Here, we present results from the most comprehensive compilation of Holocene peat soil properties with associated carbon and nitrogen accumulation rates for northern peatlands. Our database consists of 268 peat cores from 215 sites located north of 45°N. It encompasses regions within which peat carbon data have only recently become available, such as the West Siberia Lowlands, the Hudson Bay Lowlands, Kamchatka in Far East Russia, and the Tibetan Plateau. For all northern peatlands, carbon content in organic matter was estimated at 42 ± 3% (standard deviation) for Sphagnum peat, 51 ± 2% for non- Sphagnum peat, and at 49 ± 2% overall. Dry bulk density averaged 0.12 ± 0.07 g/cm3, organic matter bulk density averaged 0.11 ± 0.05 g/cm3, and total carbon content in peat averaged 47 ± 6%. In general, large differences were found between Sphagnum and non- Sphagnum peat types in terms of peat properties. Time-weighted peat carbon accumulation rates averaged 23 ± 2 (standard error of mean) g C/m2/yr during the Holocene on the basis of 151 peat cores from 127 sites, with the highest rates of carbon accumulation (25–28 g C/m2/yr) recorded during the early Holocene when the climate was warmer than the present. Furthermore, we estimate the northern peatland carbon and nitrogen pools at 436 and 10 gigatons, respectively. The database is publicly available at https://peatlands.lehigh.edu .

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Abstract. The postglacial development of peatland systems has had a strong influence on the global carbon cycle. Peatland effects on carbon cycling involve changes in both large-scale structure and community composition. The rate of C sequestration declines as a peat deposit grows, and methane emission increases as wet microhabitats expand in area. We examine the theoretical basis, underlying assumptions, and empirical evidence for two highly influential models of peatland form and development: Clymo's bog growth model (BGM) and Ingram's groundwater mound hypothesis (GMH). Our detailed theoretical analysis reveals previously unrecognized consequences of the models and, for the BGM, unrealistically stringent conditions for application. Our review of model assumptions and empirical evidence identifies the principal limitations of these models: they ignore spatial heterogeneity in peatland structure and function, fast processes occurring near the peatland surface, and interactions between peat accumulation and hydrological conditions. As a result, these models are unable to predict the effects of bog growth on hydrological conditions or peat-forming processes.We introduce five conceptual models of peatland development that differ in how bog size and shape change over time, and we examine the consequences of each model for hydrological conditions. This exercise shows how bog height growth and lateral expansion change the boundary conditions constraining peatland dynamics, and so determine the direction of ecosystem development. We link peatland properties to the four general properties of ''complex adaptive systems'' (CAS): spatial heterogeneity, localized flows, self-organizing structure and nonlinearity. We also present a framework for modeling peatlands as CAS. In this framework, the system is disaggregated, both vertically and horizontally, into a set of components that interact locally through flows of energy and resources. Both internal dynamics and external forcing drive changes in hydrological conditions and microhabitat pattern, and these autogenic and allogenic changes in peatland structure affect hydrological processes, which, in turn, constrain peatland development and carbon cycling. We conclude by outlining four areas in which further empirical research is urgently needed.

The role of peatlands in the global carbon cycle is confounded by two inconsistencies. First, peatlands have been a large reservoir for carbon sequestered in the past, but may be either net sources or net sinks at present. Second, long-term rates of peat accumulation (and hence carbon sequestration) are surprisingly steady, despite great variability in the short-term rates of peat formation. Here, we present a feedback mechanism that can explain how fine-scale and short-term variability in peat-forming processes is constrained to give steady rates of peat accumulation over longer time-scales. The feedback mechanism depends on a humpbacked relationship between the rate of peat formation and the thickness of the aerobic surface layer (the acrotelm), such that individual microforms (hummocks, lawns, hollows and pools) expand or contract vertically in response to fluctuations in the position of the water table. Hummocks (but not hollows) 'evolve' to a steady state where changes in acrotelm thickness compensate for climate-mediated variations in surface wetness. With long-term growth of a topographically confined peat deposit, the steady state gradually shifts to a thicker acrotelm (i.e. taller hummocks) and lower rates of peat formation and carbon sequestration.

The response of peatlands to changes in the climatic water budget is crucial to predicting potential feedbacks on the global carbon (C) cycle. To gain insight on the patterns and mechanisms of response, we linked a model of peat accumulation to a model of peatland hydrology, then applied these models to empirical data spanning the past 5000 years for the large mire Store Mosse in southern Sweden. We estimated parameters for C sequestration and height growth by fitting the peat accumulation model to two age profiles. Then, we used independent reconstruction of climate wetness and model reconstruction of bog height to examine changes in peatland hydrology. Reconstructions of C sequestration showed two distinct patterns of behaviour: abrupt increases associated with major transitions in vegetation and dominant Sphagnum species (fuscum, rubellum-fuscum and magellanicum stages), and gradual decreases associated with increasing humification of newly formed peat. Carbon sequestration rate ranged from a minimum of 14 to a maximum of 72 g m À2 yr À1 , with the most rapid changes occurring in the past 1000 years. Vegetation transitions were associated with periods of increasing climate wetness during which the hydrological requirement for increased seepage loss was met by rise of the water table closer to the peatland surface, with the indirect result of enhancing peat formation. Gradual decline in C sequestration within each vegetation stage resulted from enhanced litter decay losses from the near-surface layer. In the first two vegetation stages, peatland development (i.e., increasing surface gradient) and decreasing climate wetness drove a gradual increase in thickness of the unsaturated, near-surface layer, reducing seepage water loss and peat formation. In the most recent vegetation stage, the surface diverged into a mosaic of wet and dry microsites. Despite a steady increase in climate wetness, C sequestration declined rapidly. The complexity of response to climate change cautions against use of past rates to estimate current or to predict future rates of peatland C sequestration. Understanding interactions among hydrology, surface structure and peat formation are essential to predicting potential feedback on the global C cycle.