Decreased carbon accumulation feedback driven by climate‐induced drying of two southern boreal bogs over recent centuries

Abstract

Northern boreal peatlands are important ecosystems in modulating global biogeochemical cycles, yet their biological communities and related carbon dynamics are highly sensitive to changes in climate. Despite this, the strength and recent direction of these feedbacks are still unclear. The response of boreal peatlands to climate warming has received relatively little attention compared with other northern peatland types, despite forming a large northern hemisphere‐wide ecosystem. Here, we studied the response of two ombrotrophic boreal peatlands to climate variability over the last c. 200 years for which local meteorological data are available. We used remains from plants and testate amoebae to study historical changes in peatland biological communities. These data were supplemented by peat property (bulk density, carbon and nitrogen content), 14C, 210Pb and 137Cs analyses and were used to infer changes in peatland hydrology and carbon dynamics. In total, six peat cores, three per study site, were studied that represent different microhabitats: low hummock (LH), high lawn and low lawn. The data show a consistent drying trend over recent centuries, represented mainly as a change from wet habitat Sphagnum spp. to dry habitat S. fuscum. Summer temperature and precipitation appeared to be important drivers shaping peatland community and surface moisture conditions. Data from the driest microhabitat studied, LH, revealed a clear and strong negative linear correlation (R2 = .5031; p < .001) between carbon accumulation rate and peat surface moisture conditions: under dry conditions, less carbon was accumulated. This suggests that at the dry end of the moisture gradient, availability of water regulates carbon accumulation. It can be further linked to the decreased abundance of mixotrophic testate amoebae under drier conditions (R2 = .4207; p < .001). Our study implies that if effective precipitation decreases in the future, the carbon uptake capacity of boreal bogs may be threatened.

Recent Changes in Peatland Testate Amoeba Functional Traits and Hydrology Within a Replicated Site Network in Northwestern Québec, Canada

“…Although it has been shown before that lowering of water table leads to a decrease in the abundance of mixotrophs (Marcisz et al, 2016), this is not visible in our records. This might also be due to the sample collecting habitats, i.e., lawns that supply sufficient water and did not reach the threshold for changes of mixotrophy (Jassey et al, 2018;Koenig et al, 2018;Lamentowicz et al, 2019;Zhang et al, 2020). Instead, we found that increases in mixotrophy were linked to increased Sphagnum (mainly S. fuscum) abundance (Van Bellen et al, 2018).…”

“…Mixotrophy, seem to show relatively consistent increasing trends over recent decades, which are in line with the increased carbon accumulation rates, even though these increases of C accumulation rates might partly due to less decomposition of recently accumulated peats (Piilo et al, 2019). Mixotrophic testate amoebae that acquire most of their C by photosynthesis of their symbionts, essentially functioning as autotrophs (Lara and Gomaa, 2017) are suggested to play an important role in driving peatland C accumulation, with higher abundances promoting C accumulation (Marcisz et al, 2016;Zhang et al, 2020), but the opposite pattern can also be expected depending on the specific condtions. For example, in this study both positive and negative linear links between mixotrophic testate amoebae and C accumulation were derived (Figure 6), which might depend on the prey availability, i.e., mixotrophic testate amoebae either favour photosynthesis and form a positive link between C accumulation and their abundance, or predation and constitute a negative link between C accumulation and their abundance (Jassey et al, 2012(Jassey et al, , 2013Mieczan et al, 2015).…”

“…To enable a spatial comparison of our records, we collected peat cores consistently from lawn habitats, however, even in a particular site large differences between the three replicated cores were captured. It has been previously reported that different peatland habitats may also undergo different peatland-climate feedback pathways (Zhang et al, 2020). Any one site-based discussion of regional feedbacks is likely to be biased because different peatland types can experience specific successions under similar climate conditions, such as the differences observed between palsa K1P and fen K2 in region Kuujjuarapik in this study, let alone different regions that have different climate (Sim et al, 2019).…”

“…Understanding the response of peatland C cycling to climate change requires comprehensive interpretation of ecosystem-climate feedbacks. Several studies focus on the interactions between peatland hydrology and vegetation (Magnan et al, 2018;Van Bellen et al, 2018;Zhang et al, 2020), revealing spatial and temporal variations in their response to climate changes. Testate amoebae, a group of single-celled protists that are abundant and diverse in Sphagnum-dominated peatlands, are key constituents of peatland microbial communities, representing a considerable portion of microbial biomass (Gilbert et al, 1998).…”

“…Particularly, in terms of understanding peatland C cycling, more studies are needed to understand how mixotrophic testate amoebae vary temporally and spatially in response to external environmental drivers, as they contribute to photosynthetic C fixation (Gilbert et al, 1998). Peatland C fixation is generally estimated by taking into account only Sphagnum growth, without considering photosynthetic microorganisms, despite the fact that C intake during photosynthesis by mixotrophic testate amoebae may be significant at the ecosystem level (Jassey et al, 2015;Zhang et al, 2020).…”