Ar. Mosier et al., CH4 AND N2O FLUXES IN THE COLORADO SHORTGRASS STEPPE .2. LONG-TERM IMPACT OF LAND-USE CHANGE, Global biogeochemical cycles, 11(1), 1997, pp. 29-42
As part of a weekly, year-round program to measure the soil-atmosphere
exchange of nitrous oxide (N2O) and methane (CH4) in a shortgrass ste
ppe, we examined the impact of land use change on these fluxes from 19
92 through 1995, We found that conversion of grassland to croplands ty
pically decreased the soil consumption of atmospheric CH, and increase
d 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 gras
sland site, CH4 consumption decreased by about 35% and remained at the
se lower rates for the next 3 years. Although N2O fluxes were about 8
times higher for 18 months following plowing, the relative rates decli
ned to 25-50% higher than the native site after 3 years. Grasslands co
nverted to a winter wheat-fallow production system about 70 years ago
consumed about 25% less CH4 than a newly plowed site, while N2O emissi
ons 2 years after plowing were similar to the wheat fields. During the
fallow periods when soils were typically wetter and mineralized N acc
umulated, CH4 uptake rates were lower and N2O emissions were higher th
an the correspondingly active wheat fields. A wheat field that was rev
erted back to grassland in 1987 through the conservation reserve progr
am (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 den
itrification events. Another wheat field that was returned to grasslan
d in 1939 exhibited the same CH4 and N2O flux rates as comparable nati
ve pastures.