Facteur abiotique | EPFL Graph Search

vignette|Les facteurs abiotiques En écologie, les facteurs abiotiques représentent l'ensemble des facteurs physico-chimiques d'un écosystème ayant une influence sur une biocénose donnée. C'est l'action du non-vivant sur le vivant. Opposables aux facteurs biotiques, ils constituent une partie des facteurs écologiques de cet écosystème. Les facteurs abiotiques sont indépendants de la flexibilité. Bien qu'ils soient impossibles à lister de façon exhaustive, on peut les classer en plusieurs catégories :

les facteurs édaphiques (du sol) : ** la structure du sol, ** la granulométrie, ** la teneur en sels minéraux, ** la teneur en humus, ** la salinité, ** Le développement des espèces est variable en fonction de la structure, de la composition, de la teneur en humus et de la richesse en vie microbienne des sols. Par exemple, les plantes halophiles se développent en milieu salé, les orties sur sols riches en nitrates.
les facteurs climatiques : ** la quantité, la qualité de l'eau et sa

Process-based, spatially-explicit modelling of riparian forest dynamics in Central Europe

Christian Glenz

During the last decades public awareness of the limitations of traditional engineering practices and the imperative to conserve nature have led to changes in river management; including river restoration measures. The enlargement of the fluvial corridor is one of the often considered management measures. However, the high-pressure on land-use, the conflict of interests, as well as the uncertainty of vegetation and landscape development scenarios after restoration, can make their implementation difficult. In actual decision-making processes of large river restoration projects, no dynamic long-term modelling approach of potential riparian woody species development exists mainly due to the complexity of interacting driving-processes creating lateral and longitudinal gradients. So far, forest succession models applied to riparian areas are not conceived for river areas found in Central Europe and do not address explicitly environmental influences like nitrogen scarcity or drought stress important for certain riparian systems, nor they cover integrally the vegetation-hydraulics interaction. To support and enhance the decision-making processes in river restoration projects and to provide a better understanding of riparian forest dynamics and its driving-processes, the present thesis develops a coupled model of ecological and hydraulic processes to simulate riparian forest dynamics for Central European conditions, particularly for the case of enlarged fluvial corridors. The developed model RIFOD ('RIparian FOrest Dynamics') – a distribution-based forest succession model (i.e. ecological model) coupled to a quasi-2D hydraulic model – simulates short or long-term riparian forest dynamics at a yearly time step. The model, applied on a 10 times 10 m mesh grid, is spatially-explicit concerning the interactions of the ecological and hydraulic processes and integrates 65 Central European tree and shrub species. The ecological model is based on developments of different upland forest succession models, which were improved, adapted and complemented in regard to the ecological processes in riparian areas, for example concerning regeneration, nitrogen dynamics, soil water availability or flooding stress. At the basis of the modelling of physiological flooding stress response of plants, we carried out an in-depth review of the actual knowledge of the flooding stress response of Central European tree and shrub species. The review could highlight the main biotic and abiotic factors that influence species response and revealed the broad but still vague knowledge about physiological mechanisms and species-specific data of plant response. Based on the above findings, the fuzzy set theory was chosen to model flooding stress response integrating the main abiotic factors (e.g. flooding duration, -depth). The Central European tree and shrub species were classified into flooding tolerance classes by use of clustering analysis based on proxy-data, which allowed us considering indirectly the anatomical, morphological or physiological adaptations to flooding. To model mechanical flooding stress, existing mechanistic models simulating failure resistance to uprooting or stem breakage conceived for wind load studies have been adapted to the case of water flow. Required geometrical characteristics of trees and shrubs, such as crown width and crown heights, were estimated based on available field data, whereas rooting depths in dependence of the growth stage of an individual plant were simulated by developing a quasi-mechanistic vertical root growth model for Central European tree and shrub species. This root growth model allowed also a more realistic simulation of drought stress by calculating root water extraction in relation to the development stage of stand and determining species-specific and development dependent accessibility to groundwater – not integrated in the soil water balance so far. Compared to the situation in uplands, a more realistic modelling of nitrogen availability in riparian areas could be achieved by considering the loss of nitrogen via denitrifcation, as well as the loss of litter due to flooding. In opposition to existing riparian forest succession models, RIFOD considers riparian vegetation not as a purely dependent variable of flooding. Floods may affect vegetation but they are also affected by it, owing to the contribution of vegetation to hydraulic roughness. The coupling of the forest succession model to a quasi 2-D hydraulic model allowed considering this. Moreover, the quasi steady-state model approach allowed emphasizing on the ecological relevant lateral dimension and to make the model spatially explicit in the sense of vegetation-hydraulics interaction. The current version of RIFOD finds its application in riparian areas in which the geomorphological activity of the river is not a dominant process or in case of restoration projects, for widened fluvial corridors with morphologically stable stream channels. Model evaluation (validation and sensitivity analysis) revealed that RIFOD simulates plausibly the ecological gradients observed in the field and the resulting riparian forest dynamics. By applying the model at different lateral fluvial corridor designs at the River Rhone, the consequences of a restoration measure and the change of the hydrological regime for woody vegetation could be illustrated. From a management point of view, the model revealed for example that relative benefits become smaller as the width increases or that in absence of morphological activity (e.g. lateral bank erosion) the hydraulic processes alone are not sufficient for reinitiating riparian forest succession even for high energy streams such as the River Rhone. Moreover, the model allowed verifying and discussing current scientific concepts and hypotheses, as for example the intermediate stress hypothesis. Simulation results revealed that biological diversity is highest between the very low and very severe flooding stress levels confirming the intermediate stress hypothesis involving a trade-off between competitive dominant species which monopolise stable habitats and the few fugitive species that survive high levels of instability. The value of RIFOD relies in the capacity of displaying tendencies of riparian forest dynamics and associated characteristics in function of different fluvial corridor design variants. Moreover, it allows the understanding of processes and patterns in nature by allowing exploring the consequences of a set of explicitly stated assumptions that are too complex to explore by other methods. RIFOD is the first process-based riparian forest dynamics model for Central Europe and can be seen as a step forward into a more integral modelling of the riparian forest dynamics and its processes in view of a decision-aiding tool for large river restoration projects. A future integration of geomorphological processes will allow the application of RIFOD to quasi-natural river conditions.

EPFL2005

Retention of atmospherically deposited nitrogen in soil

Isabelle Morier Jaquet

Since a few decades, the balance of the nitrogen (N) cycle has been deeply disturbed by human activies. The global impact of these activities on the N cycle can be described as a doubling of the transfer from the vast and unreactive atmospheric pool to biologically available forms (N fixation). The main sources responsible for the increase of reactive N emissions are the use of artificial fertilisers (NH3) as well as the combustion processes (NOx). Reactive N is then transformed, transported by both the atmosphere and the hydrosphere and, finally, deposited on both the terrestrial and the aquatic ecosystems with potentially strong impacts. Amongst the terrestrial ecosystems, the temperate forests are particularly sensitive to reactive N increases for various reasons: they are located near to strongly anthropised areas and are thus subjected to strong depositions; they are naturally N-limited and their biochemical cycle can indeed be strongly influenced by additional N, which leads to eutrophication and potential impacts such as soil acidification, nitrate leaching and losses of biodiversity within microorganisms, plants and fauna communities. Previous studies carried out in temperate forest ecosystems have shown that the soil, namely the soil organic matter, acts as a main deposited N sink for the ecosystem. However, biogeochemical reactions responsible for N retention in the soil are still not fully understood. Moreover, the majority of the concerned studies were conducted in acidic or hydromorphous soils and very little is known today about the fate of N deposition under other soil physico-chemical conditions. Consequently, the present research aims at filling some of the gaps described above and deals with two main topics concerning the retention of atmospherically deposited N in soils that are (1) the characterisation of the retention of atmospherically deposited N in the soil, in terms of both duration and quantity, more specifically, in a well drained calcareous soil and (2) the mechanisms and processes responsible for such a retention. From a practical point of view, the retention of N deposition in the soil was characterised by means of a 15N field labelling experiment simulating N atmospheric deposition and conducted at the Grandvillard research site, in the riparian zone of La Sarine River (Swiss Prealps). The stand is a beech forest mixed with planted spruces. The soil consists of a well drained calcareous fluvisol with a fast organic matter turnover. We tracked the N tracers (15NH4+ or 15NO3-, corresponding to the main forms of deposition) from short-term (hours, days or weeks) to longer-term intervals (one year), by measuring the partitioning of 15N into different biochemical soil fractions (extractable N, microbial N, roots N and N immobilised in soil). Aiming at studying the mechanisms and processes responsible for N retention, we performed two laboratory experiments. The first one was an acid hydrolysis, in order to determine the proportion of N deposition retained in the hydrolysable fraction (i.e. the more labile N forms, including the most bioavailable ones) as well as in the non-hydrolysable fraction covering the more recalcitrant N compounds. Within the framework of this experiment, we compared the results obtained in Grandvillard and in an aditional constrasting site (Alptal). The second experiment was a short-term (one hour to one week) laboratory incubation of sterilised and not sterilised soils, which purpose was to highlight the relative importance of biotic and abiotic processes in N immobilisation. Soils were labelled with either 15NH4+ or 15NO3- and subjected to 15N CPMAS NMR spectroscopy analysis, which is a powerful technique to identify the N organic molecules in soils. Within the framework of the field labelling experiment, more than half of the tracers (either 15NO3- or 15NH4+) was recovered in the soil between one hour and one year after labelling. Therefore, the general assumption that non-fertilised temperate forest soils are the main sink for deposited N can be confirmed. Since previous studies showing the importance of the soil for the retention of deposited N were all conducted on either acidic or hydromorphous soils, our results allow us to extend the validity of their general conclusions to a wider range of unfertilised temperate forest soils. We further showed that the main forms of N deposition (i.e. NO3-and NH4+) were retained in the soil within the same range of magnitude, in spite of their different biochemical pathways. N retention occurred mainly in the soil organic layers and, consequently, our results confirme the importance of the organic fraction in the deposited N retention. Since deposited N was retained in the soil in the very short term, we confirmed the presence of very fast processes and demonstrated that the very short term dynamics determined the fate of deposited N in the longer term: the main processes were i) a loss of deposited N as extractable through lixiviation (and lateral fluxes) and ii) a retention as N immobilised in soil. Furthermore, we demonstrated that N immobilisation brought deposited N both into the hydrolysable and into the recalcitrant fractions of soil N within short-time range (i.e. one week) . We also demonstrated that the hydrolysability (hydrolysable N/ total N) was constant over the year. Within the framework of the laboratory incubation of soils subjected to the 15N CPMAS NMR spectroscopy analysis, we showed that all NMR spectra were dominated by a single signal corresponding to the amide-peptide structure, whatever the soil layer concerned (organic or organo-mineral), the form added (NO3- or NH4+) and whether the soil was sterilised or not. Such dominance of proteinaceous compounds was in agreement with the results obtained in humic substances and in various soils all over the world. Further to the extractable 15N dynamics during the incubation, we proposed that biotic processes were dominant in the short-term N immobilisation. However, an abiotic fixation of less importance was not excluded for 15NO3-. Considering our results, we propose two mechanisms responsible for the retention of deposited N in soil in the long term. The first mechanism is a rapid and stable immobilisation in the recalcitrant pool of soil organic matter. Our results showed that biotic and abiotic processes could be involved: the afore-mentioned process of abiotic nitrate fixation could lead to the N immobilisation in heterocycles whereas N incorporated in amides and then stabilised in soil organic matter seemed to be the major pathway. The second mechanism of retention we are proposing is the biological recycling within the soil or the soil-plant system: on a short scale, microbial recycling seemed to be very rapid because deposited N was mainly present under a biosynthesised form (amides), in spite of the low amounts of tracer found in the microbes. Within the soil profile, roots recycling was probably efficient: fine roots, which are often the major contributors of soil organic matter were an important sink for N deposition all over the year. The combination of various scales and techniques allowed the connection between the dynamics of N retention in soil organic layers observed and the mechanisms responsible for it (i.e. rapid immobilisation in a recalcitrant pool and biological recycling through micro-organisms and plants). The results of this multi-scale approach suggest further developments and impacts for the modelling of the long-term fate of deposited N.

EPFL2006

N-15 immobilization in forest soil: a sterilization experiment coupled with (15)CPMAS NMR spectroscopy

Claire Guenat, Isabelle Morier Jaquet

In temperate forests, soils are the main sink for atmospheric N deposition. The main processes proposed for N retention are microbial and abiotic immobilization in soil organic matter. The relative importance of these processes as well as the kind of resulting chemical compounds are not totally understood. We carried out a laboratory incubation of Hg-sterilized and non-sterilized organic and organo-mineral soil horizons, labelled with either (NO3-)-N-15 or (NH4+)-N-15. The labelled samples were incubated for 1 hour, 1 day, or 6 days, then subjected to K2SO4 extraction and analysed with N-15 CPMAS NMR spectroscopy. N immobilization was already effective in all samples and treatments after 1 hour. The corresponding NMR spectra showed that part of the immobilized N-15 was already incorporated into an amide structure. In the sterilized soils labelled with (NH4+)-N-15, the tracer was rapidly and largely immobilized by an unknown process related to the presence of Hg. In the sterilized soils labelled with (NO3-)-N-15, between one-third and one-half of the added tracer was immobilized during the first hour and only 10% more over the 6 days. These results suggest that the sterilization was incomplete at first, allowing relatively great microbial immobilization during the first hour. By contrast, over a longer time, NO3- immobilization was significantly reduced to a level corresponding to an abiotic process as Hg sterilization became more effective. Even if the low signal-to-noise ratio precluded quantitative N-15 NMR measurements, we showed that the amide-peptide signal, considered as a biotic signature, was dominant in all cases.

Wiley-Blackwell2008