Effect of acidification on leaf litter decomposition in benthic and hyporheic zones of woodland streams

Anthropogenic acidification has deleterious effects on both structure and functioning of surface water ecosystems. This study examined how it may affect the leaf decomposition rate and the community structure and activity of decomposers in both benthic and hyporheic zones of five headwater streams along an acidification gradient from highly acidic (pH 4.6) to circumneutral (pH 7.4). Overall, responses to acidification in hyporheic zones were less pronounced, but followed the same pattern as in their benthic counterparts. Leaf decomposition was much faster in the circumneutral stream, both in the hyporheic and benthic zones (k = 0.0068 and 0.0534 d−1, respectively), than in the most acidic one (k = 0.0016 and 0.0055 d−1, respectively), and correlated well with the acidic gradient in both compartments. Interestingly, leaf litter decomposition was less affected by acidification in hyporheic compared to benthic compartments, likely due to the relatively low sensitivity of fungi, which were the main decomposers of buried coarse particulate organic matter. These results argue in favour of conserving hyporheic habitats in acidified streams as they can maintain matter and species fluxes that are essential to the ecosystem

Defining Quantitative Targets for Topsoil Organic Carbon Stock Increase in European Croplands: Case Studies With Exogenous Organic Matter Inputs

The EU Mission Board for Soil Health and Food proposed a series of quantitative targets for European soils to become healthier. Among them, current soil organic carbon (SOC) concentration losses in croplands (0.5% yr(-1) on average at 20 cm depth) should be reversed to an increase of 0.1-0.4% yr(-1) by 2030. Quantitative targets are used by policy makers to incentivize the implementation of agricultural practices that increase SOC stocks. However, there are different approaches to calculate them. In this paper, we analyzed the effect of exogenous organic matter (EOM) inputs on the evolution of SOC stocks, with a particular focus on the new European targets and the different approaches to calculate them. First, we illustrated through two case-study experiments the different targets set when the SOC stock increase is calculated considering as reference: 1) the SOC stock level at the onset of the experiment and 2) the SOC stock trend in a baseline, i.e., a control treatment without EOM addition. Then, we used 11 long-term experiments (LTEs) with EOM addition in European croplands to estimate the amount of carbon (C) input needed to reach the 0.1 and 0.4% SOC stock increase targets proposed by the Mission Board for Soil Health and Food, calculated with two different approaches. We found that, to reach a 0.1 and 0.4% increase target relative to the onset of the experiment, 2.51 and 2.61 Mg C ha(-1) yr(-1) of additional C input were necessary, respectively. Reaching a 0.1 and 0.4% increase target relative to the baseline required 1.38 and 1.77 Mg C ha(-1) yr(-1) of additional input, respectively. Depending on the calculation method used, the estimated amounts of additional C input required to reach each quantitative target were significantly different from each other. Furthermore, the quality of C input as represented by the C retention rate of the additional organic material (EOM and crop residue), had a significant effect on the variation of SOC stocks. Our work highlights the necessity to take into consideration the additional C input required to increase SOC stocks, especially for soils with decreasing SOC stocks, when targets are set independently of the baseline

Toxicity of CeO2 nanoparticles on a freshwater experimental trophic chain: A study in environmentally relevant conditions through the use of mesocosms

The toxicity of CeO2 NPs on an experimental freshwater ecosystem was studied in mesocosm, with a focus being placed on the higher trophic level, i.e. the carnivorous amphibian species Pleurodeles waltl. The system comprised species at three trophic levels: (i) bacteria, fungi and diatoms, (ii) Chironomus riparius larvae as primary consumers and (iii) Pleurodeles larvae as secondary consumers. NP contamination consisted of repeated additions of CeO2 NPs over 4 weeks, to obtain a final concentration of 1 mg/L. NPs were found to settle and accumulate in the sediment. No effects were observed on litter decomposition or associated fungal biomass. Changes in bacterial communities were observed from the third week of NP contamination. Morphological changes in CeO2 NPs were observed at the end of the experiment. No toxicity was recorded in chironomids, despite substantial NP accumulation (265.8±14.1mg Ce/kg). Mortality (35.3±6.8%) and a mean Ce concentration of 13.5±3.9mg/kg were reported for Pleurodeles. Parallel experiments were performed on Pleurodeles to determine toxicity pathways: no toxicity was observed by direct or dietary exposures, although Ce concentrations almost reached 100 mg/kg. In view of these results, various toxicity mechanisms are proposed and discussed. The toxicity observed on Pleurodeles in mesocosm may be indirect, due to microorganism’s interaction with CeO2 NPs, or NP dissolution could have occurred in mesocosm due to the structural complexity of the biological environment, resulting in toxicity to Pleurodeles. This study strongly supports the importance of ecotoxicological assessment of NPs under environmentally relevant conditions, using complex biological systems

Soil organic carbon models need independent time-series validation for reliable prediction

Numerical models are crucial to understand and/or predict past and future soil organic carbon dynamics. For those models aiming at prediction, validation is a critical step to gain confidence in projections. With a comprehensive review of ~250 models, we assess how models are validated depending on their objectives and features, discuss how validation of predictive models can be improved. We find a critical lack of independent validation using observed time series. Conducting such validations should be a priority to improve the model reliability. Approximately 60% of the models we analysed are not designed for predictions, but rather for conceptual understanding of soil processes. These models provide important insights by identifying key processes and alternative formalisms that can be relevant for predictive models. We argue that combining independent validation based on observed time series and improved information flow between predictive and conceptual models will increase reliability in predictions

Ensemble modelling, uncertainty and robust predictions of organic carbon in long-term bare-fallow soils

ACKNOWLEDGEMENTS This study was supported by the project “C and N models inter-comparison and improvement to assess management options for GHG mitigation in agro-systems worldwide” (CN-MIP, 2014- 2017), which received funding by a multi-partner call on agricultural greenhouse gas research of the Joint Programming Initiative ‘FACCE’ through national financing bodies. S. Recous, R. Farina, L. Brilli, G. Bellocchi and L. Bechini received mobility funding by way of the French Italian GALILEO programme (CLIMSOC project). The authors acknowledge particularly the data holders for the Long Term Bare-Fallows, who made their data available and provided additional information on the sites: V. Romanenkov, B.T. Christensen, T. Kätterer, S. Houot, F. van Oort, A. Mc Donald, as well as P. Barré. The input of B. Guenet and C. Chenu contributes to the ANR “Investissements d’avenir” programme with the reference CLAND ANR-16-CONV-0003. The input of P. Smith and C. Chenu contributes to the CIRCASA project, which received funding from the European Union's Horizon 2020 Research and Innovation Programme under grant agreement no 774378 and the projects: DEVIL (NE/M021327/1) and Soils‐R‐GRREAT (NE/P019455/1). The input of B. Grant and W. Smith was funded by Science and Technology Branch, Agriculture and Agri-Food Canada, under the scope of project J-001793. The input of A. Taghizadeh-Toosi was funded by Ministry of Environment and Food of Denmark as part of the SINKS2 project. The input of M. Abdalla contributes to the SUPER-G project, which received funding from the European Union's Horizon 2020 Research and Innovation Programme under grant agreement no 774124.Peer reviewedPostprin

Acidification et restauration d'écosystèmes forestiers : effets sur les communautés microbiennes et sur des processus fonctionnels associés

Many terrestrial and freshwater forested ecosystems are affected by anthropogenic acidification, which can led to deleterious effects on biodiversity and ecosystem functioning. To counteract acidification, liming can be used to improve soil and water physicochemical characteristics in order to restore tree health and headwater stream functioning. In particular, liming has been shown to enhance leaf litter breakdown, which is a key ecosystem process in headwater streams. In this context, the aims of this study were, first to investigate if liming, through its effects on soil chemical characteristics, could induce changes on soil microbial communities, and second to identify what factors could be responsible, at the microbial level, of reduced leaf litter breakdown in acidified headwater streams. Results showed that moderate large-scale liming can induce sustainable changes in soil bacterial communities. Major taxonomic changes revealed notably that the ratio between Proteobacteria and Acidobacteria was higher in limed soils compared to their control counterparts, confirming that this ratio could be a microbial indicator of soil quality improvement. Results obtained in the second part of this work showed that sporulating aquatic hyphomycete diversity on leaves was strongly impaired in acidified streams, whereas fungal diversity investigated by molecular analyses was not depressed. The latter showed a lower proportion of aquatic hyphomycetes and a higher proportion of terrestrial fungi on leaves when exposed in an acidified stream compared to a circumneutral one. Microbial activity analyses bring out that Al may be an important factor that could reduce microbial leaf litter processing, this metal inducing notably a P limitation for microbial decomposers. These effects may in turn have repercussions on higher trophic levels and whole ecosystem functioningDe nombreux écosystèmes forestiers subissent les effets de l'acidification d'origine anthropique, à travers ses effets délétères sur la biodiversité et le fonctionnement de ces écosystèmes. Pour contrer ces effets, des amendements calco-magnésiens peuvent être utilisés pour améliorer les caractéristiques physico-chimiques des sols et des cours d'eau afin de restaurer l'état sanitaire des forêts et le fonctionnement des cours d'eau. En particulier, il a été démontré que les amendements, pouvaient permettre la reprise de la décomposition des litières, qui est un processus clé dans le fonctionnement des cours d'eau forestiers. Dans ce contexte, le premier objectif de cette étude était d'étudier si les amendements calco-magnésiens, à travers leurs effets sur les caractéristiques des sols, pouvaient induire des changements au sein des communautés microbiennes des sols. Le deuxième objectif était d'identifier quels facteurs pouvaient être responsables, à l'échelle microbienne, du ralentissement de la décomposition des litières dans les cours d'eau acidifiés. Les résultats ont montré que des amendements raisonnés et à grande échelle pouvaient avoir un effet durable sur les communautés bactériennes des sols. Les principaux changements taxonomiques ont notamment révélé que le ratio entre Proteobacteria et Acidobacteria était supérieur dans les sols amendés par rapport à leurs témoins, confirmant que ce ratio pouvait être un indicateur de l'amélioration de la qualité des sols. Les résultats obtenus dans la seconde partie de ce travail ont révélé que la diversité d'espèces sporulantes d'hyphomycètes aquatiques était fortement altérée dans les cours d'eau acidifiés, alors que la diversité fongique, analysée par méthodes moléculaires n'était pas affectée. Ces dernières ont révélé une plus faible proportion d'hyphomycètes aquatiques et une plus importante proportion de champignons d'origine terrestre sur les feuilles exposées dans un cours d'eau impacté. L'analyse des activités microbiennes a permis de mettre en évidence que l'aluminium était un facteur pouvant entrainer la diminution de la décomposition des feuilles, ce métal induisant notamment une limitation en phosphore pour les micro-organismes décomposeurs. Ces effets pourraient en retour avoir des répercussions sur les niveaux trophiques supérieurs et sur tout le fonctionnement de l'écosystèm

Acidification and restoration of forested ecosystems : effects on microbial communities and associated processes

De nombreux écosystèmes forestiers subissent les effets de l'acidification d'origine anthropique, à travers ses effets délétères sur la biodiversité et le fonctionnement de ces écosystèmes. Pour contrer ces effets, des amendements calco-magnésiens peuvent être utilisés pour améliorer les caractéristiques physico-chimiques des sols et des cours d'eau afin de restaurer l'état sanitaire des forêts et le fonctionnement des cours d'eau. En particulier, il a été démontré que les amendements, pouvaient permettre la reprise de la décomposition des litières, qui est un processus clé dans le fonctionnement des cours d'eau forestiers. Dans ce contexte, le premier objectif de cette étude était d'étudier si les amendements calco-magnésiens, à travers leurs effets sur les caractéristiques des sols, pouvaient induire des changements au sein des communautés microbiennes des sols. Le deuxième objectif était d'identifier quels facteurs pouvaient être responsables, à l'échelle microbienne, du ralentissement de la décomposition des litières dans les cours d'eau acidifiés. Les résultats ont montré que des amendements raisonnés et à grande échelle pouvaient avoir un effet durable sur les communautés bactériennes des sols. Les principaux changements taxonomiques ont notamment révélé que le ratio entre Proteobacteria et Acidobacteria était supérieur dans les sols amendés par rapport à leurs témoins, confirmant que ce ratio pouvait être un indicateur de l'amélioration de la qualité des sols. Les résultats obtenus dans la seconde partie de ce travail ont révélé que la diversité d'espèces sporulantes d'hyphomycètes aquatiques était fortement altérée dans les cours d'eau acidifiés, alors que la diversité fongique, analysée par méthodes moléculaires n'était pas affectée. Ces dernières ont révélé une plus faible proportion d'hyphomycètes aquatiques et une plus importante proportion de champignons d'origine terrestre sur les feuilles exposées dans un cours d'eau impacté. L'analyse des activités microbiennes a permis de mettre en évidence que l'aluminium était un facteur pouvant entrainer la diminution de la décomposition des feuilles, ce métal induisant notamment une limitation en phosphore pour les micro-organismes décomposeurs. Ces effets pourraient en retour avoir des répercussions sur les niveaux trophiques supérieurs et sur tout le fonctionnement de l'écosystèmeMany terrestrial and freshwater forested ecosystems are affected by anthropogenic acidification, which can led to deleterious effects on biodiversity and ecosystem functioning. To counteract acidification, liming can be used to improve soil and water physicochemical characteristics in order to restore tree health and headwater stream functioning. In particular, liming has been shown to enhance leaf litter breakdown, which is a key ecosystem process in headwater streams. In this context, the aims of this study were, first to investigate if liming, through its effects on soil chemical characteristics, could induce changes on soil microbial communities, and second to identify what factors could be responsible, at the microbial level, of reduced leaf litter breakdown in acidified headwater streams. Results showed that moderate large-scale liming can induce sustainable changes in soil bacterial communities. Major taxonomic changes revealed notably that the ratio between Proteobacteria and Acidobacteria was higher in limed soils compared to their control counterparts, confirming that this ratio could be a microbial indicator of soil quality improvement. Results obtained in the second part of this work showed that sporulating aquatic hyphomycete diversity on leaves was strongly impaired in acidified streams, whereas fungal diversity investigated by molecular analyses was not depressed. The latter showed a lower proportion of aquatic hyphomycetes and a higher proportion of terrestrial fungi on leaves when exposed in an acidified stream compared to a circumneutral one. Microbial activity analyses bring out that Al may be an important factor that could reduce microbial leaf litter processing, this metal inducing notably a P limitation for microbial decomposers. These effects may in turn have repercussions on higher trophic levels and whole ecosystem functionin

Modélisation de l’évolution du carbone organique dans les sols agricoles : travaux récents autour du modèle AMG

International audienc

Impact of manufactured TiO2 nanoparticles on planktonic and sessile bacterial communities

International audienceIn the present study, we conducted a 2 week microcosm experiment with a natural freshwater bacterial community to assess the effects of titanium dioxide nanoparticles (TiO2-NPs) at various concentrations (0, 1, 10 and 100 mg/L) on planktonic and sessile bacteria under dark conditions. Results showed an increase of planktonic bacterial abundance at the highest TiO2-NP concentration, concomitant with a decrease from that of sessile bacteria. Bacterial assemblages were most affected by the 100 mg/L TiO2-NP exposure and overall diversity was found to be lower for planktonic bacteria and higher for sessile bacteria at this concentration. In both compartments, a 100 mg/L TiO2-NPs exposure induced a decrease in the ratio between the Betaproteobacteria and Bacteroidetes. For planktonic communities, a decrease of Comamonadaceae was observed concomitant with an increase of Oxalobacteraceae and Cytophagaceae (especially Emticicia). For sessile communities, results showed a strong decrease of Betaproteobacteria and particularly of Comamonadaceae

Grasslands for soil organic matter storage in crop-livestock systems.

International audienc