CH4 and N2O fluxes in the Colorado shortgrass steppe .2. Long-term impact of land use change

As part of a weekly, year-round program to measure the soil-atmosphere exchange of nitrous oxide (N2O) and methane (CH4) in a shortgrass steppe, we examined the impact of land use change on these fluxes from 1992 through 1995, We found that conversion of grassland to croplands typically decreased the soil consumption of atmospheric CH, and increased the emission of N2O. Mean annual CH4 consumption and N2O efflux over 3 years in native grasslands were 35 mu g C m(-2) hr(-1) and 1.9 mu g N m(-2) hr(-1), respectively. Immediately after tilling a native grassland site, CH4 consumption decreased by about 35% and remained at these lower rates for the next 3 years. Although N2O fluxes were about 8 times higher for 18 months following plowing, the relative rates declined to 25-50% higher than the native site after 3 years. Grasslands converted to a winter wheat-fallow production system about 70 years ago consumed about 25% less CH4 than a newly plowed site, while N2O emissions 2 years after plowing were similar to the wheat fields. During the fallow periods when soils were typically wetter and mineralized N accumulated, CH4 uptake rates were lower and N2O emissions were higher than the correspondingly active wheat fields. A wheat field that was reverted back to grassland in 1987 through the conservation reserve program (CRP) continued to exhibit annual CH4 uptake and N2O emission rates similar to the wheat fields. Winter N2O emissions were, however, much higher in the CRP because of greater snow accumulation and winter denitrification events. Another wheat field that was returned to grassland in 1939 exhibited the same CH4 and N2O flux rates as comparable native pastures.