Fens and bogs are distinct in terms of their biogeochemistry, water table behavior, and net peat-accumulation regimes. While most peatlands start developing as fens, a large fraction of them eventually shift to bogs in a step-like ecosystem shift. This transition has traditionally been assumed to be primarily controlled by the ecosystem itself (autogenic control). Here we use 90 peat profiles from southernmost South America (SSA) as a case study that illustrates a synchronous, regional-scale shift from fen to bog around 4200 years ago. In light of these results, we propose and discuss conceptual models that link environmental change (allogenic control) as a trigger to the fen-bog transition (FBT). In addition, our stratigraphic analyses show that Sphagnum deposits are associated with greater peat masses, larger soil-carbon stocks, and higher rates of peat-carbon accumulation than their non-Sphagnum counterparts, with Sphagnum bogs being characterized by soil-carbon densities over twice that of non-Sphagnum peatlands (medians = 141 vs. 56 kgC/m 2). Since fens and bogs also behave differently in terms of their carbon exchanges with the atmosphere, a better appraisal of the processes involved in the FBT could help elucidate the role of this critical ecosystem shift in the past and future global carbon cycle.
Peatlands have been important terrestrial carbon (C) reservoirs throughout the Holocene, yet whether these ecosystems will become stronger or weaker C sinks in the future remains debated. While surface peat layers (acrotelm) have a greater apparent rate of C accumulation than deeper, millennial‐aged peat (catotelm), it is difficult to project how much more aerobic decomposition will take place before the younger surface cohorts join the older deeper ones. Studies have suggested that warming could lead to weakened C accumulation in peatlands due to enhanced aerobic decay in the acrotelm, which would lead to a slower transfer of peat into the catotelm, if any. Conversely, other studies have suggested increased C accumulation in the acrotelm and thus, larger long‐term C transfer into the catotelm under warming conditions because of greater plant productivity and faster peat accumulation. Improving our predictions about the rate of present and future peatland development is important to forecast feedbacks on the global C cycle and help inform land management decisions. In this study, we analyzed two peat cores from southern Patagonia to calculate their long‐ versus short‐peat C accumulation rates. The acrotelm rates were compared to the catotelm peat C legacies using an empirical modeling approach that allows calculating the future catotelm peat storage based on today's acrotelm characteristics, and thus predict if those recent rates of C accumulation will lead to greater or weaker long‐term C storage in the future. Our results indicate that, depending on local bioclimatic parameters, some peatlands may become stronger C sinks in the future, while others may become weaker. In the case of this study, the wetter site is expected to increase its C sink capacity, while our prediction for the drier site is a net decrease in C sequestration in the coming decades to centuries.
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