EFFECT OF LAND-USE CHANGE ON METHANE OXIDATION IN TEMPERATE FOREST AND GRASSLAND SOILS

Evidence is accumulating that land use changes and other human activity during the past 100 to 200 years have contributed to decreased CH4 oxidation in the soil. Recent studies have documented the effect of land use change on CH4 oxidation in a variety of ecosystems. Increased N additions to temperate forest soils in the northeastern United States decreased CH4 uptake by 30 to 60%, and increased N fertilization and conversion to cropland in temperate grasslands decreased CH4 uptake by 30 to 75%. Using these data, we made a series of calculations to estimate the impact of land use and management changes which have altered soil the CH4 sink in temperate forest and grassland ecosystems. Our study indicates that as the atmospheric mixing ratio of CH4 has increased during the past 150 y, the temperate CH4 sink has risen from approximately 8 Tg y-1 to 27 Tg y-1, assuming no loss of land cover to cropland conversion. The net effect of intensive land cover changes and extensive chronic disturbance (i.e., increased atmospheric N deposition) to these ecosystems have resulted in about 30% reduction in the CH4 sink relative to the soil sink assuming no disturbance to any of the temperate ecosystems. This will impact the global CH4 budget even more as atmospheric CH4 concentrations increase and as a result of further disturbance to other biomes. Determining the reasons for the decreased CH4 uptake due to land disturbance is necessary to understand the role of CH4 uptake in conjunction with the increasing atmospheric CH4 concentrations. Without accounting for this approximately 20 Tg y-1 temperate soil sink, the atmospheric CH4 concentration would be increasing about 1.5 times the current rate.