Geochemistry of three contrasting British peatlands: Complex patterns of cation availability and implications for microbial metabolism

ISTO = present addressInternational audienceCation availability in peat may limit CH4 production and microbial activity and thereby impact on rates of organic matter accumulation and the chemical character of the peat. We quantify total, soluble, and exchangeable cation concentrations, Exchange Site Saturation Levels (ESSLs) and organic fractions in bog-peat profiles and compare these with fen peat. Total and soluble cation concentrations are not correlated and these and exchangeable cation concentrations are lower in bog than fen peat. In all sites these vary with depth and the distribution patterns of individual cations are unique. This is explained by variation in ESSL, which is negatively correlated with Cation Exchange Capacity (CEC). Total cation concentrations in bog peat are higher in the top and bottom fractions than in the middle. Soluble concentrations in surface bog peat are low, because cations are trapped due to low ESSL. This does not occur in fen peat, with lower CEC and higher ESSL. CEC is related to total organic matter content, not just to Sphagnum, which has been invoked as the explanatory variable of high CEC in peat bogs. There is a complexity in the mechanisms controlling cation availability in peat and we suggest that total, soluble and exchangeable cation fractions need to be taken into account in studies of cation limitation of microbial activity in organic soils. CEC may also chelate exo-enzymes, further inhibiting decay processes

A water-table dependent reservoir model to investigate the effect of drought and vascular plant invasion on peatland hydrology.

International audienceThis paper investigates the water table dynamics in a peatland showing a wide range of water table fluctuations. A reservoir model of water table fluctuations in a double-porosity peat is proposed, by calculating the stored water in effective porosity of the peat from precipitation and evapotranspiration datasets. Calculations conceptualize vascular plant consumption through a crop coefficient. Changes in water storage, located in the effective porosity of the peat, are described through a maximum infiltration rate and a maximum storage capacity. Water discharges take place in runoff and percolation reservoirs. The runoff coefficient is considered to be water table dependent. This model was tested on a peatland that has experienced strong water table fluctuations caused by summer drought and/or by vascular plant water consumption. A water table dependent runoff model appeared to be adequate to describe the water table fluctuations in peatland. From this model, vascular plants were found to increase the crop coefficient and to limit percolation through the peat. The high water table depth in winter was found to change with the year and is related to an equilibrium between slow infiltration in peat versus percolation plus evapotranspiration. In this disturbed peatland, even if overland flows occurred after a drought, the re-saturation of effective porosity was slow with about 30% of air trapped in the porosity 6 months after the drought period. The effects of drought on peat saturation were observed over more than a single hydrological cycle. This can affect the biogeochemical processes controlling the C cycle in peatland

Invasion of a Sphagnum-peatland by Betula spp and Molinia caerulea impacts organic matter biochemistry. Implication for carbon and nutrient cycling.

Article AcceptedInternational audiencePeatland act as a sink of carbon (C) through the conservation of dead remains of plants. Under global changes triggered by human activities, it is not only the sink capacity of peatland that is in danger, but also the C already stored. Invasion of Sphagnum peatlands, mainly by Molinia caerulea and Betula spp, is a growing preoccupation. This study aims to assess the extent of the influence of such invasion on the biochemical characteristics of the peat. Elemental analysis and Rock Eval pyrolysis parameters were measured in 50 cm profiles collected in invaded and intact plots. The results showed that Oxygen Index (OI) can be used to detect new C substrate injection as such new materials have a lower OI than Sphagnum spp and Sphagnum peat. Differences in terms of water table level and nutrient status are suggested to be of great importance to determine the fate of surface and deep new C input. This study stimulates new investigation testing a possible priming effect triggered by the invading vegetation and using stochiomeric constrain as a theoretical framework

In situ quantification of CH4 bubbling events from a peat soil using a new infrared laser spectrometer

International audienceCH4 emissions from peatlands are space- and time-dependent. The variety of efflux routes contributes to these variabilities. CH4 bubbling remains difficult to investigate since it occurs on a timescale of seconds. The aims of this study were to test the capacity of a recently built infrared high resolution spectrometer, SPIRIT (SPectrometre Infra-Rouge In situ Troposphérique), to (1) measure in situ CH4 fluxes, (2) observe online bubbling events with quantification of CH4 emission fluxes corresponding to this very sudden degassing event. Material and methods: The closed dynamic chamber method was used and the gas concentration was measured every 1.5 seconds. Emission fluxes were calculated by the accumulation rate of CH4 against time. Measurements were undertaken during daytime in March 2009 and during day- and nighttime in May 2009, in a bare peat area, temporarily forming a shallow pool. Results and discussion: The results show that the CH4 emissions estimated with the SPIRIT ranged from 2.79 to 86.0 mg CH4-C m-2 h-1. These values are consistent with those already published. The high emissions, both through diffusion and bubbling, were on the same order as the emissions estimated in natural shallow pools. During daytime, CH4 bubbling was higher in May (56.2% of the total emission) than in March (40.7%) probably because of increased CH4 production and accumulation in peat. In May, bubbling was higher at nighttime (68.6%) than in daytime (56.2%). This has an important implication for carbon budget assessment in peatlands, particularly in boreal areas. Conclusions: The recently built infrared spectrometer, SPIRIT, was able to reliably measure CH4 fluxes and quantify CH4 flux during the degassing of a bubble. The emissions obtained are in agreement with previously published data using other measurement techniques. The results of this preliminary work highlight (1) the importance of shallow pools in peatland CH4 emissions, (2) the sensitivity of such fluxes to atmospheric pressure, a relation that has not been fully investigated or taken into account in assessing peatland carbon balance

Organic matter sources and decay assessment in a Sphagnum-dominated peatland (Le Forbonnet, Jura Mountains, France). Impact of moisture conditions.

International audienceIn a context of climate change peatlands may switch from a sink to a source of carbon (C). The induced positive feedbacks are closely linked to C dynamics, and thus to the fate of organic matter (OM) in the underlying peat. Our aims were to determine how moisture conditions in a Sphagnum-dominated peatland affect the dynamics of diagenetic sensitive OM which is thought to be the most reactive to warming effects. The approach was based on the identification of combined bulk and molecular bioindicators of OM sources and decay of peat profiles (up to 50 cm in depth) from two moisture conditions of the Le Forbonnet peatland (Jura Mountains, France). The (xylose+arabinose)/(galactose+rhamnose+mannose) ratio derived from the analysis of neutral monosaccharides was used to obtain indications on the botanical origin of the peat and tended to indicate a greater contribution of Cyperaceae in the deepest parts of the peat. Most bioindicators showed that OM decay increased with depth and was higher in the driest conditions. In these conditions, decay was shown by a loss of diagenetic sensitive oxygen-rich OM, probably leached by water table fluctuations. Decay intensity was also shown by the high ribose and lyxose contents at the peat surface, where microscopic observations also revealed relatively large quantities of fungal hyphae. The sugars could have arisen from microbial synthesis, primarily protozoan and fungal activity. These results suggest that water level changes preferentially impact biochemical changes in diagenetic sensitive OM, believed to be sensitive to drought events

CHEMICAL DIVERSITY OF SOIL LIPIDS REFLECTS SURROUNDING BIODIVERSITY IN A FRENCH PEATBOG

International audienceThe aims of this study are to characterize the spatial variability of biomarker imprints in a peatland and to test to which extent this variability can be related to the surrounding vegetation. Since the mid 1970’s La Guette peatland (région Centre, France) has been colonized by vascular plants. Nowadays it is totally colonized by Molina caerulea and, to varying extent, by Pinus sylvestris and Betula pendula. In nine stations representative of distinct vegetation cover, we analysed the lipid composition of soil developed under Molinia caerulea. The lipids of the six most abundant plants encountered in the peatland were analysed as reference samples: Sphagnum rubellum, Erica tetralix, Calluna vulgaris, Molinia caerulea, Betula pendula and Pinus sylvestris. For all these samples, lipids were ultrasonically extracted with DCM and then separated into neutral, acidic and polar compounds. The neutral fraction was further separated by flash chromatography to afford (i) aliphatics and aromatics; (ii) ethers, esters, ketones and acetates and (iii) alcohols. 5α-cholestane was added prior to analysis for quantitation. Lipid fractions were then identified and quantified by gas chromatography-mass spectrometry. In order to quantify their density, trees were counted within circles of increasing radius (giving areas of 3.14; 12.6; 28.3; 50.3 and 78.5 m2) around the sampling plot. After counting, total tree density was determined as well as individual P. sylvestris and B. pendula density. For each sample (n=9) regressions were calculated between (i) lipids and (ii) the density of the tree (n=9). This calculation was reproduced for each counting surface. Then, the resulting regression coefficient was plotted against each surface of counting. Apart from ubiquist compounds such as n-alkanes and triterpenoid ketones, we found in soils several biomarkers for which the source could be constrained. Taraxeryl, friedelanyl and moretenyl acetates were detected. Friedelanyl and moretenyl acetates were found in high amounts in open vegetation plots (9.7 and 2.9 µg.g soil-1, respectively). They were also detected in E. tetralix and C. vulgaris (Ericaceae) and, probably due to contamination, in S. rubellum. Multiflorenyl and two unknown acetates, also present in low contents, may originate from undetermined, less abundant plant. Four methoxy-serratenes (Le Milbeau et al., 2013) and seven tricyclic diterpenes were detected in both soils and P. sylvestris needles. Strong correlation (r = 0.997; p ≤ 0.0001) of their respective abundances in soils attest to their common origin and similar conservation potential. The presence of betulin and related compounds in soil samples and in B. pendula leaves are in agreement with the admitted use of these biomarkers to track the former presence Betula species in soils and sediments (i.e. Otto and Simoneit, 2001). The source of these biomarkers being defined, we then assessed their spatial distribution at the nine sampling sites. Although not abundant, triterpenyl acetates, methoxy-serratenes, diterpenes and betulin derivatives exhibit a higher coefficient of variation along sampling sites than other compound such as n-alkanes or triterpenoid ketones.The correlation between methoxy-serratenes and the number of P. sylvestris in the nine sampling sites increases with the surface used for the calculation (r = 0.93, p<0.022). These values indicate that specific biomarkers of P. sylvestris in soil samples and surrounding vegetation are spatially linked. The same applies for B. pendula and betulin derivatives (r = 0.96, p<0.008). The degradability of the betulin and its relatively high solubility may induce lost and leaching out of the sampling point. Our results provide evidence that selected biomarkers in soil samples can give a reliable picture of the surrounding vegetation. It also quantifies the area of influence of trees on the soil lipid composition, over 20 m². The approach combining tree counting and quantitation of specific biomarkers that was developed in the frame of this study proved to be efficient in evidencing the spatial control of the surrounding vegetation on lipids distribution in a peatland invaded by vascular plants.REFERENCESLe Milbeau, C., Lavrieux, M., Jacob, J., Bréheret, J.G., Zocatelli, R., Disnar, J.R., 2013. Diversity of methoxy-serratenes in a soil under a conifer forest and their potential as biomarkers of Pinaceae. Organic Geochemistry 55, 45-54.Otto, A., Simoneit, B.R.T., 2001. Chemosystematics and diagenesis of terpenoids in fossil conifer species and sediment from the Eocene Zeitz formation, Saxony, Germany. Geochimica et Cosmochimica Acta 65:20, 3505–3527

Spatial variability of soil lipids reflects vegetation cover in a French peatland

International audienceThe purpose of this study was to show how current vegetation in a peatland is imprinted in the lipid fraction of the underlying soil. La Guette is a fen peatland in Central France dominated by Sphagnum spp. and ericaceous shrubs, colonized by sedges (Molinia caerulea) and trees since the 1970s (Betula pendula and Pinus sylvestris). Lipid were identified and quantified in the most abundant plants and in samples collected at the base of M. caerulea tussocks. Among relevant biomarkers, tricyclic diterpenes and methoxy-serratenes stood out as specific for P. sylvestris, betulin derivatives as specific for B. pendula, and some pentacyclic triterpene ketones and acetates as biomarkers of Ericaceae. Multivariate analysis applied to biomarker concentration in soil samples from several sites permitted distinguishing three different vegetation types: vegetation cover: (i) closed vegetation dominated by P. sylvestris; (ii) closed vegetation dominated B. pendula and (iii) open vegetation or semi-open vegetation (early colonization by trees) constituted mainly by Ericaceae, Sphagnum and graminoids. Comparison of tree-specific biomarker concentrations with estimates of tree biomass allowed establishing of quantitative relationships that were valid over at least 80 m2 around the sampling site. Although preliminary, the results are promising in attempting to relate biomarker concentrations in geological archives to paleo-biomass

Effects of short-term ecosystem experimental warming on water-extractable organic matter in an ombrotrophic Sphagnum peatland (Le Forbonnet, France)

International audienceIn a future warmer world, peatlands may change from a carbon sink function to a carbon source function. This study tracks changes in water-extractable organic matter (WEOM) after one year of in situ experimental warming using open top chambers (OTCs). WEOM was studied in the upper peat layers (0-10 cm) through analysis of water-extractable organic carbon (WEOC), stable C isotopic composition (δ13C), specific UV absorbance at 280 nm and sugar composition of cores taken from an open bog (DRY sites) and a transitional poor fen (WET sites). At the DRY sites, the impact of OTCs was weak with respect to WEOM parameters, whereas at the WET sites, the air warming treatment led to a decrease in peat water content, suggesting that the supply of heat by OTCs was used mainly for evapotranspiration. OTCs at the WET sites also induced a relative enrichment at the surface (0 to 5 cm depth) of aliphatic and/or aromatic compounds with concomitant decrease in WEOC, as a result of decomposition. On the contrary, WEOC and sugar content increased in the deeper peat layer (7.5-10 cm depth) probably as a result of increased leaching of phenolic compounds by roots, which then inhibits microbial activity. The different response to experimental warming at DRY and WET sites suggests that the spatial variability of moisture in is critical for understanding of the impact of global warming on the fate of OM and the carbon cycle in peatlands

How to assess cutover Peatland regeneration with combined organic matter indicators ?

International audienceWhen restored, cutover peatlands can favour biodiversity and carbon (C) sequestration. Within the EU program RECIPE, we aimed to identify combinations of site physico-chemical conditions, vegetation composition and below-ground microbiological characteristics that are beneficial to the long-term biodiversity and C sink function regeneration. To unreveal these characteristics, we assessed the bioindicator value of peat organic matter (OM) physico-chemistry from cutover peatlands at various stages of regeneration. Although OM continues to reflect disturbances in the catotelm deep peat, we show that along the chronosequence the regenerated peat tends to be biochemically and physically similar to the one from the non exploited area of the same site. The combination of several indicators provides an efficient assessment of ecological conditions and makes valuable for the management of cutover peatlands

A portable infrared laser spectrometer for flux measurements of trace gases at the geosphere-atmosphere interface

International audienceA portable infrared laser absorption spectrometer named SPIRIT (SPectrom'etre Infra-Rouge In situ Troposph'erique) has been set up for the simultaneous flux measurements of trace gases at the geosphere-atmosphere interface. It uses a continuous wave distributed feedback room temperature quantum cascade laser and a patented new optical multi-pass cell. The aim of SPIRIT field studies is to get a better understanding of land and water bodies to atmosphere exchange mechanisms of greenhouse gases (GHG). The analytical procedures to derive concentrations and fluxes are described, as well as the performances of the instrument under field conditions. The ability of SPIRIT to assess space and time dependence emissions of two GHG--nitrous oxide (N2O) and methane (CH4)--for different types of ecosystems is demonstrated through in situ measurements on peatland, on fertilized soil, and on water body systems. The objectives of these investigations and preliminary significant results are reported