Abstract
Forest soils are a sink for atmospheric methane (CH(4)) and play an important role in modulating the global CH(4) budget. However, whether CH(4) uptake by forest soils is affected by global environmental change is unknown. We measured soil to atmosphere net CH(4) fluxes in temperate forests at two long-term ecological research sites in the northeastern United States from the late 1990s to the mid-2010s. We found that annual soil CH(4) uptake decreased by 62% and 53% in urban and rural forests in Baltimore, Maryland and by 74% and 89% in calcium-fertilized and reference forests at Hubbard Brook, New Hampshire over this period. This decrease occurred despite marked declines in nitrogen deposition and increases in atmospheric CH(4) concentration and temperature, which should lead to increases in CH(4) uptake. This decrease in soil CH(4) uptake appears to be driven by increases in precipitation and soil hydrological flux. Furthermore, an analysis of CH(4) uptake around the globe showed that CH(4) uptake in forest soils has decreased by an average of 77% from 1988 to 2015, particularly in forests located from 0 to 60 °N latitude where precipitation has been increasing. We conclude that the soil CH(4) sink may be declining and overestimated in several regions across the globe.