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

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.
