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

Abstract

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.

Introduction

Peatlands currently act as a major long-term carbon (C) sink ecosystem. Although these wetlands cover only 3% of the land area, they have stored a third of the global soil C since the early Holocene (Turunen et al., 2002). Most Sphagnum peatlands (up to 80%) are located at high latitudes of the northern hemisphere in the cool temperate zone in association with waterlogged, nutrient poor conditions and the presence of Sphagnum mosses (e.g. Gorham, 1991). To cope with low nutrient concentrations, Sphagnum mosses have developed mechanisms to efficiently use nutrients thanks to their high cation exchange capacity, nutrient translocation and atmospheric interception, reducing the nutrient availability to vascular plants (e.g. Turetsky et al., 2012). However, northern temperate ecosystems receive four times more airborne nitrogen (N) today than 150 years ago (Holland et al., 1999, Lamarque et al., 2005). Increased N deposition leads to a progressive N saturation of Sphagnum mosses, thus favoring the invasion of vascular plants and reducing Sphagnum moss growth (Limpens et al., 2011). Such changes seem to reduce the C sequestration rates in peatlands (Bragazza et al., 2006, Gunnarsson et al., 2008), even if they increase the vascular plants' productivity (Wu et al., 2015). However, the effect of the increase in N loads on stocks and exchanges of N and C are still understudied in peatlands, although they are known to generally increase N₂O emissions to the atmosphere (e.g. Nykänen et al., 2002, Francez et al., 2011). Peatland C-storage capacity is often considered alone to assess the effects of climate change on peatlands without considering the N stored in the ecosystems that could account for a significant N₂O source and therefore act as a positive feedback to climate change (Repo et al., 2009).

The increase in vascular plant cover due to human activities such as nutrient supply, e.g., atmospheric N deposition, or drainage, increases organic matter decomposition (Gogo et al., 2016) and modulates CO₂ and CH₄ emissions in peatlands (Ward et al., 2013, Leroy et al., 2017). The combined effects of vascular plant invasion with N deposition on both C and N cycles and stocks still remain to be elucidated. N fertilization generally stimulates the vascular plant biomass, thereby contributing to higher primary production. However, it also leads to a higher decomposition rate due to a reduction in the C/N ratio and more root exudates that generate additional respiration (Wu et al., 2015). Our aim was therefore to assess the effect of N supply on both C and N dynamics in peat mesocosms collected in a Sphagnum-dominated peatland invaded by a vascular plant, Molinia caerulea. All the peat mesocosms contained Sphagnum rubellum, and half of them also contained M. caerulea. Half of each plant community mesocosm was subjected to an increase in N deposition by a weekly amendment to reach an addition of 3.2 g N/(m²·year). Thus, the hypotheses investigated are that N deposition will lead to the following processes under the two plant communities:

• Processes involving the C cycle: (a) an increase in C fluxes by promoting ecosystem respiration (ER) due to a faster decomposition of plant tissues containing more N (Bragazza et al., 2006); (b) stimulation of the gross primary production (GPP) by an enhancement of both Sphagnum mosses and graminoid biomass (e.g. Tomassen et al., 2003, Granath et al., 2009); (c) a rise in CH₄ emissions through a higher OM decomposition and increase in root exudates.

• Processes involving the N cycle: (a) higher concentrations of the dissolved NH₄⁺ and NO₃⁻ and of the N stored by Sphagnum mosses; (b) an increase in N₂O emissions under both plant communities (Roobroeck et al., 2010).

• Processes involving M. caerulea occurrence: an increase in the C fluxes in peatlands (CO₂, CH₄) and DOC content and a decrease in the ecosystem C sink function compared to Sphagnum-dominated peatland due to the promotion of peat decomposition (Leroy et al., 2017).