LONG-TERM EFFECTS OF NITROGEN-FERTILIZATION ON METHANE OXIDATION IN SOIL OF THE BROADBALK WHEAT EXPERIMENT

Methane uptake by a temperate arable soil was investigated in incubation experiments with intact soil cores. The measurements were carried out with a soil moisture content of 16-17% (w/w) and at 25-degrees-C. The decrease of the CH4 concentrations in an amended atmosphere (10 mul CH4 l-1) was measured during a 212 h period. There was no decrease of CH4 if the soil was autoclaved showing that the disappearance of methane was entirely mediated by microbial activity. The long-term application (140 yr) of mineral nitrogen fertilizer caused significant differences in the ability of the soil to oxidize CH4: the larger the amount of fertilizer applied the lower the rate of CH4 oxidation. No significant short-term effect of mineral-N fertilization could be observed whether applied as (NH4)2SO4 or KNO3. An organic manure treatment, which has received nearly 240 kg N ha-1 each year as farmyard manure, showed almost the same ability to oxidize CH4 as an unfertilized plot and had a significantly higher CH, oxidation rate after an application of 144 kg N ha-1 as nitrate fertilizer. For the mineral-N treatments the inhibition of the CH4 oxidation increased with increasing N turnover rate but was independent of the mineral nitrogen content of the soil at the time of measurement Therefore, the continued application of mineral-N fertilizer for an extended period (at least 7 yr) caused a depletion of the bacterial methane sink in soil and may have contributed to the continuous increase in atmospheric CH4 over the past decades.