Effects of N sources and methane concentrations on methane uptake potential of a typical coniferous forest and its adjacent orchard soil

After removal of the above-ground plant debris three different soil layers were taken from a typical coniferous forest and its adjacent orchard in Numata City, Japan. The potentials of soil CH₄ uptake at two initial CH₄ concentrations were studied under aerobic conditions in the laboratory, along with inhibition of soil CH₄ oxidation by urea-N or KNO₃-N addition. Due to long-term N inputs, the CH₄ uptake of the upper mineral layer of the orchard soil was 25.4% and 87.7% lower than that of the surface forest soil at 2.4 and 12.6 l l^–1 CH₄, respectively. Methane uptake of the forest soil decreased with increasing soil depths at two CH₄ levels. However, maximal CH₄-consuming activity occurred in the 9- to 23-cm depth of the orchard soil at 12.6 l l^–1 methane. Nitrogen additions in the form of KNO₃ or urea at the rate of 200 g N g^–1 soil substantially reduced soil CH₄ uptake in the upper and sub-surface mineral layers at both sites, except that the addition of KNO₃-N had no apparent inhibitory effect on the CH₄ uptake in the 9- to 23-cm depth of the orchard soil. A strong inhibitory effect of NO₃^– addition on the CH₄ uptake, in contrast to NH₄⁺, occurred in the surface forest soil. The use of KNO₃-N, as compared to urea or urea plus a nitrification inhibitor (dicyandiamide), resulted in a lower potential to cause inhibition of CH₄ oxidation in the 0- to 23-cm depth of the orchard soil.