Functional Assemblages of Collembola Determine Soil Microbial Communities and Associated Functions

Soil processes such as decomposition are mainly performed by soil biota. Although soils worldwide are extremely biodiverse, the relationship between decomposers (fauna and microorganisms), and ecosystem function is poorly understood. Collembola are abundant and ubiquitous microarthropods that are found in terrestrial ecosystems. They can affect the amount of biomass and the activity of microbial communities, either directly through selectively feeding on fungi and bacteria, or indirectly by dissemination of microbial propagules, and the alteration of nutrient availability. However, despite the functional role they play in belowground food webs, the interactions between natural assemblages of soil microbes and Collembola receive little attention. This study, conducted in microcosm conditions, examines the effects of two distinct natural assemblages of functional groups of Collembola (ep- and euedaphic) upon microbial communities using PLFA markers and their associated soil functions (e.g., enzymatic activities and C mineralization rate) over a 2-month period. Our principal objective was to determine whether different functional groups of Collembola had varying effects on microbial soil community abundance, structure and activity, resulting in potentially important effects on ecosystem processes. Our findings show that the interactions of the functional groups of Collembola with microbial communities vary significantly whether they are alone or combined. A distinct response in the composition of the microbial communities was found at the end of the 2-month period. The communities were significantly different from each other in terms of PLFA marker composition. We found that the epedaphic species were related to and promoted Gram+ bacteria whereas euedaphic species were related to Gram- bacterial markers. This had further repercussions on soil function, such as nutrient recycling. Combining both functional groups did not lead to a complementary effect on soil microbial properties, with a drastically different outcome between the first and the second month of the experiment. Additional research dealing with the interactions between decomposers using natural assemblages will help to predict the functional outcomes of soil biota structure and composition

Changes in soil N mineralization and nitrification pathways along a mixed forest chronosequence

International audienceChanges in soil N mineralization pathways occurring along a full rotation cycle have received little attention to date, while tree uptake forNmaychange during forest ageing. The aimsof this study were (i) to characterize changes in potential net N mineralization and potential net nitrification within organic layers and the topsoil (organo-mineral horizon) along a 100-year chronosequence for a temperate oakhornbeam forest and (ii) to reveal covariances between potential netNmineralization pathways and the properties of the humic epipedon (defined as the sum of organic layers and topsoil). For that purpose, a space-for-time substitution procedure and aerobic laboratory incubation method for 28 days at 28 8C in the dark were used. In addition, acetylene and captan were used to discriminate between autotrophic and heterotrophic (bacterial and/or fungal) nitrification. Several humic epipedon properties were determined, e.g. pH, exchangeable cation concentrations, effective cation exchange capacity, total C and N, dissolved organic C and N, fungal and microbial biomass N. Potential net N mineralization and nitrification pathways changed greatly along the mixed forest chronosequence. Potential net N mineralization in the organic layers increased with stand maturation whereas potential net nitrification in the topsoil decreased significantly. Selective inhibitors revealed changes in nitrification pathways along the chronosequence, i.e. potential net nitrification was autotrophic in the topsoil while it was mainly heterotrophic within the organic layers. In the organic layer, potential net nitrification was autotrophic at the onset of the chronosequence while it appeared heterotrophic during the aggradation phase and finally fungal in mature stands. A Co-Inertia Analysis was used to reveal covariances between N mineralization pathways and humic epipedon properties. The analysis showedtwofunctionaltemporal shiftswithinNcycling along the chronosequence, one probably controlled by organic matter quality and high competition for available N resulting in the autotrophic versus heterotrophic nitrification shift in the organic layersandonemainly controlledby(i)fine organic matter abundance, allowing high N mineralization in the organic layers and (ii) acidity inhibited autotrophic nitrification in the topsoil

La manumisión de esclavos por compra y gracia en la Provincia de Antioquia, 1780-1830

This article analyzes the dynamics of the legal manumission of black slaves in the Province of Antioquia during the late colonial and early republican periods. A brief description of the effects that economic development had on the slavery in the Province is presented in the first section. Then, a review of the dynamics of manumission in Antioquia is given with base on the information extracted from primary sources including letters of freedom by purchase and by grace, lawsuits, ordinances, laws and decrees. This study shows that in spite of the influence of the particular conditions of the region on the successfulness of the strategies of freedom, the agency of the slaves must be regarded as an instrumental part of the dynamics of manumission during the period of interest.El artículo analiza las dinámicas de manumisión por gracia y por compra de los negros esclavizados en la Provincia de Antioquia durante los últimos años del periodo colonial y los primeros del republicano. En primera instancia se hace un breve esbozo del desarrollo económico de la Provincia y el impacto que generó en el sistema de explotación esclavista particular de esta región, para luego presentar un análisis de la manumisión por compra y gracia a partir de las cifras arrojadas por las fuentes primarias. Entre la documentación que se consideró en este estudio hay cartas de libertad, testamentos, pleitos judiciales, leyes, decretos, entre otros, emitidos en diferentes poblados de la Provincia durante el periodo estudiado. Este estudio muestra que a pesar de la gran importancia que tuvo el contexto particular de la Provincia en el éxito de las estrategias de libertad, la agencia de los esclavos debe ser considerada como uno de los principales motores que dinamizaron los procesos de manumisión

A comparison of permanent and fluctuating flooding on microbial properties in an ex-situ estuarine riparian system

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Biodiversity and ecosystem functions in wetlands: A case study in the estuary of the Seine River, France

The integrity of estuarine wetlands is maintained by physical connections between river and sea to floodplain. Their ecological importance can be assessed through plant biodiversity and such ecosystem functions as primary productivity and nitrate removal capacity. Multivariate analysis were used to establish a hierarchy of environmental factors related to the vegetation structure and diversity. Four different measures of plant diversity (both structural and functional) were made on a Seine River wetland. Key functions of estuarine floodplain (productivity and denitrification capacity) were either measured directly or assessed using remotely sensed data. The richest plant communities correspond to mesophilous grasslands which have an intermediate position between natural and anthropogenic disturbance regimes. These species assemblages occur in ecosystems presenting both a regular productivity in time and space and the highest denitrification potentiality

Etude de la biodisponibilité des HAP dans 5 sols naturels contrastés : Impact des caractères physico-chimiques du sol et des communautés bactériennes

National audienceLes hydrocarbures aromatiques polycycliques (HAP), polluants toxiques persistants, présentent un caractère de géo-accumulation. En fonction de leur forme chimique et des caractéristiques du sol, ils sont plus ou moins fortement séquestrés et l'efficacité des procédés de bioremédiation est alors intimement liée à la biodisponibilité des HAP pour les micro-organismes dégradeurs du sol. Le but de cette étude est donc (i) de mettre au point une technique simple et rapide afin d'évaluer la biodisponibilité des HAP dans les sols (ii) de définir les communautés bactériennes impliquées dans la dégradation des HAP et (iii) de comprendre la relation entre la biodégradation des HAP, leur biodisponibilité, la nature des sols et les communautés bactériennes dégradant le phénanthrène. (i) Les techniques d'extraction drastiques généralement mises en oeuvre pour extraire les HAP (Soxhlet, extraction assistée par micro-ondes MAE, ou par solvants sous pression) donnent la teneur pseudo-totale en HAP, mais ne permettent pas d'accéder à la fraction labile des HAP. Des méthodes d'extraction douces en phase aqueuse, utilisant des agents solubilisants, sont plus susceptibles d'accéder à cette fraction de HAP réellement biodisponibles. Des additifs tels que des agents tensioactifs synthétiques, des émulsions, des biosurfactants (produits par des bactéries), des cyclodextrines et des liquides ioniques micellaires ont été testés sur 5 sols naturels, de caractéristiques géochimiques très différentes (taux de matière organique, d'argiles, pH…). Les capacités d'extraction en phase aqueuse, en présence des différents additifs, ont ensuite été comparées aux extractions pseudo-totales effectuées par MAE, afin d'établir si le ratio entre HAP labiles et HAP totaux était corrélé aux caractéristiques de rétention des sols. (ii) Des tests de croissance en milieu minimum, en présence de Phénanthrène (PHE) comme seule source de carbone, ont été réalisés sur une centaine de souches provenant d'un des sols d'étude, permettant de mettre en évidence 2 souches significativement dégradantes, dont une produit un biosurfactant. Le dosage par HPLC-FLD du PHE résiduel dans les tubes de culture a permis de valider les résultats des tests de croissance bactérienne. Des études de la cinétique de dissipation naturelle de 7 HAP dans les 5 sols d'étude ont été parallèlement réalisées en microcosmes, et ont présenté des résultats contrastés semblant être plutôt reliés au paramètre bactérien qu'aux caractéristiques physico-chimiques des sols. (iii) Au final, une analyse en composantes principales (ACP) a permis de corréler (ou non) la biodégradation des HAP en 3 mois à certaines caractéristiques des sols, à leur biodisponibilité et au nombre de bactéries cultivables dégradant le phénanthrène

Use of dredged sediments for soil creation in the Seine estuary (France): Importance of a soil functioning survey to assess the success of wetland restoration in floodplains

International audienceThe creation and restoration of new wetlands to mitigate wetland losses is a newly developing science whose success still needs to be assessed. This study focuses on the ecological restoration of a gravel-pit in the low valley of the Seine estuary (France). Restoration consisted in filling the gravel-pit using a hydraulic technique with dredged sediments from the Seine river and covering it with alkaline peat from adjacent wet meadows. Our objectives were to survey the functions of recreated soil 3 years after the gravel-pit was filled and assess whether it regained typical wetland functionality and to determine which soil functioning parameters are the most efficient for assessing restoration success. To address these questions, an approach combining analyses of in situ and ex situ soil functioning was used. The survey was conducted on recreated soil as compared to a control soil (i.e. soil before gravel extraction). Four topographic zones were sampled corresponding to 4 types of recreated soil functioning in terms of waterlogging conditions: Hemic Histosol without waterlogged periods, Hemic Histosol with temporary waterlogged periods, Hemic Histosol with the longest waterlogged periods and Interstratified Histosol without waterlogged periods. Soil respiration and SIR results showed that large stocks of organic matter are maintained after 3 years of restoration and proved able to sequester C in recreated soils. 3 years after restoration, nitrogen removal function measured through denitrification technique was restored in the Hemic Histosol with the longest waterlogged periods. These results demonstrate that waterlogging regime maintain the C stock and accelerate the restoration of denitrification process

Effet de la densité du peuplement forestier sur les assemblages des détritivores fauniques du sol

International audienceIn the framework of the Imprebio project, our study showed that the diversity and community structure of most of the major groups of soil detritivores fauna do not respond to forest tree density manipulation over our large-scale assessment. This suggests that forest stand density management is likely to have limited impact on soil biodiversity on temperate young oak stand

GammaProteobacteria as a potential bioindicator of a multiple contamination by polycyclic aromatic hydrocarbons (PAHs) in agricultural soils.

International audienceThe impact of a multiple contamination by polycyclic aromatic hydrocarbons (PAHs) was studied on permanent grassland soil, historically presenting low contamination (i.e. less than 1 mg kg(-1)). Soil microcosms were spiked at 300 mg kg(-1) with either single or a mixture of seven PAHs. While total dissipation of the phenanthrene was reached in under 90 days, only 60% of the PAH mixture were dissipated after 90 days. Interestingly, after 30 days, the abundance of the GammaProteobacteria class (assessed by qPCR) become significantly higher in microcosms spiked with the PAH mixture. In addition, the specific abundance of the cultivable Pseudomonas spp., which belong to the GammaProteobacteria class, increased earlier and transiently (after 8 days) in the microcosms spiked with the PAH mixture. Consequently, we propose to use the GammaProteobacteria as a bioindicator to detect the impact on the bacterial community of a multiple contamination by PAHs in agricultural soils