Fresh Air for the Mire-Breathing Hypothesis: Sphagnum Moss and Peat Structure Regulate the Response of CO2 Exchange to Altered Hydrology in a Northern Peatland Ecosystem

Abstract

Sphagnum-dominated peatlands store more carbon than all of Earth’s forests, playing a large role in the balance of carbon dioxide. However, these carbon sinks face an uncertain future as the changing climate is likely to cause water stress, potentially reducing Sphagnum productivity and transitioning peatlands to carbon sources. A mesocosm experiment was performed on thirty-two peat cores collected from two peatland landforms: elevated mounds (hummocks) and lower, flat areas of the peatland (hollows). Both rainfall treatments and water tables were manipulated, and CO2 fluxes were measured. Other studies have observed peat subsiding and tracking the water table downward when experiencing water stress, thought to be a self-preservation technique termed ‘Mire-breathing’. However, we found that hummocks tended to compress inwards, rather than subsiding towards the lowered water table as significantly as hollows. Lower peat height was linearly associated with reduced gross primary production (GPP) in response to lowered water tables, indicating that peat subsidence did not significantly enhance the resistance of GPP to drought. Conversely, Sphagnum peat compression was found to stabilize GPP, indicating that this mechanism of resilience to drought may transmit across the landscape depending on which Sphagnum landform types are dominant. This study draws direct connections between Sphagnum traits and peatland hydrology and carbon cycling.

Sphagnum mosses, the impact of disturbances and anthropogenic management actions on their ecological role in CO2 fluxes generated in peatland ecosystems

“…However, studies that have used peat cores covered only by Sphagnum record higher CO 2 release during drainage (WT = -40 cm) than during slow rewetting (WT = -25 and -20 cm), due to increased R eco associated with higher microbial activity (Blodau & Moore, 2003;Strack & Price, 2009). reduces moss productivity, increases their autotrophic respiration, and increases R eco rates, generating higher CO 2 emissions to the atmosphere (Figure 6; Table 1; Table S1) (Blodau et al, 2004;O'Neill et al, 2022;Riutta et al, 2007;Tuittila et al, 2004). We generated this consensus cautiously due to some contradictory results reported in the literature, indicating increases, decreases, or low differences in Sphagnum photosynthesis rates and R eco associated with drainage (Huang et al, 2021;Kokkonen et al, 2022;Ma et al, 2022), a condition related to the complexity of the variables involved, and to the difficulty of identifying a unifying mechanism for NEE responses considering the multiple interactions between biotic and abiotic components in a peatland (Huang et al, 2021;Laiho, 2006;Laiho et al, 2004).…”

“…On the other hand, experiments with Sphagnum mesocosms obtained from different microtopographic positions of a bog located in Michigan (USA) showed that, under drainage conditions (WT = −25 cm), the GPP of mosses established in hollows was more affected than those forming hummocks, increasing CO 2 emissions in the former (O'Neill et al, 2022). Hummock-forming mosses offer greater resistance to maintain GPP under drainage conditions thanks to a high capitulum density and increased capillary transport efficiency in water retention, allowing them to respond favorably to water stress, while retaining stability in C fluxes (Adkinson & Humphreys, 2011;Bengtsson et al, 2016;O'Neill et al, 2022;Robroek et al, 2009). In general, Sphagnum gross photosynthesis exhibits a unimodal response to WT variation (Tuittila et al, 2004), in which case different studies indicate that its optimal position should be maintained between −5 and −20 cm to maximize moss GPP (Brown et al, 2017;Riutta et al, 2007;Tuittila et al, 2004) (Figure 5).…”

“…Opposite results are reported by Kokkonen et al (2022) for a poor fen, a mesotrophic fen, and a bog in Finland, where decreases in WT between −15 and −20 cm at all sites generated increases in moss P max in the fens ecosystems, especially in the mesotrophic fen. On the other hand, experiments with Sphagnum mesocosms obtained from different microtopographic positions of a bog located in Michigan (USA) showed that, under drainage conditions (WT = −25 cm), the GPP of mosses established in hollows was more affected than those forming hummocks, increasing CO 2 emissions in the former (O'Neill et al, 2022). Hummock-forming mosses offer greater resistance to maintain GPP under drainage conditions thanks to a high capitulum density and increased capillary transport efficiency in water retention, allowing them to respond favorably to water stress, while retaining stability in C fluxes (Adkinson & Humphreys, 2011;Bengtsson et al, 2016;O'Neill et al, 2022;Robroek et al, 2009).…”

“…| 5 of 22 PACHECO-CANCINO et al gross photosynthesis in a poor fen in southern Finland (Riutta et al, 2007). However, water stress caused by drainage reduces Sphagnum P max due to decreased volumetric moisture content of the moss, limiting net CO 2 uptake and GPP, which may contract the CO 2 sink (Adkinson & Humphreys, 2011;Jassey & Signarbieux, 2019;O'Neill et al, 2022;Robroek et al, 2009;Sulman et al, 2010). In this regard, Riutta et al (2007) report that decreases in WT by −14 and −22 cm caused decreases of 14% and 22%, respectively, in gross photosynthesis in an ombrotrophic peatland (bog) in southern Finland, due to reductions in Sphagnum photosynthesis over those exhibited in sedges and shrubs.…”

Assessment of Greenhouse Gas Emissions into the Atmosphere from the Northern Peatlands Using the Wetland-DNDC Simulation Model: A Case Study of the Great Vasyugan Mire, Western Siberia

“…It should be noted that sphagnum-dominated peatlands store more carbon than all the Earth's forests, thus playing an important role in the carbon balance of the planet [12]. About 90% of the peatlands of Western Siberia belong to peat bogs with a predominance of sphagnum.…”