Towards improved bottom-up inventories of methane from the European land surface

Forests and wetlands are generally seen as opposites in the methane cycle of terrestrial ecosystems. Wetlands are sources for atmospheric methane and forest soils sinks. However, this greenhouse gas is also emitted by wet forest soils, which is commonly disregarded due to lacking information on their spatial distribution. Here, we estimated the potential bias made for the European methane budget of terrestrial ecosystems when neglecting wet forest ecosystems but including rice paddies and latest estimates for lakes. We appointed distinct annual methane rates for individual land use types based on a literature survey and weighted them according to their European area. This was performed separately for four major ecozones (cold, temperate, continental and Mediterranean). Three approaches were applied: (1) the mean values for forests and wetlands were calculated in three different scenarios, (2) assuming that boreal needle-leaved evergreen forest with a low tree cover (<40%) is predominately forested wetland (3) assuming different shares of wet forest ecosystems in individual forest areas. For the net balance 2.8 Tg CH4-C a(-1) were calculated which includes emissions from rice paddies (0.2 Tg CH4-C a(-1)) and from lakes (2.5 Tg CH4-C a(-1)). The different approaches for the net balances that included wet forest ecosystems mainly ranged between 4.6 and 6.7 Tg CH4-C a(-1). The results suggest that wet forest ecosystems are approximately as important as wetlands for the European methane balance. European bottom-up inventories are improved best by more accurate mapping of wetlands both within and outside forests and more flux data for lakes and continental wetlands. (C) 2012 Elsevier Ltd. All rights reserved.