Résultat de la recherche
Earthworms, Plants, and Soils
The importance of engineers is increasingly recognized in soil science because of their implication in most important pedological processes. Furthermore, they contribute to ecological functions provided by soils in both natural and human‐modified environments. In this review, we focus on the role of two ecosystem engineers: (1) plants, their root system, and associated microorganisms and (2) earthworms. First, we explain why they are considered as major soil engineers, and which variables (texture, porosity, nutrient, and moisture dynamics) control their activities in space and time (hotspots and hot moments). Then, their roles in three processes of soil formation are reviewed, namely, rock and mineral weathering, soil structure (formation, stabilization, and disintegration), and bioturbation. For each of them, the involved mechanisms that occur at different spatial scales (from local to landscape) are presented. On one hand, tree uprooting plays a key role in rock weathering and soil profile bioturbation. In addition, living and dead roots also contribute to rock alteration and aggregation. On the other hand, earthworms are mainly involved in the formation of aggregates and burrows through their bioturbation activities and to a less extent in weathering processes. The long‐term effects of such mechanisms on soil heterogeneity, soil development, and pathways of pedogenesis are discussed. Finally, we show how these two main ecosystem engineers contribute to provisioning and regulating services. Through their physical activities of burrowing and soil aggregation, earthworms and plants increase plant productivity, water infiltration, and climate warming mitigation. They act as catalysts and provide, transform, and translocate organic matter and nutrients throughout the soil profile. Finally, due to inter‐ and intraspecific interactions and/or symbiosis with microorganisms (arbuscular fungi, bacteria), they enhance soil fertility, decrease parasitic action, and bioremediate some pollutants. Future research is, however, still needed for a better understanding of the relationships between adequate soil management, agricultural practices, and soil biota in a perspective of relevant maintenance and durability of ecological services.
Composition and superposition of alluvial deposits drive macro-biological soil engineering and organic matter dynamics in floodplains
Soil structure formation in alluvial soils is a fundamental process in near-natural floodplains. A stable soil structure is essential for many ecosystem services and helps to prevent river bank erosion. Plants and earthworms are successful soil engineering organisms that improve the soil structural stability through the incorporation of mineral and organic matter into soil aggregates. However, the heterogeneous succession of different textured mineral and buried organic matter layers could impede the development of a stable soil structure. Our study aims at improving the current understanding of soil structure formation and organic matter dynamics in near natural alluvial soils. We investigate the effects of soil engineering organisms, the composition, and the superimposition of different alluvial deposits on the structuration patterns, the aggregate stability, and organic matter dynamics in in vitro soil columns, representing sediment deposition processes in alluvial soils. Two successions of three different deposits, silt–buried litter–sand, and the inverse, were set up in mesocosms and allocated to four different treatments, i.e. plants, earthworms, plants+earthworms, and a control. X-ray computed tomography was used to identify structuration patterns generated by ecosystem engineers, i.e. plant root galleries and earthworm tunnels. Organic matter dynamics in macro-aggregates were investigated by Rock- Eval pyrolysis. Plant roots only extended in the top layers, whereas earthworms preferentially selected the buried litter and the silt layers. Soil structural stability measured via water stable aggregates (%WSA) increased in the presence of plants and in aggregates recovered from the buried litter layer. Organic matter dynamics were controlled by a complex interplay between the type of engineer, the composition (silt, sand, buried litter) and the succession of the deposits in the mesocosm. Our results indicate that the progress and efficiency of soil structure formation in alluvial soils strongly depends on the textural sequences of alluvial deposits.
Rock-Eval pyrolysis discriminates soil macro-aggregates formed by plants and earthworms
Plants and earthworms, as soil ecosystem engineers, play a crucial role during stabilisation of organic matter in soil through its incorporation into soil aggregates. It is therefore essential to better understand the mechanisms and interactions of soil engineering organisms regarding soil organic matter stabilisation. Several methods have already been successfully applied to differentiate soil aggregates by their origin, but they cannot specify the degree of organic matter stability within soil aggregates. Rock-Eval pyrolysis has already been proved to be pertinent for analyses of soil organic matter bulk chemistry and thermal stability, but it has not yet been directly applied to identify biogenic organic matter signatures within soil aggregates. In this study, Rock-Eval pyrolysis was used for the identification of the soil aggregate origin as well as for the determination of the soil organic matter bulk chemistry and thermal stability in a controlled experiment. Mesocosms were set up, containing treatments with a plant, an earthworm species, or both. Water stable soil macro-aggregates > 250 μm were sampled and tested with Rock-Eval pyrolysis after a two-month incubation period. Rock-Eval pyrolysis was able to differentiate soil macro-aggregates by their origin, and to identify a specific signature for each treatment. Macro-aggregates from the plant and earthworm treatment were characterized by a mixed signature incoming from the two soil engineers, indicating that both engineers contribute concomitantly to soil aggregate formation. Organic matter thermal stability was not positively affected by earthworms and even tends to decrease for the plant treatment, emphasising that organic matter was mainly physically protected during the incubation period, but not stabilised. However, future research is required to test if signatures for the tested organisms are species-specific or generally assignable to other plant and earthworm species.
Use of X-ray microcomputed tomography for characterizing earthworm-derived belowground soil aggregates
Soil structure is closely linked to biological activities. However, identifying, describing and quantifying soil aggregates remain challenging. X-ray microcomputed tomography (X-ray μCT) provides a detailed view of the physicalstructure at a spatial resolution of a few microns. It could be a useful tool todiscriminate soil aggregates, their origin and their formation processes for a better comprehension of soil structure properties and genesis. Our study aims to (a) determine different X-ray μCT-based aggregate parameters for differentiating earthworm casts belowground (earthworm aggregates) from aggregates that are not formed by earthworms (non-earthworm aggregates), and (b) to evaluate if these parameters can also serve as specific “tomographic signatures” for the studied earthworm species. For this purpose, we set up a microcosm experiment under controlled conditions during 8 weeks, including three species of earthworms tested separately: the epigeic Lumbricus rubellus, the anecic Lumbricus terrestris and the endogeic Allolobophora chlorotica. Our results show that X-ray μCT analysis helps distinguish earthworm aggregates from non-earthworm ones using (a) the relative volume of the components within aggregates and (b) the volumetric mass of aggregates and their global volume. In particular, the volume ratio of mineral grains within the aggregates is significantly different according to earthworm species. So, X-ray μCT is a powerful and promising tool for studying the composition of earthworm casts and their formation. However, future research is needed to take into account the shapes and spatial distribution of the aggregates' components, in particular the different states of organic matter decomposition.
Ecosystem engineers’ contribution to soil structure formation in floodplains
La formation de la structure du sol est un processus primordial en zone alluviale semi-naturelle et revitalisée. Une structure du sol stable protège les berges de l’érosion et contribue à la préservation des services écosystémiques de ce type de milieux. Cependant, la mise en place de la structure des sols alluviaux est délicate pour plusieurs raisons. La dynamique alluviale très marquée engendre régulièrement des engorgements et rajeunit continuellement les sols au travers de dépôts de sédiments non consolidés, ce qui impacte les macroorganismes tels que les ingénieurs du sol. Les plantes et les vers de terre sont des organismes ingénieurs très performants capables de façonner une structure cohésive au moyen de macro-agrégats stables. Ces derniers peuvent contenir des teneurs conséquentes de matière organique, stabilisée par les particules minérales, ce qui contribue à sa séquestration dans le sol. Malgré son aspect crucial, le rôle des plantes et des vers de terre dans la mise en place de la structure du sol et la stabilisation de la matière organique au sein d’agrégats reste peu méconnu en zones alluviales. Plus particulièrement, l’influence de l’hydrologie à l’échelle du paysage ainsi que celle des paramètres physico-chimiques sur les plantes et les vers de terre est encore peu étudié, et notamment leur capacité à améliorer la stabilité structurale des sols alluviaux. De plus, les mécanismes de formation des macro-agrégats ainsi que la stabilisation de la matière organique par les plantes et/ou les vers de terre selon le type de sédiment est encore mal connu. Enfin, les connaissances manquent sur l’efficacité de ces acteurs dans la réalisation d’une structure stable à court terme, et sous l’effet d’une dynamique fluviale intense. Pour toutes ces raisons, une expérimentation en trois étapes a été menée afin : I) d’analyser la stabilité structurale des sols en fonction des plantes et des communautés lombriciennes, de l’hydrologie du milieu et des paramètres physicochimiques à l’échelle du terrain, II) de comprendre les mécanismes de formation des macro-agrégats et de la stabilisation de la MO au travers d’une étude en mésocosmes, à l’échelle des processus, en sélectionnant des ingénieurs du sol, à savoir la baldingère faux-roseau Phalaris arundinacea et un ver de terre endogé Allolobophora chlorotica. Ce chapitre a été divisé en deux parties, IIa) tester la pyrolyse Rock Eval pour discriminer les macro-agrégats issus de P. arundinacea et A. chlorotica, IIb) analyser les mécanismes de formation des macro-agrégats et de stabilisation de la MO lors d’une superposition de couches de différents matériaux minéraux et organiques, en simulant ainsi des sols alluviaux reconstitués en mésocomes. Dans le troisième chapitre, III) l’efficacité de P. arundinacea et des communautés lombriciennes à créer une structure du sol stable sur le court terme a été déterminée au sein d’un système expérimental semi-contrôlé, exposé à la dynamique alluviale naturelle in situ. La structure du sol a été analysée au moyen des indicateurs pédologiques traditionnels combinés à des techniques modernes d’imagerie. L’abondance des plantes a été démontrée comme étant drastiquement impactée par la fluctuation des niveaux d’eau, mais elle contribue toutefois très fortement à la stabilisation des horizons de surface. P. arundinacea a largement amélioré la structure du sol dans les dépôts sableux sur le court terme et a contribué à la fabrication de macro-agrégats stables en 8 semaines en mésocosmes. Sur le terrain, l’abondance des vers de terre n’est ni corrélée à la stabilité structurale des horizons de surface, ni à aucun des paramètres physico-chimiques ou fluctuations des niveaux d’eau. Cependant, les communautés lombriciennes, incluant A. chlorotica, ont amélioré la porosité su sol sur le court terme, mais la stabilité de leurs structures biogéniques n’a jamais augmenté, que ce soit en mésocosmes ou en conditions semi-naturelles. Toutefois, A. chlorotica augmente de manière efficace la stabilité thermique de la matière organique dans les macro-agrégats formés à partir de sédiments limoneux. Sur le long terme, les vers de terre, dont A. chlorotica, contribuent à la formation de la structure du sol et à la séquestration du carbone quand leurs structures biogéniques gagnent en stabilité avec le temps. Ces résultats laissent supposer des interactions entre plantes et vers de terre dans la formation des macro-agrégats, mais celles-ci n’ont pas été clairement établies avec les techniques utilisées. Les méthodes, qui ont permis de déterminer la formation de la structure du sol et la stabilisation de la MO, ont été très utiles mais les procédures standards nécessitent encore d’être définies pour notamment la préparation des échantillons et le traitement des données. En conclusion, les plantes et les vers de terre possèdent un grand potentiel pour favoriser la réussite des projets de revitalisation en zone alluviale, les plantes sur le court terme et les vers de terre sur un plus long terme., Soil structure formation constitutes an extremely important process in near-natural and restored floodplains. A stable soil structure protects riverbanks from erosion and contributes to the preservation of ecosystem services. However, developing a soil structure in alluvial soils is difficult for several reasons. Extensive alluvial dynamics cause periodic waterlogging and continuously rejuvenate soils by the deposition of unconsolidated sediments which affect soil macro-organisms acting as soil engineers. Plants and earthworms are highly successful soil engineering organisms being able to build up a stable soil structure through the formation of stable macro-aggregates. Macro-aggregates may contain significant amounts of organic matter which can be efficiently stabilised through associations to mineral particles thus contributing to the sequestration of organic matter in the soil. Despite its importance, the role of plants and earthworms in soil structure formation in floodplain soils and in organic matter stabilisation in macro-aggregates is still poorly investigated. In particular, the influence of the landscape hydrology and soil physicochemical parameters on plants and earthworms and their capacity to improve the structural stability of floodplain soils are widely unexplored. Moreover, the mechanisms of macro-aggregate formation and organic matter stabilisation by plants and earthworms including interaction effects in different alluvial sediments are poorly understood. Third, little is known about the efficiency of plants and earthworms to create a stable soil structure in the short term under extensive alluvial dynamics. For this purpose, a three stage experiment was designed: I) analysing the structural stability of soils as a function of plant and earthworm communities, the landscape hydrology and soil physicochemical parameters at the field scale, II) understanding the mechanisms of macro-aggregate formation and OM stabilisation in mesocosms by means of two selected soil engineers, e.g. the red canary grass Phalaris arundinacea and the endogeic earthworm Allolobophora chlorotica. This chapter was divided in two parts, IIa) testing Rock-Eval pyrolysis to discriminate macro-aggregates formed by P. arundinacea and A. chlorotica and IIb) analysing the mechanisms of macro-aggregate formation and OM stabilisation for a succession of different mineral and organic layers similar to alluvial soils reconstructed in mesocosms. In the third chapter III), the efficiency of P. arundinacea and earthworm communities to create a stable soil structure in the short term was determined in semi-controlled field plots exposed to natural alluvial dynamics. Soil structure was analysed using different traditional pedologic indicators combined with modern imaging techniques. Plant abundance was demonstrated to be crucially affected by fluctuating water levels, but nevertheless strongly contributed to the stabilisation of the topsoils. Especially P. arundinacea was highly efficient to improve the soil structure in sandy alluvial deposits in the short term and to build up stable macro-aggregates within 8 weeks in mesocosms. Earthworm abundance neither correlated to the structural stability of topsoils nor responded to any soil physicochemical parameters or fluctuating water levels in the field. However, earthworm communities, including A. chlorotica increased the porosity in the short term, but the stability of their structures was neither increased mesocosms nor in semi-controlled field plots. Nevertheless, A. chlorotica efficiently increased the thermal stability of organic matter in macro-aggregates in silty alluvial layers. In the long term, earthworms including A. chlorotica contribute to soil structure formation and the sequestration of carbon when their structures gain in stability with aging. Based on the results, interactions between plants and earthworms in macro-aggregate formation and OM stabilisation was assumed, but could not be clearly demonstrated with the applied methods. The methods used to determine soil structure formation and OM stabilisation were highly useful, but standard procedures still need to be defined for data processing and sample preparation. Conclusively, plants and earthworms have great potential to increase the success of river restoration projects, whereby plants in the short term and earthworms in the long term.
Coupling X-ray computed tomography and freeze-coring for the analysis of fine-grained low-cohesive soils
This paper presents the coupling of freeze-core sampling with X-ray CT scanning for the analysis of the soil structure of fine-grained, low-cohesive soils. We used a medical scanner to image the 3D soil structure of the frozen soil cores, providing X-ray CT data at a millimetric resolution over freeze-cores that are up to 62.5 cm long and 25 cm wide. The obtained data and the changes in gray level values could be successfully used to identify and characterize different soil units with distinctly different physical properties. Traditional measurements of soil bulk density, carbon and particle size analyses were conducted within each of the identified soil units. These observations were used to develop a 3D model of soil bulk density and organic matter distribution for five freeze-cores obtained at a restored floodplain in Switzerland. The millimetric X-ray CT scanning was applied to detect the impact of freeze-coring on the soil structural integrity. This allows identifying undisturbed zones, a critical precondition for any subsequent assessment of soil structure. The proposed coupling is thought to be applicable to a wide range of other low-cohesive soil types and has a large potential for applications in hydrogeology, biology or soil science.
Pioneer plant Phalaris arundinacea and earthworms promote initial soil structure formation despite strong alluvial dynamics in a semi-controlled field experiment
Soil structure formation is among the most important processes in river floodplains which are strongly influenced by alluvial dynamics. In the context of river restoration projects, a better understanding of soil structure formation in habitats adjacent to the river can help to prevent damages caused by riverbank erosion. Ecosystem engineers such as pioneer herbaceous plants and earthworms likely contribute to soil structure formation even despite less favourable environmental conditions. This study aims to assess the capacity of the herbaceous perennial and native species Phalaris arundinacea and earthworm communities to promote a stable soil structure in alluvial sediments, in particular fresh alluvial deposits, in the short term. Delimited plots were set-up in a restored floodplain adjacent to the Thur River in NE Switzerland and exposed to natural alluvial dynamics for 19 months. Four treatments were replicated in a randomised complete block design: (i) plots with Phalaris arundinacea as only vegetation, (ii) plots with all vegetation constantly removed, (iii) and (iv) the earthworm community reduced by mustard treatment, otherwise as (i) and (ii), respectively. Soil structure formation was analysed at the end of the experiment using different indicators: aggregate stability, field-saturated hydraulic conductivity and the porosity calculated from X-ray CT reconstructions of freeze cores. Phalaris arundinacea was capable of improving the porosity and aggregate stability of both alluvial sediments present at the beginning of the experiment but also of sediments freshly deposited during the observation period. The latter indicates a structuring effect within only one vegetation period. Earthworm abundance was as a whole very low, most likely due to the large proportion of sand. There was a small earthworm effect on soil structure formation, and only in combination with Phalaris.arundinacea. Our findings highlight the ability of Phalaris arundinacea in efficiently structuring sandy alluvial sediments in the short term even under strong alluvial dynamics. Phalaris arundinacea can therefore play a key role in the early stage of river restoration projects. Thus, facilitating the colonisation by such native pioneer herbaceous plants is a suitable step to improve the success of river restoration projects.
Topsoil structure stability in a restored floodplain: Impacts of fluctuating water levels, soil parameters and ecosystem engineers
Ecosystem services provided by floodplains are strongly controlled by the structural stability of soils. The development of a stable structure in floodplain soils is affected by a complex and poorly understood interplay of hydrological, physico-chemical and biological processes. This paper aims at analysing relations between fluctuating groundwater levels, soil physico-chemical and biological parameters on soil structure stability in a restored floodplain.Water level fluctuations in the soil are modelled using a numerical surface-water–groundwater flow model and correlated to soil physico-chemical parameters and abundances of plants and earthworms. Causal relations andmultiple interactions between the investigated parameters are tested through structural equation modelling (SEM). Fluctuatingwater levels in the soil did not directly affect the topsoil structure stability, but indirectly through affecting plant roots and soil parameters that in turn determine topsoil structure stability. These relations remain significant for mean annual days of complete and partial (N25%)water saturation. Ecosystemfunctioning of a restored floodplainmight already be affected by the fluctuation of groundwater levels alone, and not only through complete flooding by surface water during a flood period. Surprisingly, abundances of earthworms did not showany relation to other variables in the SEM. These findings emphasise that earthworms have efficiently adapted to periodic stress and harsh environmental conditions. Variability of the topsoil structure stability is thus stronger driven by the influence of fluctuatingwater levels on plants than by the abundance of earthworms. This knowledge about the functional network of soil engineering organisms, soil parameters and fluctuating water levels and how they affect soil structural stability is of fundamental importance to define management strategies of near-natural or restored floodplains in the future
Topsoil structure stability in a restored floodplain: Impacts of fluctuatingwater levels, soil parameters and ecosystem engineers
Ecosystem services provided byfloodplains are strongly controlled by the structural stability of soils. The developmentof a stable structure infloodplain soils is affected by a complex and poorly understood interplay of hydrological,physico-chemical and biological processes. This paper aims at analysing relations betweenfluctuating groundwaterlevels, soil physico-chemical and biological parameters on soil structure stability in a restoredfloodplain. Water levelfluctuations in the soil are modelled using a numerical surface-water–groundwaterflow model and correlated tosoil physico-chemical parameters and abundances of plants and earthworms. Causal relations and multiple interactionsbetween the investigated parameters are tested through structural equation modelling (SEM). Fluctuating water levelsin the soil did not directly affect the topsoil structure stability, but indirectly through affecting plant roots and soil pa-rameters that in turn determine topsoil structure stability. These relations remain significant for mean annual days ofcomplete and partial (N25%) water saturation. Ecosystem functioning of a restoredfloodplain might already be affectedby thefluctuation of groundwater levels alone, and not only through completeflooding by surface water during afloodperiod. Surprisingly, abundances of earthworms did notshow any relation to other variables in the SEM. Thesefindingsemphasise that earthworms have efficiently adapted to periodic stress and harsh environmental conditions. Variabilityof the topsoil structure stability is thus stronger driven by the influence offluctuating water levels on plants than by theabundance of earthworms. This knowledge about the functional network of soil engineering organisms, soil parametersandfluctuating water levels and how they affect soil structural stability is of fundamental importance to define man-agement strategies of near-natural or restoredfloodplains in the future.