNA gene and internal transcribed spacer (ITS) fragments amplified from total community DNA were studied by quantitative real-time PCR (qPCR) and denaturing gradient gel electrophoresis (DGGE), respectively. The formation of macro-aggregates (>2 mm), the total organic carbon (OC) and nitrogen (N) contents, the OC and N contents in particle size fractions and the CO2 respiration were determined. The second manure input resulted in higher CO2 respiration rates, 16S rRNA gene and ITS copy numbers, indicating a stronger response of the microbial community in the matured soil-like system. The type of clay minerals was identified as the most important factor determining the composition of the bacterial communities established. The additional OM and longer incubation time led to a re-formation of macro-aggregates which was significantly higher when montmorillonite was present. Thus, the type of clay mineral was decisive for both microbial community composition as well as macro-aggregation, whereas the addition of other components had a minor effect. Even though different bacterial communities were established depending on the artificial soil composition, the amount and quality of the OM did not show significant differences supporting the concept of functional redundancy.

Biocontrol inoculants often show inconsistency in their efficacy at field scale and the reason for this remains often unclear. A high rhizosphere competence of inoculant strains is assumed to be a key factor for successful biocontrol effects as the biocontrol strain has to compete with the indigenous microbial community in the rhizosphere. It is known that many factors, among them plant species and soil type shape the rhizosphere microbial community composition. However, microbial community composition in the rhizosphere can also be influenced by the presence of a pathogen. We hypothesized that plant species, soil type, and a pathogen affect the rhizosphere competence of a biocontrol strain and its biocontrol effect against a soil-borne pathogen. To test the hypothesis, we used an experimental plot system with three soil types (diluvial sand, alluvial loam, loess loam) kept under similar agricultural management at the same field site for 12 years. We investigate the rhizosphere competence of Pseudomonas sp. RU47 in two plant species (potato and lettuce) and its biocontrol effect against Rhizoctonia diseases. The colonization density of a rifampicin resistant mutant of RU47 in the rhizosphere of both crops was evaluated by plate counts. Bacterial community compositions were analyzed by denaturing gradient gel electrophoresis (DGGE) of 16S rRNA gene fragments amplified from total community DNA. The inoculant RU47 was able to colonize the rhizosphere of both model crops in a sufficient density and to reduce disease severity of black scurf on potato and bottom rot on lettuce in all three soils. DGGE indicated that RU47 affected the bacterial community composition stronger in the rhizosphere of lettuce than in the potato rhizosphere. In contrast, the effect of the pathogen Rhizoctonia solani on the bacterial community was much stronger in the rhizosphere of potato than in the lettuce rhizosphere. A significant effect of RU47 on the Pseudomonas-specific gacA fingerprints of the rhizosphere was only observed in lettuce in alluvial soil. The soil type and plant species independent biocontrol effects of RU47 and its minor influence on the indigenous bacterial community composition might be important criteria for the registration and use of RU47 as biocontrol strain.

Phenoxyacetic acids can be degraded by diverse soil microorganisms. Nevertheless, we miss information about the succession of 4-chloro-2-methylphenoxyacetic acid (MCPA) degraders in micro-environments of soils as well as specific functions of different microbial groups during MCPA degradation. We studied MCPA degradation at the soil-litter interface in a microcosm experiment and followed the succession of different degrader populations by quantifying the abundance of 16S rRNA genes as well as, the fungal ITS fragment and the functional genes tfdA (in total and divided into three classes) and cadA. Adjacent to the litter layer, a dynamic depletion zone of MCPA indicated that the litter effect on MCPA degradation depends on substrate availability and the affected soil volume. The increase of the tfdA class III and cadA genes was linked to MCPA mineralisation. Total abundance of tfdA genes was dominated by class I MCPA degraders and did not reflect MCPA degradation potential of the soil. Litter addition induced the development of pioneer and late-stage fungal communities, which were probably both involved in MCPA degradation. The results underline the importance of the ecological behaviour of different degrader populations for the understanding of herbicide degradation in soils.

To study the influence of the clay minerals montmorillonite (M) and illite (I), the metal oxides ferrihydrite (F) and aluminum hydroxide (A), and charcoal (C) on soil bacterial communities, seven artificial soils with identical texture provided by quartz (Q) were mixed with sterilized manure as organic carbon source before adding a microbial inoculant derived from a Cambisol. Bacterial communities established in artificial soils after 90 days of incubation were compared by DGGE analysis of bacterial and taxon-specific 16S rRNA gene amplicons. The bacterial community structure of charcoal-containing soils highly differed from the other soils at all taxonomic levels studied. Effects of montmorillonite and illite were observed for Bacteria and Betaproteobacteria, but not for Actinobacteria or Alphaproteobacteria. A weak influence of metal oxides on Betaproteobacteria was found. Barcoded pyrosequencing of 16S rRNA gene amplicons done for QM, QI, QIF, and QMC revealed a high bacterial diversity in the artificial soils. The composition of the artificial soils was different from the inoculant, and the structure of the bacterial communities established in QMC soil was most different from the other soils, suggesting that charcoal provided distinct microenvironments and biogeochemical interfaces formed. Several populations with discriminative relative abundance between artificial soils were identified.

Microbial communities in soil reside in a highly heterogeneous habitat where diverse mineral surfaces, complex organic matter and microorganisms interact with each other. This study aimed to elucidate the long-term effect of the soil mineral composition and charcoal on the microbial community composition established in matured artificial soils and their response to phenanthrene. One year after adding sterile manure to different artificial soils and inoculating microorganisms from a Cambisol, the matured soils were spiked with phenanthrene or not and incubated for another 70 days. 16S rRNA gene and internal transcribed spacer fragments amplified from total community DNA were analyzed by denaturing gradient gel electrophoresis. Metal oxides and clay minerals and to a lesser extent charcoal influenced the microbial community composition. Changes in the bacterial community composition in response to phenanthrene differed depending on the mineral composition and presence of charcoal, while no shifts in the fungal community composition were observed. The abundance of ring-hydroxylating dioxygenase genes was increased in phenanthrene-spiked soils except for charcoal-containing soils. Here we show that the formation of biogeochemical interfaces in soil is an ongoing process and that different properties present in artificial soils influenced the bacterial response to the phenanthrene spike.

The fate of polycyclic aromatic hydrocarbons (PAHs) in soil is determined by a suite of biotic and abiotic factors, and disentangling their role in the complex soil interaction network remains challenging. Here, we investigate the influence of soil composition on the microbial community structure and its response to the spiked model PAH compound phenanthrene and plant litter. We used long-term matured artificial soils differing in type of clay mineral (illite, montmorillonite) and presence of charcoal or ferrihydrite. The soils received an identical soil microbial fraction and were incubated for more than two years with two sterile manure additions. The matured artificial soils and a natural soil were subjected to the following spiking treatments: (I) phenanthrene, (II) litter, (III) litter + phenanthrene, (IV) unspiked control. Total community DNA was extracted from soil sampled on the day of spiking, 7, 21, and 63 days after spiking. Bacterial 16S rRNA gene and fungal internal transcribed spacer amplicons were quantified by qPCR and subjected to denaturing gradient gel electrophoresis (DGGE). DGGE analysis revealed that the bacterial community composition, which was strongly shaped by clay minerals after more than two years of incubation, changed in response to spiked phenanthrene and added litter. DGGE and qPCR showed that soil composition significantly influenced the microbial response to spiking. While fungal communities responded only in presence of litter to phenanthrene spiking, the response of the bacterial communities to phenanthrene was less pronounced when litter was present. Interestingly, microbial communities in all artificial soils were more strongly affected by spiking than in the natural soil, which might indicate the importance of higher microbial diversity to compensate perturbations. This study showed the influence of soil composition on the microbiota and their response to phenanthrene and litter, which may increase our understanding of complex interactions in soils for bioremediation applications.