Travel Time and Source Variation Explain the Molecular Transformation of Dissolved Organic Matter in an Alpine Stream Network

Streams and rivers are important components of the carbon cycle as they transport and transform dissolved organic matter (DOM). Using high resolution Fourier transform ion cyclotron resonance mass spectrometry, we studied the spatial distribution of DOM at the molecular level at more than 100 sites across a stream network during summer and winter basefow. We developed a model approximating the time DOM spent in the fluvial network, a key constraint on the biogeochemical processing of DOM. Discharge-weighted travel time explained the compositional changes of DOM, which differed markedly in summer and winter. We attribute these seasonal differences to variation in source material, putatively reflecting the dynamics of freshly produced DOM in summer and DOM with an imprint of leaf litter in winter. Hydrological mixing was an important driver of the spatial dynamics of DOM. From the convergence rate of DOM compound intensities to the network wide average, we inferred the spatial distribution of sources within the catchment. Finally, we estimated network-wide apparent mass transfer coefficients (vf app) of individual DOM compounds, which describe the vertical velocity at which DOM compounds are removed by biotic and abiotic processes. We identified the oxidative state of carbon as an important factor explaining vf app , which we consequently attribute to biological uptake of thermodynamically favorable DOM compounds. This work contributes to our understanding of the spatial processes, temporal constraints, and chemical properties of DOM that regulate the transformation and diagenesis of DOM at the fluvial network scale.

Peatland vascular plant functional types affect dissolved organic matter chemistry

Remy Jean Henry Albrecht, Luca Bragazza, Alexandre Buttler, Samuel Thomas Gabriel Hamard, Vincent Eric Jules Jassey, Adrian Pulgarin, Bjorn Jozef Maria Robroek

Northern peatlands are large repositories of carbon. Peatland vascular plant community composition has been functionally associated to a set of biogeochemical processes such as carbon cycling. Yet, we do not fully understand to what extent vascular plant functional types (PFTs) affect the quality of dissolved organic matter, and if there is any feedback on soil microbial activity. Using a longer-term plant removal experiment in a boreo-nemoral peatland in Southern Sweden, we relate the dominance of different vascular plant functional types (i.e. ericoids and graminoids) to the chemistry of the dissolved organic matter (DOM) and microbial enzymatic activities (fluorescein diacetate hydrolysis, FDA). Our results show that PFTs modifies the composition of DOM moieties, with a decrease of low molecular weight organic compounds after vascular plant removal. The decrease of enzymatic activity by up to 68 % in the plant removal plots suggests a reduction in DOM mineralization in the absence of vascular plants. Our results show that plant-derived low molecular organic compounds enhance peatland microbial activity, and suggest that an increase of vascular plant cover in response to climate change can potentially destabilize the OM in peatlands, leading to increased carbon losses.

Springer2016

Carbon fluxes and productivity regimes in Alpine streams

Marta Boix Canadell

High-altitude catchments have a major role in the transport of organic matter to streams due to the storage of dissolved organic carbon (DOC) in soils and glacier ice and the subsequent mobilization during the melting processes. Yet, stream function goes beyond the conveying downstream of the terrestrial and glacier derived DOC since they are also highly active in the mineralization, retention and production of organic matter. Stream ecosystem metabolism integrates the processes that regulate the conversion between the organic and inorganic forms of carbon in streams and is a fundamental measure to determine whether carbon accumulates or it is lost within the ecosystem. In a global warming scenario, high-mountain stream ecosystems are faced to modifications in their structure and function as a result of climate-driven hydrological changes. Thus far, despite the active role of alpine streams in the global carbon cycle, studies on the impacts from changes in snowmelt, glacier ice melt and groundwater contribution to streamflow have been largely focusing on alterations in hydrological regimes, water availability, geomorphology and biodiversity. To fill this gap, the aim of this thesis is to examine DOC fluxes dynamics and productivity regimes across a range of glacierized and non-glacierized alpine catchments to anticipate the possible consequences of projected hydrological changes on alpine stream biogeochemistry and ecosystem functioning. This thesis is supported by the collection and analysis of high-frequency time series of physicochemical parameters, geomorphological data and streamwater samples from different Alpine streams with contrasting glacier coverage. The first part of the thesis investigates the response across all the streams of the annual DOC export to runoff primarily driven by snowmelt and glacier melt. In the second part, with the use of dissolved oxygen time series, the ecosystem energetics regimes in a glacier-, groundwater- and snowmelt-fed stream are explored during two years and related to their physical template. The third and last part is focused on providing estimates of gross primary production (GPP) rates for the energetic regimes established in the three Alpine stream types. The obtained results show a varied response of DOC export to runoff across the catchments which was related to the degree of glaciation and vegetation cover. Our findings also reveal different stream ecosystem energetic regimes among the streams and highlight discharge as the major modulator of drivers, such as light, gas exchange rate or stability, on the seasonal and daily dissolved oxygen dynamics. Lastly, the magnitude and the temporal patterns of ecosystem GPP are the result of the light and streambed disturbance conditions that are largely determined by the hydrological and turbidity regime. I argue that glacier shrinkage together with changes in snowmelt hydrology will alter the response of DOC yield to discharge, with consequent impact on the timing and magnitude of the lateral DOC fluxes from terrestrial to stream ecosystems. Also, an eventual reduction in glacier runoff and snowpack duration and content will alter the physical template for primary producers, which may lead to a greater production of autochthonous organic matter across high-mountain streams. Overall, I assume a shift in the magnitude and temporal patterns of energetic inputs, with consequences for the stream biogeochemistry and function.

EPFL2020

Modelling of mechanisms affecting nitrogen and carbon cycles in soils subject to land use change

David Andrew Barry, Jordi Batlle-Aguilar, Alessandro Brovelli

In order to meet demands for crops, pasture and firewood, the rate of land use change from forested to agricultural uses steadily increased over several decades, resulting in an increased release of nutrients towards groundwater and surface water bodies. In parallel, the continuous degradation of natural ecosystems such as riparian zones, contribute to diminish their capacity to provide ecological services, i.e. reduce the impacts of deleterious human activities on both surface and groundwater. Land use changes and restoration practices are known to affect both soil nutrient dynamics and their transport to neighbouring areas. Biogeochemical transformations in soils are heavily influenced by microbial decomposition of soil organic matter. Carbon and nutrient cycles are in turn strongly sensitive to environmental conditions, primarily to soil moisture. This physical variable affects the reaction rate of almost all soil biogeochemical transformations, including microbial and fungal activity, nutrient uptake and release from plants, etc. Soil water saturation is not constant neither in time nor in space, thus further complicating the picture. In order to interpret field experiments and elucidate the different mechanisms taking place, numerical tools are beneficial. The impact of hydroclimatic variability (soil moisture in particular) on soil carbon and nitrogen cycles was highlighted along with the consequences of land use changes from forest to pasture and/or agriculture. To this end, a mechanistic model based on the compartment model of Porporato et al. (2003) was developed. The predictive capabilities of the model to forecast the effects of land use changes over carbon and nitrogen dynamics were shown, modelling four different scenarios, intertwining two different climate conditions (with and without seasonality) with two contrasting soils having physical properties that are representative of forest and agricultural soils. Synthetic time series of precipitation, and therefore soil moisture evolution in time, were generated using a stochastic approach. The model was subject to isothermal conditions and other considerations such as average values of carbon and nitrogen concentrations over the rooting depth. As well an average microbial organic decomposition rate was considered. Simulation results demonstrated higher concentrations of carbon and nitrogen in forests soils as a result of higher litter decomposition than in agricultural soils (Figure 1). High frequency changes in water saturation under seasonal climate scenarios were shown to be commensurate with carbon and nitrogen concentrations in agricultural soils. Forest soils have different properties to agricultural soils, leading to an attenuation of the seasonal climate-induced frequency changes in water saturation, with accompanying changes in carbon and nitrogen concentrations. The model was shown to be capable of simulating the long term effects of modified physical properties of agricultural soils as a consequence of tillage, ploughing and harvesting. It is, thus, a promising tool to predict carbon and nitrogen turnover changes due to changes in land use. These results encourage further model improvements aimed at accounting for the complexity of the soil system and its processes for better understanding soil nutrients turnover.