Soil-atmosphere exchange of methane in adjacent cultivated and floodplain forest soils

The soil-atmosphere exchange of methane was measured in adjacent cultivated (corn) and forest (upper floodplain, mixed hardwood) habitats of the south eastern U.S. piedmont for a period of 3 years using closed chambers. We hav e evaluated the effect of the following factors on soil-atmosphere methane exchange: (1) interannual variability of climatic conditions, (2) landscape position (i.e., river levee versus terrace), and (3) disturbance ranging f rom intense (cultivation) through moderate (approximately annual flooding e vents that last from weeks to months) to subtle (approximately annual flood ing of a few days duration). We found that mean methane consumption in the cultivated and forested terrace sites was <0.3 mg CH4 m(-2) d(-1), whereas the mean consumption rate in forested levee sites was about 1.4 mg CH4 m(-2 ) d(-1) over the course of the 3 years. Moisture levels in the upper soil ( 0-5 cm) appear to exert little control of methane exchange in any of the ha bitats. We observed little seasonal variation in methane flux in the levee sites, in contrast to results observed by others in higher-latitude and tro pical forests, Our results suggest that very subtle differences in landscap e position and disturbance impact the strength of the soil methane sink. We cannot conclude that agricultural development destroyed the methane sink c apacity of these floodplain terrace soils because it was probably already q uite low due to periodic disturbance by flooding. Limited measurements of n itrogen cycling suggest that methane flux differences observed among the di fferent habitats are not obviously related to differences in N mineralizati on or nitrification as in other ecosystems.