Short-term effects of nitrogen on methane oxidation in soils

The short-term effects of N addition on CH4 oxidation were studied in two s oils. Both sites are unfertilized,one has been under long-term arable rotat ion, the other is a grassland that has been cut for hay for the past 125 ye ars. The sites showed clear differences in their capacity to oxidise CH4, t he arable soil oxidised CH4 at a rate of 0.013 mu g CH4 kg(-1) h(-1) and th e 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 a tmospheric CH4 oxidation approximately in inverse proportion to the amount of NH4+ added. The addition of KNO3 caused no change in the rate of CH4 oxi dation 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 nitri fiers in regulating CH4 oxidation. The arable soil showed a low rate of gro ss N mineralisation (0.67 mg N kg(-1) day(-1)). but a relatively high propo rtion of the mineralised N was nitrified. The grassland soil had a high rat e of gross N mineralisation (18.28 mg N kg(-1) day(-1)), but negligible nit rification activity. It is hypothesised that since there was virtually no n itrification in the grassland soil then CH4 oxidation at this site must be methanotroph mediated.