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Are wetlands a net source or net sink of GHGs? On the one hand, they store a lot of carbon. On the other hand, they emit methane, a potent green house gas.

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Depends on a number of factors. The main issue is the balance of carbon sequestration in peat versus methane emission if they're trapping more carbon dioxide through growth than their equivalent emissions to rot/burning then they're a net sink. Most undisrupted wetlands are large net sinks taking in many times more carbon than they emit even taking into account the relatively high impact of methane measured against carbon dioxide. The big problem is that many wetlands have been disrupted in one or more ways leading to their degradation and changing the balance of uptake/storage vs emission those that are worse effected have become net emitters of carbon.

There are of course other issues to consider, like the alternative land use, a waterlogged grazing pasture can be a much larger emitter of methane per acre than the bog that occupied the same site until it was drained and it sequesters minimal carbon. Peat land turned over to managed leguminous hardwood forestry may be able to sequester just as much carbon but with far less methane.

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Carbon neutral, but overall massive carbon sink. Rotting vegetation produces CO2, Growing vegetation consumes it. However many wetland soils are oxygen deprived, so soil chemistry preserves carbon mass and buries under sediments. This is how coal formed.

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