Short-term effects of nitrogen on methane oxidation in soils

The short-term effects of N addition on CH4 oxidation were studied in two soils. Both sites are unfertilized,one has been under long-term arable rotation, the other is a grassland that has been cut for hay for the past 125 years. The sites showed clear differences in their capacity to oxidise CH4, the arable soil oxidised CH4 at a rate of 0.013 mu g CH4 kg(-1) h(-1) and the grassland soil approximately an order of magnitude quicker. In both sites the addition of (NH4)(2)SO4 caused an immediate reduction in the rate of atmospheric CH4 oxidation approximately in inverse proportion to the amount of NH4+ added. The addition of KNO3 caused no change in the rate of CH4 oxidation in the arable soil, but in the grassland soil after 9 days the rate of CH4 oxidation had decreased from 0.22 mu g CH4 kg(-1) h(-1) to 0.13 mu g CH4 kg(-1) h(-1) in soil treated with the equivalent of 192 kg N ha(-1). A N-15 isotopic dilution technique was used to investigate the role of nitrifiers in regulating CH4 oxidation. The arable soil showed a low rate of gross N mineralisation (0.67 mg N kg(-1) day(-1)). but a relatively high proportion of the mineralised N was nitrified. The grassland soil had a high rate of gross N mineralisation (18.28 mg N kg(-1) day(-1)), but negligible nitrification activity. It is hypothesised that since there was virtually no nitrification in the grassland soil then CH4 oxidation at this site must be methanotroph mediated.