Reduction‐Oxidation Potential and Dissolved Organic Matter Composition in Northern Peat Soil: Interactive Controls of Water Table Position and Plant Functional Groups

Abstract

Globally important carbon (C) stores in northern peatlands are vulnerable to oxidation in a changing climate. A growing body of literature draws attention to the importance of dissolved organic matter (DOM) in governing anaerobic metabolism in organic soil, but exactly how the reduction-oxidation (redox) activities of DOM, and particularly the phenolic fraction, are likely to change in an altered climate remain unclear. We used large mesocosms in the PEATcosm experiment to assess changes in peatland DOM and redox potential in response to experimental manipulations of water table (WT) position and plant functional groups (PFGs). WT position and PFGs interacted in their effects on redox potential and quantity and quality of DOM. Phenolics were generally of higher molecular weight and more oxidized with sedges in lowered WTs. Altered DOM character included changes in dissolved nitrogen (N), with higher N:[phenolics] with higher E4:E6 (absorbance ratio λ = 465:665) DOM in the lowered WT and sedge PFG treatments. Conversely, biomolecular assignments to amino-sugars were largely absent from low-WT treatments. Low WT resulted in the creation of unique N compounds, which were more condensed (lower H:C), that changed with depth and PFG. The accumulation of oxidized compounds with low WT and in sedge rhizospheres could be very important pools of electron acceptors beneath the WT, and their mechanisms of formation are discussed. This work suggests the effects of changes in vegetation communities can be as great as WT position in directly and interactively mediating peat redox environment and the redox-activity of DOM. Plain Language Summary Peatlands are important ecosystems in both the global carbon (C)cycle and the Earth's climate system owing to their ability to store vast quantities of C taken from the atmosphere. Peat C stays locked up in these ecosystems largely owing to cool and wet conditions, and as such, these C stores are vulnerable to release back to the atmosphere if the climate or water levels change. Water table level and plant species composition have a combined effect on C and nitrogen (N) cycling in peatlands. We manipulated both factors in an experimental setting composed of 24 large bins into which we put intact peatland miniecosystems. Sedges play a big role in producing N compounds below the peat surface, and this work suggests these compounds can actually be synthesized into larger, less accessible compounds. In addition, activities of sedge roots and other dominant plant communities (such as shrubs in the heath family of plants) may interact in the synthesis of oxidized, larger molecules. These molecules can allow microbes to continue to decompose peat even in the absence of oxygen. This could increase the release of greenhouse gas carbon dioxide to the atmosphere, while reducing inputs of the stronger greenhouse gas methane.

Peatland microbial community responses to plant functional group and drought are depth‐dependent

“…These patterns can be explained by abiotic and biotic factors: the intolerance of most fungi to anoxic conditions (Kavanagh, 2011) constraining most taxa to shallow peat, and the colocation of the dominant ErMF with their shallowly rooted Ericaceae hosts (Moore et al, 2002; Wallén, 1987). In contrast, the broad range of moisture niches, metabolic pathways and redox tolerance among soil prokaryotes (e.g., Bodelier & Dedysh, 2013; Lennon et al, 2012) and the strong sensitivity of prokaryote communities to changes in soil moisture (e.g., Bapiri et al, 2010; Barnard et al, 2013) explain their shift with WT treatments in both drier surface peat as well as at the acrotelm–catotelm boundary where redox conditions are most dynamic (Kane et al, 2019; Tfaily et al, 2018). These depth‐dependent effects indicate that WT and PFG are among the key shapers of the vertical physicochemical gradients that structure peatland microbial communities (Andersen et al, 2013; Artz et al, 2007; Lin et al, 2014), the activities of which then feed back to modulate carbon cycling along the peat profile (Chanton et al, 2009; Kane et al, 2019; Lin et al, 2014; Tfaily et al, 2018).…”

“…PEATcosm was a mesocosm experiment designed to test the influence of seasonal drought and PFG on peatland ecosystems. Detailed descriptions of the experimental design, peat characteristics, porewater chemistry and vegetation can be found in Kane et al (2019), Lamit et al (2017) and Potvin et al (2015). The experiment contained 24 ~1‐m 3 peat monoliths excavated from an oligotrophic acidic (pH = ~4) Sphagnum peatland in Minnesota, USA (47.07278°N, 92.73167°W), in May 2010.…”

“…Next, we hypothesized, (H2) WT manipulation will shift microbial community structure, with these effects also being strongly depth‐dependent. Although WT impacts are probably broad, the most distinct effects of WT should occur at depths where the WT is the most dynamic and distinct between the treatments, and is known to manifest a strong influence on peat and porewater chemistry (Kane et al, 2019; Lin et al, 2014). Given that the depth‐dependent effects of each PFG are in part a consequence of the different ways that sedges and Ericaceae interact with oxic vs. anoxic conditions, we further hypothesized, (H3) the responses of microbial communities to manipulation of one factor (PFG or WT) will be dependent on the level of the other factor and these interactive effects will in turn be dependent on depth in the peat profile.…”

Effects of water level alteration on carbon cycling in peatlands

“…A general pattern of DOC quality under water level alteration is that drainage increases DOC aromaticity and decreases DOC biodegradability (Wickland, Neff, and Aiken 2007;Hribljan et al 2014;Hansen et al 2016;Jassey et al 2018). Specifically, water table drawdown increases the occurrence and molecular weight of phenolics in DOC (Blodau and Siems 2012;Frank et al 2014;Lou et al 2014;Kane et al 2019), while rising water table decreases aromaticity and makes DOC more labile (Holl et al 2009;Hribljan et al 2014;Dieleman et al 2016b).…”

“…These species also transport oxygen into saturated peat enhancing microbial decomposition, resulting in relatively higher CO 2 emissions with sedge dominance (Shannon and White 1994;Silvan et al 2005;Garnet et al 2005;Strack et al 2006a;Fritz et al 2011;Green and Baird 2012;Robroek et al 2015;Jordan et al 2016;Marushchak et al 2016;Rupp et al 2019). A recent study found sedge roots are conducive to the generation of oxidized, larger compounds that serve as electron acceptors in anaerobic microbial decomposition, which increases CO 2 emissions (Kane et al 2019).…”