Methane fluxes from differentially managed grassland study plots: the important role of CH4 oxidation in grassland with a high potential for CH4 production

Methane oxidation fluxes were monitored with the closed chamber method in e ight treatment plots on a semi-wet grassland site near Giessen, Germany. Th e management regimes differed in the amount of nitrogen (NH4NO3) fertilizer applied and in the height of the in-ground water table. No inhibition of C H4 oxidation occurred, regardless of the amount of annual N fertilizer appl ied. Instead, the mean CH4 consumption rates were correlated with the mean soil moisture of the plots. However, the correlation between daily soil wat er content and corresponding CH4 oxidation rate was always weak. During dro ught period (late summer) water stress was observed to restrict CH4 oxidati on rates. The findings led to the question whether methane production with soil depth might modify the CH4 fluxes measured at the surface. Therefore, two new methods were applied: (1) soil air sampling with silicone probes; a nd (2) anaerobic incubations of soil cores to test for the methane producti on potential of the grassland soil. The probe measurements revealed that th e CH4 sink capacity of a specific site was related to the vertical length o f its CH4 oxidizing column, i.e. the depth of the CH4 producing horizon. An aerobically incubated soil cores produced large amounts of CH4 comparable w ith tropical rice paddy soil. Under field conditions, heavy autumnal rain i n 1998 led to a dramatic increase of soil CH4 concentrations upto 51 mul 1( -1) at a depth of 5 cm. Nevertheless, no CH4 was released when soil surface CH4 fluxes were measured simultaneously. The results thus demonstrate the high CH4 oxidation potential of the thin aerobic topsoil horizon in a non-a quatic ecosystem. (C) 2001 Elsevier Science Ltd. All rights reserved.