Cell-wall polysaccharides play an important role in decay resistance of Sphagnum and actively depressed decomposition in vitro

Abstract

Sphagnum-dominated peatlands head the list of ecosystems with the largest known reservoirs of organic carbon (C). The bulk of this C is stored in decomposition-resistant litter of one bryophyte genus: Sphagnum. Understanding how Sphagnum litter chemistry controls C mineralization is essential for understanding potential interactions between environmental changes and C mineralization in peatlands. We aimed to separate the effects of phenolics from structural polysaccharides on decay of Sphagnum. We measured aerobic microbial respiration of different moss litter types in a lab. We used chemical treatments to step-wise remove the chemical compounds thought to be important in decay-resistance in three taxonomically distant moss genera. We also focused on the effect of Sphagnum-specific cell-wall pectin-like polysaccharides (sphagnan) on C and N mineralization. Removing polymeric lignin-like phenolics had only negligible effects on C mineralization of Sphagnum litter, but increased mineralization of two other bryophyte genera, suggesting a minor role of these phenolics in decay resistance of Sphagnum but a major role of cell-wall polysaccharides. Carboxyl groups of pectin-like polysaccharides represented a C-source in non-Sphagnum litters but resisted decay in Sphagnum. Finally, isolated sphagnan did not serve as C-source but inhibited C and N mineralization instead, reminiscent of the effects reported for phenolics in other ecosystems. Our results emphasize the role of polysaccharides in resistance to, and active inhibition of, microbial mineralization in Sphagnum-dominated litter. As the polysaccharides displayed decay-inhibiting properties hitherto associated with phenolics (lignin, polyphenols), it raises the question if polysaccharide-dominated litter also shares similar environmental controls on decomposition, such as temperature or nutrient and water availability.

Biochemical determinants of litter quality in 15 species of Sphagnum

“…Since both carbohydrates (Turetsky et al 2008), notably sphagnan (Painter 1991;Børsheim et al 2001;Painter 2003;Hájek et al 2011), as well as insoluble phenolics (Tsuneda et al 2001) and soluble phenolics (Freeman et al 2001;Bragazza et al 2006) have been suggested as determinants for decay resistance of Sphagnum, it would be possible to envisage that species allocate carbon to production of either primary or secondary metabolites, i.e. carbohydrates or phenolics.…”

“…Similar to true lignin, the lignin-like phenolics in Sphagnum are thought to shield the polysaccharides by forming an amorphous coat on the cell wall surface (Tsuneda et al 2001). Removal of polymeric phenolics from the litter did not substantially increase the rate of Sphagnum decomposition in vitro (Hájek et al 2011); however, their concentration seems to correlate with Sphagnum decomposability in the field (Turetsky et al 2008;Hájek et al 2011). Some studies found structural, i.e.…”

“…cell-wall polysaccharides (Turetsky et al 2008), especially the pectic cell-wall polysaccharide Bsphagnan^ (Painter 1991;Børsheim et al 2001;Painter 2003;Hájek et al 2011), to be the main compound influencing litter quality in Sphagnum. Although the antimicrobial properties of sphagnan were attributed to acidification (Ballance et al 2007;Stalheim et al 2009), sphagnan addition has been found to block nitrogen mineralisation of plant litters (Hájek et al 2011). Hájek et al (2011) interpreted such active inhibition of microbial decay as electrostatic inactivation of microbial extracellular enzymes by the negatively charged sphagnan polymer (Ballance et al 2008).…”

“…Although the antimicrobial properties of sphagnan were attributed to acidification (Ballance et al 2007;Stalheim et al 2009), sphagnan addition has been found to block nitrogen mineralisation of plant litters (Hájek et al 2011). Hájek et al (2011) interpreted such active inhibition of microbial decay as electrostatic inactivation of microbial extracellular enzymes by the negatively charged sphagnan polymer (Ballance et al 2008). The negative charge is provided by unesterified carboxyl groups of uronic acid units (Ballance et al 2007), which determine the cation exchange capacity (CEC) of Sphagnum shoots (Clymo 1963).…”

Shifting environmental controls on CH<sub>4</sub> fluxes in a sub-boreal peatland

“…Throughout the growing season, days with high daily NEE were associated with increased emissions of CH 4 . The peat at this study site is primarily composed of Sphagnum moss which decomposes slowly (Hajek et al, 2011;Turetsky et al, 2008a;van Breemen, 1995). However, during the summer months, plant productivity increases considerably as daily NEE increases to −7.7 g m −2 d −1 .…”