Soil respiration | EPFL Graph Search

Soil respiration refers to the production of carbon dioxide when soil organisms respire. This includes respiration of plant roots, the rhizosphere, microbes and fauna. Soil respiration is a key ecosystem process that releases carbon from the soil in the form of CO2. CO2 is acquired by plants from the atmosphere and converted into organic compounds in the process of photosynthesis. Plants use these organic compounds to build structural components or respire them to release energy. When plant respiration occurs below-ground in the roots, it adds to soil respiration. Over time, plant structural components are consumed by heterotrophs. This heterotrophic consumption releases CO2 and when this CO2 is released by below-ground organisms, it is considered soil respiration. The amount of soil respiration that occurs in an ecosystem is controlled by several factors. The temperature, moisture, nutrient content and level of oxygen in the soil can produce extremely disparate rates of respiration.

Silver nanoparticle effects on simple stream food webs and ecosystem processes

Carmen Omayra Gil Allué

With the global nanotechnology market growing rapidly, nanomaterials are being increasingly released into aquatic environments, where they can undergo modifications and sedimentation, which will put benthic organisms at risk. Of particular interest is the study of nanomaterial effects on periphyton, a community of auto- and heterotrophic microorganisms embedded in an extracellular polymer matrix that covers submerged surfaces in aquatic ecosystems. Although periphyton assumes important functions in ecosystems, like primary production, little information is available on its sensitivity to nanomaterials and how these might be transferred to higher trophic levels in food webs. In the present thesis, effects of short- and long-term exposures to citrate-coated silver nanoparticles (AgNP, 35 nm) and silver ions (Ag+, dosed as AgNO3) were assessed on periphyton in microcosm studies for periods between 2 hours and 21 days. After exposure, primary production, secondary production, photosynthetic yield, microbial respiration and potential extracellular enzyme activity were measured. Moreover, addition of an aquatic snail, Physa acuta, feeding on periphyton allowed the assessment of effects of AgNP on an aquatic herbivore and trophic transfer efficiency. Contaminated (AgNP and AgNO3) and uncontaminated periphyton was offered to snails in a 5-day experiment to determine silver uptake and the mortality, feeding rate and reproduction of the snails. Our findings show that all endpoints measured in periphyton were affected by both AgNP and AgNO3 during short-term exposure, but AgNO3 was more toxic on a total silver mass basis than AgNP. Concentrations causing a 50% reduction in the different endpoints (EC50) ranged from 17 to 83 µM AgNP and from 2 to 4 µM AgNO3. Exposure of periphyton in the presence of silver ion ligands to complex Ag+ (ca. 1% of total silver in AgNP suspensions) prevented AgNP toxicity to most endpoints, confirming that the observed toxic effects were caused by the bioavailable Ag+. An exception was the extracellular enzyme leucine aminopeptidase, which was directly affected by AgNP. Concentrations of AgNP and AgNO3 that caused low toxicity during short-term exposure also caused effects on periphyton functions upon long-term exposure, with algae and bacteria being differently affected. Periphyton exposed to 10 µM AgNP for 21 days showed a 63% decrease in primary production and a 246% increase in bacterial secondary production per unit of total periphyton biomass. The community composition exposed to AgNP or AgNO3 differed from that of the control and also from each other. Furthermore, AgNP-exposed communities were more tolerant to subsequent AgNP exposure. Trophic transfer of silver from contaminated periphyton to snails occurred efficiently, independently of the silver form. Feeding rate of snails on AgNO3-exposed communities was 50% lower than on controls, and disturbance of digestion was evident upon feeding on both AgNP- and AgNO3-exposed communities. No changes in the mortality or egg production of the snails were observed. However, the hatching success of eggs from snails that fed on contaminated periphyton was reduced by more than 79%, and 28-day old embryos showed malformations. Overall, this study clearly identified damaging effects of AgNP on periphyton, with consequent indirect impacts on a consumer, trophic relationships and stream ecosystem processes.

EPFL2015

Winter ecology of a subalpine grassland: Effects of snow removal on soil respiration, microbial structure and function

Michael Bahn, Alexandre Buttler, Konstantin Svetlozarov Gavazov, Robert Thomas Edmund Mills

Seasonal snow cover provides essential insulation for mountain ecosystems, but expected changes in precipitation patterns and snow cover duration due to global warming can influence the activity of soil microbial communities. In turn, these changes have the potential to create new dynamics of soil organic matter cycling. To assess the effects of experimental snow removal and advanced spring conditions on soil carbon (C) and nitrogen (N) dynamics, and on the biomass and structure of soil microbial communities, we performed an in situ study in a subalpine grassland in the Austrian Alps, in conjunction with soil incubations under controlled conditions. We found substantial winter C-mineralisation and high accumulation of inorganic and organic N in the topsoil, peaking at snowmelt. Soil microbial biomass doubled under the snow, paralleled by a fivefold increase in its C:N ratio, but no apparent change in its bacteria-dominated community structure. Snow removal led to a series of mild freeze-thaw cycles, which had minor effects on in situ soil CO2 production and N mineralisation. Incubated soil under advanced spring conditions, however, revealed an impaired microbial metabolism shortly after snow removal, characterised by a limited capacity for C-mineralisation of both fresh plant-derived substrates and existing soil organic matter (SOM), leading to reduced priming effects. This effect was transient and the observed recovery in microbial respiration and SOM priming towards the end of the winter season indicated microbial resilience to short-lived freeze-thaw disturbance under field conditions. Bacteria showed a higher potential for uptake of plant-derived C substrates during this recovery phase. The observed temporary loss in microbial C-mineralisation capacity and the promotion of bacteria over fungi can likely impede winter SOM cycling in mountain grasslands under recurrent winter climate change events, with plausible implications for soil nutrient availability and plant-soil interactions. (C) 2017 Elsevier B.V. All rights reserved.

Elsevier Science Bv2017

Improved framework to estimate travel time and derived distributions in hydrological control volumes

Mitra Asadollahi

Crossing properties of soil saturation, defined as the duration and excursion of soil saturation below and above certain thresholds, are key variables to ecosystem functioning and evolution by primarily influencing the plant and soil microbes physiological dynamics. Under climate change, the variability in the precipitation patterns is expected to be more ubiquitous and directly translate to a shift in soil saturation levels. This, in turn, will influence crossing properties of soil saturation and thus be expected to leave a mark on ecosystems. However, the lack of a deep understanding of the links between biological processes and the change in soil saturation patterns hinders foretelling the impact of climate variability on ecohydrological systems even at the smallest scale, that is, the level of a single plant.To that end, this thesis presents experimental results from laboratory soil column experiments and extensive computational studies on the interactions between soil saturation and biological activities, that is, plants and microbes. To this aim, the depth-time soil saturation patterns are mapped by means of modeling the transport processes of relevance and experimental tools at a minimal hydrological control volume in search for upscaling methods. In this context, two modeling approaches can be employed. Physically based models that explicitly represent transport processes with high costs of application at larger scales. Age-based models that rely on generic functions with a focus on time past since arrival, that is, the age of event water in the storage. This thesis work combines models (HYDRUS-1D as physically based and tran-SAS as age-based) with soil column experimental data and Bayesian inference methods to explore the links between age characteristics and transport processes. In this thesis, data from four different experimental designs on monthly to yearly timescale under varied environmental conditions are examined in terms of tracer type, precipitation pattern, vegetation, soil type, resolution and type of collected data. Of the modeled data, two sets of results from specific experiments were carried out within the framework of this thesis.This thesis addresses also the role of the balance between dispersive and convective forces, known as Péclet number, in controlling the shape of age functions in drainage. While sole reliance on drainage flow and tracer data (reflective of Péclet number) suffice to capture age dynamics in drainage, un- certainties may arise for predictions on (evapo)transpiration. This thesis deals also with the role of root distributions as controls in determining the age to root uptake and shows plants with similar uniform-equivalent root distribution render very similar age dynamics. It was shown in this thesis that nitrate consumption by microbes can be linked to age characteristics of a conservative tracer and that the relation between microbial respiration and soil saturation may bear more complexity than is currently integrated in most models. Overall, this thesis lays steps to the development of modeling tools with high predictive power for soil saturation by originating upscaling methods and bridging the gaps between physically based and age-based models with a view on the consequences of (possi- bly changing) variable soil saturation on adaptations of microbial communities and their metabolic product.

EPFL2022