Moisture controls on CO₂ exchange in a Sphagnum‐dominated peatland: results from an extreme drought field experiment

Abstract

Peatlands store globally significant quantities of soil carbon, and Sphagnum moss is the main peat forming vegetation type in bogs. Sphagnum moss productivity is driven by the moisture content of its apical cluster of branches, the capitulum. Capitulum moisture content is dependent on the arrangement of leaves, branches and stems for a given species and also on hydrological conditions of the underlying peat. Despite this link, the response of CO 2 exchange in Sphagnum-dominated peatlands to extreme drought is still unclear, particularly under field conditions.We used drainage to expose Sphagnum rubellum to extreme drought and monitored water table, volumetric water content (VWC), gross ecosystem photosynthesis (GEP), ecosystem respiration (ER) and net ecosystem exchange of CO 2 (NEE) at plots in a 25 m transect perpendicular to a deep drainage ditch and compared results to an undrained site. VWC in the upper 10 cm of peat was strongly related to water table at depths shallower than 55 cm. Below this depth, near surface VWC remained relatively constant between 25 and 28% and Sphagnum GEP was effectively shut down. This also resulted in decreased ER at these locations. The combined effect was a linear relationship between VWC and NEE with moist sites acting as net CO 2 sinks (up to 5 g CO 2 m 2 day 1 ) whereas sites closest to the ditch were consistently small carbon sources. We suggest that understanding how climate change will alter peatland hydrology relative to the moisture thresholds of Sphagnum mosses is critical to determining the fate of their carbon sink function.

Evaluating effects of land management on greenhouse gas fluxes and carbon balances in boreo-temperate lowland peatland systems

“…Studies varied considerably in their findings, shown clearly in the forest plot and by the presence of significant heterogeneity. Studies were undertaken over 4 [43], 6 [44], 12 [45], 15 [35], 16 [36,42], and 32 [46] months (study period not stated in [47] [49], and 32 [46] months (study period not stated in [47]). …”

Rapid deglacial and early Holocene expansion of peatlands in Alaska

“…Saturated soils are necessary for the existence of peatlands, but the role of moisture in peatland C accumulation remains unclear. On relatively short time scales, water table depth manipulations have not produced consistent results (10,11), and numerous studies have shown stronger responses of C dynamics to temperature than moisture changes (10)(11)(12)(13).…”

Arctic Ecosystem Responses to Changes in Water Availability and Warming: Short and Long-Term Responses

“…Gross primary productivity responses to warming and water table changes are directly related to the ecosystem structure ). For instance, vascular and non-vascular plants respond differently to warming and drying (Hollister et al 2005, Strack et al 2009); the lack of roots and stomata in many non-vascular plants limit their ability to control water loss, negatively affecting their photosynthetic potential under warm and dry conditions.…”

“…Drying and warming are likely to increase microbial activity, increasing ER, while flooding is likely to have the opposite effect (Oberbauer et al 1992). Net ecosystem exchange is affected by the differential responses of vascular and non-vascular plants and microbes to warming and water changes (Sjogersten et al 2006, Strack et al 2009). The combined effect of warming and drying could reduce NEE as a result of decreased bryophyte productivity and increased microbial activity, but a potential increase in vascular productivity as a result of warmer temperatures could offset the increase in ER, maintaining NEE rates .…”