Biogeochemical plant-soil microbe feedback in response to climate warming in peatlands

Peatlands act as global sinks of atmospheric carbon (C) through the accumulation of organic matter(1), primarily made up of decay-resistant litter of peat mosses(2). However, climate warming has been shown to promote vascular plant growth in peatlands, especially ericaceous shrubs(3). A change in vegetation cover is in turn expected to modify above-ground/below-ground interactions(4), but the biogeochemical mechanisms involved remain unknown. Here, by selecting peatlands at different altitudes to simulate a natural gradient of soil temperature, we show that the expansion of ericaceous shrubs with warming is associated with an increase of polyphenol content in both plant litter and pore water. In turn, this retards the release of nitrogen (N) from decomposing litter, increases the amount of dissolved organic N and reduces N immobilization by soil microbes. A decrease of soil water content with increasing temperature promotes the growth of fungi, which feeds back positively on ericaceous shrubs by facilitating the symbiotic acquisition of dissolved organic N. We also observed a higher release of labile C from vascular plant roots at higher soil temperatures, which promotes the microbial investment in C-degrading enzymes. Our data suggest that climate-induced changes in plant cover can reduce the productivity of peat mosses and potentially prime the decomposition of organic matter by affecting the stoichiometry of soil enzymatic activity.

Indirect effects of experimental warming on dissolved organic carbon content in subsurface peat

Luca Bragazza, Alexandre Buttler, Vincent Eric Jules Jassey

The peatland carbon store is threatened by climate change and is expected to provide positive feedback on air temperature. Most studies indicate that enhanced temperature and microbial activities result in a rise of dissolved organic carbon (DOC) as a consequence of higher peat decomposition. Few of them, however, have investigated the impact of in situ experimental warming on DOC response. We studied the response of DOC, dissolved organic nitrogen (DON), phenol oxidase, and fluorescein diacetate activities (FDA) to a 3-year in situ experimental warming using open-top chambers (OTCs) in a Sphagnum-dominated peatland. No significant warming of soil was recorded, implying that the simultaneous decrease in DOC and DON and the rise in FDA at the depths of 25 and 40 cm were not caused by the direct effect of OTCs on water temperature, but might instead have been mediated by plant root exudates. The water chemistry suggests that DOC production was compensated by in situ mineralization. We hypothesize that an increased hydrolysis of organic matter (OM) was counterbalanced by the mineralization of dissolved organic matter (DOM) and that microorganisms preferentially used labile compounds originating from increased root exudates. This trade-off between production of DOC through hydrolysis and consumption in the process of mineralization shows (1) the limitation of using only DOC as an indicator of the sensitivity of peat decomposition to climate warming and (2) the need to improve our understanding of the indirect impact of root exudates.

Springer Heidelberg2014

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

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.