Effects of Sphagnum Leachate on Competitive Sphagnum Microbiome Depend on Species and Time

Plant specialized metabolites play an important role in soil carbon (C) and nutrient fluxes. Through anti-microbial effects, they can modulate microbial assemblages and associated microbial-driven processes, such as nutrient cycling, so to positively or negatively cascade on plant fitness. As such, plant specialized metabolites can be used as a tool to supplant competitors. These compounds are little studied in bryophytes. This is especially notable in peatlands where Sphagnum mosses can dominate the vegetation and show strong interspecific competition. Sphagnum mosses form carpets where diverse microbial communities live and play a crucial role in Sphagnum fitness by regulating C and nutrient cycling. Here, by means of a microcosm experiment, we assessed to what extent moss metabolites of two Sphagnum species (S. fallax and S. divinum) modulate the competitive Sphagnum microbiome, with particular focus on microbial respiration. Using a reciprocal leachate experiment, we found that interactions between Sphagnum leachates and microbiome are species-specific. We show that both Sphagnum leachates differed in compound richness and compound relative abundance, especially sphagnum acid derivates, and that they include microbial-related metabolites. The addition of S. divinum leachate on the S. fallax microbiome immediately reduced microbial respiration (-95%). Prolonged exposition of S. fallax microbiome to S. divinum leachate destabilized the food web structure due to a modulation of microbial abundance. In particular, leachate addition decreased the biomass of testate amoebae and rotifers but increased that of ciliates. These changes did not influence microbial CO2 respiration, suggesting that the structural plasticity of the food web leads to its functional resistance through the replacement of species that are functionally redundant. In contrast, S. fallax leachate neither affected S. divinum microbial respiration, nor microbial biomass. We, however, found that S. fallax leachate addition stabilized the food web structure associated to S. divinum by changing trophic interactions among species. The differences in allelopathic effects between both Sphagnum leachates might impact their competitiveness and affect species distribution at local scale. Our study further paves the way to better understand the role of moss and microbial specialized metabolites in peatland C dynamics.

Carbon and nitrogen dynamics in a soil profile: Model insights and application to a restored Swiss riparian area

David Andrew Barry, Jordi Batlle-Aguilar, Alessandro Brovelli, Pascal Niklaus

The key environmental importance of natural, healthy ecosystems has been progressively recognized and restoration of degraded lands towards their former natural state has become an area of active research worldwide. During restoration, environmental conditions (such as vegetation type and water availability) are manipulated to create ecological conditions suitable for the successful establishment of a target composition of species. Often, ecological restoration induces changes to adjacent ecosystems. This is the case of riparian ecosystems, and their restoration to their original undisturbed situation is likely to cause changes in nutrient cycles. For example, following the restoration of a riparian zone, microbial communities adapted to one set of environmental conditions have to acclimatize to another, and the subsequent changes in the composition of the biomass populations might induce changes in soil organic matter mineralization and soil respiration rates. Since the biogeochemical cycles are tightly interconnected, these changes can trigger nutrient storing or release, therefore inducing changes in nutrient cycles of adjacent ecosystems. Overall, the effects of the restoration activities on the hydrologic regime, soil properties and vegetation are still largely unknown and poorly understood. Within the RECORD project (http://www.cces.ethz.ch/projects/nature/Record), a large collaborative research effort undertaken to monitor and understand the changes in ecosystem functioning in riparian areas undergoing restoration, a numerical model has been developed to simulate the vertical transport of the mobile C and N components in a soil profile (model development discussed in the companion submitted abstract Batlle-Aguilar et al.). In the model, microbial decomposition of the soil organic matter drives biogeochemical transformations of C and N, while the activity of the soil biota is primarily controlled by the soil moisture content. The temporal evolution of the soil properties measured at one location of the RECORD experimental site, in a mixed forest dominated by ash and maple characteristic for the transition from riparian to upland forest, was used to validate the model and to gain insights into the key factors controlling the nutrient turnover. The site is located next to the Thur River where a revitalization project involving removal of levees has been implemented to create more natural conditions in the riparian zone. Soil water content and temperature at several depths were monitored continuously between October 2008 and October 2009. In October 2008, January 2009 and in biweekly frequency between April and October 2009, topsoil and soil solution at several depths were sampled. The soil solution samples were analysed for major carbon and nitrogen species, and the soil samples for denitrification enzyme activity, potential nitrification and related properties. In addition, soil respiration and N2O emissions were measured at each sampling event. Preliminary modelling results are shown, together with a discussion of the most influential parameters and processes controlling C and N turnover in riparian soils.

2010

Carbon and nitrogen dynamics in a soil profile: Model insights and application to a restored Swiss riparian area

David Andrew Barry, Jordi Batlle-Aguilar, Alessandro Brovelli, Pascal Niklaus

2010

Effects of Subordinate Plant Species in Plant and Soil Community Structure and Ecosystem Functioning

Pierre Rémi Mariotte

The relevance of biodiversity to human health is an increasing international political issue as it causes concern for ethical and aesthetic reasons, but also has a strong impact on ecosystem properties and ecological goods and services utilised by humanity. However, humans have applied increasing pressure on worldwide biodiversity through pollution, land-use and climate change, over exploitation and the introduction of invasive species which can lead to major alterations of biological communities and a consequent decrease in biodiversity. When communities are assembled at random from a pool of species, more diverse mixtures have a higher probability to contain species or species-groups with high capacity to drive ecosystem processes. Therefore, it is relevant to ask "how may we classify species in a community in terms of functionality?", and "which species-group is important to maintain the productivity and stability of ecosystems?" In this thesis, I chose to classify species according their frequency and their cumulative relative cover in a plant community (i.e. plant hierarchy) and to differentiate three species-groups: dominant, subordinate and transient, which contribute towards biodiversity. Dominant species are clearly very important for ecosystem functioning, due to the large amount of biomass they produce, but there is growing evidence that subordinate species, which represent a low amount of plant biomass in grassland ecosystems, are also of considerable importance for ecosystem functioning. Given this recognition, a further objective was to explore the ecosystem level effects of these subordinate plant species, which is currently not well known. This research focused on two aspects: the persistence of subordinate species in the community, and their role in grassland ecosystems. The approach to satisfy these objectives encompassed four years of experimental field studies and glasshouse microcosm experiments. In order to explain the persistence of subordinate species in semi-natural grasslands, we performed two greenhouse experiments to test the effects of root competition exclusion and the inoculation of arbuscular mycorrhizal fungi (AMF) on the competitiveness of dominant and subordinate species, directly measured by their biomass production. The effects of subordinate species on ecosystem functioning were assessed through long-term field experiment in two sites in the Swiss Jura Mountains, where dominant, subordinate and transient species were preliminarily determined. At both site (Les Amburnex, La Frétaz), a removal experiment was carried out in randomly replicated plots with three different treatments: control without perturbations, removal of all subordinate species and partial removal of dominant biomass. In one site, a summer drought treatment was added using rainout shelters to simulate an extreme climatic event. Biomass production, litter decomposition and soil respiration were monitored during four years in each plot and complemented in some cases by measurements of soil dissolved inorganic nitrogen concentrations, soil microbial communities (bacteria and AMF) and carbon and nitrogen isotopes in plant leaves. The results of the different experiments conducted in this thesis highlight the importance of cattle activity and AMF on the persistence of subordinate plant species in semi-natural grasslands. Indeed, cattle activities (i.e. trampling) provide spatial heterogeneity, through gap creation (areas of root competition reduction), and thus favours the growth of less competitive subordinate species. While AMF acted as a parasite in the greenhouse experiment, the current findings suggest that the competitiveness of dominant species was reduced through the action of the fungus, which indirectly enhanced subordinate species. The interactions between subordinate species and soil organisms were confirmed in the field as bacterial and AMF communities shifted in plots where subordinate species were removed compared to the control treatments. Moreover, the absence of subordinate species had many negative effects on ecosystem functioning by reducing litter decomposition, soil respiration, nitrogen mineralization and community above-ground production. These findings suggest that plant-soil feedbacks explain the importance of subordinate species on ecosystem functioning despite only representing a very low quantity of biomass in plant communities. During the summer drought, subordinate species increased the resistance of the plant community and maintained productivity. Therefore, these outcomes confirm the role of drought-resistant subordinates in the functioning of grassland ecosystems threatened by climate change. The synthesis of this thesis has important implications for the way grasslands should be managed for biodiversity and confirms that extensive grazing must be maintained as it promotes species coexistence and the persistence of key subordinate species. Moreover, this work demonstrates the important role of subordinate species in ecosystem functioning, showing that not only dominant species determine ecosystem properties. This thesis serves as a contribution to the advances in our understanding of ecosystem functioning and above and below-ground linkages, and provides basis for future research in this domain.

EPFL2012

Carbon and nitrogen dynamics in a soil profile: Model insights and application to a restored Swiss riparian area

David Andrew Barry, Jordi Batlle-Aguilar, Alessandro Brovelli, Pascal Niklaus

2010

Effects of Subordinate Plant Species in Plant and Soil Community Structure and Ecosystem Functioning

Pierre Rémi Mariotte

EPFL2012

Carbon and nitrogen dynamics in a soil profile: Model insights and application to a restored Swiss riparian area

David Andrew Barry, Jordi Batlle-Aguilar, Alessandro Brovelli, Pascal Niklaus

2010