The impact of nitrogen, CO2 and pH on ethylene and methane metabolism was investigated in two kinds of forest soils, an alpine spruce forest soil (pH 2.9) and a mixed deciduous hardwood forest soil (pH 5.2). The two sieved (< 2mm) and deep-frozen soils were thawed and adjusted to pF 2.5 or 1.8. Nitrogen was added as (NH4)2SO4 or KNO3 at concentrations of 0, 10, 100, and 500 Myg N g-1 dw. The headspace of soil samples was enriched with CO2 to give concentrations of 0.03, 1, 5, 10, and 20%. For changing the pH, a buffer solution or lime was added to the soil and the investigated pH-values were 3, 4, 5, 6, and 7. To differentiate between ethylene production and decomposition, one part of the samples was treated with low levels of acetylene to inhibit ethylene decomposition. To another part of samples, ethylene was added to give a final concentration of 20 Myl l hoch-1. The changes in concentration of the investigated gases was measured by gaschromatography with a flame ionization detector. In most cases, the addition of nitrogen and increasing CO2 concentrationsled to increasing net production rates and decreasing uptake rates of ethylene and methane. Production rates in the presence of acetylene did not show a clear trend with increasing nitrogen or CO2 concentrations. We therefore concluded that the microbial uptake of ethylene or methane is more sensitive to those factors than their production. The inhibition of methane uptake by ammonium can be due to the inhibition of the methane monooxygenase by intermediates of the ammonium oxidation. Only in the spruce forest soil the uptake of ethylene and methane was enhanced by moderate additions of ammonium. Ammonium seems to have a positive effect on microorganisms by lowering the high C:N ratio of this soil. In the pH-experiment, the highest ethylene and methane production in the alpine spruce forest soil was measured at pH 6. The highest ethylene uptake in the deciduous forest soil was also at pH 6, methane metabolism in the deciduous forest soil was negligible. It can be concluded that mineral nitrogen additions in the form of fertilizers or wet deposition and increasing CO2 concentrations seem to lower the potential of forest soils to oxidize ethylene and methane. Liming tends to increase ethylene and methane production in acid coniferous forest soils.
