Chemosphere, Vol.26,Nos.1-4,pp 675-685, 1993 0045-6535/93 $6.00 + 0.00 Printed in Great Britain PergamonPressLtd. EFFECT OF LAND USE CHANGE ON METHANE OXIDATION IN TEMPERATE FOREST AND GRASSLAND SOILS D.S. Ojima ~*, D.W. Valentine ~'z, A.R. Mosier ~, W.J. Parton 1, and D.S. SchimeP ~ tNamral Resource Ecology Laboratory, Colorado State University, Fort Collins, CO 80523, USA 2USDA/ARS, P.O. Box E, Fort Collins, CO 80522, USA 3Climate System Modeling Program, University Corporation for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307-3000, USA (Received in USA 13 November 1991; accepted 14 April 1992) ABSTRACT Evidence is accumulating that land use changes and other human activity during the past 100 to 200 years have contributed to decreased CI~ oxidation in the soil. Recent studies have documented the effect of land use change on ~ oxidation in a variety of ecosystems. Increased N additions to temperate forest soils in the northeastern United States decreased CI-L uptake by 30 to 60%, and increased N fertilization and conversion to cropland in temperate grasslands decreased CI- L 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 CI- L 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.t to 27 Tg y-t, 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 CI- L 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 CI-I 4 concentrations increase and as a result of further disturbance to other biomes. Determining the reasons for the decreased CI-I 4 uptake due to land disturbance is necessary to understand the role of CI-L uptake in conjunction with the increasing atmospheric CI-L concentrations. Without accounting for this approximately 20 Tg y.t temperate soil sink, the atmospheric CH 4 concentration would be increasing about 1.5 times the current rate. INTRODUCTION During the past century, global methane (CI-L) concentrations have more than doubled and during the last 30 y have increased at an annual rate of about 1% (Rasmussen and Khalil, 1981; Bolle et al., 1986; Steele et al., 1987; Khalil et al., 1989). Although atmospheric CI- L concentration is about 200 times less than CO2, it contributes approximately 15% to current "greenhouse" forcing in the atmosphere (Hansen et al., 1989). This is because CI-L is about 30 times more effective as CO2 in absorbing IR radiation (Wuebbles and Edmonds, 675