METHANE OXIDATION IN SOIL AS AFFECTED BY LAND-USE, SOIL-PH AND N-FERTILIZATION

Net uptake of CH4 was measured in intact soil cores (6.4 cm dia, 12 cm deep) collected from an arable wheat field, from three sites left unc ultivated for more than 110 years following arable cropping and from a permanent grassland with different mineral N treatments subdivided in to four pH levels. Soil cores were incubated in sealed 1 litre jars at 250 degrees C for 48 h with a CH4-amended atmosphere of 10 mu l 1(-1) at the start of incubation. The decrease in CH4 concentration followe d first-order-kinetics and by log-transformation individual uptake rat es could be calculated for each treatment. Soil from a calcareous site (PH 7.4) under deciduous woodland (Broadbalk Wilderness wooded sectio n) oxidized CH4 6 times faster than the arable plot (pH 7.8) with the highest activity in the adjacent Broadbalk Wheat Experiment (with upta ke rates of - 80 and - 13 nl CH4 1(-1) h(-1), respectively). The CH4 u ptake rate was only 20% of that in the woodland in an adjacent area th at had been uncultivated for the same period but kept as rough grassla nd by the annual removal of trees and shrubs and, since 1960, grazed d uring the summer by sheep. It is suggested that the continuous input o f urea through animal excreta was mainly responsible for this differen ce. Another undisturbed woodland area with an acidic soil reaction (pH 4.1) did not oxidize any CH4. On a permanent grassland site (Park Gra ss Continuous Hay Experiment), the plot without N fertilization showed a distinct pH effect: CH4 consumption decreased from -67 to -35 nl CH 4 1(-1) h(-1) with decreasing pH in the range 6.3-5.6 and declined to zero between pH 5.6 and 5.1. Mineral N applied annually as (NH4)(2)SO4 , at either 96 or 144 kg N ha(-1) for 130 years, completely inhibited CH4 oxidation, even where lime was applied to maintain a soil pH of ab out 6. By contrast, the long-term application of N as NaNO3 (96 kg N h a(-1) a(-1)) caused no decline in CH4 oxidation compared to unfertiliz ed grassland at the same pH and, in some cases, caused a small increas e. Withholding NH4-N for 3 years caused no significant recovery of CH4 -oxidizing activity; withholding NO3-N caused a slight decline. Thus, land use (arable, cut grassland, grazed grassland or woodland), soil p H, N fertilizer inputs and form of N (NH4 or NO3) all have marked and interacting effects on the extent to which aerobic soil acts as a sink for CH4. The mechanisms through which the factors operate are not kno wn but some possibilities are discussed. The results have important im plications for the planning of land use and agricultural practices tha t will maximize the extent to which aerobic soils can act as a sink fo r CH4.