Minimal-time bioremediation of natural water resources

We study minimal time strategies for the treatment of pollution of large volumes, such as lakes or natural reservoirs, with the help of an autonomous bioreactor. The control consists in feeding the bioreactor from the resource, the clean output returning to the resource with the same flow rate. We first characterize the optimal policies among constant and feedback controls, under the assumption of a uniform concentration in the resource. In a second part, we study the influence of an inhomogeneity in the resource, considering two measurements points. With the help of the Maximum Principle, we show that the optimal control law is non-monotonic and terminates with a constant phase, contrary to the homogeneous case for which the optimal flow rate is decreasing with time. This study allows the decision makers to identify situations for which the benefit of using non-constant flow rates is significant

A modeling approach of the chemostat

Population dynamics and in particular microbial population dynamics, though they are complex but also intrinsically discrete and random, are conventionally represented as deterministic differential equations systems. We propose to revisit this approach by complementing these classic formalisms by stochastic formalisms and to explain the links between these representations in terms of mathematical analysis but also in terms of modeling and numerical simulations. We illustrate this approach on the model of chemostat.Comment: arXiv admin note: substantial text overlap with arXiv:1308.241

The buffered chemostat with non-monotonic response functions

We study how a particular spatial structure with a buffer impacts the number of equilibria and their stability in the chemostat model. We show that the occurrence of a buffer can allow a species to persist or on the opposite to go extinct, depending on the characteristics of the buffer. For non-monotonic response functions, we characterize the buffered configurations that make the chemostat dynamics globally asymptotically stable, while this is not possible with single, serial or parallel vessels of the same total volume and input flow. These results are illustrated with the Haldane kinetic function.Comment: 9th IFAC Symposium on Nonlinear Control Systems (NOLCOS 2013), Toulouse : France (2013

Cycle de l'azote et rôle des légumineuses d'ombrage dans les systèmes caféiers au Costa Rica

En Amérique Centrale, les systèmes agroforestiers représentent une alternative à la monoculture intensive de café fondée sur l'utilisation importante d'intrants et qui peine à concilier à la fois production, respect de l'environnement et rentabilité économique. Les systèmes agroforestiers ont un potentiel de stockage de carbone mais aussi de réduction du lessivage de nitrates et d'émissions d'oxyde nitreux, un important gaz à effet de serre. Néanmoins, au Costa Rica, beaucoup de plantations caféières en monoculture ou sous ombrage de légumineuses fortement taillées sont encore gérées conventionnellement et reçoivent de grandes quantités d'engrais azoté (200 kg N/ha/an). Des pratiques alternatives utilisant des engrais organiques ainsi qu'une gestion plus raisonnée des légumineuses d'ombrage sont toutefois en expansion. Comprendre les mécanismes et quantifier les changements dans la dynamique de l'azote en fonction de l'espèce ligneuse associée, de sa conduite, des apports d'engrais et des conditions de site est de première importance pour évaluer l'efficience d'utilisation de l'azote et la contamination des nappes par les nitrates et les émissions d'oxyde nitreux. Pour évaluer l'impact des pratiques sur la dynamique de l'azote, l'UR " Ecosystèmes de Plantations " du CIRAD (UMR Eco&Sols) a établi en collaboration avec ses partenaires, le CATIE et l'ICAFE des bilans de flux d'azote dans différents systèmes agroforestiers caféiers de référence comparés à des monocultures. Les études ont montré pour les systèmes conventionnels en général une faible efficience d'utilisation de l'engrais par le caféier et l'arbre d'ombrage ainsi qu'un fort potentiel de pertes de nitrates dans les eaux de drainage et des émissions d'oxyde nitreux non négligeables. Les travaux ont montré également par rapport à la monoculture un effet très positif de la légumineuse d'ombrage (Erythrina poeppigiana) sur la disponibilité en azote du sol, et la production de café en particulier dans les systèmes organiques. Ainsi la légumineuse d'ombrage permet un niveau élevé de fixation d'azote atmosphérique et un recyclage important de cet azote lors de l'élagage des branches feuillées. (Texte intégral

Nitrogen fixing shade trees in coffee agroforestry: Quantification of nitrogen transfer to the coffee plant

Nitrogen-fixing shade trees in coffee agroforestry systems are assumed to provide an alternative nitrogen source for the coffee crop when fertilizer applications are low, but the transfer of nitrogen from shade trees to the coffee crop has not yet been quantified directly. We present a case study for coffee agroforestry systems with Erythrina poeppigiana as a shade tree. The transfer of nitrogen from the N2 fixing tree to coffee plants was measured through a stable isotope pulse and chase experiment. Shade trees that had been labelled with a 15N-enriched nitrate solution, were pruned, and the prunings were subsequently laid out below coffee crops under conventional or organic management. Significant fractions of nitrogen ended up in the coffee plants 5 months after the deposition of 15N labelled prunings on the plantation floor (figure 1). More nitrogen from prunings was found in the soil under organic management than under conventional management. This finding was associated with higher macrofauna abundance, particularly earthworms, in the organic system. Coppicing of the shade tree and subsequent decomposition of the pruned material was the dominant mechanism for nitrogen transfer to the coffee plant, while other mechanisms only affected coffee plants directly neighbouring the tree. In conclusion, the pruning of shade trees in coffee agroforestry systems is an important pathway for the transfer of fixed N to the coffee plants and seems to be essential in organic systems

Giant spin-dependent photo-conductivity in GaAsN dilute nitride semiconductor

A theoretical and experimental study of the spin-dependent photoconductivity in dilute Nitride GaAsN is presented. The non linear transport model we develop here is based on the rate equations for electrons, holes, deep paramagnetic and non paramagnetic centers both under CW and pulsed optical excitation. Emphasis is given to the effect of the competition between paramagnetic centers and non paramagnetic centers which allows us to reproduce the measured characteristics of the spin-dependent recombination power dependence. Particular attention is paid to the role of an external magnetic field in Voigt geometry. The photoconductivity exhibits a Hanle-type curve whereas the spin polarization of electrons shows two superimposed Lorentzian curves with different widths, respectively related to the recombination of free and trapped electrons. The model is capable of reproducing qualitatively and quantitatively the most important features of photoluminescence and photocurrent experiments and is helpful in providing insight on the various mechanisms involved in the electron spin polarization and filtering in GaAsN semiconductors.Comment: 10 pages, 5 figure