Coarse and Fine Particulate Organic Matter Transport by a Fourth-Order Mountain Stream to Lake Bourget (France)

Transport of coarse particulate organic matter (CPOM) derived from forest litterfall has been hardly studied in rivers, unlike fine particulate organic matter (FPOM) or dissolved organic matter (DOM). Yet, many rivers are dammed or run into lakes, and there is growing evidence that CPOM accumulation in river delta participates substantially in ecological processes such as greenhouse gas emissions of lakes and reservoirs. We investigated the transport of CPOM and FPOM by the Leysse River (discharge from 0.2 to 106 m(3) s(-1)) to Lake Bourget (France) in relation to aerial litter deposition, river network length, and discharge. Over a 19-month study period, the volume-weighted mean CPOM and FPOM concentrations were 1.3 and 7.7 g m(-3,) respectively. Most CPOM and FPOM transport occurred during major flood events, and there were power relationships between maximum discharge and particulate organic matter (POM) transport during these events. The annual export of CPOM (190 t AFDM) was 85% of the litter accumulation in autumn on permanent sections of the riverbed (224 t AFDM), which suggests that export is a major process compared to breakdown. Export of CPOM was 1.25 t yr(-1) km(-2) of the forested catchment area. This study highlights the need to account for long-range CPOM transport to describe the fate of litter inputs to streams and to quantify the organic matter input and processing in lakes and reservoirs.

Ecological Controls of Aerobic and Anaerobic Microbial Activities and Potential Impacts on Greenhouse Gases in Regenerating Peatlands

Andy Siegenthaler

Peatlands represent massive global carbon (C) pools and sinks. Carbon accumulation depends on the ratio between net primary production and decomposition of organic matter, both of which can change under projected increases of atmospheric carbon dioxide (CO2) and N deposition or after peat cutting. Significant areas of peatlands have been damaged by drainage and peat harvesting worldwide. After abandoning exploitations and spontaneous regeneration, these secondary peatlands can become major C-sinks but also important methane (CH4) sources. As the atmospheric concentration of methane, a greenhouse gas (GHG) of major importance, has more than doubled during the past 200 years, identifying factors regulating the flux of CH4 and CO2 into the atmosphere is crucial. This research focuses on the microbiological and ecological aspects of biodegradation (decay) occurring in abandoned cutover peatlands. This work is based on complementary fields such as plant eco-physiology, soil biology and biochemistry, microbial and molecular ecology and biogeochemistry. Initially, we described the gas exchange occurring between these ecosystems and the atmosphere and concluded that peatlands could sequester carbon during their regeneration, although not without emitting important quantities of methane. This brought us to consider the aerobic decomposition of plant litter. Decomposition was not only influenced by changes in produced plant litter quality but also more directly through micro-environmental conditions. Aerobic biodegradation creates the substrate for methanogenic microorganisms responsible for methane production. On the whole, these used acetate as a substrate for methanogenesis and were numerous in early peatland regeneration stages. In pioneering stages of regeneration, acetate production could be maintained by important contribution of homoacetogenic bacteria. Methane diffuses through the waterlogged peat and is turned into CO2 by methane oxidising bacteria in the upper layers. These bacteria, with various oxidation efficiencies, were shown to have contrasting ecologies in relation to vegetation type. Their abundance and distribution is also strongly linked to the emission of greenhouse gases. The common denominator of all these processes is the nature of the organic matter itself which plays a dominating role in the carbon transfer, energy transduction, and control of its own biodegradation.

EPFL2011

Interactive effects of depth and temperature on CH4 and N2O flux in a shallow podzol

Robert Thomas Edmund Mills

Measuring and modelling the efflux of greenhouse gases from soils is crucial for gauging ecosystem responses to climate and land-use change, and potential contributions and feedbacks to gas emissions. Upland soils with high amounts of organic matter can produce large effluxes of CH4 and potentially N2O, and therefore understanding the sensitivity of such fluxes to changes in climate (e.g. temperature) is of importance. Here we consider the role of shallow podzols in the temperature response of CH4 and N2O efflux using a simple laboratory incubation. Such soils have a shallow peat layer overlain by coarse organic matter, and by splitting and incubating these layers across a 1-30 degrees C temperature ramp, we observed a significant negative temperature response for both gases, and a gas-dependent effect on the presence of a between-layer difference. Given these observations, there is a need to consider the temperature sensitivity of near surface layers as distinct, and to recognise the potential for shallow podzols to have a strong source sink transition across temperature ranges. (C) 2013 Elsevier Ltd. All rights reserved.

Pergamon-Elsevier Science Ltd2013

Composition and superposition of alluvial deposits drive macro-biological soil engineering and organic matter dynamics in floodplains

Claire Guenat, Renée-Claire Le Bayon, Rodolphe Schlaepfer, Andreas Cedric Schomburg, Pascal Turberg

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.