Geochemical and microbial resilience of ferralsol associated to aeration changes and rum vinasse input

L épandage de déchets liquides d'agro-industries permet leur élimination tout en apportant au sol certains nutriments pour les plantes. Il favorise toutefois l'anoxie et les activités microbiennes anaérobies, et peut entraîner diverses évolutions géochimiques portant sur les minéraux et la mobilité des métaux, ainsi que des changements de diversités microbiennes taxonomique et fonctionnelle. Se pose alors la question de la résilience du sol après retour à des conditions oxygénées. Les objectifs de cette thèse étaient (i) de consolider les acquis portant sur les évolutions géochimiques et microbiennes du sol au cours d'un épisode anoxique débutant par un apport de vinasse, (ii) d'évaluer les résiliences géochimique et microbienne du sol après retour à des conditions oxygénées, et (iii) d'étudier la survie des bactéries anaérobies en conditions oxiques. Des incubations de boues d un ferralsol de l île de la Réunion ont été réalisées pour diverses successions de conditions oxiques et anoxiques, de la vinasse de rhumerie étant apportée au début des périodes anoxiques. A plusieurs dates, des mesures ont permis de caractériser la phase gazeuse, la solution (pH, composés organiques et minéraux dont les métaux), les phases solides (états d oxydation de Fe et Mn, CEC du sol) et les communautés microbiennes (biomasse fongique, densités en nombre des bactéries, des archées, des fermentaires et des réducteurs de Fe(III) cultivables, diversité moléculaire bactérienne). En conditions anoxiques, les biotransformations ont inclus des fermentations, de l acétogénèse vraie, des réductions de Fe(III) et Mn(IV), de la sulfato-réduction et, au-delà de 14 jours d'anoxie, de la méthanogénèse acétoclastique. Elles ont entrainé une forte mobilisation de Fe et Mn, et la mobilisation de divers ETM, ainsi qu'une augmentation de la CEC du sol. Les nombres de bactéries et d'archées ont augmenté pendant respectivement les fermentations et la méthanogénèse, alors que la biomasse fongique a été stable pour des périodes anoxiques allant jusqu'à 28 jours. La diversité bactérienne a été modifiée avec apparition dès 2 jours d'anoxie de quelques pics dominants en CE-SSCP, pour ultérieurement se complexifier. Le retour à des conditions oxygénées a provoqué l'oxydation et l'immobilisation rapides de Fe(II), l'échange du Fe(II) adsorbé par d autres métaux dont la mobilité a brutalement diminué (Ca, Mg, K, Na, Mn, Pb, Ni ). Il n'y a pas eu d'oxydation de Mn(II) en solution et en phases solides pour des périodes oxiques allant jusqu'à 28 jours. La CEC est resté plus élevée qu'à l'origine et les phases solides contenant Fe ont évolué vers des formes différentes des formes initiales et des celles en fin de périodes anoxiques. La biomasse fongique a augmenté transitoirement, tandis que le nombre des bactéries est resté stable et que le nombre d'archées à légèrement diminué pour les situations où la méthanogénèse avait permis leur croissance. La diversité bactérienne a été à nouveau fortement modifiée avec apparition après 2 jours d'oxygénation de quelques pics dominants en CE-SSCP dont certains avaient déjà été observés après installation de conditions anoxiques. La persistance de quelques pics apparus en conditions anoxiques et l absence d autres pics disparus en conditions anoxiques suggèrent que la résilience microbienne n est pas totale pour une période d anoxie de 28 jours. Une aération intermédiaire de 7 jours n a affecté ni la croissance des bactéries+archées et des fermentaires cultivables, ni les fonctions de fermentation et de réduction de FeThe spreading of liquid by-products of food industry enables their elimination and supplies the soil with nutrients for plants. Unfortunately, it simultaneously favours anoxia and anaerobic microbial activities that may lead to various geochemical changes, including mineral alterations/neo-formations and metal mobilizations. It also favours changes in microbial taxonomic and functional diversities. The soil resilience has not been checked until now. The goals of this work were therefore (i) to confirm already obtained results dealing with microbial and geochemical changes in anoxic conditions, (ii) to assess for microbial and geochemical resiliences after the return to oxic conditions, and (iii) to describe the survival of anaerobic microorganisms in oxic conditions. Incubations of slurries of a ferralsol of Reunion Island were performed with successively oxic and anoxic conditions, rum vinasse being supplied at the beginning of each of the anoxic periods. Measurements were performed at some dates to characterize the composition of the gaseous headspace of the flasks, the slurry solution (pH, organic and mineral compounds, including metal elements), solid phases (iron (Fe) and manganese (Mn) oxidation level, soil CEC) and microbial communities (fungal biomass, bacteria and archaea numbers, numbers of cultivable fermentative bacteria and Fe(III) reducers, bacteria molecular diversity). Under anoxic conditions, main biotransformations included fermentations, true acetogenesis, Fe(III) and Mn(IV) reductions, SO42- reduction and acetoclastic methanogenesis. They led to a great mobilization of Fe and Mn, as well as an increase in soil CEC. Bacteria and Archaea numbers increased during fermentations and methanogenesis, respectively, whereas fungal biomass remained constant during 28 days in anoxic conditions. Microbial molecular diversity was changed with the appearance of dominant peaks in CE-SSCP characterizations after 2 days of anoxia, but this diversity became more complex for longer anoxic periods. The return to oxic conditions induced the oxidative immobilization of Fe, the exchange of adsorbed Fe(II) with other metals (Ca, Mg, K, Na, Mn, Pb, Ni ) from which the mobility decreased suddenly. Mn(II) in solution and solid phases was not oxidised for 28 day oxic periods. The soil CEC remained higher than its initial value and Fe solid phases evolved to phases that differed from initial phases and phases at the end of the anoxic period. The fungal biomass transiently increased, whereas the bacteria number didn't vary and the archaea number slightly decreased for treatments in which methanogenesis previously enabled their growth. The bacteria molecular diversity was modified again with the appearance of peaks in CE-SSCP, some ones being already observed after the beginning of anoxic conditions. The persistence of some peaks that appeared in anoxic conditions and the definitive disappearance of other peaks in anoxic conditions suggest that microbial resilience was only partial even after 28 days in oxic conditions. A period in oxic conditions between 2 periods in anoxic conditions did neither affect the growth of bacteria, archaea and cultivable fermentative bacteria, nor the microbial fermentations and Fe(III) reduction during the second anoxic periodAVIGNON-BU Centrale (840072102) / SudocSudocFranceF

Variations in the cation exchange capacity of a ferralsol supplied with vinasse, under changing aeration conditions. Comparison between CEC measuring methods

International audienceThe spreading of vinasse may favour soil anoxia and affect cation exchange capacity (CEC). Our aims were to assess the reliability of CEC measurements in reducing conditions, quantify CEC variations in a ferralsol under changing aeration conditions, and understand the mechanisms involved. In a first experiment, soil slurry was incubated for a succession of 7 oxic, 28 anoxic and 28 oxic days, vinasse being supplied when anoxia began. CEC was measured at intervals by exchanging Mg2+, Cu-trien or Co(NH3)63+, the solutions being characterized before and after exchange. In a second experiment, soil CEC was measured after the elution of NH4+ solutions of pH 5 to 7. In a third experiment, we assessed the reduction of Co(NH3)63+ by Fe2+ and checked for induced changes by geochemical simulations. During the first oxic period, CEC varied according to method, in the order: Cu-trien < Mg2+<Co(NH3)63+<NH4+. It increased by 2 cmolc kg− 1 when pH increased from 5 to 7. After 28 days of anoxia, CEC estimates were 2.75, 1.43 and 5.1 times their initial values for the 0.05 M Mg2+, Cu-trien and Co(NH3)63+ methods respectively; pH after exchange was about the same as initial slurry pH (6.9), except after Co(NH3)63+ exchange when it was 9.4. This complex was partly reduced by Fe2+ leading to NH3 release, pH increase, Fe(OH)3 and Co(OH)2 precipitations, Co(II) adsorption and overestimation of CEC. After the return to oxic conditions, the CEC did not return to its initial value. At all dates, CEC measured by 0.05 M Mg2+ method agreed with the exchangeable cation measurement

Geochemical resilience of a ferrasol subjected to anoxia and organic matter amendment

The spreading of vinasse on soils may favor not only anoxia but also mineral alterations and metal mobilization. Our aims were to record the geochemical changes occurring in a ferralsol subjected to vinasse input and anaerobiosis, and to check for soil resilience after a return to aerobic conditions. Soil slurries were therefore incubated under successive 7 d of aerobic, 0 to 28 d of anaerobic, and 28 d of aerobic conditions, vinasse being supplied at one of three levels when anaerobiosis began. At several dates, the slurry solution was characterized (pH and organic and mineral compounds) and Mn and Fe oxidation states in solids were assessed. Before incubations, about 20% of the Fe in solids was already reduced, whereas almost all Mn was found as Mn(IV). During the first aerobic period, mobilized metals included Pb, Zn, Ni, Cu, and Cr. After vinasse input during the first 14 d of anaerobiosis, the principal biotransformations were fermentation and true acetogenesis; Fe and Mn were mobilized during this period (up to 4.05 and 6.2 mmol L−1, respectively), and most Mn and an unknown but small fraction of Fe in solids were reduced. During the subsequent 14 d of anaerobiosis, only acetoclastic methanogenesis was observed. The return to aerobic conditions led to rapid oxidative immobilization of Fe, desorption of exchangeable Fe(II), and the partial immobilization of other metals (Ca, Mg, K, Na, Mn, Pb, and Ni). Manganese was not oxidized, and there was no return to the initial conditions for Fe and Mn in solid