Soil Microbial Components Research Articles

Retention Levels and Years-After-Harvesting Influence over Soil Microbial Activity and Biomass in Southern Patagonian Forests

Variable retention harvesting (VRH) was designed for timber purposes and biodiversity conservation in natural forests. This system was globally tested, but few studies are related to soil microbial components. The objective was to evaluate different retention types (aggregated and dispersed retention) considering different years-after-harvesting (6, 9, 16 YAH) on soil microbial community attributes compared with unmanaged primary forests (PF) in Nothofagus pumilio forests of Tierra del Fuego (Argentina). This study also evaluated the influence of climate, soil, and understory vegetation. Results showed that aggregated retention increased microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), and soil basal respiration (SBR) compared to dispersed retention, but with similar values than PF. However, harvested areas decreased MBC/MBN values compared with PF. The results showed an overall decrease in microbial biomass and activity in 9 YAH stands, with a positive recovery at 16 YAH. Soil pH, mean annual temperature, and understory vegetation cover showed a positive relationship with MBC, MBN, and SBR. The recovery after 16 YAH reached to different microbial communities. Therefore, the maintenance of retention components in managed stands for longer periods is needed. The results highlight some advantages of VRH as a tool for conservation of forest-dwelling soil microorganisms, including microbial biomass and activity.

Soil health indicators reveal that past dairy manure applications create a legacy effect

AbstractUnderstanding the long‐term effects of manure applications on the soil microbial component in semiarid climates will be key to sustain essential processes that affect their productivity and soil health. In this paper, soil health indicators encompassed both selected chemical and biological indicators. From 2004 to 2009, solid dairy manure treatments were applied to plots at cumulative rates of 0, 134, and 237 dry Mg ha−1 (34–56 dry Mg ha−1 year−1) in a randomized complete block with three replicates. Soil samples were taken from each manure rate in the spring of 2020 at 0–15 and 15–30 cm. Eleven years after manure applications ceased, many of the soil chemical and biological indicators were different between the manure and control treatments. In general, soil organic carbon and biological indicators were significantly greater in the 134 and 237 Mg ha−1 treatments as compared to the 0 Mg ha−1 treatment.

Use and evaluation of new organic fertilizers in the cultivation of Abelmoschus esculenthus (Okra)

Research objective: The objective of this work was to evaluate the properties and effect of certain organic fertilizers that can be used in organic farming in comparison with fertilizers of inorganic origin on Abelmoschus esculenthus plants. In addition, the interactions that can develop with microorganisms in the rhizosphere and the possible beneficial effects on transplant stress, nutrient uptake by roots and reduction of plant mortality were investigated. Materials and Methods: The experiments, which began in May 2022, were conducted in the greenhouses of CREA-OF in Pescia on plants placed in pots 12 cm in diameter and on which fertilizers of organic and inorganic origin were evaluated. In September 2022, plant height, number of leaves, leaf surface area, vegetative weight, root volume, root length, seeds per plant, number, weight, girth and length of pods, total substrate bacteria, substrate pH and number of dead plants were analysed. Results and Discussion: The experiment showed that the application of organic nitrogen and potassium fertilizers can significantly improve the quality and yield of Abelmoschus esculenthus plants. In general, a significant increase in height and vegetative and root growth of the plants was observed, especially in nitrogen and potassium synthesis, with an improvement also in the number of fruits and seeds per plant. Plant mortality also decreased significantly compared to the control with synthetic fertilizers, and microbial colonisation of the substrate increased, which probably affected the plants by influencing both vegetative and root growth and defence against biotic stressors. Conclusions: Organic fertiliser trials showed that these products can significantly improve plant growth by positively affecting the soil microbial component. This is an additional opportunity, especially for those farmers who not only seek to reduce the use of synthetic fertilizers, but also want to use sustainable techniques that better preserve plants and soils, while guaranteeing the production of quality products.

ИНДИКАЦИЯ ФРАГМЕНТОВ ГЕНОВ ФЕРМЕНТОВ У БАКТЕРИЙ ВИДА BACILLUS MEGATERIUM

The soil microbial component plays a critical role in maintaining the soil functionality, performing several vital functions such as soil formation, decomposition of dead and decayed organic matter, circulation of macro and micro elements and transformation of toxic chemicals into non-toxic forms. Various Bacillus species, including B. megaterium, are known to fix atmospheric nitrogen. To ensure nitrogen fixation and diversity analysis of diazotrophs, the most widely used gene marker is nifH, which is responsible for synthesis of proteins of the nitrogenase complex. Various soil microorganisms, including B. megaterium, are endowed with the ability to solubilize phosphorus inaccessible form to accessible one and / or mineralize organic phosphorus to accessible phosphorus form. Phosphatases and phytases are two groups of enzymes that catalyze transformation of organic phosphates to inorganic ones. It was found that Bacillus megaterium exhibits acidic and alkaline phosphatase activity and produces phytases. The aim of this work was to determine by RT-PCR method the presence of genes responsible for enzyme synthesis of phytase, nitrogenase, and alkaline phosphatase in strains of previously isolated bacteria of Bacillus megaterium species. To specify the presence of genes encoding synthesis of the desired enzymes in Bacillus megaterium, an in-silico analysis of the annotated genomes of this bacterial species, presented in the NCBI information database, was carried out. Further on, the selection of primers for screening the target parts was made. According to the results of the study, 7 out of 16 isolated B. megaterium strains contained all three required DNA parts responsible for synthesis of phytase, nitrogenase, and alkaline phosphatase enzymes.

Can repeated soil amendment with biogas digestates increase soil suppressiveness toward non-specific soil-borne pathogens in agricultural lands?

AbstractSoil suppressiveness which is the natural ability of soil to support optimal plant growth and health is the resultant of multiple soil microbial components; which implies many difficulties when estimating this soil condition. Microbial benefits for plant health from repeated digestate applications were assessed in three experimental sites surrounding anaerobic biogas plants in an intensively cultivated area of northern Italy. A 2-yr trial was performed in 2017 and 2018 by performing an in-pot plant growth assay, using soil samples taken from two fields for each experimental site, of which one had been repeatedly amended with anaerobic biogas digestate and the other had not. These fields were similar in management and crop sequences (maize was the recurrent crop) for the last 10 yr. Plant growth response in the bioassay was expressed as plant biomass production, root colonization frequency by soil-borne fungi were estimated to evaluate the impact of soil-borne pathogens on plant growth, abundance of Pseudomonas and actinomycetes populations in rhizosphere were estimated as beneficial soil microbial indicators. Repeated soil amendment with digestate increased significantly soil capacity to support plant biomass production as compared to unamended control in both the years. Findings supported evidence that this increase was principally attributable to a higher natural ability of digestate-amended soils to reduce root infection by saprophytic soil-borne pathogens whose inoculum was increased by the recurrent maize cultivation. Pseudomonas and actinomycetes were always more abundant in digestate-amended soils suggesting that both these large bacterial groups were involved in the increase of their natural capacity to control soil-borne pathogens (soil suppressiveness).

Resilience of the microbial communities of semiarid agricultural soils during natural climatic variability events

The microbial community carries out important processes that support plant growth and maintain soil health. However, extreme climatic variability such as higher temperatures and prolonged drought may cause fluctuations in the microbial community abundance, composition, and activity, thus affecting essential functions they govern including nutrient cycling and soil organic matter (SOM) dynamics. This study evaluated five agricultural lands under continuous cotton (Gossypium hirsutum L.) production from the Texas High Plains semi-arid region during the growing season and post-harvest from 2014 to 2018. Although sites differed in irrigation practices and soil textural class (e.g. from sandy loam to clay), all responded similarly to a decrease in precipitation, and increase in temperatures from 2015 to 2016. Microbial abundance via ester linked-fatty acid methyl ester (EL-FAME) analysis during the growing season of 2016 decreased by 82% in the sandiest soil, and by 38% in the soil with highest clay, when compared to 2015, a year of record precipitation for this region (e.g., 693 vs. 465 mm). Similarly, enzyme activities were reduced by as much as 87% (depending on the site) in the 2016 growing season when compared to 2015. The significant decreases in the soil microbial component could be explained by extremely low soil moisture (e.g., gravimetric water content averaged 2% in drylands, and 6% in irrigated sites) associated to low precipitation and high soil temperatures during the 2016 growing season (6.7 °C warmer than 2015). Furthermore, these conditions resulted in shifts in microbial community composition, especially significant decrease (p < 0.05) of saprophytic fungal markers by 59% (e.g., from an average of 26.8 to 10.9 nmol g−1 soil). However, after 2016 all biological parameters showed increasing trends, thus demonstrating a remarkable recovery and higher resilience than expected for semiarid soils, which experience extremes in temperature, frequent droughts and have low SOM (<1%).

Mikania micrantha invasion enhances the carbon (C) transfer from plant to soil and mediates the soil C utilization through altering microbial community

Exotic plant invasion alters the structure and coverage of terrestrial vegetation and affects the carbon (C) stocks in ecosystems. Previous studies have shown the increases in the C stocks with increasing invasive plants, but these results remain contentious. Soil microbial communities are usually altered by plant invasion, which potentially influences the C cycling underground. We hypothesized that the plant invasion-caused dynamic changes in soil microbes would lead to the corresponding change in soil C accumulation. Using greenhouse experiments we simulated different invader intensities through varying the relative abundance of invasive species Mikania micrantha and its co-occurring native species Paederia scandens. By analyzing 13C-phospholipid fatty acid we found the invasive M. micrantha assimilated more 13C and transferred faster the fixed 13C through different tissues to soils, as compared to native P. scandens. Soil microbial components, i.e., i15:0, 16:0, 10Me16:0, 18:1w9c and 18:2w6,9 were mainly using the photo-assimilated 13C. In addition, we found a hump-shaped relationship between soil net 13C accumulate rate and rhizosphere microbial biomass, indicating that the soil C accumulation may be either enhanced or reduced in invaded ecosystems, depending on microbe abundance.

Soil health indicators as affected by diverse forage species and mixtures in semi-arid pastures

‘WW-B.Dahl’ Old World bluestem [OWB, Bothriochloa bladhii (Retz) S.T. Blake] is a persistent pasture grass in the semi-arid Texas High Plains. Some growers are transitioning their irrigated continuous cotton (Gossypium hirsutum L.) land to dryland or low-irrigation production of WW-B.Dahl owing to diminished water supply. This grass strongly deters soil-dwelling ants (Hymenoptera: Formicidae), presumably via release of unidentified phytochemical repellants in the soil. Our aim was to determine whether the inhibitory effects of OWB extend to soil microbial community structure and function in relation to other adapted forages. We compared soil chemical and microbial properties in a Pullman clay-loam soil for OWB relative to OWB-alfalfa (Medicago sativa L.), alfalfa, and native mixed-grass pastures at 0–5 cm and 10–15 cm depth in June and December of 2016. The pastures ranged in age from 7 to 17 yr. Most measurements of organic forms of C and N showed greatest amounts in OWB-alfalfa; however, not always significantly. Enzyme activities linked to C, N, and S transformations were greatest (P ≤ 0.03) with OWB-alfalfa. This trend was similar for alkaline phosphatase activity, involved in P cycling, but not always significantly. Soil organic C in OWB was 1.2–1.4 fold greater than in the native mix depending on soil depth and sampling date, whereas SOC in OWB-alfalfa was 1.4–1.6 fold greater than in the native mix. Enzyme activities and microbial biomass N (MBN) were likewise greater in OWB and OWB-alfalfa than in the native mix, likely because the latter received no irrigation or N inputs. The microbial community structure demonstrated greater total FAMEs, bacterial markers for Gram+, Gram−, and actinomycetes in OWB-alfalfa compared to OWB, alfalfa, and native mix. Similar results were found in fungal markers for arbuscular mycorrhizal fungi (AMF) and saprophytic fungi. Protozoan indicators were found only in December, of which the native mix had the highest levels in the 0–5 cm depth. In general, selected soil properties, soil microbial biomass, community and enzyme activities were greater in December than in June except AMF, which was greater in June. The stimulating effect of OWB growing with alfalfa on the soil microbial community suggested this forage combination as favorable for soil health in relation to OWB, alfalfa alone, and native mixed-grass pastures. The previously reported inhibitory effect of OWB on soil-dwelling ants did not carry over to a depressing effect on the soil microbial component.

The added value of including key microbial traits to determine nitrogen‐related ecosystem services in managed grasslands

Abstract Despite playing central roles in nutrient cycles and plant growth, soil microbes are generally neglected in the study of ecosystem services (ES), due to difficulties to assess their diversity and functioning. However, to overcome these hurdles, new conceptual approaches and modern tools now provide a means to assess the role of micro‐organisms in the evaluation of ES. In managed grasslands, soil microbes are central in providing nitrogen (N)‐related ES such as maintenance of soil fertility and retention of mineral forms of N. Here, we applied state‐of‐the‐art techniques in microbial ecology and plant functional ecology to uncover the intrinsic link between N‐related bacterial functional groups, important plant functional traits, environmental factors and three proxies of maintenance of soil fertility and potential for N‐leaching across managed grasslands in three regions of Europe. By constructing well‐defined structural equation modelling, we showed that including key microbial traits improve on average more than &gt;50% of the total variances of ES proxies, that is, ammonium () or nitrate () leaching, and soil organic matter content. Geographic differences arose when considering the direct relationships of these ES proxies with specific microbial traits: nitrate leaching was positively correlated to the maximum rate of nitrification, except in the Austrian site and potentially leached –N was negatively correlated to the fungi/bacteria ratio, with the exception of the French site. Synthesis and applications. The integration of soil microbial functional traits in the assessment of nitrogen‐related grassland ecosystem services has direct contributions for understanding sustainable management of grassland ecosystems. The fundamental aspects of this study suggest that integrating a soil microbial component in grassland management may enhance sustainability of such grass‐based agroecosystems.

Lasting effects of soil health improvements with management changes in cotton-based cropping systems in a sandy soil

The soil microbial component is essential for sustainable agricultural systems and soil health. This study evaluated the lasting impacts of 5 years of soil health improvements from alternative cropping systems compared to intensively tilled continuous cotton (Cont. Ctn) in a low organic matter sandy soil. Our previous study (phase I) evaluated soil health microbial indicators (microbial biomass, community composition and enzyme activities) during 5 years (2003-2007) when more plant residue was returned to the soil by rotating cotton (primary cash crop) with grain sorghum (secondary cash crop) with and without a rye winter cover crop (Ctn-Rye-Sg and Sg-Ctn) or with a system that involved no cash crop and returned maximum biomass to the soil (3 times residue vs Cont. Ctn) with a sorghum x sudangrass hybrid with winter rye cover (SSd-Rye). The current study (phase II) addressed what happens to the microbial component (same microbial indicators) once the management is changed to more cotton production the following 3 years (i.e., cotton was planted 2 out of 3 years in SSd-Rye and was grown for 3 years in Ctn-Rye-Sg). During the first year of phase II (2008), all plots were planted in cotton, and higher cotton yields in SSd-Rye (108–150%) corresponded to microbial trends found in the original rotations (SSd-Rye > Sg-Ctn = Ctn-Rye-Sg). In regard to microbial indicators, the most significant trends with reintroduction of cotton and increase in tillage were a reduction in fungal FAME (fatty acid methyl esters) indicators with a lower ratio of fungi to bacteria and much lower AMF (15.1 to 3.98% of arbuscular mycorrhizal fungi), and less biogeochemical cycling potential of soil according to several enzyme activities (by as much as 50%). At the end of phase II (2010), Ctn-Rye-Sg and SSd-Rye were still found to have higher microbial biomass and enzyme activities (1.5 times both measurements except for phosphodiesterase) compared to continuous cotton. This study demonstrates the vulnerability of microbial communities in sandy soils, which requires agroecosystems that will support biomass incorporation (e.g., sorghum and cover crops) when possible in order to sustain essential functions and overall soil health.

The impact of legume and cereal cover crops on rhizosphere microbial communities of subsequent vegetable crops for contrasting crop decline

Crop decline is a progressive soil health reduction commonly associated to the specialized cultivations. As a series of biotic factors are involved in this phenomenon, it can be controlled with essentially agronomic methods, the impact of winter cover crops on early growth at subsequent vegetable crops was investigated in a two-year field study. Barley and hairy vetch were incorporated into the soil following two mechanical terminations of cover crops (green manure and green mulching). Immediately after, tomato and zucchini seedlings were transplanted and grown for 28days. The above-ground biomass of the vegetables was taken as indicator of crop response to pre-plant treatments, whilst root colonizing fungi and rhizosphere bacteria were the two analyzed soil microbial components. Root-colonizing fungi were evaluated using culture-based methods, bacteria were analyzed by amplification of rhizosphere soil DNA with 16S rDNA, then processed with PCR-DGGE. Tomato and zucchini growth response after vetch was always significantly higher than after barley regardless of mechanical termination. Rhizosphere bacterial communities differed significantly between cover crops and this differences was maintained also in the subsequent vegetable crops. Root-colonizing fungi differed between barley and vetch, although they shared most species (70%) such as Pythium spp., binucleate Rhizoctonia AG-A and several Fusarium spp. The latter three fungal groups were also found most abundant in tomato and zucchini roots. Pathogenicity test showed that tomato was more susceptible than zucchini to Pythium, Rhizoctonia solani, Fusarium oxysporum and Fusarium spp. and that their relationship with vegetable plants varied from pathogenic to neutral, up to mutualistic in the case of Rhizoctonia AG-A. These findings indicate difficulty to identify specific biotic agents responsible of crop decline. Tomato and zucchini showed an undoubted growth improvement after legume; however, a certain specificity of Cylindrocarpon-like fungi and Phoma spp. to hairy vetch suggests that, when exploiting benefit of this cover crop for mitigating yield decline of vegetable crops, legume should alternate cereal and other botanically distant genera in rotation.

Soil microbial component and its respiratory activity in coniferous forests of the Northwest Priladozhje area

The microbial biomass carbon (Cmic) by substrate-induced respiration method (SIR), the basal (microbial) respiration rate (BR) and the metabolic quotient (q CO2 = BR / Cmic) were calculated from samples of plant litter and mineral layers of forest soils (Podburs, Sod-podburs, Gleyzems, Litozems) of four catenas of northern and southern exposure of selgovy landscape in the Northwest Priladozhje area. Sample sites were located at different distances (300, 500 and 2000 m) from a source of aerotechnogenic pollution (a granite quarry). High levels of biogenecity were found in plant litter, low ones in corresponding mineral layers of the soil profile in the pine forests examined. Soils of accumulative parts of the landscape were characterized by higher Cmic and BR compared to transit and autonomic ones. The impact of aerotechnogenic pollution from the granite quarry on soil microbial component was found, it was expressed by decreased Cmic and BR mainly in forest plant litter. Refs 13. Fig. 1. Tables 6.

Evidence for proteolysis of a recombinant prion protein in a lamb brain-amended loamy soil

Soils contaminated by prions, the infectious agents responsible for transmissible spongiform encephalopathy diseases, remain infectious to grazing animals for many years. In this study, the ability of enzymes produced by soil microbes to degrade a recombinant prion protein (recPrP) was investigated in a loamy soil. A 15N-labelled recPrP was added to soil in which microbial biomass and soil proteolytic activity had been increased by either simultaneous or prior amendment with lamb brain, and distribution of 15N among soil solid particles, soluble molecules and bacterial biomass was determined. After 1 day the proportions of recovered recPrP-N associated with microbial biomass and soluble molecules were 6–9 and 15–19%, respectively, which is consistent with the hypothesis of degradation. A greater incorporation of 15N-derived β-sheeted recPrP into the microbial biomass pool occurred when the soil proteolytic activity was pre-stimulated by a lamb brain amendment, suggesting that the recPrP degradation in soil is mediated by the activity level of proteolytic enzymes produced by the microbial biomass. The majority (35–87%) of the recovered recPrP-N was associated with the soil particles. An observed partial degradation of recPrP deposited on a mica surface by soil soluble enzymes indicated a sorption-related resistance to proteolysis. In conclusion, integration of the stimulation and turnover of the soil microbial component, after an input of a large amount of animal organic matter with the sorption properties of prion protein, is required to model and predict prion survivability, transformation and transmissibility in soil.

Rhizosphere microbiota interfers with plant-plant interactions

Diversity, structure and productivity of above-ground compartment of terrestrial ecosystems have been generally considered as the main drivers of the relationships between diversity and ecosystem functioning. More recently it has been suggested that plant population dynamics may be linked with the development of the below-ground community. The biologically active soil zone where root-root and rootmicrobe communications occur is named Rhizosphere where root exudates play active roles in regulating rhizosphere interactions. Root exudation can regulate the soil microbial community, withstand herbivory, facilitate beneficial symbioses, modify the chemical and physical soil properties and inhibit the growth of competing plant species. In this review, we explore the current knowledge assessing the importance of root exudates in plant interactions, in communications between parasitic plants and their hosts and how some soil microbial components could regulate plant species coexistence and change relationships between plants. This review will be focussed on several well documented biological processes regulating plant-plant communications such as exotic plant species invasions, negative root-root communication (allelopathy) and parasitic plant / host plant interactions and how some soil microbial components can interfere with signal traffic between roots. The reported data show that the overall effect of one plant to another results from multiple interacting mechanisms where soil microbiota can be considered as a key component. (Resume d'auteur)

Rhizosphere microbiota interfers with plant-plant interactions

Diversity, structure and productivity of above-ground compartment of terrestrial ecosystems have been generally considered as the main drivers of the relationships between diversity and ecosystem functioning. More recently it has been suggested that plant population dynamics may be linked with the development of the below-ground community. The biologically active soil zone where root-root and root-microbe communications occur is named “Rhizosphere” where root exudates play active roles in regulating rhizosphere interactions. Root exudation can regulate the soil microbial community, withstand herbivory, facilitate beneficial symbioses, modify the chemical and physical soil properties and inhibit the growth of competing plant species. In this review, we explore the current knowledge assessing the importance of root exudates in plant interactions, in communications between parasitic plants and their hosts and how some soil microbial components could regulate plant species coexistence and change relationships between plants. This review will be focussed on several well documented biological processes regulating plant-plant communications such as exotic plant species invasions, negative root-root communication (allelopathy) and parasitic plant / host plant interactions and how some soil microbial components can interfere with signal traffic between roots. The reported data show that the overall effect of one plant to another results from multiple interacting mechanisms where soil microbiota can be considered as a key component.

Evaluating the effect of fertilizer application on soil microbial community structure in rice based cropping system using fatty acid methyl esters (FAME) analysis

The present study was undertaken to observe the effects of chemical fertilizers and compost amendments on microbial community structure in rice based cropping system under long term fertilizer application experiment. Changes in microbial community structure assessed by the relative abundance of fatty acid methyl ester (FAME) profiles revealed that compost amendments led to promoting the abundance of Gram-positive bacterial FAMEs. Multivariate analysis of FAME profiles separated the treatments primarily on the basis of the amount of added compost. A link between different fertilizer application treatment and soil microbial components was tentatively established, but it needs to be verified in further studies.

Influence of red mud, zeolite and lime on heavy metal immobilization, culturable heterotrophic microbial populations and enzyme activities in a contaminated soil

In this study we compared the efficiency of different amendments, notably red mud (a bauxite residue), natural zeolite and lime, to immobilize the heavy metals present in a polluted acidic soil [Pb (3266 mg kg − 1 d.m.), Cd (35.4 mg kg − 1 d.m.) and Zn (1495 mg kg − 1 d.m.)] and to influence several microbiological properties. The addition of all the amendments decreased significantly ( P < 0.05) the solubility of Pb, Cd and Zn and the increase of soil pH was identified as a common mechanism of action for both red mud and lime. Despite reducing the heavy metal solubility, the addition of zeolite and lime did not affect the number of culturable fast-growing heterotrophic bacteria whereas the red mud caused a significant increase ( P < 0.05) of this soil microbial component. All the amendments reduced the number of heterotrophic fungi ( P < 0.05) while only the lime addition influenced negatively the number of free-living N 2-fixing bacteria. Furthermore, the addition of red mud and lime caused a significant change of the dominant culturable bacterial community. Partial sequencing of the 16S rDNA gene of randomly selected bacterial isolates revealed that dominant strains from the lime and red mud-soils were mostly belonging to β-Proteobacteria and Bacteroidetes/Chlorobi group whilst only Actinobacteria and Firmicutes were identified in control and zeolite soils. These latter were largely colonized by Arthrobacter and Bacillus while Ralstonia and Pedobacter prevailed in soils treated with lime and red mud. Moreover, the Biolog Ecoplate assay revealed that the number of substrates utilized (OD 590 > 0.15) by the different microbial communities (richness) was following the order: red mud-soil > lime-soil > zeolite-soil = control-soil. The same trend was observed for the urease activity but not for dehydrogenase and β-glucosidase. It is concluded that all the amendments considered influenced with a different extent the heavy metal mobility in soil as well as the structure and function of the resident culturable microbial communities. Red mud was the most effective at reducing the “mobile” form of the metals considered and at promoting bacterial abundance and soil enzyme activity. Nevertheless, red mud and lime addition caused a dramatic shift of the culturable bacterial population from Gram positive to Gram negative forms. The poor catabolic activity detected with the Biolog assay for zeolite and control-soil suggested the inability of Arthrobacter and Bacillus strains to oxidize the substrates in the Ecoplate rather than reflect the actual catabolic versatility of the microbial communities.

Additional experiences to elucidate the microbial component of soil suppressiveness towards strawberry black root rot complex

SummarySeveral studies were carried out to investigate the soil microbial components involved in suppressing strawberry black rot root which occurs throughout the Italian strawberry growing region. Quantitative and qualitative evaluation of fungi involved in black root rot were combined with several soil microbial parameters involved in soil suppressiveness towards black root rot agents. The first survey, carried out in an intensively cultivated area of northern Italy, identified Rhizoctonia spp. as the main root pathogen together with several typical weak pathogens belonging to the well‐known black rot root complex of strawberry crop: Cylindrocarpondestructans, Fusarium oxysporum, F. solani, Pestalotia longiseta and others. The root colonisation frequency of strawberry plants increased strongly from autumn to spring at harvesting stage. Rhizoctonia spp. were the only pathogens which followed the rising trend of root colonisation with relative frequency; all the weak pathogens of strawberry black root rot complex did not vary their frequency. Only non‐pathogenic fungi decreased from autumn to spring when at least 60% of colonising fungi were represented by Rhizoctonia. These data suggested that the late vegetative stage was the best time to record the soil inoculum of root rot agents in strawberry using root infection frequency as a parameter of soil health. A further study was performed in two fields, chosen for their common soil texture and pH, but with significant differences in previous soil management: one (ALSIA) had been subjected to strawberry monoculture without organic input for several years; the other (CIF) has been managed according to a 4‐year crop rotation and high organic input. In this study Pythium artificially inoculated was adopted as an indicator for the behaviour of saprophytically living pathogens in bulk soil. Pythium showed a sharp, different response after inoculation in bulk soil from the two soil systems evaluated. Pythium was suppressed only in the CIF field where the highest levels of total fungi and fluorescent bacteria and highest variability were observed. The suppressiveness conditions towards Pythium, observed in the CIF and absent in the ALSIA field, corresponded with the root infection frequency recorded at the late vegetative stage on strawberry plants grown in the two fields: strawberry plants from the CIF field showed lower root colonisation frequency and higher variability than that recorded on those coming from the ALSIA field.

Soil microbial dynamics in maize-growing soil under different tillage and residue management systems

The long-term impact of tillage and residue management on soil microorganisms was studied over the growing season in a sandy loam to loamy sand soil of southwestern Quebec, growing maize ( Zea mays L.) monoculture. Tillage and residue treatments were first imposed on plots in fall 1991. Treatments consisted of no till, reduced tillage, and conventional tillage with crop residues either removed from (−R) or retained on (+R) experimental plots, laid out in a randomized complete block design. Soil microbial biomass carbon (SMB-C), soil microbial biomass nitrogen (SMB-N) and phospholipid fatty acid (PLFA) contents were measured four times, at two depths (0–10 and 10–20 cm), over the 2001 growing season. Sample times were: May 7 (preplanting), June 25, July 16, and September 29 (prior to corn harvest). The effect of time was of a greater magnitude than those attributed to tillage or residue treatments. While SMB-C showed little seasonal change (160 μg C g −1 soil), SMB-N was responsive to post-emergence mineral nitrogen fertilization, and PLFA analysis showed an increase in fungi and total PLFA throughout the season. PLFA profiles showed better distinction between sampling time and depth, than between treatments. The effect of residue was more pronounced than that of tillage, with increased SMB-C and SMB-N (61 and 96%) in +R plots compared to −R plots. This study illustrated that measuring soil quality based on soil microbial components must take into account seasonal changes in soil physical and chemical conditions.

Kinetic analysis of soil microbial components under perturbed and steady-state conditions in a gray luvisol

We found that cycling of cystine- 14C through solutions of soil samples at steady-state was influenced by two kinetically-defined biontic components. We report work aimed at further characterizing these two components. Surface horizons from a cultivated Gray Luvisol and a Dark Brown Chemozemic soil were sampled. They were incubated with cystine at 60 ng g −1 to maintain steady-state conditions and with glucose at 1000 μg g −1 to perturb the system. Soil solution 14C and 14CO 2 were monitored. The data were analyzed for their fit to several alternate models. The kinetic model which described 14C cycling through the solution of perturbed soil samples is consistent with C allocation into a pool of small intermediate molecules of biosynthesis and into macromolecules such as proteins and RNA. The protein component did not degrade measurably during the period of study. A second kinetic model described non-perturbed soil conditions in which 14C-cystine was partitioned between microbial cytoplasmic and protein components. Both models are consistent with pure culture studies. Neither model can be considered to distinguish between microbial taxa. We have inferred that taxonomic controls do not determine short-term 14C kinetics and that a physiologic basis for the kinetic distinctions may be most reasonable. This appears to hold both under conditions which favor growth (i.e. excess glucose) and which preserve steady-state (small cystine additions) in soils incubated under laboratory conditions.