Dissolved organic matter concentration and composition discontinuity at the peat–pool interface in a boreal peatland

Abstract

Abstract. Pools are common features of peatlands and can represent from 5 % to 50 % of the peatland ecosystem's surface area. Pools play an important role in the peatland carbon cycle by releasing carbon dioxide and methane to the atmosphere. However, the origin of this carbon is not well constrained. A hypothesis is that the majority of the carbon emitted from pools predominantly originates from mineralized allochthonous (i.e., plant-derived) dissolved organic matter (DOM) from peat rather than in situ primary production. To test this hypothesis, this study examined the origin, composition, and degradability of DOM in peat porewater and pools of an ombrotrophic boreal peatland in northeastern Quebec (Canada) for 2 years over the growing season. The temporal evolution of dissolved organic carbon (DOC) concentration, the optical properties, molecular composition (THM-GC-MS), stable isotopic signature (δ13C-DOC), and degradability of DOM were determined. This study demonstrates that DOM, in both peat porewater and pools, presents a diverse composition and constitutes highly dynamic components of peatland ecosystems. The molecular and isotopic analyses showed that DOM in pools was derived from plants. However, DOM compositions in the two environments were markedly different. Peat porewater DOM was more aromatic, with a higher molecular weight and DOC : DON (dissolved organic nitrogen) ratio compared to pools. The temporal dynamics of DOC concentration and DOM composition also differed. In peat porewater, the DOC concentration followed a strong seasonal increase, starting from 9 mg L−1 and reaching a plateau above 20 mg L−1 in summer and autumn. This was explained by seasonal peatland vegetation productivity, which is greater than microbial DOM degradation. In pools, DOC concentration also increased but remained 2 times lower than in the peat porewaters at the end of the growing season (∼ 10 mg L−1). Those differences might be explained by a combination of physical, chemical, and biological factors. The limited hydraulic conductivity in deeper peat horizons and associated DOM residence time might have favored both DOM microbial transformation within the peat and the interaction of DOM aromatic compounds with the peat matrix, explaining part of the shift of DOM compositions between peat porewater and pools. This study did not report any photolability of DOM and only limited microbial degradability. Thus, it is likely that the DOM might have been microbially transformed at the interface between peat and pools. The combination of DOM quantitative and qualitative analyses presented in this study demonstrates that most of the carbon present within and released from the pools originates from peat vegetation. These results demonstrate that pools represent a key component of the peatland ecosystem ecological and biogeochemical functioning.

Peatlands Versus Permafrost: Landscape Features as Drivers of Dissolved Organic Matter Composition in West Siberian Rivers

“…This indicates that peatland cover acts as a control not only on DOC concentration in surface waters (Xenopoulos et al, 2003) but also with respect to DOM composition, specifically through contributions of more terrestrial DOM with higher average mass. This agrees with other observations from both peat porewater, which has been shown to have high aromaticity and molecular weight (Prijac et al, 2022;Tfaily et al, 2013), and from data collected from the largest Arctic rivers, where the Ob' River, which contains the greatest peatland area of all six major Arctic watersheds, is observed to export the most aromatic DOM (Amon et al, 2012;Behnke et al, 2021). Furthermore, Arctic watersheds such as the Severnaya Dvina and Onega that have extensive peatland coverage have been shown to export DOM that is even more aromatic in nature and exhibit higher average mass DOM than that found in the major Arctic rivers with comparatively less peatland cover in their watersheds (Johnston et al, 2018;Starr et al, 2023).…”

Climate‐driven spatial and temporal patterns in peatland pool biogeochemistry

“…It is also possible that higher TN and TP concentrations were related to dry and wet deposition of dust in more continental parts of southern Patagonia (Kleinebecker et al, 2008) or to the positive effect volcanic ash has on OM decomposition and nutrient cycling and loading (Broder et al, 2012; Modenutti et al, 2013). Similarly, spatial trends in pH and SUVA may reflect differences in plant diversity among regions, as pool acidity and the aromaticity of DOC have been related to the composition of vegetation surrounding the pools (Arsenault et al, 2019; Prijac et al, 2022). Pools in our dataset represented a wide range of depth, area, and vegetation, and the 20 peatlands that were compared cover large gradients of altitudinal and topographic parameters.…”

“…Temperature and precipitation are broad‐scale (>10 km) climate drivers of spatial and temporal variability in the biogeochemistry of freshwater bodies (Collins et al, 2019), but such factors often vary in response to small scale (<1 km) land use and terrain differences (Cao et al, 2020). In permafrost‐free temperate and boreal peatland pools, C and nutrient processing have been shown to be partly driven by local temperature and summer precipitation variations due to their effect on microbial activity dynamics, and on mass concentration, dilution, and transfer from the surrounding soil (Arsenault et al, 2018; Prijac et al, 2022). At larger scales, for a given set of terrain characteristics, spatial and possibly temporal patterns in biogeochemistry that follow climatic gradients may emerge in lakes (Soranno et al, 2019), but to our knowledge, this has not yet been studied in more structurally homogeneous systems like peatland pools.…”

“…In ombrotrophic settings, pools from both raised (i.e., dome-shaped) or blanket (i.e., peatlands that follow the landscape morphology) peatlands have little to no hydrological and biogeochemical connectivity with the surrounding landscape and groundwater (Fraser, Roulet, & Lafleur, 2001;Holden & Burt, 2003). Locally, pool biogeochemistry, therefore, relies almost entirely on intrinsic controls, related to their morphology, the composition of the surrounding vegetation, and soil chemistry (Arsenault et al, 2018(Arsenault et al, , 2019Prijac et al, 2022). These factors are constrained at a larger scale by altitudinal, topographical, and hydrological influences (Belyea, 2007;Rydin & Jeglum, 2013).…”

A Carbon Source in a Carbon Sink: Carbon Dioxide and Methane Dynamics in Open‐Water Peatland Pools

“…The 25 largest pools (15% of the total number of pools) represented 80% of the total pool area, while the 118 smallest pools (72% of the total number of pools) only represented 10% of the total pool area. Research documenting C-GHG dynamics in the draining stream and dissolved organic matter (DOM) composition and exchange along the peat-pool interface within this study site has previously been published (Prijac et al, 2022(Prijac et al, , 2023Taillardat et al, 2022).…”