Contrasting vulnerability of drained tropical and high‐latitude peatlands to fluvial loss of stored carbon

Abstract

Carbon sequestration and storage in peatlands rely on consistently high water tables. Anthropogenic pressures including drainage, burning, land conversion for agriculture, timber, and biofuel production, cause loss of peat-forming vegetation and exposure of previously anaerobic peat to aerobic decomposition. This can shift peatlands from net CO 2 sinks to large CO 2 sources, releasing carbon held for millennia. Peatlands also export significant quantities of carbon via fluvial pathways, mainly as dissolved organic carbon (DOC). We analyzed radiocarbon ( 14 C) levels of DOC in drainage water from multiple peatlands in Europe and Southeast Asia, to infer differences in the age of carbon lost from intact and drained systems. In most cases, drainage led to increased release of older carbon from the peat profile but with marked differences related to peat type. Very low DOC-14 C levels in runoff from drained tropical peatlands indicate loss of very old (centuries to millennia) stored peat carbon. High-latitude peatlands appear more resilient to drainage; 14 C measurements from UK blanket bogs suggest that exported DOC remains young (<50 years) despite drainage. Boreal and temperate fens and raised bogs in Finland and the Czech Republic showed intermediate sensitivity. We attribute observed differences to physical and climatic differences between peatlands, in particular, hydraulic conductivity and temperature, as well as the extent of disturbance associated with drainage, notably land use changes in the tropics. Data from the UK Peak District, an area where air pollution and intensive land management have triggered Sphagnum loss and peat erosion, suggest that additional anthropogenic pressures may trigger fluvial loss of much older (>500 year) carbon in high-latitude systems. Rewetting at least partially offsets drainage effects on DOC age.

The impact of ditch blocking on fluvial carbon export from a UK blanket bog

“…Constraints on the amount of aerated peat in ditched blanket bogs may also help to explain differences in DOC response observed between studies in different regions. A comparison of DOC radiocarbon ( 14 C) measurements from intact, drained, and re‐wetted peatlands from the boreal to tropical climate zones (which included our study catchments) showed highly variable sensitivity to drainage‐induced loss of DOC from deeper peat (Evans et al, ), with tropical peatlands showing the greatest sensitivity and blanket bogs the least. These differences are reflected in literature showing clear evidence of DOC response to drainage and re‐wetting in boreal, continental temperate, and tropical peatlands (Glatzel et al, ; Haapalehto et al, ; Menberu et al, ; Moore et al, ; Strack et al, ; Urbanová et al, ), but not in blanket peatlands (see above).…”

“…One possibility, given the observed subsidence at our site, is that blanket bog drainage causes an initial flush of DOC, which dissipates as the peat surface subsides and ditches infill. Elsewhere, if ditches develop into erosional features and water‐table drawdown is maintained or intensified, more sustained DOC increases and loss of old carbon via this pathway may occur (e.g., Evans et al, ; Stimson, Allott, Boult, & Evans, ).…”

In the line of fire: the peatlands of Southeast Asia

“…This exposes the upper part of the peat column to aerobic mineralization triggering the loss of peat C from long-term storage, principally as a direct flux of CO 2 to the atmosphere (e.g. [10][11][12]) but also as CH 4 from drainage ditches [13] and as dissolved organic C (DOC) in drainage waters [14,15]. Across the world, peatlands have been drained to enable a range of land uses including livestock grazing, crop cultivation, forestry and peat extraction.…”

The role of waterborne carbon in the greenhouse gas balance of drained and re-wetted peatlands

“…Roulet and Moore, 1995;Minkinnen and Laine, 2006;Sirin et al, 2013) to over 0.2 in some grasslands (Van den Pol-Van Dasselaar, 1999;Schrier-Uijl et al, 2010;Vermaat et al, 2011), although such high values appear unusual outside the Netherlands, and indicative values of 0.05 were suggested for grasslands and extraction sites in the IPCC Wetland Supplement (IPPC, 2014a). Differences in drainage ditch density between categories reflect differences in the characteristics of the peat; for example, fibrous tropical peatlands can be effectively drained by a few large but widely spaced canals, whereas highly humified blanket bogs require a much higher density of smaller ditches to effectively lower water tables (Evans et al, 2014b). Grasslands in some flat former fen landscapes, such as the Netherlands, tend to contain large areas of open water.…”