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International audience; Algal assemblages have been widely used as an ecological indicator of aquatic ecosystem health conditions because of their specific sensitivity to a wide variety of environmental conditions. In turbid rivers, as in other aquatic systems, phytoplankton structure plays an important role in structuring aquatic food webs. Worldwide, phytoplankton is less studied in turbid, large tropical rivers compared to temperate river systems. The present study aimed to describe the phytoplankton diversity and abundance in a turbid tropical river (the Red River, northern part of Vietnam from 20 degrees 00 to 25 degrees 30 North; from 100 degrees 00 to 107 degrees 10 East) and to determine the importance of a series of environmental variables in controlling the phytoplankton community composition. Phytoplankton community was composed of 169 phytoplankton taxa from six algal groups including Bacillariophyceae, Chlorophyceae, Cryptophyceae, Euglenophyceae, Dinophyceae and Cyanobacteria. Community composition varied both spatially and with season. Sixteen measurement environmental variables were used as input variables for a three-layer backpropagation neural network that was developed to predict the phytoplankton abundance. Phytoplankton abundance was successfully predicted using the tagsig transfer function and the Levenberg-Marquardt backpropagation algorithm. The network was trained to provide a good overall linear fit to the total data set with a slope (R) and mean square error (MSE) of 0.808 and 0.0107, respectively. The sensitivity analysis and neutral interpretation diagram revealed that total phosphorus (TP), flow discharge, water temperature and P-PO43- were the significant variables. The results showed that the developed ANN model was able to simulate phytoplankton abundance in the Red River. These findings can help for gaining insight into and the relationship between phytoplankton and environmental factors in this complex, turbid, tropical river.

This study aimed to evaluate the decomposition of three exogenous organic amendments (buffalo manure, its compost and vermicompost), mixed with biochar or not, under tropical climate conditions. Litterbags containing pure substrates or mixtures were exposed to soil under natural rainfall conditions during one year. Most of C and N of exogenous organic matters (EOMs) and biochar were removed rapidly in the first 16 weeks of incubation. After one year, in case of applying alone, only around 30% of C of EOMs and 40% of C of biochar remained. Our data highlighted that in mixture, the presence of biochar led to about 40% higher amounts of C remaining, most probably related to aggregate formation preventing physical disintegration and/or leaching losses. Consequently, this study demonstrated the interest of mixing organic substrates with biochar for preventing C removal from soil immediately after field exposure. (C) 2016 Published by Elsevier B.V.

International audience; Background: Exogenous organic matter may be valuable amendments for degraded tropical soil in Northern Vietnam. Their effect on carbon storage and composition need to be evaluated. Method: The study investigates soil from mesocosms cultivated with maize and amended with four different kinds of amendments: only mineral fertilizers (control), buffalo manure, compost or vermiconnpost as well as biochar. Results: All organic amendments showed a positive effect on soil carbon storage and significantly influenced soil organic matter quality. The presence of biochar increased lignin-derived compounds abundance as well as both plant and microbial sugar content of soil amended with vermicompost but had no effect in the case of control soil. Conclusion: Synergistic effects between plants and different organic amendments influence carbon storage and soil organic matter composition.

International audience; The Nui Coc reservoir (water surface: 25.2 km(2); catchment area: 567 km(2)), typical of medium sized reservoirs in Vietnam and in South East Asia, was built in the 1970s to stimulate social and economic development in the surrounding area. The nitrogen and phosphorus budgets were established in this typical reservoir catchment to determine the degree of human-induced alteration of nitrogen and phosphorus cycles. The results showed that both nitrogen and phosphorus inputs are higher than their outputs in the agricultural soil system, indicating the excess of fertilizer utilization in the catchment. The budget of the hydrosystem revealed that the nitrogen and phosphorus fluxes from agricultural and forest soils accounted for 50% and 51% of the total inputs to the hydrosystem, respectively. About 66% of the annual total nitrogen and 79% of the annual total phosphorus inputs to the hydrosystem were deposited/eliminated in the reservoir.

International audience; Field transfer experiments of periphytic diatom assemblages developed on artificial substrates were set up to assess the responses of those communities to environmental disturbances. The glass slides were positioned for colonization at the relatively unpolluted site (Red, in the Red River) and at the heavily polluted site (TL, in the To Lich River) in the beginning of the experiment. After a period of 2 weeks, the colonized glass slides were concomitantly transferred from the unpolluted Red site to the heavily polluted TL site and to the moderate polluted site (NT 2, in the Nhue River) and, conversely, from the TL site to the Red site, and then to the NT 2 site. The responses and the adapting capacity of periphytic diatom communities to environmental changes were assessed through the cell density, diversity index, species richness, taxonomic composition, and diatom indices after 2 and 4 weeks of transfer periods. For all transfers except for the transfer from the Red to the TL site in which the growth inhibition of diatom cells was found, the diatom density significantly increased until the end of the experiment. Thus, the diatom communities have expressed their pollution tolerance or sensitivities by changing their composition to adapt themselves to environmental changes. Characteristic species of the Red site (Gyrosigma scalproides, Navicula recens) were replaced by Nitzschia palea, Nitzschia umbonata, Aulacoseira granulate typical species of the NT 2 site, in the biofilm transferred from the Red site to the NT 2 site. The relative abundances of typical diatom species of the Red site proliferated in the biofilm transferred from the TL site to the Red site. The replacement of periphytic diatom communities appeared after the transfer from the second week at the different sites. The slow shift of the species towards the typical species at the TL site could result from the organized structure of diatoms within biofilm before the transfer from the Red site to the TL site. The shifts in values of the Index of Specific Polluosensitivity and Diatom Assemblage Index to organic pollution throughout the experiment indicated the clear sensitivity of these indices to water quality changes.