Fabien Leroy

Litter decomposition in peatlands is promoted by mixed plants

PurposeThe carbon sink function of peatlands is primarily driven by a higher production than decomposition of the litter Sphagnum mosses. The observed increase of vascular plants in peatlands could alter the decomposition rate and the carbon (C) cycle through a litter mixing effect, which is still poorly studied. Here, we examine the litter mixing effect of a peat moss (Sphagnum fallax) and two vascular plants (Pinus uncinata and Eriophorum vaginatum) in the field and laboratory-based experiment.Materials and methodsDuring the laboratory incubation, mass loss, CO2 production, and dissolved organic carbon concentration were periodically monitored during 51xa0days. The collected data were then processed in a C dynamics model. The calculated enzymatic activity was correlated to the measured β-glucosidase activity in the litter. In the field experiment, mass loss and CO2 production from litter bags were annually measured for 3xa0years.Results and discussionBoth laboratory and field experiments clearly show that the litter mixture, i.e., Sphagnum-Pinus-Eriophorum, had a synergistic effect on decomposition by enhancing the mass loss. Such enhanced mass loss increased the water extractable C and CO2 production in the litter mixture during the laboratory experiment. The synergistic effect was mainly controlled by the Sphagnum-Eriophorum mixture that significantly enhanced both mass loss and CO2 production. Although the β-glucosidase activity is often considered as a major driver of decomposition, mixing the litters did not cause any increase of the activity of this exo-enzyme in the laboratory experiment suggesting that other enzymes can play an important role in the observed effect.ConclusionsMixing litters of graminoid and Sphagnum species led to a synergistic effect on litter decomposition. In a context of vegetation dynamics in response to environmental change, such a mixing effect could alter the C dynamics at a larger scale. Identifying the key mechanisms responsible for the synergistic effect on litter decomposition, with a specific focus on the enzymatic activities, is crucial to better predict the capacity of peatlands to act as C sinks.

Response of C and N cycles to N fertilization in Sphagnum and Molinia-dominated peat mesocosms

Plant communities play an important role in the C-sink function of peatlands. However, global change and local perturbations are expected to modify peatland plant communities, leading to a shift from Sphagnum mosses to vascular plants. Most studies have focused on the direct effects of modification in plant communities or of global change (such as climate warming, N fertilization) in peatlands without considering interactions between these disturbances that may alter peatlands C function. We set up a mesocosm experiment to investigate how Greenhouse Gas (CO2, CH4, N2O) fluxes, and dissolved organic carbon (DOC) and total dissolved N (TN) contents are affected by a shift from Sphagnum mosses to Molinia caerulea dominated peatlands combined with N fertilization. Increasing N deposition did not alter the C fluxes (CO2 exchanges, CH4 emissions) or DOC content. The lack of N effect on the C cycle seems due to the capacity of Sphagnum to efficiently immobilize N. Nevertheless, N supply increased the N2O emissions, which were also controlled by the plant communities with the presence of Molinia caerulea reducing N2O emissions in the Sphagnum mesocosms. Our study highlights the role of the vegetation composition on the C and N fluxes in peatlands and their responses to the N deposition. Future research should now consider the climate change in interaction to plants community modifications due to their controls of peatland sensitivity to environmental conditions.

CO2 and CH4 budgets and global warming potential modifications in Sphagnum-dominated peat mesocosms invaded by Molinia caerulea

Plant communities play a key role in regulating greenhouse gas (GHG) emissions in peatland ecosystems and therefore in their ability to act as carbon (C) sinks. However, in response to global change, a shift from Sphagnumdominated to vascular-plant-dominated peatlands may occur, with a potential alteration in their C-sink function. To investigate how the main GHG fluxes (CO2 and CH4) are affected by a plant community change (shift from dominance of Sphagnum mosses to vascular plants, i.e., Molinia caerulea), a mesocosm experiment was set up. Gross primary production (GPP), ecosystem respiration (ER) and CH4 emission models were used to estimate the annual C balance and global warming potential under both vegetation covers. While the ER and CH4 emission models estimated an output of, respectively, 376± 108 and 7± 4 g C m−2 yr−1 in Sphagnum mesocosms, this reached 1018± 362 and 33± 8 g C m−2 yr−1 in mesocosms with Sphagnum rubellum and Molinia caerulea. Annual modeled GPP was estimated at −414±122 and−1273±482 g C m−2 yr−1 in Sphagnum and Sphagnum + Molinia plots, respectively, leading to an annual CO2 and CH4 budget of−30 g C m−2 yr−1 in Sphagnum plots and of−223 g C m−2 yr−1 in Sphagnum + Molinia ones (i.e., a C sink). Even if CH4 emissions accounted for a small part of the gaseous C efflux (ca. 3 %), their global warming potential value makes both plant communities have a climate warming effect. The shift of vegetation from Sphagnum mosses to Molinia caerulea seems beneficial for C sequestration at a gaseous level. However, roots and litter of Molinia caerulea could provide substrates for C emissions that were not taken into account in the short measurement period studied here.