Declines in methane uptake in forest soils

Abstract

Forest soils are a sink for atmospheric methane (CH₄) and play an important role in modulating the global CH₄ budget. However, whether CH₄ uptake by forest soils is affected by global environmental change is unknown. We measured soil to atmosphere net CH₄ 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 CH₄ 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 CH₄ concentration and temperature, which should lead to increases in CH₄ uptake. This decrease in soil CH₄ uptake appears to be driven by increases in precipitation and soil hydrological flux. Furthermore, an analysis of CH₄ uptake around the globe showed that CH₄ 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 CH₄ sink may be declining and overestimated in several regions across the globe.